Advances in Organometallic Chemistry

VOLUME SIXTY THREE

ADVANCES IN ORGANOMETALLIC CHEMISTRY

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VOLUME SIXTY THREE

ADVANCES IN ORGANOMETALLIC CHEMISTRY Edited by

PEDRO J. PÉREZ Laboratorio de Catálisis Homogénea CIQSO-Centro de Investigación en Química Sostenible and Departamento de Química y Ciencia de los Materiales Universidad de Huelva-Huelva Spain Founding Editors

F. GORDON A. STONE ROBERT WEST

AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier

Academic Press is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA 525 B Street, Suite 1800, San Diego, CA 92101-4495, USA 125 London Wall, London, EC2Y 5AS, UK The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK First edition 2015 © 2015 Elsevier Inc. All rights reserved No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-802269-6 ISSN: 0065-3055 For information on all Academic Press publications visit our website at store.elsevier.com

CONTENTS Contributors

vii

1. Coordination and Activation of EdH Bonds (E 5 B, Al, Ga) at Transition Metal Centers

1

Ian M. Riddlestone, Joseph A.B. Abdalla, and Simon Aldridge 1. Introduction 2. Borane (BdH) σ-Complexes 3. Alane (AldH) σ-Complexes 4. Gallane (GadH) σ-Complexes 5. Conclusions References

2. Singular Metal Activation of Diboron Compounds

2 4 19 26 31 32

39

Stephen A. Westcott and Elena Fernández 1. Introduction: The Activation of Cl2B–BCl2 and Beyond 2. General Trends on Activation of Symmetrical Diboron Compounds 3. Precise Activation of Unsymmetrical Dialkoxy-diamino-diboron Compounds 4. Summary and Outlook Acknowledgments References

3. Catalytic Oxidation of Alcohols: Recent Advances

39 42 77 79 80 80

91

Maximilian N. Kopylovich, Ana P.C. Ribeiro, Elisabete C.B.A. Alegria, Nuno M.R. Martins, Luísa M.D.R. S. Martins, and Armando J.L. Pombeiro 1. Introduction 2. Aerobic and Peroxidative Oxidations 3. Acceptorless Dehydrogenative Oxidations 4. Oxidative Desymmetrizations 5. Cascade and Sequential Reactions 6. Conversion of Renewable Sources and Hydrogen Production 7. Irradiation-Promoted Oxidations 8. Catalysts Recyclization Acknowledgments References

93 94 112 120 125 130 134 144 157 157

v

vi

Contents

4. Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of

175

Michael Limbach 1. Introduction 2. Shedding Light into the Dark: Lactone Formation 3. Lactone Cleavage and Final Ligand Exchange 4. Catalytic Reactions 5. Catalysts Gone Astray 6. Conclusions and Outlook Acknowledgments References

5. Poly-NHC Complexes of Transition Metals: Recent Applications and New Trends

175 178 182 189 194 195 195 196

203

Andrea Biffis, Marco Baron, and Cristina Tubaro 1. Introduction 2. Novel Poly-NHC Ligands 3. Novel Poly-NHC Metal Complexes 4. Applications 5. Conclusions and Outlook References Index

203 204 222 232 272 272 289

CONTRIBUTORS Joseph A.B. Abdalla Inorganic Chemistry Laboratory, Oxford, United Kingdom Simon Aldridge Inorganic Chemistry Laboratory, Oxford, United Kingdom Elisabete C.B.A. Alegria Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, and Chemical Engineering Department, ISEL, R. Conselheiro Emı´dio Navarro, Lisboa, Portugal Marco Baron Dipartimento di Scienze Chimiche, Universita` di Padova, Padova, Italy Andrea Biffis Dipartimento di Scienze Chimiche, Universita` di Padova, Padova, Italy Elena Fernańdez Departament Quı´mica Fı´sica i Inorga`nica, University Rovira i Virgili, Tarragona, Spain Maximilian N. Kopylovich Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal Michael Limbach CaRLa (Catalysis Research Laboratory), Heidelberg, and BASF SE, Synthesis & Homogeneous Catalysis, Ludwigshafen, Germany Luı´sa M.D.R.S. Martins Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, and Chemical Engineering Department, ISEL, R. Conselheiro Emı´dio Navarro, Lisboa, Portugal Nuno M.R. Martins Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal Armando J.L. Pombeiro Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal Ana P.C. Ribeiro Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal Ian M. Riddlestone Inorganic Chemistry Laboratory, Oxford, United Kingdom

vii

viii

Contributors

Cristina Tubaro Dipartimento di Scienze Chimiche, Universita` di Padova, Padova, Italy Stephen A. Westcott Department of Chemistry and Biochemistry, Mount Allison University, Sackville, New Brunswick, Canada

CHAPTER ONE

Coordination and Activation of EdH Bonds (E 5 B, Al, Ga) at Transition Metal Centers Ian M. Riddlestone, Joseph A.B. Abdalla, Simon Aldridge* Inorganic Chemistry Laboratory, Oxford, United Kingdom *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 2. Borane (BdH) σ-Complexes 2.1 σ-Complexes Featuring Tetra-Coordinate Boranes 2.2 σ-Complexes Featuring Tri-Coordinate Boranes 3. Alane (AldH) σ-Complexes 4. Gallane (GadH) σ-Complexes 5. Conclusions References

2 4 5 9 19 26 31 32

ABBREVIATIONS ArCl C6H3Cl2-2,6 Arf C6H3(CF3)2-3,5 cat catecholato, O2C6H4-1,2 Catf O2C6H3-1,2-F-3 cdt 1,5,9-cyclododecatriene cht 1,3,5-cycloheptatriene cod 1,5-cyclooctadiene Cp cyclopentadienyl Cp0 methylcyclopentadienyl Cy cyclohexyl dcype 1,2-bis(dicyclohexylphosphino)ethane Dipp 2,6-diisopropylphenyl hppH 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine i Bu isobutyl ¼ CH2CH(CH3)2 i Pr isopropyl ¼ CH(CH3)2 Mes mesityl ¼ 2,4,6-trimethylphenyl NHC N-heterocyclic carbene p-cym para-cymene ¼ 4-iPrC6H4Me

Advances in Organometallic Chemistry, Volume 63 ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.02.003

#

2015 Elsevier Inc. All rights reserved.

1

2

Ian M. Riddlestone et al.

pin pinacolato, OCMe2CMe2O quin quinuclidine ¼ N(CH2CH2)3CH tbe tertbutylethene (3,3-dimethylbutene), tBuCHCH2 t Bu tertbutyl ¼ C(CH3)3 thf tetrahydrofuran

Mes

N

N

IMes

Mes

Mes

N

N

5-Mes

Mes

Mes

Mes Dipp N

N

6-Mes

Dipp N

N

6-Dipp

1. INTRODUCTION Over the last 20 years, significant research effort has been expended on probing the mode(s) of interaction of group 13 hydrides with transition metal centers. An initial driver for such work—notwithstanding issues relating to chemical reversibility—was potential applications of boron/nitrogen based materials containing a high weight% of hydrogen, as hydrogen storage materials; more recent work has focussed on such systems as building blocks in the construction of inorganic polymers.1 Applications of alanes in hydrogen storage media have also received some attention,2 but in general, the hydrides of the heavier group 13 elements have not been in the spotlight, primarily due to their lower percentages by weight of hydrogen and their greater thermal instability.3 Control of dihydrogen release or of polymer formation, for example, means that a significant part of this effort has been directed toward metalmediated processes; the interaction of EdH bonds with transition metal centers and their potential modes of activation are therefore of key importance.4,5 As a consequence, the factors underpinning the fundamental bonding interactions (σ-donation, π-acceptance) of BdH, and to a lesser extent AldH and GadH bonds, with transition metals have been explored (Fig. 1). The resulting data offer comparison with much more widely established families of σ-complex, featuring coordinated HdH, CdH, or SidH bonds.6–8 Even leaving aside complexes containing anionic borohydride-type ligands,9 the coordination of BdH bonds at transition metal centers is well precedented, and many BdH σ-complexes featuring three- and fourcoordinate boranes have been reported.5 Examples abound involving either

3

Coordination and Activation of EdH Bonds

ERn

LxM

H

Donation from filled EJH σ-bonding orbital

LxM

ERn H

LxM

ERn H

Back-bonding into EJH σ∗ antibonding orbital ERn Lx M

H

More ⬙side-on⬙ alignment allows for greater π back-bonding

Figure 1 Key σ-donor/π-acceptor interactions in EdH σ-complexes.

κ1 or κ2 binding—that is featuring interactions with the metal center through one or two BdH bonds, respectively. Within this sphere, there has been a wealth of investigation into the interaction of the BdH bonds in amine- and aminoboranes with transition metals, with such σ-complexes having been postulated as intermediates in BdN dehydrocoupling reactions.1,4 In-depth mechanistic details of such processes are not the focus of this chapter and will not be examined in depth beyond consideration of the modes of ligation of relevant species. Activation of one BdH bond, generally involving oxidative addition at a metal center, is very well established, in part reflecting the implication of such a step in a number of processes leading to the borylation of both saturated and unsaturated hydrocarbons.10,11 However, activation of two BdH bonds, leading ultimately to dehydrogenation and the formation of a subvalent borylene complex, is much less common.12,13 The related chemistry of the heaver group 13 elements aluminium and gallium is much less developed than that of boron.14,15 The interaction of AldH bonds with transition metal centers is limited to the formation of a relatively small number of σ-complexes featuring bridging MdHdAl motifs, with no reported propensity toward oxidative addition of AldH bonds or dehydrogenation at a transition metal center.16 σ-Alane complexes themselves are rare; the first example of an unsupported AldH σ-complex, (cdt)Ni[κ1-HAlMe2quin], was reported by Po¨rschke and coworkers in 1990.14 There is an even greater paucity of examples of GadH coordination at transition metals, in part due to the much greater thermal fragility of GadH bonds.3 Ueno reported the first structurally characterized examples of GadH σ-complexes, involving coordination of the amine-stabilized gallane quinGaH3 at [Cp0 Mn(CO)2] and [M(CO)5] fragments (M ¼ Cr, Mo, W).15 However, with the +1 oxidation state more readily accessible

4


for gallium (and the GadH bond being inherently weaker than its lighter congeners),3,17 the potential for novel and interesting chemistry initiated by the dehydrogenation of Ga(III) hydride complexes at transition metal centers is not only foreseeable but beginning to be realized in practice.15,18 This chapter will primarily focus on reviewing the coordination and activation of AldH and GadH bonds at transition metal centers, making reference to key examples of related BdH σ-complexes in order to put fundamental issues of electronic structure and bonding into appropriate context. In the interests of space, and with a view to comparing the intrinsic electronic/geometric properties of the coordinated σ-bond, “tethered” systems in which the coordinated EdH bond forms part of an existing metal-bound ligand are not as a rule included.

2. BORANE (BdH) σ-COMPLEXES The long-standing desire to use an organometallic system to selectively functionalize CdH bonds,19 and the inherent challenges related to this process, have seen the development of a number of systems to model the CdH bond activation process (involving, for example, silanes and boranes as alkane mimics).5,8 The isoelectronic relationship between CH4 and [BH4] led to much interest in the formation of transition metal, lanthanide, and actinide borohydride complexes featuring κ1, κ2, and κ3 coordination modes (Fig. 2).9,20 Although isoelectronic with CH4, the inherent negative charge possessed by the [BH4] anion provides a significant electrostatic contribution to bonding.9 Consequently, the coordination chemistry of neutral boranes is perceived as a better potential model for alkane coordination, and this area has been the subject of significant interest in the recent chemical literature. Two distinct classes of borane coordination complex have emerged, involving three- and four-coordinate boranes, respectively. The formation of Lewis-base adducts of BH3 is well established, and being charge neutral, LBH3 systems perhaps more closely resemble alkanes in their potential ligating properties, than does the borohydride H LnM

H

B H

H

H H

LnM

B

LnM H

H H

H

B

H

H

Figure 2 Coordination modes of BH4 at a single transition metal center.

5


anion. Of key importance in rationalizing the bonding in complexes containing LBH3 ligands is the coordinative saturation of the boron center, and crucially the population of the vacant boron pz-orbital by the Lewis base (L). Consequently, the LBH3 moiety has a limited role as a π-acceptor, as the BdH σ*-orbital is typically too high in energy to interact significantly with metal-based d-orbitals.21 By contrast, three-coordinate boranes, archetypally represented by HBcat, possess a formally vacant pz-orbital at the boron center, which is potentially of suitable energy to interact with transition metal d-orbitals allowing for π back-bonding.22,23

2.1 σ-Complexes Featuring Tetra-Coordinate Boranes The isoelectronic relationship between neutral LBH3 adducts and alkanes means that they offer model systems for the study of CdH activation parameters. A number of σ-complexes of such adducts have been reported featuring a variety of Lewis bases, L. κ1-Complexes bound through a single BdH bond can be formed via the in situ generation of a 16-electron transition metal fragment, for example, following photolytic ejection of a CO ligand. Accordingly, the groups of Shimoi and, more recently, Braunschweig have synthesized a number of Group 6 metal σ-borane complexes of the type M(CO)5(κ1-H3BL) via this route (Scheme 1).21,24 Structural characterization of these systems by single-crystal X-ray diffraction shows that in each case, the borane exhibits a significantly more “end on” binding motif than that seen in σ-silane and dihydrogen systems.6,8 This binding motif is consistent with the BdH σ-bond acting primarily as a σ-donor with essentially negligible π-acceptor capabilities. The coordinated BdH bond thus shows very little activation, and the complex therefore represents minimal progression along the oxidative addition pathway. Rapid L

H H

CO

hu L˙BH3

CO

−CO

OC

CO OC

H

M OC CO

B

CO M CO

OC CO

M = Cr; L = PMe3 1, PPh3 2, NMe3 3, IMe 4 M = W; L = PMe3 5, PPh3 6, NMe3 7, IMe 8

Scheme 1 Photolytic generation of four-coordinate borane σ-complexes of chromium and tungsten.21,24

6


exchange of bridging and terminal BdH bonds typically results in a statistically averaged 1H NMR chemical shift with the associated fluxional process unable to be frozen out for 1–8, even at 80 °C.21 A convenient bonding comparison between LBH3 and silane σ-complexes can also be drawn from systems featuring the [CpMn(CO)2] fragment, again reported by Shimoi and by Braunschweig (Scheme 2).24,25 The structures of borane complexes 9–11 (determined by X-ray diffraction) are also consistent with the coordinated BdH bond acting almost exclusively as a σ-donor. As such, the measured MdHdB angles [e.g., 142(3)° for 9] are wide, and the Mn B separations are significantly longer than those found in related silane complexes [e.g., 2.682(3) A˚ ˚ for Cp0 Mn(CO)2(H2SiPh2)].25,26 In addition, the orifor 9 cf. 2.391(12) A entation of the coordinated BdH bond deviates significantly from the close-to-horizontal orientation observed in silane σ-complexes in which back-bonding from the metal-based HOMO is significant (Fig. 3).25 This close-to-vertical orientation of the BdH bonds in 9–11 attests to negligible π-contributions to binding in each case, since the highest-lying orbital of the 16-electron [CpMn(CO)2] fragment of suitable symmetry for π back-bonding lies essentially coplanar with the Cp ring.27 As a consequence, the infrared-measured stretching frequencies for the CO ligands are

hu L˙BH3

Mn OC

H Mn

−CO

CO

H

OC

OC

H

B

OC L L = NMe3 9, PMe3 10, IMe 11

Scheme 2 Photolytic generation of complexes of the type CpMn(CO)2{κ1-H3BL}.24,25

Mn OC

H

ERn H CO OC

OC

CO

Mn

Mn ERn CO

H

ERn

Figure 3 Schematic diagram showing limiting torsional alignments of the EdH bond in half-sandwich sigma complexes: (left) vertical orientation, (center) horizontal orientation, and (right) overlap of HOMO of a 16-electron [CpM(CO)2] fragment with EdH σ*-orbital in horizontal alignment.

7


significantly lower than those found, for example, in the corresponding complex of the much better π-acceptor H2 [e.g., 1927/1820, 1918/1839, and 1986/1922 cm1 for 9, 10, and CpMn(CO)2(H2), respectively].15b,25 The dominant role of σ-donation implies that complex stability is strongly influenced by the energy match between the BdH σ-bonding orbital and the LUMO of the transition metal fragment. Thus, the elevation of the former (e.g., by the use of more strongly σ-donating Lewis bases, L) and/or the stabilization of the latter (e.g., by the use of a cationic metal center) would be expected to lead to higher binding affinities for the LBH3 ligand. Thus, Shimoi and coworkers have also reported on use of the half-sandwich ruthenium chlorides CpRu(PMe3)2Cl and Cp*Ru(PMe3)2Cl as sources of cationic transition metal centers via halide abstraction, and their ability to coordinate boranes of the type Me3EBH3 (E ¼ P or N) (Scheme 3). These complexes feature more elongated BdH bonds than the corresponding neutral manganese systems, an observation which is thought to reflect the polarization of the BdH bond brought about by coordination to the more electrophilic positively charged ruthenium center.28 The minor extent of back-bonding is confirmed by relatively long Ru B contacts, with the closer approach of the borane ligand in 13 [d(RudB) ¼ 2.586(5) A˚ vs. ˚ for 12] presumably reflecting the enhanced donor capabilities 2.648(3) A of the phosphine (vs. amine) donor at L.28 Stronger binding of the borane ligand might also be envisaged by the adoption of a κ2 mode of binding, and therefore by employing a metal fragment with an electron count of 14 (or lower). A number of such systems have been reported, primarily featuring group 9 metal fragments of the types [L2M(H)2]+ and [L2M]+ (M ¼ Rh, Ir; e.g., 16, 17, Scheme 4).13a,29,30 Much of this work has focussed on amine borane complexes due to their potential relevance in the catalytic dehydrocoupling of ammonia borane and related amine-containing systems.1 While model complexes featuring tertiary amine boranes (chiefly H3BNMe3) have been used to probe issues of R5

R5

H

Na[BAr f4] Ru Me3P Me3P

Cl

−NaCl

Ru Me3P Me3P

H

[BAr f4]

B H

EMe3

R = H; E = N 12, P 13 R = Me; E = N 14, P 15

Scheme 3 Formation of cationic κ1-H3BL complexes by halide abstraction.28

8


F Rh iPr

iPr

3P

H

Rh

i

−C6H5F

PiPr3

3P

NMe

H3B.NMe3

Pr3P

3

B

H

H

16 PiPr3

PiPr3 H

H3B.NMe3

L

H

Rh H

PiPr3

L

Rh

H

1,2-C6H4F2

NMe3 H

B

H

H

PiPr3 17

L = agostic/solvent interaction

Scheme 4 Syntheses of group 9 metal complexes featuring κ2 amine borane ligands.29a,b

IMes

H3B.NtBuH2 Na[BArf4]

H Rh

Cl

H

C6H5F

IMes

NtBuH2

IMes H

H

Rh

H

B

H

H

Rh

H H

H PCy3

H

20

H

Me

H PCy3 H H2 Ir H H2 PCy3

B H

H3B.NMeH2 Me

−H2 H

Ir

H

NMeH2

H

PCy3

N H

N

19

NMeH2 H

Ir

Bu

B

IMes

18 PCy3

t

H

H

−H2

H

IMes

6h

IMes 48 h

H

H

H

H

−H2

H H

PCy3

B

B

B

B

H

NMeH2

N

H

H

21

Scheme 5 Dehydrogenation of secondary amine boranes at group 9 metal centers leading to the formation of a coordinated aminoborane (upper)13a or a linear borazane chain (lower).29e

electronic and geometric structure, systems featuring NdH bonds are additionally amenable to dehydrogenation—a process which has been found to be facile at a range of transition metal centers (Scheme 5). In particular, a number of borane complexes thought to be likely intermediates in metal-catalyzed dehydrogenation/dehydrocoupling have been isolated and characterized. Weller and coworkers, for example, have investigated the dehydropolymerization of methylamine borane, H3BNMeH2, showing that the growth of a linear borazane chain can occur by a


9

dehydrogenation reaction occurring between the H3BNMeH2 monomer and the κ2-aminoborane complex [(Cy3P)2Ir(H)2(κ2-H3BNMeH2)]+ (20). This dehydrocoupling process yields [(Cy3P)2Ir(H)2(κ2H3BNMeHBH2NMe2H)]+ (21), which can be synthesized independently from H3BNMeHBH2NMe2H and [(Cy3P)2Ir(H)2(H2)2]+ (Scheme 5).29e A number of systems featuring H3BNR2X fragments bound in this fashion have been characterized by X-ray crystallography: predictably, the κ2 mode of coordination brings the metal and boron centers into closer proximity than is observed for κ1 complexes. Thus, for example, even allowing for the differing radii of Rh(III) and Ru(II), the M B distance measured ˚ ] is markedly shorter for [(IMes)2Rh(H)2(κ2-H3BNMe3)]+ [23, 2.327(3) A 1 than that found in [CpRu(PMe3)2(κ -H3BNMe3)]+ [12, 2.648(3) ˚ ].13a,28 Given the relatively high-lying nature of the BdH σ*-orbital in A H3BNMe3, such differences presumably reflect the geometric constraints of the four-membered M(μ-H)2B bridge, rather than any underlying electronic factors (similar effects having previously been reported for κ1 and κ2 borohydride complexes).9 Definitive determination of hydrogen atom positions in amine borane complexes by neutron diffraction techniques has been less readily forthcoming. Very recently however, the neutron structure of the iridium cation [Ir(5-Mes)2(H)2(κ2-H3BNHMe2)]+ (24, as the [BArf4] ˚ ] and Ir BdN salt) has been reported.30j The Ir B distance [2.21(4) A angle [125(2)°] determined from the heavy-atom skeleton are in line with previous literature reports on related κ2 amine borane coordination13; in addition, the structure reveals an essentially planar Ir(μ-H)2B ring and IrdH and BdH distances of 1.75(7)/1.87(6) and 1.21(6)/1.29(8) A˚, respectively. The latter can be contextualized by the corresponding terminal BdH dis˚. tance of 1.13(6) A

2.2 σ-Complexes Featuring Tri-Coordinate Boranes Mechanistic investigations by Hartwig and coworkers into Cp2TiMe2catalyzed hydroboration led to the isolation of the first σ-complex of a tri-coordinate borane.22,23 Thus, the reaction of dimethyltitanocene with three equivalents of HBcat yields the thermally fragile bis-borane complex Cp2Ti(κ1-HBcat)2 (25) together with methane and MeBCat (Scheme 6). Characterization by 11B NMR spectroscopy is consistent with an appreciable Ti B interaction in these systems (e.g., δB ¼ 45 ppm for 25 cf. 28 ppm for free HBcat), while the IR-measured BdH stretching frequencies (e.g., 1611 cm1 for 25, cf. 2660 cm1 for free HBcat) suggest

10


H Me

3 HBcat

Ti

Bcat Bcat

Ti Me

−MeBcat −MeH

Cp2Ti(PMe3)2

PMe3 Ti

H 25

Bcat H

26

Scheme 6 Synthesis of Cp2Ti(κ -HBcat)2 (25) from Cp2TiMe2 and conversion Cp2Ti(PMe3)(κ1-HBcat) (26) via ligand redistribution with Cp2Ti(PMe3)2.23,31 1

significant activation of the BdH bond. The solid-state structure of 25 shows that the boron atom lies in a plane which also contains both catechol oxygen atoms and the titanium center, with the hydride (located in the difference map) sitting above this plane.23 The Ti B separation [2.335(5) A˚] is longer than those found in related metallocene boryl complexes,10a but consistent with a structurally significant interaction between the two atoms. As such, a model of bonding is proposed based on σ-donation from the BdH bonding orbital, and back-bonding into the formally vacant boron-centered pz-orbital. A related mono-borane complex Cp2Ti(PMe3)(κ1-HBCat) (26) could also be prepared via a ligand redistribution reaction between Cp2Ti (PMe3)2 and 25, and a solid-state structure determined for the closely related fluorocatechol derivative Cp2Ti(PMe3)(κ1-HBcatf) (27).31 The BdH bond in 27 appears to be further along the oxidative addition pathway in comparison to the bis-borane 25, as reflected in the relative BdH and TidH bond ˚ , BdH 1.35(5) A ˚ ; 25: TidH 1.74(4) A ˚ , BdH lengths [27: TidH 1.61(5) A 1.25(3) A˚]. The greater extent of activation can be understood in terms of the greater electron-donating capabilities of the phosphine coligand, as well as slightly enhanced electrophilicity of the borane due to fluorine incorporation. Hartwig and coworkers have also investigated three-coordinate σ-borane complexes which provide close comparison with archetypal manganese silane σ-complexes.8,32,33 Thus, the synthesis of a range of σ-borane complexes featuring the [Cp0 Mn(CO)2] fragment has also led to an understanding of the effects of the borane substituents on the progression toward BdH oxidative addition. Of the complexes reported, Cp0 Mn(CO)2(κ1-HBcat) (28), Cp0 Mn(CO)2(κ1-HBpin) (29), and Cp0 Mn(CO)2(κ1-HBCy2) (30),32 28 and 29 can be synthesized via the photolytic reaction of Cp0 Mn(CO)3 with the respective borane. Substantially increased yields are obtained by using the salt metathesis reaction between K[Cp0 Mn(CO)2H] and the

11


corresponding chloroborane (Scheme 7). This route proves to be particularly apposite in the formation of 30, since the photolytic reaction of Cp0 Mn(CO)3 with HBCy2 does not proceed, presumably due to the dimeric nature of the parent borane. The solid-state structures of 28–30 show that the center of the BdH bond remains essentially equidistant from the manganese center in all three complexes, and that the oxidative addition process is reflected in a “pivoting” around this point, the extent of which determines the MndB and MndH distances. Of the three boranes, HBcat gives rise to the most “side-on” binding, bearing closest resemblance to that of dihydrogen, whereas HBCy2 is more “end on” and more closely resembles the binding observed in Lewis base adducts of tetra-coordinate boranes LBH3 (Fig. 4). The HBpin complex lies between those of HBcat and HBCy2. The structural data show that the π-acceptor capability of the boranes follows the order HBcat > HBpin > HBCy2, and that the reverse trend exists for their σ-donating capabilities.32 As seen with other families of EdH σ-complex, the incorporation of electron-withdrawing substituents (e.g., alkoxy or aryloxy groups) makes the LUMO of the borane ligand a better energy match to the HOMO of the transition metal fragment, thereby enhancing π back-bonding.8 Consequently, a shorter Mn B interaction is observed

K R2BX Mn H

OC

- KX

OC

H

Mn OC OC

R 2B

R2 = cat 28, pin 29, Cy2 30

Scheme 7 Formation of manganese σ-borane complexes by salt metathesis.32 Mn HBcat (28) H

HBpin (29) HBCy2 (30)

B

Figure 4 Overlaid positions of Mn, B, and H atoms from the crystal structures of 28–30. Adapted with permission from J. Am. Chem. Soc. 122 (2000) 9435–9443. Copyright (2000) American Chemical Society.

12


for the catechol (aryloxy) borane than pinacol (alkoxy) borane, and dicyclohexylborane has the longest Mn B separation. The differences in the σ-donor and π-acceptor capabilities of the three boranes are also reflected in the infrared stretching frequencies of the CO ancillary ligands in 28–30, with the frequencies for the HBCy2 complex (30) being lower than those for both the HBpin (29) and HBcat (28) complexes (30: 1967/1901 cm1; 29: 1981/1924 cm1; 28: 1995/1937 cm1). As such, these data reflect a sequentially more electron-rich metal fragment resulting from a combination of stronger σ-donation and weaker π-acceptor properties for HBCy2 over HBpin over HBcat.32 The coordination of three-coordinate boranes at 14-electron metal centers through two BdH bonds has also been reported, with the first example being characterized by Sabo-Etienne and coworkers.34 The reaction of Ru(H2)2(H)2(PCy3)2 with H2BMes results in the displacement of the two dihydrogen ligands and the formation of the κ2-complex (Cy3P)2(H)2Ru(κ2-H2BMes) (31) (Scheme 8). The Ru–B distance deter˚ ], due—at mined for 31 in the solid state is exceptionally short [1.938(4) A least in part—to the geminal bis(BdH) coordination mode. Nonetheless, such a geometry appears better preorganized for population of the boron pz-orbital through π back-donation than does that seen in κ1-complexes, a supposition backed up by computational studies. Stradiotto and coworkers have demonstrated a similar coordination mode for H2BMes using the 16-electron precursor Cp*Ru(Cl)(PiPr3) to generate chloroborohydride complex 32, with subsequent halide abstraction giving a cationic system [Cp*Ru(PiPr3)(κ2-H2BMes)]+ (33) featuring κ2coordination of H2BMes in accordance with an 18-electron count (Scheme 9).35 Here too, the Ru B distance is short [1.921(2) A˚] and the RuBdC angle almost linear [172.2(2)°], and DFT calculations also show the population of the formally vacant boron-centered p-orbital as a result of π back-donation from ruthenium.

PCy3 H Ru H PCy3

PCy3 H2

H2BMes

H

H2

- 2H2

H

H B

Ru

Mes

H PCy3 31

Scheme 8 Displacement of two dihydrogen ligands by the borane H2BMes to form κ2-complex 31.34

13


Me5

iPr P 3

Me5 H2BMes

Ru Cl

Me5 Ru

H

i

Pr3P

Li[BArf4]

H

B

Mes

- LiCl

iPr

Ru 3P

H H

B Mes

32

Cl

33

Scheme 9 Formation of [Cp*Ru(PiPr3){κ2-H2BMes}][BArf4] (33). Counterion omitted for clarity.35

With much interest being focussed on the catalytic dehydrogenation of ammonia/amine boranes, the chemistry of their dehydrogenated counterparts, aminoboranes (H2BNRR0 ), has received concerted attention. Interest in developing catalysts for the efficient release of hydrogen from ammonia/ amine boranes has meant that transition metal systems have featured prominently in the literature,1,29 and the interaction of aminoboranes with catalytically relevant transition metal centers therefore represents an area of research relevant to both hydrogen storage chemistry and inorganic polymer formation. The aminoborane monomer is formally isoelectronic with an alkene and therefore has the potential not only to bind through the BdH bonds but also in an analogous manner to an alkene through the BdN π-system.13a,30c,g,j,36,37 Sabo-Etienne and coworkers have used the [(Cy3P)2Ru(H)2] fragment to trap a number of aminoboranes (resulting from dehydrogenation of the parent amine boranes) as κ2-complexes. The structure of (Cy3P)2Ru(H)2(κ2-H2BNH2) (34) shows that the parent aminoborane H2BNH2 binds through both BdH bonds.36a Calculations to determine the difference in energy between this mode of coordination and the classical “side-on” binding showed the bis(σ-BH) motif to be at least 14 kcal mol1 more stable than binding through the BdN π-system. Electronic divergence from alkene donors, notably the more hydridic nature of the BdH bonds (cf. CdH) and the lower-lying nature of the B]N π system,38 presumably contributes to these findings. Independently, Aldridge and coworkers observed the bis(σ-BH) binding motif for a number of aminoborane complexes involving carbene-stabilized rhodium and iridium cations (35–38, Fig. 5).13,30c The groups of Weller and Sabo-Etienne subsequently prepared the related and isoelectronic aminoborane complexes (Cy3P)2Ru(H)2(κ2H2BNiPr2) (39) and [(Cy3P)2M(H)2(κ2-H2BNiPr2)]+ (M ¼ Rh 40, or Ir 41).36c These complexes provide a useful platform on which to study the

14


PCy3

IMes H

H

H

B

Ru H

H PCy3 34

B

M H

H

R

H

H

N

H

N R

IMes M = Rh; R = iPr 35, Cy 36 M = Ir; R = iPr 37, Cy 38

Figure 5 Initially reported κ2-aminoborane complexes. Counterions omitted for clarity.13,30c,36a

impact of the metal fragment on the electronic/geometric structures of aminoborane complexes. All three complexes feature the bis(σ-BH) coordination mode, with back-bonding into the BdN π*-orbital (the LUMO of the aminoborane ligand) also conceivable. The results of NBO analyses indicate that the extent of this π back-donation decreases in the order Ru(II) > Ir(III) > Rh(III). Greatest interaction is observed with the neutral ruthenium complex 39, with the lower energy orbitals associated with the cationic group 9 fragments resulting in less effective overlap with the BdN π*-orbital. The greater radial extension of the 5d-orbitals in Ir(III) partially compensates for this and results in greater π back-donation than in Rh(III). While the observation of the bis(σ-BH) coordination of aminoboranes is therefore well precedented,13a,30c,g,j,36,37 it should be noted that these systems universally feature a 14-electron transition metal center, which presumably distorts the potential energy landscape toward the adoption of the (4-electron donor) κ2-coordination mode. Investigation of the intrinsic ligating properties of aminoboranes as 2-electron donors (e.g., mono-BH σ-coordination vs. BN π-bonding, for example) has therefore been carried out using 16-electron fragments of the type [CpRu(PR3)2]+ (PR3 ¼ PPh3 or 1/2dcpe).39 The resulting complexes of H2BNCy2 uniformly feature the aminoborane acting as a 2-electron donor through the coordination of a single BdH σ-bond (Scheme 10). Two fluxional processes have been shown by VT-NMR to operate in both [CpRu(PPh3)2(κ1-H2BNCy2)][BArf4] (42) and [CpRu(dcpe)2(κ1H2BNCy2)][BArf4] (43) involving (i) rotation about the Rud(BH) centroid vector; and (ii) interconversion between bound and unbound BdH bonds, with the latter having an intrinsically higher activation barrier. It is also clear that π back-donation provides a chemically significant contribution to bonding. Thus, the more electron-rich complex 43 is notably more stable

15


Na[BArf4]/N2

H

then H2BNCy2

Ru

H

R3P R3P

Cl - NaCl

R3P R3P

Ru B

Cy

N Cy

PR3 = PPh3 42, 1/2 dcpe 43

Scheme 10 Formation of κ1-aminoborane complexes of the type [CpRu(PR3)2{κ1H2BNCy2}]+. Counterion omitted for clarity.39

i

i

Pr3P Me

H

H

B

Ru H

H i

Cl

N

Pr3P

Me

H B

Cl Os

Me H

H i

44

B

Ru H

i

Me H

N Me

Pr3P

Pr3P

N Me

H i

Pr3P

Pr3P

45

46

Figure 6 Differing degrees of BdH bond activation in group 8 complexes containing the H2BNMe2 fragment.36b,e

than 42, a finding consistent with the DFT calculated binding energies for the aminoborane ligand at the two metal fragments (43: 26 kcal mol1; 42: 15 kcal mol1).39 BdH bond activation in aminoborane complexes has been shown in an elegant study from Sabo-Etienne and coworkers to be intrinsically linked to the extent of back-bonding from the metal fragment.36e Thus, a series of group 8 metal complexes (44–46, Fig. 6) have been characterized which features sequentially: a symmetrically bound H2BNMe2 ligand at Ru(II) (44), an unsymmetrically bound ligand featuring appreciable activation of one BdH bond (45), and an Os(IV) center bound to distinct hydride and (α-agostic) primary boryl ligands (46). The latter description, while consistent with the expectation of greater oxidative activation at the 5d metal, contrasts with the borinium formulation originally proposed by Estereulas and coworkers for this system.36b Formal BdH oxidative addition (as proposed in 46) is a very widespread fundamental mode of reactivity, being confirmed as a pathway for the construction of direct MdB bonds as long ago as 1990,40 and being implicated in a number of metal-catalyzed processes for the borylation of organic substrates.10,11 As such, its complete coverage is beyond the scope of this chapter, and only selected examples of BdH activation are examined here.

16


H

Me

H Ir

B H

H

iPr

3P

IMes

IMes

H Rh

N Me

B IMes

IMes 47

Me

48

Rh

N Me

H

Cl

iPr P 3

H B(OR)2

(OR)2 = cat 49, pin 50

Figure 7 Varying degrees of BdH bond activation in group 9 borane complexes. Counterions omitted for clarity.41a,36f

Related to systems 44–46 are cationic group 9 complexes formed by dehydrogenation of dimethylamineborane, H3BNMe2H, and featuring markedly different degrees of activation of the boron-containing ligand (47 and 48; Fig. 7).36f Thus, the 18-electron iridium complex [(IMes)2Ir (H)2(κ2-H2BNMe2)]+ (47) can be viewed as a straightforward κ2 adduct of the aminoborane dehydrogenation product, while the 14-electron rhodium species [Rh(IMes)2(H)(B(H)NMe2)]+ (48) (which results from formal loss of a second equivalent of dihydrogen) is formulated as a rhodium aminoboryl hydride, analogous to 46. The bent RhdBdN frame˚ for work [132.6(10)°], short RhdB bond (1.960(9) A˚ cf. 2.204(4) A 2 i + 13 [(IMes)2Rh(H)2(κ -H2BN Pr2)] ), and RhdH/BdH locations determined by single-crystal neutron diffraction measurements for 48 provide strong evidence for effectively complete BdH bond cleavage.36f Thus, the metal-bound hydrogen atom is characterized (in the solid state) by sep˚ (to boron) and 1.40(2) A ˚ (to rhodium). These can be arations of 2.53(3) A compared with the analogous distances measured by Marder and coworkers (also by neutron diffraction) for Rh(PiPr3)2H(Cl)Bcat (49) and Rh(PiPr3)2H ˚ ; BdH: 1.53(1) and 1.57 (Cl)Bpin (50) [RhdH: 2.004(10) and 2.013(5) A ˚ (1) A, respectively]—compounds that are thought to possess “modest” residual BdH interactions.41a The degree of activation of the BdH bond in 48 can be further put in context when compared with the half-sandwich rhodium systems reported by Hartwig and coworkers. Thus, the short BdH contacts determined in Cp*Rh(Bpin)2(H)2 (51) and Cp*Rh(Bpin)3(H) (52) (12% shortening of the MndGa distance [from 2.643(2) for 84 ˚ for 85]. A similar tendency toward metal-mediated dehydroto 2.311(2) A genation is observed with group 6 metal carbonyls of the type M(CO)4(cod) (M ¼ Cr W). Thus, Cr(CO)4[κ2-H2Ga{(NDippCMe)2CH}] (86) is the only simple κ2 adduct of the Nacnac-stabilized gallane which can be trapped, albeit as a cocrystallite with the (dehydrogenated) gallylene system Cr(CO)5[Ga{(NDippCMe)2CH}] (87). Reactions with Mo(CO)4(cod) and W(CO)4(cod) by contrast exclusively generate the gallylene complexes

Dipp Dipp H

Me

N

Mn CO

H Dipp

84

N

hn Ga

Mn

Ga OC

-H2

N

Me

OC

N

CO Dipp

Me

Me 85

Scheme 23 Manganese-mediated dehydrogenation of a gallane σ-complex.18

30


M(CO)5[Ga{(NDippCMe)2CH}] (and H2), with the additional carbonyl ligand apparently being sequestered from the M(CO)4(cod) starting material. The corresponding alane, {HC(MeCDippN)2}AlH2, on the other hand, consistently generates robust κ2 σ-complexes of the type M(CO)4[κ2-H2Al{(NDippCMe)2CH}] (71–73; Scheme 17) in its reactions with M(CO)4(cod) (M ¼ Cr W).49a,d The reactivity of {HC(MeCDippN)2}GaH2 toward homoleptic monoand dinuclear metal carbonyls is also characterized by loss of dihydrogen and the formation of donor/acceptor complexes featuring the Ga(I) carbenoid ligand {HC(MeCDippN)2}Ga. By consideration of the differing products, Co2(CO)7Ga{(NDippCMe)2CH} (88) and (H)Mn(CO)4Ga {(NDippCMe)2CH} (89), obtained from the reactions with Co2(CO)8 and Mn2(CO)10, two alternative dehydrogenation pathways are revealed, with either H2 or MdH bonds acting as the ultimate hydrogen sink (Scheme 24).18 In the case of the cobalt system 89, the reaction is thought (on the basis of computational studies) to proceed via extremely facile GadH oxidative addition to one metal center of the photolytically generated [Co2(CO)7] fragment. This yields a (hydrido)cobalt gallyl complex with a single hydrogen atom attached to both cobalt and gallium atoms. Subsequent rotation about the CodGa bond precedes rate-limiting loss of H2, which occurs with a free-energy barrier of ca. 27 kcal mol1. This mechanism is very similar to that proposed for the analogous reaction with Fe(CO)5, which generates the closely related (albeit mononuclear) iron gallylene complex Dipp

Me N

H Ga H

N Mn2(CO)10 / hn -HMn(CO)5 Dipp

Me

H CO

Me

Dipp

Co2(CO)8 / hn -H2

N

Me

OC CO

CO

N Ga

N

Dipp

Me

OC Dipp

88

Mn

CO

Ga

Co

Co

CO

N CO

Me

Dipp

CO

OC CO

89

Scheme 24 Activation of the GadH bonds in {HC(MeCDippN)2}GaH2 by Co2(CO)8 and Mn2(CO)10.18


31

Fe(CO)4Ga{(NDippCMe)2CH} (90).18 In the case of Mn2(CO)10, however, the corresponding activation reaction at [Mn2(CO)9] would generate a more sterically encumbered metal center in the initial step, and MndH reductive elimination is presumably therefore favored over H2 loss, leading to the observed formation of HMn(CO)5 and the monometallic manganese gallylene complex 88. Finally, it is of note that Himmel and coworkers have recently reported the catalytic dehydrogenation of an amine gallane with bicyclic guanidine ligands using an iridium-based catalyst {η3-1,3-(OPtBu2)2C6H3}IrH2.2c Thus, H3GaNMe3 and the guanidine-stabilized gallane H3GahppH can be dehydrocoupled using this catalyst to form [H2Ga(μ-hpp)]2 with evolution of H2. There is, however, no direct evidence for a mechanism involving activation of the GadH bond at the iridium center directly, nor any suggestion of an intermediate featuring bridging IrdHdGa interactions.

5. CONCLUSIONS The significantly weaker nature of AldH and GadH bonds compared to their borane counterparts is one of the reasons why the coordination chemistry of the heavier congeners has remained for many years relatively underdeveloped. With the availability of alanes and gallanes featuring strongly donating and/or sterically demanding substituents (such as quinuclidine, N-heterocyclic carbenes, β-diketiminates), this situation has begun to be addressed, and fundamental features of AldH and GadH containing ligands begun to emerge. Here too, the respective EdH bond strengths apparently play a key role in defining aspects of structure/bonding and reactivity. Thus, coordination of four-coordinate boranes and gallanes at [CpMn(CO)2] fragments, for example, reveals a more side-on alignment for the latter donor, consistent with greater back-bonding into the (lower energy) GadH σ*-orbital.15a,25 Intrinsic bond strengths might also play a role in the markedly differing capacities of boranes, alanes, and gallanes to undergo activation at transition metal centers. Superficially, the successively weaker EdH bonds (ca. 89, 67, and 62 kcal mol1, respectively)3,17 might signal a greater propensity for activation for the heavier elements. In practice, while GadH and BdH bonds are prone to activation, AldH bonds are largely inert to oxidative addition, with one recent example being reported utilizing a highly reducing main group metal center (and even then giving rise to a reversible activation process).16a Presumably what must also be considered here is the strength of the MdE bond being

32


formed (E ¼ B, Al, Ga), with the greater propensity of boron to undergo BdH bond cleavage, reflecting the very strong nature (and potential π-augmentation) of metal–boron bonds.56 Mechanistically, the fundamental processes implicit in EdH bond activation have also begun to be illuminated, with studies of the interaction of {HC(MeCDippN)2}GaH2 with cationic rhodium bis(phosphine) fragments, [(R3P)2Rh]+, for example, showing that the degree of GadH bond activation can be “tuned” by variation in the phosphine ligands, to provide snapshots of the activation process leading to gallium-to-rhodium transfer of the two hydrogen atoms.57

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(OC)5W(η1-GaH3(quinuclidine)): the first gallane-coordinated transition-metal complex. Organometallics. 2002;21:2347–2349; See also: (b) Muraoka T, Ueno K. Gallane-coordinated transition metal complexes and related compounds. Coord Chem Rev. 2010;254:1348–1355. For Al-H activation at a main group metal centre see: (a) Chu T, Korobkov I, Nikonov GI. Oxidative addition of σ bonds to an Al(I) center. J Am Chem Soc. 2014;136:9195–9202; and (b) Xiong Y, Yao S, Driess M. Versatile conversion of N-heterocyclic silylene to silyl metal compounds by insertion of divalent silicon into metal-carbon and metal-hydrogen bonds. Chem Eur J. 2012;18:3316–3320. Reductive elimination of RH in the formation of Al(I) species has been reported by Fischer and Frenking: (c) Ganesamoorthy C, Loerke S, Gemel C, Jerabek P, Winter M, Frenking G, Fischer RA. Reductive elimination: a pathway to low-valent aluminium species. Chem Commun. 2013;49:2858–2860. Aldridge S, Downs AJ. The Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities. Chichester, UK: Wiley; 2011. Turner J, Abdalla JAB, Bates JI, et al. Formation of sub-valent carbenoid ligands by metal-mediated dehydrogenation chemistry: coordination and activation of H2Ga {(NDippCMe)2CH}. Chem Sci. 2013;4:4245–4250. For a collection of recent reviews, see Acc Chem Res. 2012;45(6):777–958, C–H functionalization, notably: (a) Gaillard S, Cazin CSJ, Nolan SP. N-Heterocyclic carbene gold(I) and copper(I) complexes in C–H bond activation. Acc Chem Res. 2012;45:778–787; (b) Colby DA, Tsai AS, Bergman RG, Ellman JA. Rhodium catalyzed chelation-assisted C–H bond functionalization reactions. Acc Chem Res. 2012;45:814–825; (c) Hartwig JF. Borylation and silylation of C–H bonds: a platform for diverse C–H bond functionalizations. Acc Chem Res. 2012;45:864–873; (d) Hashiguchi BB, Bischof SM, Konnick MM, Periana RA. Designing catalysts for functionalization of unactivated C–H bonds based on the CH activation reaction. Acc Chem Res. 2012;45:885–898; (e) Boisvert L, Goldberg KI. Reactions of late transition metal complexes with molecular oxygen. Acc Chem Res. 2012;45:899–910; (f ) Roizen JL, Harvey ME, Du Bois J. Metal-catalyzed nitrogen-atom transfer methods for the oxidation of aliphatic C–H bonds. Acc Chem Res. 2012;45:911–922; (g) Neufeldt SR, Sanford MS. Controlling site selectivity in palladium-catalyzed C–H bond functionalization. Acc Chem Res. 2012;45:936–946; (h) Haibach MC, Kundu S, Brookhart M, Goldman AS. Alkane metathesis by tandem alkane-dehydrogenation– olefin-metathesis catalysis and related chemistry. Acc Chem Res. 2012;45:947–958. See also Chem Rev. 2010;110(2):575–1211, Selective functionalization of C–H bonds. (a) Bernskoetter WH, Schauer CK, Goldberg KI, Brookhart M. Characterization of a rhodium(I) σ-methane complex in solution. Science. 2009;326:553–556. For an example of a crystallographically characterized alkane complex see: (b) Pike SD, Thompson AL, Algarra AG, Apperley DC, Macgregor SA, Weller AS. Synthesis and characterization of a rhodium(I) σ-alkane complex in the solid state. Science. 2012;337:1648–1651. Shimoi M, Nagai S, Ichikawa M, et al. Coordination compounds of monoboraneLewis base adducts: syntheses and structures of [M(CO)5(η1-BH3L)] (M ¼ Cr, Mo, W; L ¼ NMe3, PMe3, PPh3). J Am Chem Soc. 1999;121:11704–11712. He XM, Hartwig JF. True metal-catalyzed hydroboration with titanium. J Am Chem Soc. 1996;118:1696–1702. Hartwig JF, Muhoro GN, He XM, Eisenstein O, Bosque R, Maseras F. Catecholborane bound to titanocene. Unusual coordination of ligand σ-bonds. J Am Chem Soc. 1996;118:10936–10937. Bissinger P, Braunschweig H, Kupfer T, Radacki K. Monoborane NHC adducts in the coordination sphere of transition metals. Organometallics. 2010;29:3987–3990.


35

25. Kakizawa T, Kawano Y, Shimoi M. Syntheses and structures of manganese complexes of boraneLewis base adducts, [CpMn(CO)2(η1-BH3L)] (L ¼ NMe3, PMe3). Organometallics. 2001;20:3211–3213. 26. (a) Schubert U, Ackermann K, Woerle B. A long silicon-hydrogen bond or a short silicon-hydrogen nonbond? Neutron-diffraction study of (η5CH3C5H4)(CO)2(H)MnSiF(C6H5)2. J Am Chem Soc. 1982;104:7378–7380. (b) Scherer W, Eickerling G, Tafipolsky M, McGrady GS, Sirsch P, Chatterton NP. Elucidation of the bonding in Mn(η2-SiH) complexes by charge density analysis and T1 NMR measurements: asymmetric oxidative addition and anomeric effects at silicon. Chem Commun. 2006;2986–2988. 27. Schilling BER, Hoffmann R, Lichtenberger DL. CpM(CO)2(ligand) (Cp ¼ cyclopentadienyl, M ¼ metal) complexes. J Am Chem Soc. 1979;101:585–591. 28. Kawano Y, Hashiva M, Shimoi M. Syntheses and reactivity of cationic boraneruthenium complexes [(η5-C5R5)Ru(PMe3)2(η1-BH3EMe3)][BArf4] (R ¼ H, Me; E ¼ N, P; BArf4 ¼ [B{3,5-C6H3(CF3)2}4]). Organometallics. 2006;25:4420–4426. 29. See for example: (a) Staubitz A, Sloan ME, Robertson APM, et al. Catalytic dehydrocoupling/dehydrogenation of N-methylamine-borane and ammonia-borane: synthesis and characterization of high molecular weight polyaminoboranes. J Am Chem Soc. 2010;132:13332–13345; (b) Staubitz A, Soto AP, Manners I. Iridium-catalyzed dehydrocoupling of primary amine–borane adducts: a route to high molecular weight polyaminoboranes, boron–nitrogen analogues of polyolefins. Angew Chem Int Ed Engl. 2008;47:6212–6215; (c) Sewell LJ, Lloyd-Jones GC, Weller AS. Development of a generic mechanism for the dehydrocoupling of amine-boranes: a stoichiometric, catalytic, and kinetic study of H3BNMe2H using the [Rh(PCy3)2]+ fragment. J Am Chem Soc. 2012;134:3598–3610; (d) Huertos MA, Weller AS. Intermediates in the Rh-catalysed dehydrocoupling of phosphine–borane. Chem Commun. 2012;48:7185–7187; (e) Sewell LJ, Huertos MA, Dickinson ME, Weller AS, LloydJones GC. Dehydrocoupling of dimethylamine borane catalyzed by Rh(PCy3)2H2Cl. Inorg Chem. 2013;52:4509–4516; (f ) Hooper TN, Huertos MA, Jurca T, Pike SD, Weller AS, Manners I. Effect of the phosphine steric and electronic profile on the Rh-promoted dehydrocoupling of phosphine–boranes. Inorg Chem. 2014;53:3716–3729; (g) Johnson HC, Leitao EM, Whittell GR, Manners I, Lloyd-Jones GC, Weller AS. Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine–boranes using a [Rh(Xantphos)]+ catalyst. J Am Chem Soc. 2014;136:9078–9093. 30. (a) Douglas TM, Chaplin AB, Weller AS. Amine-borane σ-complexes of rhodium. Relevance to the catalytic dehydrogenation of amine-boranes. J Am Chem Soc. 2008;130:14432–14433. (b) Douglas TM, Chaplin AB, Weller AS, Yang X, Hall MB. Monomeric and oligomeric amine–borane σ-complexes of rhodium. Intermediates in the catalytic dehydrogenation of amine–boranes. J Am Chem Soc. 2009;131:15440–15456. (c) Tang CY, Thompson AL, Aldridge S. Rhodium and iridium aminoborane complexes: coordination chemistry of BN alkene analogues. Angew Chem Int Ed Engl. 2010;49:921–925. (d) Chaplin AB, Weller AS. Amine- and dimeric amino-borane complexes of the {Rh(PiPr3)2}+ fragment and their relevance to the transition-metal-mediated dehydrocoupling of amine-boranes. Inorg Chem. 2010;49:1111–1121. (e) Johnson HC, Robertson APM, Chaplin AB, et al. Catching the first oligomerization event in the catalytic formation of polyaminoboranes: H3BNMeHBH2NMeH2 bound to iridium. J Am Chem Soc. 2011;133:11076–11079. (f ) Dallanegra R, Robertson APM, Chaplin AB, Manners I, Weller AS. Tuning the [L2Rh. . .H3BNR3]+ interaction using phosphine bite angle. Demonstration by the catalytic formation of polyaminoboranes. Chem Commun. 2011;47:3763–3765. (g) Tang CY, Phillips N, Kelly MJ, Aldridge S. Hydrogen shuttling: synthesis and

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reactivity of a 14-electron iridium complex featuring a bis(alkyl) tethered ligand. Chem Commun. 2012;48:11999–12001. (h) Johnson HC, McMullin CL, Pike SD, Macgregor SA, Weller AS. Dehydrogenative boron homocoupling of an amine-borane. Angew Chem Int Ed Engl. 2013;52:9776–9780. (i) Algarra AG, Sewell LJ, Johnson HC, Macgregor SA, Weller AS. A combined experimental and computational study of fluxional processes in sigma amine–borane complexes of rhodium and iridium. Dalton Trans. 2014;43:11118–11128. (j) Phillips N, Tang CY, Tirfoin R, et al. Modulating reactivity in iridium bis(N-heterocyclic carbene) complexes: influence of ring size on E-H bond activation chemistry. Dalton Trans. 2014;43:12288–12298. Muhoro CN, Hartwig JF. Synthesis, structure, and reactivity of [Cp2Ti(HBcat)(PMe3)]: a monoborane σ complex. Angew Chem Int Ed Engl. 1997;36:1510–1512. Schlecht S, Hartwig JF. σ-Borane complexes of manganese and rhenium. J Am Chem Soc. 2000;122:9435–9443. McGrady GS, Sirsch P, Chatterton NP, et al. Nature of the bonding in metal-silane σ-complexes. Inorg Chem. 2009;48:1588–1598. (a) Alcaraz G, Clot E, Helmstedt U, Vendier L, Sabo-Etienne S. Mesitylborane as a Bis(σ-BH) ligand: an unprecedented bonding mode to a metal center. J Am Chem Soc. 2007;129:8704–8705. See also: (b) Gloaguen Y, Benac-Lestrille G, Vendier L, et al. Monosubstituted borane ruthenium complexes RuH2(η2:η2-H2BR)(PR0 3)2: a general approach to the geminal bis(σ-B–H) coordination mode. Organometallics. 2013;32:4868–4877; (c) Buil ML, Cardo JJF, Esteruelas MA, Fernandez I, Onate E. Hydroboration and hydrogenation of an osmium–carbon triple bond: osmium chemistry of a bis-σ-borane. Organometallics. 2015;34:547–550. (a) Hesp KD, Rankin MA, McDonald R, Stradiotto M. Synthesis and characterization of a cationic ruthenium complex featuring an unusual bis(η2-BH) monoborane ligand. Inorg Chem. 2008;47:7471–7473. (b) Hesp KD, Kannemann FO, Rankin MA, McDonald R, Ferguson MJ, Stradiotto M. Probing mesitylborane and mesitylborate ligation within the coordination sphere of Cp*Ru(PiPr3)+: a combined synthetic, X-ray crystallographic, and computational study. Inorg Chem. 2011;50:2431–2444. (a) Alcaraz G, Vendier L, Clot E, Sabo-Etienne S. Ruthenium bis(σ-B-H) aminoborane complexes from dehydrogenation of amine–boranes: trapping of H2B-NH2. Angew Chem Int Ed Engl. 2010;49:918–920. (b) Esteruelas MA, Fernandez-Alvarez FJ, Lopez AM, Mora M, Onate E. Borinium cations as σ-BH ligands in osmium complexes. J Am Chem Soc. 2010;132:5600–5601. (c) Alcaraz G, Chaplin AB, Stevens CJ, et al. Ruthenium, rhodium, and iridium bis(σ-BH) diisopropylaminoborane complexes. Organometallics. 2010;29:5591–5595. (d) Stevens CJ, Dallanegra R, Chaplin AB, et al. [Ir(PCy3)2(H)2(H2B-NMe2)]+ as a latent source of aminoborane: probing the role of metal in the dehydrocoupling of H3BNMe2H and retrodimerisation of [H2BNMe2]2. Chem Eur J. 2011;17:3011–3020. (e) Beńac-Lestrille G, Helmstedt U, Vendier L, Alcaraz G, Clot E, Sabo-Etienne S. Probing mesitylborane and mesitylborate ligation within the coordination sphere of Cp*Ru(PiPr3)+: a combined synthetic, X-ray crystallographic, and computational study. Inorg Chem. 2011;50:11039–11045. (f ) Tang CY, Phillips N, Bates JI, Thompson AL, Gutmann MJ, Aldridge S. Dimethylamine borane dehydrogenation chemistry: syntheses, X-ray and neutron diffraction studies of 18-electron aminoborane and 14-electron aminoboryl complexes. Chem Commun. 2012;48:8096–8098. (g) Addy DA, Bates JI, Kelly MJ, et al. Synthesis and reactivity of half-sandwich ruthenium κ2-aminoborane complexes. Aus J Chem. 2013;66:1211–1218. (h) Joost M, Alcaraz C, Vendier L, Poblador-Bahamonde A, Clot E, Sabo-Etienne S. Synthesis of a ruthenium bis(diisopropylamino(isocyano) borane) complex from the activation of an amino(cyano)borane. Dalton Trans. 2013;42:776–781. (i) Esteruelas MA, Fernandez I, Lopez AM, Malka M, Onate E. Osmium-promoted dehydrogenation of amine–boranes and B–H bond activation of


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the resulting amino–boranes. Organometallics. 2014;33:1104–1107. (j) Cassen A, Vendier L, Daran J-C, et al. B–C bond cleavage and Ru–C bond formation from a phosphinoborane: synthesis of a bis-σ borane aryl-ruthenium complex. Organometallics. 2014;33:7157–7163. For an early report examining aminoborane interactions with transition metals see: Schmid G. Metall-bor-verbindungen, VIII. Aminoborane als π-liganden in metallkomplexen. Chem Ber. 1970;103:528–534. Westwood NPC, Westiuk NH. Aminoborane: helium (He I) photoelectron spectrum and semiempirical/ab initio investigation of the ground and first excited cationic states. J Am Chem Soc. 1986;108:891–894. Vidovic D, Addy DA, Kra¨mer T, McGrady J, Aldridge S. Probing the intrinsic structure and dynamics of aminoborane coordination at late transition metal centers: mono(σ-BH) binding in [CpRu(PR3)2(H2BNCy2)]+. J Am Chem Soc. 2011;133:8494–8497. (a) Knorr JR, Merola JS. Synthesis and structure of a [(1,2-phenylenedioxy)boryl] iridium hydride complex: a model system for studying catalytic hydroboration. Organometallics. 1990;9:3008–3010. (b) Baker RT, Ovenall DW, Calabrese JC, et al. Boryliridium and boraethyliridium complexes fac-[IrH2(PMe3)3(BRR0 )] and fac-[IrH (PMe3)3(η2-CH2BHRR0 )]. J Am Chem Soc. 1990;112:9399–9400. For other neutron diffraction studies of coordinated B–H bonds, see: (a) Lam WH, Shimada S, Batsanov AS, et al. Accurate molecular structures of 16-electron rhodium hydrido boryl complexes: low-temperature single-crystal X-ray and neutron diffraction and computational studies of [(PR3)2RhHCl(Boryl)] (Boryl ¼ Bpin, Bcat). Organometallics. 2003;22:4557–4568; (b) Hebden TJ, Denney TJ, Pons V, et al. σ-Borane complexes of iridium: synthesis and structural characterization. J Am Chem Soc. 2008;130:10812–10820. Hartwig JF, Cook KS, Hapke M, et al. Rhodium boryl complexes in the catalytic. Terminal functionalization of alkanes. J Am Chem Soc. 2005;127:2538–2552. Braunschweig H, Forster M, Kupfer T, Seeler F. Borylene transfer under thermal conditions for the synthesis of rhodium and iridium borylene complexes. Angew Chem Int Ed Engl. 2008;47:5981–5983. See, for example: (a) Belsky VK, Erofeev AB, Bulychev BM, Soloveichik GL. Synthesis and molecular structure of solvated hydride complexes of aluminium and di-η5cyclopentadienylyttrium. J Organomet Chem. 1984;265:123–133; (b) Belskii VK, Bulychev BM, Erofeev AB, Soloveichik GL. Alumohydride complex of yttrium with three-coordinated hydrogen atoms. The crystal and molecular structure of {[(η5C5H5)2Y(μ3-H)][μ2-H)AlH2OC4H8]}2. J Organomet Chem. 1984;268:107–111; (c) Khan K, Raston CL, McGrady JE, Skelton BW, White AH. Hydride-bridged heterobimetallic complexes of zirconium and aluminum. Organometallics. 1997;16:3252–3254; (d) Plois M, Wiegand T, Wolf R. Novel ruthenium(II) aluminate anions: building blocks of unique cage structures. Organometallics. 2012;31:8469–8477. (a) Lobkovskii B, Soloveichik GL, Erofeev AB, Bulychev BM, Bel’ski VK. The crystal and molecular structure of the complex of yttrium biscyclopentadienyl chloride with aluminium hydride monoetherate [((η5-C5H5))2YCl]2AlH3—OEt2. J Organomet Chem. 1982;235:151–159. (b) Lobkovskii EB, Soloveichik GL, Sisov AI, Bulychev BM, Gusev AI, Kirillova NI. Structural chemistry of bimetallic hydride complexes of titanium and aluminium: I. Crystal and molecular structure of [(η5-C5H5)2, Ti(μ-H)2AlH2]2 (CH3)2NCH2CH2N(CH3)2C6H6. J Organomet Chem. 1984;265:167–173. (a) Burlakov VV, Kaleta K, Beweries T, et al. Reactions of five-membered metallacyclocumulenes Cp2M(η4-t-Bu-C4-t-Bu) (M ¼ Ti, Zr) with diisobutylaluminum hydride. Organometallics. 2011;30:1157–1161. (b) Arndt P, Spannenberg A, Baumann W, et al. Reactions of zirconocene 2-vinylpyridine complexes with diisobutylaluminum hydride and fluoride. Organometallics. 2004;23:4792–4795.

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47. Yow S, Gates SJ, White AJP, Crimmin MR. Zirconocene dichloride catalyzed hydrodefluorination of Csp2-F bonds. Angew Chem Int Ed Engl. 2012;51:12559–12563. 48. (a) Steinke T, Gemel C, Cokoja M, Winter M, Fischer RA. AlCp* as a directing ligand: C-H and Si-H bond activation at the reactive intermediate [Ni(AlCp*)3]. Angew Chem Int Ed Engl. 2004;43:2299–2302. (b) Steinke T, Cokoja M, Gemel C, et al. C-H activated isomers of [M(AlCp*)5] (M ¼ Fe, Ru). Angew Chem Int Ed Engl. 2005;44:2943–2946. 49. (a) Riddlestone IM, Edmonds S, Kaufman PA, et al. σ-Alane complexes of chromium, tungsten, and manganese. J Am Chem Soc. 2012;134:2551–2554. (b) Abdalla JAB, Riddlestone IM, Tirfoin R, Phillips N, Bates JI, Aldridge S. Al-H σ-bond coordination: expanded ring carbene adducts of AlH3 as neutral bi- and tri-functional donor ligands. Chem Commun. 2013;49:5547–5549. (c) Riddlestone IM, Urbano J, Phillips N, et al. Salt metathesis for the synthesis of M–Al and M–H–Al bonds. Dalton Trans. 2013;42:249–258. (d) Abdalla JAB, Riddlestone IM, Turner J, et al. Coordination and activation of Al-H and Ga-H bonds. Chem Eur J. 2014;20:17624–17634. 50. (a) Cordero B, Go´mez V, Platero-Prats AE, et al. Covalent radii revisited. Dalton Trans. 2008;2832–2838. (b) Emsley J. The Elements. 3rd ed. Oxford: OUP; 1998. 51. (a) Nako AE, Gates SJ, White AJP, Crimmin MR. Preparation and properties of a series of structurally diverse aluminium hydrides supported by β-diketiminate and bis(amide) ligands. Dalton Trans. 2013;42:15199–15206. (b) Nako AE, Tan QW, White AJP, Crimmin MR. Weakly coordinated zinc and aluminum σ-complexes of copper(I). Organometallics. 2014;33:2685–2688. 52. Cui C, Roesky HW, Schmidt H-G, Noltemeyer M, Hao H, Cimpoesu F. Synthesis and structure of a monomeric aluminum(I) compound [{HC(CMeNAr)2}Al] (Ar ¼ 2,6iPr2C6H3): a stable aluminum analogue of a carbene. Angew Chem Int Ed Engl. 2000;39:4274–4276. 53. (a) Ruff JK, Hawthorne MF. The amine complexes of aluminum hydride. I. J Am Chem Soc. 1960;82:2141–2144. (b) Greenwood NN, Storr A, Wallbridge MGH. Trimethylamine adducts of gallane and trideuteriogallane. Inorg Chem. 1963;2:1036–1039. (c) Shriver DF, Parry RW. Preparation and characterization of trimethylamine gallane and bistrimethylamine gallane’. Inorg Chem. 1963;2:1039–1042. 54. (a) Pulham CR, Downs AJ, Goode MJ, Rankin DWH, Robertson HE. Gallane: synthesis, physical and chemical properties, and structure of the gaseous molecule Ga2H6 as determined by electron diffraction. J Am Chem Soc. 1991;113:5149–5162. (b) Downs AJ, Pulham CR. The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation. Chem Soc Rev. 1994;23:175–184. 55. Atwood JL, Bott SG, Elms FM, Jones C, Raston CL. Tertiary amine adducts of gallane: gallane-rich [{GaH3}2(TMEDA)] (TMEDA ¼ N,N,N0 ,N0 -tetramethylethylenediamine) and thermally robust [GaH3(quinuclidine)]. Inorg Chem. 1991;30:3792–3793. 56. Rablen PR, Hartwig JF, Nolan SP. First transition metal-boryl bond energy and quantitation of large differences in sequential bond dissociation energies of boranes. J Am Chem Soc. 1994;116:4121–4122. 57. Abdalla JAB, Edwards AJ, Aldridge S. Unpublished results.

CHAPTER TWO

Singular Metal Activation of Diboron Compounds Stephen A. Westcotta, Elena Fernándezb,*

a Department of Chemistry and Biochemistry, Mount Allison University, Sackville, New Brunswick, Canada b Departament Quı´mica Fı´sica i Inorga`nica, University Rovira i Virgili, Tarragona, Spain *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction: The Activation of Cl2B–BCl2 and Beyond 2. General Trends on Activation of Symmetrical Diboron Compounds 2.1 Metal Activation of Tetra(alkoxy)diborons 2.2 Metal Activation of Halide- or Amine-Substituted Diboron Compounds 2.3 Nanoparticle Activation 3. Precise Activation of Unsymmetrical Dialkoxy-diamino-diboron Compounds 4. Summary and Outlook Acknowledgments References

39 42 43 70 73 77 79 80 80

1. INTRODUCTION: THE ACTIVATION OF Cl2B–BCl2 AND BEYOND Compounds containing direct boron–boron bonds have been of considerable interest for many years owing to their unusual electron-deficient nature. A successful proof of concept was the eminent reactivity between Cl2B–BCl2 and ethylene representing the first addition of a borane reagent to an alkene,1 even before any hydroboration reaction between B2H6 and olefins was described.2 Despite the fact that the diborated compound formed at 80 °C was characterized as Cl2B(CH2)(CH2)BCl2, the authors deferred to assign a name to the new organoboron compound until “a committee, considering nomenclature of boron compounds in 1954, had reached a final decision.” Looking at this incipient novel reactivity, the addition of Cl2B– BCl2 was carried out to a wider range of alkenes, including diboration of cyclopropene and double addition to butadiene.3 For ethyne, cis-addition Advances in Organometallic Chemistry, Volume 63 ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.02.001

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Stephen A. Westcott and Elena Fernández

of B2Cl4 was defined and later extended to other terminal alkynes.4 A first hypothesis for the activation of the diboron tetrachloride was suggested by Feeney, Holliday, and Marsden in 1961, indicating that the initial fission of BdB bond and then addition to olefins were unlikely.5 Based on the observed rapid reaction of Cl2B–BCl2 with ethylene at low temperature, these authors postulated as a first stage the π-donation from the olefin into the vacant p-type orbital of the boron atoms, since these being on adjacent bonded atoms might constitute a “vacant π-orbital.” As a consequence, this interaction would give a π-complex (1) which might either dissociate again or suffer fission of the B–B to give the product 2 (Scheme 1). In parallel, Fox and Wartik evidenced a slow interaction between Cl2B– BCl2 and aromatic compounds, leading to a single electrophilic substitution product C6H5BCl2 from benzene, but to a double addition product from naphthalene toward complete saturation of one ring.6 Subsequently, Zeldin and Wartik assumed cis-addition of Cl2B–BCl2 to the π-bonds of the conjugated systems in 1,3-cyclohexadiene and naphthalene.7 Again, the rapidity of the reaction between the diboron tetrachloride and the carbon–carbon π-bond of the conjugated system, even at very low temperatures, weighed in the author’s criteria against B–B rupture prior to addition. Besides this intuitively favored activation of the diboron tetrachloride, Rudolph simultaneously proposed a four-center transition state 3 (Fig. 1).8 The favored orientation in the suggested transition state (Fig. 1) would seem to require that Cl2B–BCl2 assumes a near-planar configuration,9 the driving force for which should be the maximum orbital overlap between the vacant p-orbitals on the boron atoms and the basic site of the π-orbital on the hydrocarbon, represented in structure 4 (Fig. 1).10 In contrast to the above mechanism, which must give rise to a cis product, a process involving prior homolytic cleavage of the boron–boron bond to generate free dichloroboryl radicals would, because of the opportunity for free rotation about the carbon–carbon single bond, not be expected π-complex Cl

Cl B

Cl R2C

Cl

B

+

Cl B

Cl CR⬘2

B

Cl

Cl

R2C

CR⬘2

BCl2

Cl2B R2C

CR⬘2 2

1

Scheme 1 Postulated activation of Cl2B–BCl2 with olefins toward diboration reaction.

41

Singular Metal Activation of Diboron Compounds

Four-center transition state Cl

Cl B

B

Cl

C

C

B

B

Cl H

H C

C R

R 3

4

Figure 1 Suggested concerted interaction of Cl2B–BCl2 with olefins.

Cl B Cl R2C

Cl

R2C

Cl

+

B

+

CR⬘2 B Cl2

CR⬘2

−

Cl B Cl

5

Scheme 2 Unlikely heterolytic cleavage of Cl2B–BCl2.

to proceed in a stereospecific manner. Furthermore, calculation carried out on bond energy does not favor dissociation as an initiating step (ΔΗB–B ¼ 79.0 kcal/mol).11 The activation of Cl2B–BCl2 involving the formation of a cyclic organoboronium ion 5 (Scheme 2) was also discarded because of the lack of trans-diborated compounds observed. The activation of Cl2B–BCl2 with ferrocene has also been suggested to proceed through a π-complex, 6 to form the ferrocenyldichloroborane 7, discarding other intermediates of lower stability where Fe directly interacts with B (Scheme 3).12 The slower reactivity observed for the addition of Cl2B–BCl2 to the haloolefins might be a consequence of the less favored formation of the π-complex in this case. The diborated product suffers an elimination of trihaloborane, and subsequent addition of Cl2B–BCl2 to the resulting vinyltrihaloborane gives as a final product the poly(dihaloboryl)ethane.13 The π-acidity of Cl2B–BCl2 and the fact that it was the first isolated compound containing a localized two-center-two-electron (2c/2e) BdB bond gave rise to tremendous interest into routes to prepare organoboron compounds from this tetra(chloro)diboron.14–17 This issue was further revisited by Morrison highlighting the possibility to use Cl2B–BCl2 as the precursor for the synthesis of Br2B–BBr2, F2B–BF2, (R2N)2B–B(NR2)2, and (RO)2B–B(OR)2 when reacted with BBr3, SbF3, dialkylamines, and

42


Cl

Cl B

Cl

+

Cl

Cl

B Cl

Cl

Cl B

B

Cl

BCl2

Cl

B Fe

Fe

B Cl

Fe

Fe

6

7

+

HBCl2

Cl

Scheme 3 Activation of Cl2B–BCl2 with ferrocene.

alcohols, respectively.18 However, at that stage, progress became limited by the instability and restricted availability of Cl2B–BCl2, which was originally produced by an electric discharge through BCl3 at low temperatures,19–21 and a further improved method from copper vapor and BCl3.22,23 A more accessible diboron compound was then required to continue with the expected applicability on organoborane synthesis although the activation model should be conceptually changed.

2. GENERAL TRENDS ON ACTIVATION OF SYMMETRICAL DIBORON COMPOUNDS The stability of diboron compounds X2B–BX2 depends on the nature of the substituent on B, decreasing from X ¼ R2N, RO > F > aryl, alkyl, Cl, whereupon Cl2B–BCl2 being the most prone toward disproportionation. Organodiboron compounds, R2B–BR2, are stable only when sterically demanding R groups, such as t-Bu, CH2-t-Bu, and mesityl, are protecting the B atom.24–26 On the contrary, tetra(amino)diborons21,27,28 and tetra(alkoxy)diborons21,29–32 show a remarkable stability due to the extensive π-type overlap with the neighboring nitrogen or oxygen atoms which stabilize the boron atom. The concomitant consequences are lower reactivity against unsaturated substrates, but the resulting organoboron compounds become generally air and water stable and amenable to chromatography. The new scenario for tetra(amino)diboron and tetra(alkoxy)diboron compounds shows a less π-acidic B–B system that might justify the unlikely formation of the π-complex between (R2N)2B–B(NR2)2 or (RO)2B–B(OR)2 and the olefins, diminishing the direct reactivity toward diboration. The BdB bond in tetrakis(dimethylamino)diboron is so stable that this diboron compound is generally used to prepare other boron–boron linkages.30,33

43


O

B

O

B

O

O B2pin2 9

O

B

O

B

O

Stability

Reactivity

O

B2cat2

8 Cl

B B

Cl

Cl

Cl Cl2B−BCl2

Figure 2 Relative Lewis acidity of Cl2B–BCl2 and tetra(alkoxy)diboron compounds B2cat2 and B2pin2.

However, tetra(alkoxy)diboron compounds, such as B2cat234 (cat ¼ 1,2O2C6H4) (8) and B2pin235 (pin ¼ 1,2-O2C2Me4) (9) (Fig. 2), have emerged as the synthetic reagents of choice to react with unsaturated substrates, because of the appropriate balance between stability and reactivity parameters. In particular, the pinacolato derivative 9, despite the fact that is slightly less Lewis acidic than 8 and therefore less reactive, provides the widest range of organoboron compound synthesis because of their great stability (Scheme 4).36,37

2.1 Metal Activation of Tetra(alkoxy)diborons Taking into consideration that boron–boron dissociation energy in tetra(alkoxy)diboron compounds has been estimated to be slightly higher than that of the corresponding data for other diboron compounds,38,39 together with the low Lewis acidity of the boron centers, a new model to activate the (RO)2B–B(OR)2 has to be considered. In fact, an external activator of the tetra(alkoxy)diboron is required to accomplish its addition to unsaturated substrates. The activators can be considered transition metals or Lewis bases (LBs), and the mode of interaction can be displayed as a homolytic or as a heterolytic B–B cleavage, as it is illustrated for B2pin2 in Scheme 5. When a transition metal complex, characterized by a lowvalent metal, interacts with the nonpolar tetra(alkoxy)diboron species, a

44


Bpin

+ XBpin

CO + O(Bpin)2 X CO2 Bpin

Bpin

R

R

R

R R

H

B2pin2

H

R

Bpin

R

Bpin

R R

R

O

Bpin

R

R

R

Bpin

Bpin O

Bpin

R Bpin

Bpin

Scheme 4 Selected synthetic applications of B2pin2.

A

B

O

O

M B O

M

(n−2)+

B O O

O

O

O

O

X

LB

B

B

B

B O

n+

+

B O

LB O

B

B

B

O

O

O

O B

X

M

M O

C

n+

n+

O

O

O

O

+ O

− O B O

Scheme 5 Activation modes for B2pin2. (A) Activation by oxidative addition, (B) activation by σ-bond metathesis, and (C) activation by Lewis bases.

three-centered σ-complex is invoked followed by intramolecular B–B cleavage to form the corresponding oxidized metal complex (Scheme 5A). As a product of this oxidative addition, the resulting two boryl ligands will be mutually cis, although subsequent isomerization may occur. Alternatively, a transition metal species that includes a MdX bond (X ¼ preferentially OR group) can perform a σ-bond metathesis with the

45


tetra(alkoxy)diboron compound, promoting a heterolytic cleavage of the B–B with the concomitant formation of a M-boryl species and the resulting X–B(OR)2 (Scheme 5B). The formation of the tris(alkoxy)borane compound can be considered as the driving force in a sequence where the metal does not involve any change in its oxidative state. Finally, the addition of an appropriate LB establishes a selective interaction with one of the B(OR)2 unit from the tetra(alkoxy)diboron, delivering a new Lewis acid–base adduct, MeO ! Bpin–Bpin, that is characterized by a quaternized B(sp3) moiety and an enhanced nucleophilic B(sp2) unit (Scheme 5C). Within the sections 2 and 3, several examples are disclosed to illustrate the ability of transition metal complexes to transform the unreactive (RO)2B–B(OR)2 compounds into very reactive species toward the addition to unsaturated substrates. 2.1.1 Activation by Metal Complexes of Group 10 Miyaura and Suzuki published in 1993 their groundbreaking work40 on the diboration of alkynes with B2pin2, using a platinum(0) complex [Pt(PPh3)4] as the activator of the diboron. They were the first to propose a mechanism involving oxidative addition of the pinBdBpin bond to Pt as the initial step (Scheme 6) followed by coordination of the alkyne to the bis(boryl)-Pt complex cis-[Pt(Bpin)2(PPh3)2] (10a) (Scheme 6), provoking an insertion of the unsaturated substrate into the PtdB bond and generating eventually the cis-1,2-diborated alkene by reductive elimination. The authors demonstrated the activation of B2pin2 by NMR analysis of the reaction mixture R Bpin

B2pin2

Pt(0)

R⬘ Bpin

Bpin R

R⬘

Bpin

Pt

Pt

Bpin

Bpin

Bpin Pt R

Bpin

R

R⬘

R⬘

Scheme 6 Proposed mechanism for the Pt(0)-diboration of alkynes with B2pin2.

46


of Pt(PPh3)4 with 10 equiv. of B2pin2 in toluene at room temperature for 12 h or at 100 °C for 1 h. The 11B NMR spectra exhibited a new signal at 21.5 ppm besides that for the starting B2pin2 (29.9 ppm). The same authors exhibited a comparative study on the activation of (MeO)2B–B(OMe)2 and (Me2N)2B–B(NMe2)2 with the platinum(0) complex [Pt(PPh3)4], and further application to the diboration of 1-octyne. The results clearly demonstrated that both the tetra(alkoxy)diboron performed in a similar way (quantitative conversion on the cis-1,2-diborated product, under 80 °C), but the tetrakis(dimethylamino)diboron was only added in 7% to the 1-octyne, even at 120 °C.41 The oxidative addition of (Me2N)2B–B(NMe2)2 to Pt(0) was less efficient, as it was expected, due to the enhanced stability of the diboron by the π-donation of the amino groups to the B atoms. To complement the activation study of tetra(alkoxy)diboron with Pt(0) complexes, Smith and Iverson disclosed their findings on the reactivity of B2cat2 with [Pt(η2-CH2]CH2)(PPh3)2], which resulted in the loss of ethylene, even at 80 °C, followed by oxidative addition of the BdB bond to give the bis(boryl) complex cis-[Pt(Bcat)2(PPh3)2] (10b) (Scheme 7).42,43 The 11B NMR data for 10b provided a broad signal at δ 47.2 ppm, and reactivity studies confirmed that the stoichiometric addition of alkynes led to the corresponding diborated products. Interestingly, mechanistic studies favored a mechanism where phosphine dissociation generated a three-coordinate intermediate that mediated the alkyne insertion. The molecular structure of similar bis(boryl) complexes cis-[Pt(B-4Butcat)2(PPh3)2] (10c) and 10d along with a number of diphosphine derivatives (10e–g), monophosphines (10h–j), or even mixed phosphines (10k) (Scheme 7) was subsequently reported by Marder and Norman who provided conclusive evidence about the activation of tetra(alkoxy) diboron with Pt(0) complexes, through the oxidative addition of the BdB bond, and further reactivity with alkynes and diynes.44,45 It is noteworthy that the addition of 1,2-bis(dimethylphosphino)methane (dppm) to cis-[Pt(Bcat)2(PPh3)2] proceeded to give predominantly cis-[Pt(Bcat)2(dppm)] (10g) along with minor amounts of an unusual binuclear product [Pt2(μ-Bcat) (Bcat)(μ-dppm)2(PPh3)] (Scheme 7). The same authors concluded that either Pt(II)bis(boryl) or Pt(0)-ethylene complexes are more efficient catalyst precursors than [Pt(PPh3)4] for the diboration of alkynes and that B2cat2 reacts faster than B2pin2, which in turn is much faster than B2(4-Butcat)2. When the basic phosphine PMe3 is involved in the precursor [Pt(PMe3)4], the activation of B2cat2 takes place smoothly.46

47


The platinum precursor species [Pt(η2-CH2]CH2)(PPh3)2], postulated by Smith and Iverson, hold out an inducement since the olefin ligand did not always serve as an innocent “mask” for the low-valent metal center. In fact, tris(bicyclo[2.2.1]heptene)platinum(0) instantaneously reacts with B2cat2 at room temperature to give a bis(boryl)bicyclo-[2.2.l]-heptane as the chief organic product (Scheme 8).42 It indicated that metal–olefin complexes

O

O

O

O

O

B B O

Ph3P Ph3P

Pt

Ph3P

PPh3

Ph3P

Ph3P

Pt

Ph3P

Ph3P

Pt

O

PPh3

10a: B(OR)2 = Bpin

Cy3P

O B B

B(OR)2 B(OR)2

10b: B(OR)2 = Bcat t 10c: B(OR)2 = B(4-Bu cat) 10d: B(OR)2 = B(4-X-cat) dppm dppe

PCy3

Bcat

Pt

10e

Bcat

Ph2 P Pt

10k PMe3

P Ph2

PMe2Ph PhMe2P PhMe2P

Pt

Ph2 P Pt P Ph2

Bcat Bcat

Bcat Bcat

10f

Bcat Ph2 P Pt

Bcat 10j

PEt3

PMePh2

Ph2MeP Ph2MeP

Bcat

Et3P

Bcat

Et3P

Pt 10i

Pt

P Ph2

Me2P

Bcat Bcat

10g

+

Bcat

Pt

Me2P

PMe2 PPh3 Pt Bcat PMe2

Bcat Bcat

10h

Scheme 7 Multiple strategies to activate tetra(alkoxy)diboron with Pt(0) and related transformations.

Pt Bcat Bcat

+ O O

B

B

O O

Scheme 8 Diboration of olefins with B2cat2 by Pt(0)-olefin complexes.

48


can rapidly mediate diboration of olefins which are otherwise unreactive with BdB bonds. This first example was published in 1995 and opened a interesting new perspective to activate tetra(alkoxy)diboron to add them to olefins. In general, the diboration of alkenes with [Pt(dba)2] required 50 °C,47 while the use of [Pt(NBE)2] or [Pt(COD)2] (NBE ¼ norbornene, COD ¼ 1,5-cyclooctadiene) as precursors could diborate alkenes at room temperature.48 The reactions proceed smoothly to give 1,2-diborylalkanes in high yield, and the catalysts were compatible with common functional groups. Further developments in the area by Baker showed that [Pt(COD)Cl2] could efficiently activate B2cat2 and promote the diboration of terminal alkenes, vinylarenes, alkynes, and aldimines (Scheme 9). The last example was a great advance because it represented the first direct approach toward α-aminoboronate esters.49 A theoretical study has been carried out by Morokuma for the mechanism of Pt(0)-catalyzed alkyne and alkene diboration reactions with the B3LYP density functional method.50 The complexation energy between (OH)2B–B(OH)2 and Pt(PH3)2 has been calculated to be 3.7 kcal/mol, where the B–B and P–Pt–P axes are perpendicular to each other. The next step is the activation of the BdB bond, and the activation barrier has been calculated to be 12.5 kcal/mol, relative to the molecular complex (OH)2B– B(OH)2–Pt(PH3)2 or 8.8 kcal/mol relative to the reactants.50 In parallel, Sakaki theoretically investigated the insertion of Pt(PH3)2 into X2B–BX2 (X ¼ H or OH) with the ab initio MO/MP4SDQ, SD-CI, and coupled cluster with double substitution methods. They found that this reaction proceeds with a moderate Ea of 15 kcal/mol and a considerable Eexo of 20 kcal/mol for (OH)2B–B(OH)2. They noted that the BdB bond undergoes the insertion reaction of Pt(PH3)2 much more easily than does the CdC bond.51 Nowadays, the Pt-catalyzed diboration of terminal

N O

O B B

O

O

Pt(COD)Cl2

+ rt

N

O B O B O O

Scheme 9 Diboration of aldimines with B2cat2 activated by [Pt(COD)Cl2] complex.

49


alkenes can be accomplished in an enantioselective fashion in the presence of chiral phosphinite ligands. Reaction progress kinetic analysis and kinetic isotope effects suggest that the stereodefining step in the catalytic cycle is the olefin migratory insertion into a PtdB bond.52 The addition of diboranes to 1,3-dienes was first addressed by Miyaura, through the activation of B2pin2 with [Pt(PPh3)4], forming selectively 1,4bis(allyl)boronates as single Z isomers.53 Interestingly, however, if the phosphine-free platinum species [Pt(dba)2] (dba ¼ dibenzylideneacetone) was used, the diborated products were those resulting from diene dimerization.53 Further advances in this issue provided the design of chiral tetra(alkoxy)diboron compounds to be activated by Pt and used as asymmetric inductors in the diboration of 1,3-dienes.54 The oxidative addition products derived from reactions of novel chiral diboranes with [Pt(η2-CH2]CH2)(PPh3)2] were studied by single-crystal X-ray diffraction studies (Scheme 10). Unfortunately, their application in the platinumcatalyzed diborations of a range of prochiral 1,3-dienes showed that chirality transfer from the diborane to the diene was not efficient in these cases. The B2pin2 activation by cis-[Pt(η2-CH2]CH2)(PPh3)2] was also found to be an effective catalyst precursor for the 1,4-diboration of α,β-unsaturated ketones at 80 °C for 12 h (Scheme 11),55 while [Pt(PPh3)4] as catalyst precursor required 20 h at 110 °C to add B2pin2 to the same activated olefins.56A second generation of platinum system

MeO2C

O

O B B

MeO2C

O

O

O

CO2Me CO2Me

O

Ph

B B Ph

O

Ph Ph

Ph3P Ph3P

O O

Ph3P Pt Ph3P

11a

B O O

Ph

O

Ph

CO2Me

O

Ph

CO2Me

CO2Me

Ph

Pt

CO2Me O B O

O B B

O

Ph3P Pt Ph3P

O Ph3P Pt Ph3P

B O B O O 11b

Ph

Ph

B O B O O

Ph Ph

11c

Scheme 10 Activation of chiral diboron compounds by Pt(0)-olefin complexes.

50


RO

RO

RO

RO O B O

R

O O

R

O

O

O

R

O

O

B B

B

O

O Ph3P Ph3P

O B O

O

R

O B O

Pt N Pt N

CH(CO2Me) CH(CO2Me) 12

Scheme 11 Influence of ligand nature on Pt(0) complex to activate diboron and add to α,β-unsaturated esters.

based on Pt(0) diimine, [Pt(BIAN)(DMFU)] (BIAN ¼ bis(phenylimino) acenaphthene, DMFU ¼ dimethylfumarate) (12), resulted efficient in the activation of B2pin2 to provide exclusively the unexpected 3,4-diborated addition products for α,β-unsaturated esters (Scheme 11), but selective 1,4-addition for α,β-unsaturated ketones and aldehydes.57 Interestingly, this performance was even better than that promoted by [Pt(NBE)3] leading to mixtures of 1,4- and 3,4-diborated products after activation of B2pin2.57 The mechanism of this reaction has been studied with the aid of density functional theory (DFT) by calculating important intermediates and transition states.58 The catalyzed diborations are believed to proceed by oxidative addition of diborons to the Pt(0) center followed by 1,4-conjugate addition of a PtdB bond to give an O-bound enolate intermediate containing a PtdCdC]CdOdB linkage. Reductive elimination would then generate the corresponding 1,4-addition product. In the case of methyl acrylate, a 1,3-shift of the O-bound boryl group provides the experimentally observed and thermodynamically favored 3,4-addition product (Scheme 12). To complete this picture, Srebnik was able to react B2pin2 and diazomethane with the aid of [Pt(PPh3)4] (Scheme 13A),59 which afforded a new carbenoid insertion reaction into the BdB bond. The same authors extended the scope of substrates to open the possibility of preparing C1-bridged bis-boronates with a quaternary carbon atom (Scheme 13B). The issue has recently been revisited by Wommack and Kingsbury developing a method to construct doubly C-substituted 1,1-diborons by diazoalkyl insertion into cis-[Pt(Bpin)2(PPh3)2] (Scheme 13C).60

51


N Pt N

(H)MeO O O

O

N

B O

O

O

O B

Pt

O

OMe B O

1,4-diborated

O O B O

3,4-diborated

O B

N

B

N

O

O

O

O

O B B

O

OMe(H)

OMe(H)

O

O

O

N

O B

Pt

O B O

OMe(H) N

Pt N O

(H)MeO

O B O

N

(H)MeO

Pt

OMe(H)

N

O

O

(H)MeO

O O B O

Scheme 12 Postulated diboration of methylacrylate and acrolein with Pt.

C Bpin

Bpin C

A O

O B B

O

O

R1

Ph3P PPh3 Pt Ph3P PPh3

+

CH2N2 Et2O, 0 °C

H2 C O B O

B O O

B

PPh3 R1

N O

O B B

O

O

+

Ph3P PPh3 Pt PPh3 Ph3P

+ N −

O

toluene, 80 °C

Pt

Bpin Ph3P Pt Bpin Ph3P

+

R2 Bpin N2

PPh3

R2

N2

R1

Ph3P

Pt

Bpin Bpin

R2

C R1

−

N2

R1

O B O

O B

R2 R1

Bpin

PPh3 Bpin Ph3P Pt Bpin Ph3P

PPh3

Ph3P

B2pin2

Ph3P PPh3 Pt Ph3P PPh3

R2

PPh3 R1

R2 Bpin PPh3

Bpin

Pt R2

N2

Scheme 13 Activation of B2pin2 by [Pt(PPh3)4] and further mechanistic proposal for formal carbon insertion.

The activation of diboron by platinum complexes and further reactivity with unsaturated substrates or nucleophiles have been always regarded to involve three- or four-coordinated Pt intermediates. Recently, Braunschweig has been instrumental in furthering our understanding of the

52


platinum-mediated B–B oxidation reaction.61 The reactivity of trans-[Pt(B (4-Butcat))(Me)(PCy3)2] (13) with B2cat2 leads to the formation of both MeBcat and MeB(4-Butcat), and two plausible mechanisms were invoked to rationalize this mixture. The first proposed mechanism involved the associative formation of a six-coordinate intermediate via oxidative addition of the B2cat2 group with a subsequent reductive elimination. This proposal required the formation of short-lived hypercoordinate Pt species (Scheme 14). The second pathway simply suggested a σ-bond metathesis of the BdB bond with the PtdC bond. Bis(boryl) species cis-[Pt (Bcat)2(PCy3)2] (14) was also examined by X-ray diffraction studies, and the short BdB bonds suggested that the boryl ligands were only loosely bound to the metal center, presumably arising from the steric hindrance caused by the bulky phosphine ligands. The activation of the diboron bond by σ-bond metathesis between B2cat2 and a PtdC bond was already observed by Iverson and Smith.42 They found that metallocyclopentane cis-[Pt(CH2)4(PPh3)2] could react cleanly with 2 molar equiv. of B2cat2 at 95 °C to form cis-[Pt(Bcat)2(PPh3)2] (10b) in 68% yield and catB(CH2)4Bcat in 47% (Scheme 15). Although a wealth of research has focussed on the use of platinum complexes in diboron activation chemistry, much less is known about the analogous palladium chemistry.51,62 Indeed, early theoretical studies have claimed that although the barrier for oxidative addition of B2(OH)4 to Pd(PH3)2 is smaller than for the analogous Pt complex (8.6 vs. 14.0 kcal/mol), the reaction is endothermic for Pd but exothermic for Pt. It has been postulated that [Pd(B(OH)2)2(PH3)2] complex resides higher

+ +

Me

B

O

+

O

O

B B

O

O

Me Cy3P

PCy3 Me

Pt

B

13

B

O

PCy3

Me

O O

Cy3P

Pt

14 O

O

B

O O

O

B

O Me

B

+ +

O O PCy3

Pt

O B O B O O

O O O B B O + +

Cy3P Pt Cy3P

B

+

Cy3P Pt Cy3P

O B O B O O

O O

Cy3P

Scheme 14 Activation of B2cat2 by trans-[Pt(B(4-Butcat))(Me)(PCy3)2] and further mechanistic proposal for MeBcat and Me B(4-Butcat) formation.

53


O

2 eq

O

+

B B O

O

Ph3P Pt Ph3P

Ph3P Pt Ph3P

O B O B O

+

O

O B O

O B O

10b

Scheme 15 Activation of B2cat2 by σ-bond metathesis with cis-[Pt(CH2)4(PPh3)2].

BAr

F

BAr

4

PPh2 O

O B B

O

O

+

N

Pd PPh2

F 4

PPh2 THF

cat B

N

Pd PPh2

Bcat

MLn

Z

+

B B

1,2-addition

MLn

Z

B

B

15

Scheme 16 Activation of B2cat2 by [Pd(II)(PNP)]+.

in energy than the analogous Pt complex, and this energy difference persists for all the catalytic intermediates. Morken has carried out some exceptional diboration chemistry with palladium complexes and has claimed that electron-donating ligands would stabilize diborated intermediates that are in high oxidation states and thus might facilitate Pd catalysis. Isotopelabeling experiments, kinetic analysis, and computational and stereodifferentiating experiments all suggest that the catalytic cycle for the enantioselective diboration of allenes with [Pd2(dba)3]/ligand and B2pin2 involves a rate-determining step involving oxidative addition of the diborane(4) species to palladium.63,64 Several years later, Ozerov has found that B2cat2 heterolytically adds to a [Pd(II)(PNP)]+ fragment (PNP ¼ diarylamido/bisphosphine ligand) to give the corresponding palladium-boryl species 15 (Scheme 16).65 The activation of BdB bond across a PddN bond to form the NdB and PddB bonds is unique. Although mechanistic implications for this reaction were not addressed, the authors suggest the possibility that the reaction is initiated by the formation of a σ-complex of the B2cat2 with [Pd(II)(PNP)]+, followed by a 1,2-addition. Previous computational and experimental investigations by Bo, Peris, and Fernańdez argued in favor of a viable B–B σ-complex when [Pd(II) (NHC)(Br)]+ (NHCs ¼ N-heterocyclic carbenes) promotes the heterolytic splitting of B2cat2.66 Direct oxidative addition of B2cat2 to [Pd(II)(NHC) Br]+ was first calculated as an endothermic process by 22.0 kcal/mol (Scheme 17A), while the barrier of the reverse process is extremely low. These results are in agreement with previous studies by Morokuma and

54


Scheme 17 Activation of B2cat2 by [Pd(II)(NHC)(Br)]+ and [Pd(II)(NHC)]2+.

Sakaki.51,62 In fact, the product from the oxidative addition is a saturated hexacoordinated Pd(IV) complex which would require creation of a vacant site in order to enable the coordination of an alkene. The activation of B2cat2 by [Pd(II)(NHC)Br]+ resulted more favorably through σ-bond metathesis providing [Pd(NHC)(Bcat)]+ and BrBcat (Scheme 17B) with 3.4 kcal/mol above the reactants (Scheme 17B). Alternatively, the dicationic complex [Pd(II)(NHC)]2+ could form a very stable σ-complex with B2cat2, with 32.9 kcal/mol below the two isolated reagents, which promoted the oxidative addition of the diboron (Scheme 17C). It seems that palladium complexes with high oxidative states and basic ligands can be involved in oxidative addition as well as in σ-bond metathesis. Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes.67,68 This reaction is completely regioselective and highly

55


stereoselective affording diborated products with mainly Z stereochemistry. This Pd-catalyzed reaction proceeds via a previously unknown mechanism involving the oxidative addition of an IdB bond into the palladium center instead of the oxidative addition of a BdB bond to metal (Scheme 18). After the insertion of the allene substrate to the PddB bond, a Pd-allyl species is formed with the boryl attached to the central carbon of the π-allyl group. Eventually, the transmetalation of B2pin2 with the Pd–C species regenerates the IdB bond and gives the metal intermediate B–Pd-allyl species that forms the diborated product by reductive elimination. The activation of the diboron by the Pd-allyl species is a key step for the success of the diboration of allenes. Since palladium complexes have proven to be efficient in activating diboron compounds throughout transmetalation, a wide range of applications have been considered in the last decade, such as palladium-catalyzed transformation of allylic alcohols to allylboronates,69,70 borylation of allylic halides71 or allylic acetates,71,72 and the β-boration of α,β-unsaturated carbonyl substrates.73 Interestingly, both palladium and nickel showed to be similarly efficient to activate B2pin2 and catalyze the addition to unsaturated substrates. Oshima postulated that Ni(0) species react with substrate α,βunsaturated esters and amides to generate the η2-coordinated complex, which activates the B2pin2 to favor the formation of η3-coordinated

R1

O B O

R2

B

Pd(0) O O O O O B O

R1

O R1

R2 O B O

B Pd I

O

B I

R2 Pd

transmetalation O O

B B

O

O B O

R1 R2 I

Pd

O

Scheme 18 1,2-Diboration of allenes with B2pin2 by phosphine-free Pd complexes.

56


boryl-nickel(II) complex (Scheme 19A).74 An eventual reductive elimination provides the boryl enolate product, which is susceptible to protonolysis and affords the β-boryl ester product. Westcott and Fernańdez demonstrated that the use of chiral ligands to modify the nickel complex can generate asymmetric induction in the CβdB bond.73 Morken has also studied the 1,4-diboration of conjugated dienes by activating B2pin2 with Ni(0) and postulated an alternative mechanism where the oxidative addition of B–B to Ni(0) is not involved.75 Instead, they postulated an initial association of Ni(0) with the diene to form a Ni–olefin complex which is responsible to react with B2pin2 providing the least hindered NidC bond that releases the desired diborated product by reductive elimination (Scheme 19B).

O

A Bpin OBpin R

R

Ni(0)

R⬘

R⬘ O R Ni O O B O R

O R⬘

Ni

O O

−O

B B

B B

O O

O

R Ni

B

O

+O

O B O

Bpin

R⬘

R⬘

Bpin R

R

Ni(0)

Ni R

O O

B–B

O O

O O B Ni

O B O R

Scheme 19 Proposed mechanism for Ni(0) activation of B2pin2 throughout Ni–olefin species.

57


A i

i

O

O B B

O

O

Pr2P

+

Ni i

Pr2P

P Pr2

i

N Ni N

P Pr2

O N

B

Ni

i

P Pr2

O

i

P Pr2 O

B

O i

i

P Pr2 O

P Pr2 O O

O B B

O

+

N

Ni i

P Pr2

OtBu

B B

N

Ni i

P Pr2

B OtBu

O O O

O Bu

t

Scheme 20 Proposed mechanism for activation of B2cat2 through (A) binuclear oxidative addition reaction and (B) σ-bond metathesis.

A process that involves an unprecedented BdB bond cleavage in B2cat2 promoted by two Ni(I) centers in the dimer [Ni(μ2-PNP)]2 has been described by Meyer and Mindiola to proceed plausibly via a binuclear oxidative addition reaction (Scheme 20A). The 11B NMR spectrum of [Ni(Bcat)(PNP)] clearly reveals the formation of a rare example of a nickel-boryl (47 ppm).76 Alternatively, the [Ni(Bcat)(PNP)] complex can be prepared by σ-bond metathesis from [Ni(OtBu)(PNP)] and B2cat2 (Scheme 20B).77 A proposed intermediate could be detected by NMR spectroscopy and confirmed by DFT calculations as the isomer that conducts the σ-bond metathesis. The weak interaction between the nickel center and the tethered borane fragment in B2cat2 is rationalized as a consequence of the high energy required to access the empty dx2 y2 -orbital in a sterically encumbered square planar environment. Moving from the Pt to Pd and Ni complexes, it can be noted a progressive change on the activation mode of the tetra(alkoxy)diboron compounds from oxidative addition of the B–B within Pt(0) complexes to σ-bond metathesis with Pd or Ni. Going further to the right of the periodic table, the activation of diboron by σ-bond metathesis becomes more frequent. 2.1.2 Activation by Metal Complexes of Group 11 Miyaura made another proof of concept when he demonstrated the activation of B2pin2 with CuCl in the presence of KOAc (Scheme 21).78,79 Analogously, Ito and Hosomi postulated that [(CuOTf )2C6H6] itself did not consume bis(catecholato)diboron (B2cat2) in the absence of an α,β-enone.80 The presence of PBu3 enhanced the β-boration of the enone, presumably

58


O

CuCl/KOAc

O B B

O

O

DMF/rt

O B

Cu·KCl

O

+

O B

OAc

O

Scheme 21 Suggested copper(I) activation of B2pin2 via σ-bond metathesis.

O O B

Ar N Cu

Ar B O O

O O B

O

Cu

O

Ar

Ar

PPh2

O B

Cu N

O Bu

t

N

N

16

PPh2 B O O

Cu

O

O O

O Bu

t

PPh2

B

PPh2 17

Scheme 22 Influence of ligands in copper(I) activation of B2pin2 via σ-bond metathesis.

because the PBu3 coordinates to (CuOTf )2C6H6 and the resulting copper system activates the B2pin2. The activation of diboron reagents with Cu(I) opened a new platform of copper-mediated borylation reactions as the borylation of functionalized allyl acetates by Ramachandran.81 Sadighi provided evidences of the σ-bond metathesis interaction by reacting the known complex [(IPr)Cu(OtBu)] with B2pin2, forming a product identified as [(IPr)Cu(Bpin)] (16) (Scheme 22). The nature of the N-heterocyclic ligand IPr seems to favor the isolation and full characterization of the Cu(I)-boryl complex, which resulted very active in reduction of CO2 to CO,82,83 or insertion of aldehydes84 and alkenes.85 Ito and Sawamura also observed that ligands with large bite angle, such as Xantphos, would activate Cu(I)-OR for σ-bond metathesis with B2pin2 to form a Cu–B species 17 useful as a “formal boryl nucleophile” (Scheme 22).86 Then, the formal SN20 attack of the Cu–B species on an allylic carbonate would allow γ-selective formation of an allylboron compound along with a copper carbonate that would undergo decarboxylation to regenerate the Cu–OR.

59


The expansion of this chemistry came by the work of Yun,87,88 since the copper borylation of α,β-unsaturated carbonyl compounds could be achieved by the activation of B2pin2 with CuCl in the presence of base and MeOH to favor the σ-bond metathesis. Remarkably, the use of chiral diphosphines to modify the Cu(I) salt was the key to deliver the Bpin moiety with asymmetric induction.89 A more detailed mechanism for the borylation of α,β-unsaturated carbonyl compounds was described by Lin with the aid of DFT calculations.90 The σ-bond metathesis step has also been elucidated, experimentally and theoretically, in the copper-catalyzed borylation of aryl halides with B2pin2 by Marder and Kleeberg.91 The activation of B2pin2 could also be carried out in the presence of Cu(II) salts to promote β-boration of α,β-unsaturated carbonyl compounds in water as solvent. Santos has proposed a mechanism to activate the B2pin2 where amine/H2O could contribute to the preactivation of the diboron to form the Cu(II)-boryl species (Scheme 23A).92 In a similar context, Kobayashi has postulated a σ-bond metathesis step between B2pin2 and Cu(OAc)2 to form a Cu(II)-boryl species by assistance of a chiral 2,20 -bipyridine ligand (Scheme 23B), which resulted in a very efficient catalyst for the asymmetric β-boration of α,β-unsaturated carbonyl compounds in water as solvent.93 The first example of copper-catalyzed diboration of alkenes with Cu complexes involved NHC ligands. Fernańdez and Pe´rez studied the nature of the interaction between the unsaturated complex [Cu(NHC)(NCMe)]+ and B2cat2 through a theoretical DFT study with the B3LYP functional in order to understand the activation mode.94 The results were conclusive, in favor of the [Cu(NHC)(σ-catB–Bcat)]+ (18) description (Fig. 3), where the A

N open to air O

H

H2O

O B B

O

-

O B

O

O

Cu(II)-picoline

O

+

H

CuCl

O B

B O O

Cu

O N

+

O HO B O

HCl

+

B O

O B B

O

O

N

+

N

t

t

Bu OH AcO

H2O

Bu

Cu

HO OAc

Scheme 23 Activation of B2pin2 with Cu(II) in water.

N

N

t

t

Bu

Cu

HO O

B O

O H

Bu

60


Cu

2.265

B2 1.721

2.268

B1

18

Figure 3 Formation of the sigma adduct [Cu(NHC)(σ-catB–Bcat)]+.

unbroken BdB bond coordinates Cu as a B–B sigma adduct. The presence of a base seems to favor the heterolytic cleavage of the diboron in the complex [Cu(NHC)(σ-catB–Bcat)]+, to generate the catalytically active copper boryl complex. The difference on reactivity between B2cat2 and B2pin2 in the diboration reaction of alkenes catalyzed by carbene-ligated copper(I) complexes was further studied by Lin and Marder.95 The higher reactivity of B2cat2 versus B2pin2 in this reaction results largely from the enhanced electrophilicity/ Lewis acidity of the former, which significantly lowers the barrier in the product-forming σ-bond metathesis step (Fig. 4). The relative barriers in the reactions of B2cat2 and B2pin2 with (NHC)CuOMe are much closer than with analogous CuR systems. The reagent B2pin2 is, after all, the most used diboron reagent in copper borylation reactions and also in multicomponent processes whereupon the σ-bond metathesis step to form the CudBpin bond is the initial key step.96 When moving to tetra(alkoxy)diboron activation by silver complexes, we found only two significant attempts to activate B2cat2 and B2pin2. In both cases, the common feature is that the silver complex is modified with NHC ligands. Fernańdez and Peris found that the dimer [(mentimid)2Ag]AgCl2 (mentimid ¼ 1-methyl-3-(+)-methylmenthoxide imidazolium chloride)

0.0 (−0.0) L

Cu

O B O

O +

L

CH3

O O B B O O

O B O Cu O CH3 −9.0 (−25.0)

0.0 (−0.0) L

Cu

O B B O O L Cu O O

O +

CH3

1.5 (−16.1)

O O B B O O

TS

−1.8 (−18.7)

−15.6 (−31.5) O O B

O B O

O L

Cu

L −26.3 (−27.1)

O

Cu

O B

CH3

TS

−21.7 (−23.5)

O B O

O CH3

O L

Cu

B O

+

CH3 L Cu

O B O

+ O B O CH3 O

Figure 4 Calculated σ-bond metathesis barriers involving B2cat2 and B2pin2.

O B O CH3 O

62


O

O

O

B B O

N Ag Cl N

O

O

N Ag

t

KO Bu

N

B

N B

tO

Bu

O

O

O Ag

N

B O

+

O

B OtBu

O

19

Scheme 24 Activation of B2pin2 via σ-bond metathesis with Ag(I) complexes.

could react with B2cat2 and promote the diboration of alkenes.97,98 Almost one decade later, Yoshida used [(IMes)AgCl] to activate B2pin2 and apply the corresponding (IMes)Ag-boryl species in a formal hydroboration of alkynes.99 While in the diboration reaction a base was not required, in the case of hydroboration, the authors justify the use of the KOtBu to assist the σ-bond metathesis between [(IMes)AgCl] and B2pin2 (Scheme 24). The use of gold(I) complexes to activate diboron reagents is even less prominent, although the scarce examples represent a nonexisting goldcatalyzed diboration of alkenes with a concomitant high selectivity toward the diborated product.98,100 Westcott, Baker, and Marder found that an electron-rich phosphane gold complex could be used in the first catalyzed diboration of alkenes. The reaction of B2cat2 with alkenes in the presence of [AuCl(PEt3)] and ethane-1,2-diylbis(dicyclohexylphosphane) yielded exclusively the 1,2-bis(boronate) ester.100 2.1.3 Activation by Metal Complexes of Group 9 Although the gold standard for diboration chemistry described in the last example represented the first highly selective diboration of alkenes by transition metal complexes, Marder, Westcott, and Baker also described in the same work that [RhCl(PPh3)3] was found to catalyze the diboration of vinyl arenes using B2cat2; however, product selectivities were complicated by a competing β-hydride elimination route.100 Further attempts have improved only slightly the chemoselectivity on the diborated product but have made possible to run the reaction in a enantioselective manner by the selection of the appropriate chiral phosphine modifying the Rh(I) complex.101–103 Although the oxidative addition of the BdB bond in B2cat2 was proposed as a key step in the catalytic cycle, evidence for this reaction came from the groups of Marder and Norman. A number of different tetra(alkoxy)diboron reagents were activated by either [RhCl(PPh3)3] (Scheme 25) or [Rh(μ2-Cl) (PPh3)2]2, as evidenced by multinuclear NMR spectroscopy.104 The authors

63


O

O

O

O

O

B B O

O

O

O

O

B B

O B B O

+

+

+

PPh3 Cl Ph3P Rh PPh3

PPh3 Cl Ph3P Rh PPh3

Cl PPh3 Ph3P Rh PPh3

O

O B

O O

PPh3

PPh3

PPh3

O

O B

Cl B O Ph3P Rh PPh3

Cl B O Ph3P Rh PPh3

20a

20b

O B

O Cl Rh B O Ph3P PPh3

O

20c

Scheme 25 Activation of B2cat2 and its 4-Bu and 3,5-Bu analogs by oxidative addition to [RhCl(PPh3)3]. t

t

realized that B2cat2 and its 4-But and 3,5-But diborane analogs, which have shorter BdB bonds than B2pin2, are nonetheless easier to oxidatively add to Rh(I). It has to be noted that the bis(boryl) complex [RhCl(PPh3)2(Bcat)2] (20a) was previously detected by the slow reactivity of [Rh(μ2-Cl)(PPh3)2]2 with an excess of HBcat through the intermediate [RhCl(H)(PPh3)2 (Bcat)].105 Likewise, addition of B2cat2 to the electron-rich complex [RhMe(PMe3)4] was thought to proceed via initial dissociation of a phosphine ligand followed by oxidative addition of the BdB bond. A subsequent reductive elimination step generated MeBcat along with the boryl complex [Rh(Bcat)(PMe3)4] (21), which has trapped the dissociated ligand (Scheme 26). Addition of a second equivalent of B2cat2 proceeded smoothly to give the tris(boryl)rhodium(III) complex fac-[Rh(Bcat)3(PMe3)3] (22) (Scheme 26). The fac-arrangement of the boryl groups was attributed to the strong trans-influence of these ligands, arising from their significant σ-donating ability.106 DFT calculations suggest that the boryl ligands prefer to occupy sites that have minimal metal boryl σ*-antibonding character.107 Oxidative addition of BdB bonds has also been observed in Rh-alkene complexes. For example, [Rh(η5-C5H5)(PMe3)(C2H4)], [Rh(η5-C5H5) (PPh3)(C2H4)], and [Rh(η5-C5H4CF3)(PMe3)(C2H4)] led to B–B oxidative addition of B2pin2 by photolysis in hexane at 10 °C to form [Rh(η5C5H4CF3)(PR3)(Bpin)2] (23) (Scheme 27).108 Interestingly, competition

64


Bcat O O

B B

O

+

O

PMe3

[RhMe(PMe3)4]

Rh

Me3P

PMe3 MeBcat

PMe3 21

Bcat O O

B B

O

+

O

Bcat

Me3P

Bcat

Me3P

PMe3

Rh

Rh PMe3 PMe3

Bcat

Me3P

PMe3

PMe3

21

22

Scheme 26 Activation of B2cat2 with [RhMe(PMe3)4].

R⬘ R⬘ O

O B B

O

O

Rh

+

Rh

R3P

R3 P

B O O

B O

O

23

Scheme 27 Activation of B2pin2 with [Rh(η5-C5H5)(PR3)(C2H4)].

experiments using HBpin and B2pin2 were performed with these rhodium complexes and results suggested a slight preference for B–B oxidative addition over BdH bond activation. Braun and coworkers have investigated the reactivity of [Rh(Bpin) (PEt3)3] toward a number of ketones and imines. The resulting insertion (into the RhdB bond) products were subsequently treated with B2pin2 to give unique diborated products through a possible mechanism involving B–B oxidative addition and reductive elimination (Scheme 28).109 B2pin2 has also been activated by [Rh(Bpin)(PEt3)3] in the presence of functionalized arenes to accomplish their selective borylation.110 Bis(pinacolato)diboron and bis(neopentyl glycolato)diboron have been added to α,β-unsaturated ketones, esters, nitriles, and aldehydes through [RhCl(PPh3)3] activation, presumably via oxidative addition.111 However, Nishiyama has suggested a σ-bond metathesis step to form Rh(III)-boryl species, from B2pin2 and [Rh(III)(OAc)2(OH)(Phebox)] complex, which

65


R PEt3 N O

O B B

O

+

O

Et3P

O B O

R PEt3

Rh

Et3P

PEt3

O

Rh

B

+

O

O O B

N

O B

O

PEt3

Scheme 28 Activation of B2pin2 with a rhodium aryl species.

O

R

O

O O

B B

O O

O B O Rh OH2 OAc

OAc Rh OH2 OAc

+

O

+

O

B

OAc

N

N O

R N

N

R

O

R

Scheme 29 Activation of B2pin2 with [Rh(III)(OAc)2(H2O)(Phebox)].

resulted in useful catalytic systems in the asymmetric β-boration of α,β-unsaturated carbonyl compounds (Scheme 29) from B2pin2 and [Rh(III)(OAc)2(H2O)(Phebox)] complex.112 Formation of a boryl acetate by-product was confirmed by the authors using 11B NMR spectroscopy at 60 °C. The same authors used this strategy to promote asymmetric diboration of terminal alkenes with high enantioselectivities and chemoselectivities. In that case, the use of a base, such as NaOtBu, accelerated the reaction, and the authors attributed the beneficial influence to the preactivation of the [Rh(III)(OAc)2(H2O)(Phebox)] complex toward the σ-bond metathesis step.113 Alternatively, Fernańdez and Bo demonstrated that Rh(III) complexes modified with NHC ligands could catalyze the diboration of cyclic systems, postulating the oxidative addition of B2pin2, or bis(neopentylglycolato) diboron (B2neop2) to the Rh(III) system as the initial key step (Fig. 5).114 Moving from Rh to Ir activation of diboron compounds, it is somewhat surprising that only a few studies have focussed on the stoichiometric addition of diborane compounds to iridium complexes, considering the wealth of information on the iridium-catalyzed borylation of alkanes and arenes. Early studies have shown that B2cat2 adds cleanly to [IrCl(PEt3)3] to give bis(boryl) complex [IrCl(Bcat)2(PEt3)2] (24) with the concomitant decoordination of PEt3 (Scheme 30).115 A single-crystal X-ray diffraction study confirmed that this complex was isomorphous with its rhodium analog. The iridium complex differs in solution from the rhodium species since

+

N N +

N N

HN

NH

HN

Beg

Rh

Rh Beg

+

N N

NH

NH

HN Beg

Rh

21.7 19.5

B

Beg Beg B O

HN Beg

21.6 dB---B=2.15 Å

O

O +

dB---B=2.14 Å

+

N N

Beg

NH Rh Beg

10.6

B O

dB–B=1.70 Å

5.4

dB---B=2.18 Å

C

0.0

+

Beg (eg = ethyleneglycolato = –OCH2CH2O–) Beg

A +

N N

Beg

NH

HN Rh

+2

H N

Rh

N N

HN dB---B=2.89 Å

Figure 5 Energy profile for the formation of cationic bisboryl Rh(III)-NHC complex. Electronic energy as kcal/mol. Distances between B atoms are given in Å.

67


Bcat O O BB O O

+ [IrCl(PEt3)3]

Et3P Cl

Ir 24

Bcat PEt3

PEt3

Bcat Et3P Cl

Ir PEt3

Bcat PEt3

25

Scheme 30 Activation of B2cat2 with [IrCl(PEt3)3] through oxidative addition.

the saturated octahedral cis,mer-[IrCl(Bcat)2(PEt3)3] (25) can be formed from 24 by coordination of PEt3 (Scheme 30). Similar reactivity of B2cat2 with [IrCl(η2-COE)(PMe3)3] (COE ¼ cyclooctene) was reported to give cis,mer-[IrCl(Bcat)2(PMe3)3].116 Theoretical calculations, using the DFT method, examined the iridiumcatalyzed borylation of benzene with diboron source B2eg2 (eg ¼ ethyleneglycolato, dOCH2CH2Od). This study found that oxidative addition of the BdB bond occurred at a lower energy than the corresponding CdH bond activation step.117 A tris(boryl)iridium(III) complex was also concluded to be catalytically active, and an unusual seven-coordinate iridium(V) species is believed to be involved as a key intermediate in the catalytic cycle.118 A tris(boryl)iridium(III) complex has been previously prepared and structurally characterized by Marder from the reaction between [Ir(η5-indenyl)(η4-COD)] (COD ¼ cyclooctadiene) and excess of catecholborane.119 While electron-rich second and third row metals are known to readily activate the strong BdB bond in most starting diborane(4) compounds, a few studies have suggested that this step may also be carried out using selected first row metals. For instance, Marder, Norman, and coworkers have shown that B2cat2 reacts with a cobalt(0) complex containing strong σ-donor phosphine ligands, [Co(PMe3)4], via oxidative addition to give the 17-electron bis(boryl)cobalt complex cis-[Co(Bcat)2(PMe3)3] (26) (Scheme 31).120 The relatively short B–B interaction in 26 (it is only ˚ longer than the BdB bond in B2cat2) suggests that this complex 0.507 A can be viewed as lying part way along an oxidative addition reaction coordinate.

68


PMe3 O O

B B

Me3P

O

+ [Co(PMe3)4]

O

Co

Me3P PMe3

O B O 2.185 B O O

26

PMe3 Me3P

Co

Me3P

O B O 2.192

PhMe2P

PMe2Ph O B O Co 2.271

PhMe2P

B O O 27

B O O 28

Scheme 31 The activation of B2cat2 with [Co(PMe3)4] to generate bis(boryl)cobalt complex.

O

Me O

O B O

B O

Me3P

+ Me3P

O B

PMe3 Co

PMe3 Me3P

+

Co

PMe3

PMe3

O

B

Me

O

PMe3 29

Scheme 32 The activation of B2cat2 with [Co(Me)(PMe3)4] through σ-bond metathesis.

The activation of B2(4-Mecat)2 (4-Mecat ¼ 1,2-O2-4-MeC6H3) by [Co(PMe3)4], or B2cat2 with [Co(PMe2Ph)4] afforded the complexes [Co (PMe3)3(B-4-Mecat)2] (27) and [Co(PMe2Ph)3(Bcat)2] (28), respectively. For these complexes, the acute B–Co–B angle and short B–B distance are similar to complex 26 and fundamental features for this type of Co(II) bisboryl complex (Scheme 31). A molecular orbital analysis, on the basis of DFT calculations, for the model complex [Co(PH3)3(B(O2C2H2))2] does reveal the presence of a three-center CoB2 interaction consistent with some degree of weak BdB bonding involving one of the Co d-orbitals and the in-phase combination of the two “empty” boron p-orbitals from the two B(O2C2H2) ligands, which is consistent with these observed structural features. Interestingly, an activation of B2cat2 by [Co(Me)(PMe3)4] through a σ-bond metathesis step could be observed resulting in the production and isolation of the Co(I) species 29 and MeBcat (Scheme 32),121 accommodating the boryl group on the axial position.107 The activation of B2pin2 with [Co(PNP)(OtBu)] afforded a new compound [Co(PNP)(Bcat)], which represents the first example of a square planar Co(II)-boryl complex (Scheme 33).77 Interestingly, the mechanism suggests an activation of the diboron by σ-bond metathesis instead of an oxidative addition pathway.

69


PiPr2 O

O B B

O

+

N

PiPr2 O

OtBu

Co

Co B

N

O PiPr2

O

O

PiPr2

t

B O Bu O

30

Scheme 33 The activation of B2pin2 with [Co(PNP)(O Bu)] through σ-bond metathesis. t

CO

CO

CO O

O O

B B

O O

+

OC

[Fe(CO)5]

B Fe

OC CO

B CO

O

O

O

OC

O

OC

B Fe B

O CO

O

OC

O

OC

O

B

O

B

O

Fe

CO

O

31

Scheme 34 The activation of B2cat2 with [Fe(CO)5] through oxidative addition.

2.1.4 Activation by Metal Complexes of Group 8 Hartwig successfully demonstrated the photolytic activation of the BdB bond in B2cat2, and related substituted catechol derivatives, with iron pentacarbonyl.122 These oxidative addition reactions of the diborane reagents to the presumably photochemically in situ generated [Fe(CO)4] were rapid. Full conversion of [Fe(CO)5] gave the thermally sensitive complex cis-[Fe(Bcat)2(CO)4] (31) and derivatives (Scheme 34). Although the roles of ruthenium and osmium complexes in diboration and borylation have not yet been studied in significant detail, elegant stoichiometric studies with electron-rich osmium complexes have been carried out by Roper and coworkers.123,124 Indeed, addition of B2cat2 to [Os(CO)2(PPh3)2], arising from the reductive elimination of o-tolylBcat in a starting boryl complex, resulted in the bis(boryl) complex trans,cis,cis[Os(CO)2(PPh3)2(Bcat)2] (32) (Scheme 35). Likewise, the analogous ruthenium complex trans,cis,cis-[Ru(CO)2(Bcat)2(PPh3)2] (33) was readily prepared by the addition of B2cat2 to [Ru(CO)2(PPh3)3], along with the loss of phosphine. Although the analogous bis(boryl) species [OsCl(H) (Bcat)2(PiPr3)2] (34) has been prepared and structurally characterized, this compound was generated from the addition of excess catecholborane to [OsCl2(H)2(PiPr3)2].125 The short H–Os–B angles of 54–58° in 34 suggest that a type of σ-bonding interaction involving the HdB bond is present. Indeed, complex 34 reductively eliminated HBcat under an atmosphere of CO to give the boryl complex [OsCl(Bcat)(CO)2(PiPr3)2].

70


OC OC

PPh3 Bcat Os R PPh3

R = o-tolyl

Ru

[Os(CO)2(PPh3)2]

Ph3P RBcat O O B B O O

CO Ph3P

CO Ph3P

O CO B O 32

CO PPh3

Ph3P CO

Os

PPh3

O B O

Ph3P

Ru

Ph3P

CO

O B O B O O

33

Scheme 35 Formation of the bis(boryl) complex trans,cis,cis-[Os(CO)2(PPh3)2(Bcat)2] (32) and trans,cis,cis-[Ru(CO)2(PPh3)2(Bcat)2] (33).

2.2 Metal Activation of Halide- or Amine-Substituted Diboron Compounds The diboron tetrafluoride, F2B–BF2, can react with olefins in a direct way, but slightly slower in comparison with Cl2B–BCl2, providing relatively stable diborated products. In 1959, Schlesinger noted that the reaction of acetylene with diboron tetrafluoride did not proceed to give addition products even at 100 °C.3 To obtain the diborated alkene product, a mixture of the reagents was heated at 120–140 °C for 24 h in a sealed tube of such dimensions that the estimated initial pressure was about 7 atm. In that particular case, the activation of the diboron tetrafluoride by a transition metal complex could also offer a better performance toward the diboration of alkenes and alkynes. Therefore, in 1998, it was demonstrated that the reaction between [Pt(η2-C2H4)(PPh3)2] and a slight excess of F2B–BF2 in toluene solution afforded, after workup, pale-yellow crystals of the complex cis-[Pt(BF2)2(PPh3)2] (35) (Scheme 36).126 There was no decisive crystallographic evidence for platinum-to-boron π-backbonding. The reaction between complex cis-[Pt(BF2)2(PPh3)2] and two equivalents of the alkyne di-p-tolylethyne was investigated, but little reaction in dichloromethane was observed. An analogous activation of F2B–BF2 with [Pt(η2-C2H4) (DPPB)] [DPPB ¼ 1,4-bis(diphenylphosphino)butane] afforded the complex cis-[Pt(BF2)2(DPPB)] (36) (Scheme 36).127 The reaction between Vaska’s compound, trans-[IrCl(CO)(PPh3)2], and F2B–BF2 afforded a complex characterized by X-ray crystallography as the iridium(III) (tris)boryl species fac-[Ir(BF2)3(CO)(PPh3)2] (37) (Scheme 36). The molecular structure of 37 comprises an octahedral iridium(III) center bonded to three

71


F F

F B B

F

Pt

+

F

PPh3

Ph3P

PPh3

Ph3P

Pt

F

F B B

F

+

Cl

F Ph3P

Ir

CO

Ph3P

Pt 36

B

F F B F

Ir

B

F

F

PPh2

F

35

PPh3

Ph2 P

B F B F

B F B F F

F F

OC Ph3P 37

Scheme 36 Activation of F2B–BF2 through oxidative addition with [Pt(η2-C2H4)(PPh3)2], [Pt(DPPB)(η2C2H4)], and trans-[IrCl(CO)(PPh3)2].

BF2 groups in a fac configuration, two cis phosphines, and one carbonyl ligand. A degree of positional disorder involving the carbonyl group and the trans-related BF2 ligand was present. The group of Norman studied the reactivity of B2(NMe2)2Cl2 toward [Pt(η2-C2H4)(PPh3)2]. In this case, exclusively products arising from BdB bond activation were verified, that is, cis-[Pt{B(NMe2)(Cl)}2(PPh3)2] and trans-[Pt(Cl){B(NMe2)(Cl)}(PPh3)2].128 Bearing the experience of Norman in mind, the Braunschweig group reconsidered the reactivity of halide-substituted diboranes toward low-valent platinum complexes evaluating the question whether a selective oxidative addition of BdX bonds is possible in the presence of a BdB bond.129,130 Indeed, the addition of 2 equiv. of B2Mes2Br2 to [Pt(PEt3)3] leads to an unusual diboran(4)yl ligand containing a dative PtdB bond, along with the concomitant formation of the Lewis acid–base adduct (Et3P)B2Mes2Br2. This chemistry was expanded to include a number of diborane(4) derivatives and all proceeded via activation of the BdX and not the BdB bond.131–134 Conversely, oxidative addition of the BdB bond within [2]borametalloarenophanes 38 was reported to proceed smoothly and in high yields with platinum(0) phosphine complexes to give the corresponding bis(boryl)platinum(II) complexes 39 (Scheme 37).135–137 Subsequent addition of these complexes to alkynes gave the ansa-bis(boryl)alkene products. An interesting study by Braunschweig and Koster involved the activation of 1,2-diaminodichlorodiboranes B2(NC4H8)2Cl2 and B2(NC5H10)2Cl2 with Na[C5H5Fe(CO)2] at ambient temperature, with concomitant

72


tBuLi / tmeda

Cr

B

NMe2

[Pt(PEt3)4]

Cr

B

NMe2 Pt(PEt3)2

Cr

Br(Me2N)B–B(NMe2)Br

B

B

NMe2

NMe2

38

39

Scheme 37 Activation of [2]borametalloarenophanes with platinum(0) phosphine complexes.

N

N N N B N N N

Cl B H

H M

[M(μ2-Cl)(COD)]2 M = Rh, Ir

H

N

N

N

B

B N

N

CO

N H

[M(CO)6]

N B

M = Cr, Mo, W

N

B H

H

CO M CO

N CO

N

N

Scheme 38 Activation of doubly base-stabilized diborane(4) [HB(hpp)]2.

formation of the iron diborane(4)yl complexes [Cl(R2N)B–B(NR2)Fe (C5H5)(CO)2] and NaCl.138 It is plausible that cleavage may arise from an oxidative addition pathway as these 1,2-diaminodichlorodiboranes have been subsequently activated by platinum(0) complex [Pt(PEt3)4]. Himmel has launched an exceptional study on the activation of specific doubly base-stabilized diborane [HB(HPP)]2(HPP ¼ 1,3,4,6,7,8hexahydro-2H-pyrimido-[1,2-a]pyrimidinate). The two guanidinate bridges prevent oxidative addition reactions with complete cleavage of the BdB bond and formation of a diboryl complex. Nevertheless, the engagement of the BdB bond in the direct metalddiborane bond should lead to significantly elongated BdB bond distances in the diborane ligand. Complexes [M{HB(HPP)}2(CO)4] (M ¼ Cr, Mo, or W) were prepared by photolysis of a reaction mixture of [HB(HPP)]2 and [M(CO)6]. Reaction between [HB(HPP)]2 and [MCl(COD)]2 furnished the complexes [M(cod){HB(HPP)}2Cl] (M ¼ Rh or Ir) (Scheme 38).139 The experimental results show that the bonding in the complexes [M(COD){HB(HPP)}2Cl] (M ¼ Rh or Ir) involves predominantly the BdB bonding electrons, as a consequence of the B2–M three-center bond converse to the bonding in the group 6 complexes. The BdB bond is significantly weakened, but the two bridging hpp substituents prevent complete cleavage with formation of a diboryl metal complex. The authors define this situation as a frozen intermediate at an early stage of an oxidative addition. A quantum-chemical

73


investigation of the bonding of these diborane systems with a series of transition metal complexes has also been carried out by the same authors.140 A key step in many transition metal-catalyzed diboration reactions is oxidative addition of the BdB bond of a diborane(4) compound (R2B–BR2) to a low-valent transition metal center affording a metal bisboryl complex of the form LnM(BR2)2.

2.3 Nanoparticle Activation Within the last decade, another source of diboron activation has been promoted in basis to the efficient application to catalysis. This is the use of nanoparticles which can interact with the diboron reagent and develop an enhanced performance toward diboration reaction. Fernańdez observed in 2008 that the in situ formation of Au nanoparticles, from Au(I) complexes, could not only activate the B2cat2 but also deliver the boryl units on alkenes with total chemoselectivity.141 The gold nanoparticles were estimated to have a mean crystallite size of 10.5 0.3 nm. The gold nanoparticles were stabilized by 2,20 -bis-(diphenylphosphino)-1-10 binaphthyl (BINAP), diphenylphosphinoethane (DPPE), and L-glutathione. The core size and size distribution of BINAP-Au nanoparticles were examined by transition electron microscopy (TEM), and the image shows disperse nanoparticles 6.9 3.0 nm in diameter (Fig. 6). Pure BINAP-Au nanoparticles with smaller diameter (1.7 0.3 nm) were alternatively synthesized following the Fujihara protocol,142 from HAuCl4 + BINAP in the presence of NaBH4, and eventually used in the catalytic diboration reaction. The activity and chemoselectivity observed were 50

Distribution

40

30

20

10

0 5

10

15

20

25

30

Particle diameter (nm)

Figure 6 In situ BINAP-stabilized gold nanoparticles: TEM (500,000) and core size distribution.

74


similar to those of the in situ BINAP-stabilized gold nanoparticles. Interestingly, the recovered reddish solid BINAP-stabilized gold nanoparticles were reused in a new catalytic diboration reaction of styrene. Conversion and total chemoselectivity toward the diborated product are maintained high. This shows that the catalytic system was stable and that it could be recycled to activate new diborons and promote diboration on olefins. Unfortunately, the use of chiral stabilizers of the gold nanoparticles did not contribute to induce asymmetry in the new CdB bond formed. It was suggested that the activation of the B2cat2 could be involved in a base-mediated σ-bond metathesis instead of oxidative addition. This hypothesis gained support from the cross-addition experiment, in which B2cat2 and B2((4-Me)cat)2 were simultaneously added to styrene and cross-diborated product was observed. Further work in this area came from Corma et al.143 about magnesiasupported Cu(II) or copper oxide nanoparticles which exhibited a unique regio- and stereoselectivity in the catalyzed monoborylation of alkynes with bis(pinacolato)diboron, assisted by triphenylphosphine. In the same work, supported platinum on magnesia exhibited higher catalytic activities even in the absence of triphenylphosphine, but the products formed were the bis-boronated alkenes. Ceria was also a suitable support for the platinum nanoparticles. Because the organoborate products obtained with CuO/MgO and Pt/MgO were different, it was proposed that the activation of B2pin2 took place in a different way. The active sites formed by platinum atoms of the metal nanoparticles on the solid surface could interact with B2pin2 to promote the oxidative addition (Scheme 39A). The supported copper catalyst only leads to one CdB bond, and it is particular relevant that requires the assistance of triphenylphosphine as promoter. In that case, a Lewis acid (boron)/base(phosphorous) adduct was suggested to participate in the activation process (Scheme 39B). Braunschweig et al. also reported Pt sponge- or Pd/C-activated borametalloarenophanes to conduct catalyzed diboration of propyne and 2-butyne in a heterogeneous fashion.136 More recently, Jin et al. prepared PtNPore from Pt–Cu alloy and conducted the activation of B2pin2 toward the catalytic diboration of terminal alkynes affording the corresponding 1,2diborylalkene as cis-adduct in 99% yield within 40 h.144 However, the leaching experiments and inductively coupled plasma (ICP-MS) showed that Pt atoms in PtNPore catalyst were leached to the reaction solution. The same authors prepared the analog AuNPore system from Au30Ag70 forming a thin film that possesses a bicontinuous porous structure with an average diameter of 30 nm for both ligaments and nanopores. When the authors

75


A

R1

C

R2

C

Bpin

Pt B2pin 2

Bpin

R1

C

Support

R2

C

Bpin

Bpin

Bpin

Bpin

Pt

Pt Support Support

R1

C

C

B

B2pin2 R1

Bpin

R2

OH

R2 C

+

C

Bpin

O

O B B

PPh3

O

O PPh3

Cu

H

Support H2O

+

R2 R1 C

Bpin

PPh3

Bpin

C

Cu

Bpin Cu

–

Support

–

Support R2 R1 C

C Bpin Cu

–

R1

C

C

R2

Support

Scheme 39 Suggested mechanism for B2pin2 activation and reactivity with alkynes by Pt and Cu nanoparticles.

studied the activation of B2pin2 and the catalytic diboration of terminal alkynes, they found a remarkably increased activity with completed diborated reactions in 1.5 h, giving the corresponding cis-adduct as the major isomer with a small amount of trans-adduct. It is worth noting that AuNPore was active only in toluene as solvent. The leaching experiments and ICP-MS analysis indicated that no gold catalyst was leached.

76


The recovered AuNPore catalyst exhibited high recyclability without decreasing the catalytic activity after reusing for two more cycles. However, other diboranes such as bis(neopentylglycolato)diboron (B2neop2) and bis(catecholato)diboron (B2cat2) were not activated by AuNPore. Other nanoporous metal catalysts such as nanoporous palladium (PdNPore), nanoporous copper (CuNPore), and nanoporous silver (AgNPore) were totally inactive for the studied diboration. From a mechanistic perspective of the activation mode, as it happened previously with the stabilized gold nanoparticles described by Fernańdez, a cross-addition experiment of B2pin2 and B2hex2 to phenylacetylene showed the formation of a cross-addition product, suggesting that the reaction did not involve the oxidative addition of diborons to Au(0). Although the detailed driving force for the cleavage of the BdB bond in B2pin2 by the AuNPore surface remains uncertain, on the basis of the experimental observations, the authors suggested that B2pin2 gets absorbed onto the lowcoordinated Au atoms on the stepped surface of AuNPore. The BdB bond is cleaved on the surface of AuNPore to give [Au-Bpin] species under the reaction conditions. Next, the adsorbed alkyne reacts rapidly with two [AuBpin] species either through the simultaneous addition path to form the corresponding cis-adduct (Scheme 40) or through the stepwise addition, in which the formation of a vinyl cation intermediate is involved. There might exist an interaction between the vinyl cation and the electron-rich Au atoms. Commercially available and inexpensive γ-Fe2O3 magnetic nanoparticles (particle size 58 nm) also efficiently activate B2pin2 and promote a direct borylation of alkenes.145 The mechanism of this unusual nano-Fe2O3catalyzed aromatic borylation reaction is not clear. The kinetic isotope effect was measured to be 1.3, indicating that a CdH bond activation by oxidative addition to the iron catalyst is not likely. An electrophilic metalation by Fe–B species, followed by reductive elimination, seems conceivable.

O O

B B

O O

AuNPore

O O B R1 C

O C R2 B O

AuNPore

O O B

O B O

R2

R1

+

B O O

O O B

R1

+

H

AuNPore

AuNPore R2 = H

Scheme 40 Suggested mechanism for B2pin2 activation and reactivity with alkynes by AuNPore.

77


3. PRECISE ACTIVATION OF UNSYMMETRICAL DIALKOXY-DIAMINO-DIBORON COMPOUNDS The number of examples of unsymmetrical diboron reagents containing B(OR)2 and B(NR2)2 moieties is less abundant than the corresponding tetra(alkoxy)diborons. However, the possibility of synthesizing mixed (RO)2B–B(NR2)2 compounds increases the potential application toward difunctionalized organoboranes. Suginome was a pioneer in this issue and he was able to prepare the unsymmetrical diboron (pin)B–B (dan) compound,146 in which one of the two boron atoms carries the naphthalene-1,8-diaminato (dan) ligand that behaves as an effective protecting group for the boronyl group.147–150 Several transition metal complexes, such as [Pt(dba)2], [Pd(dba)2], [Ni(cod)2], [RhCl(cod)]2, and [IrCl(cod)]2, were used to activate (pin)B–B(dan). Despite the lack of information about the metal boryl species resulting from this activation, their application in the regioselective diboration of alkynes leads to the addition of the Bpin moiety in the internal position (Scheme 41). A recent paper by Borner and Kleeberg discussed the synthesis and reactivity of unsymmetrical diborane(4) compounds (pin)B–B(dmab) (dmab ¼ 1,2-di(methylamino)benzene) and (pin)B–B(dbab) (dbab ¼ 1,2di(benzylamino)benzene) also derived from diaminoboryl ligands.151 They found that reactions of [Pt(η2-CH2]CH2)(PPh3)2] with these new diborane(4) species gave selective and unprecedented formation of unsymmetrical bis(boryl)platinum(II) complexes (Scheme 42). In the same study, (pin)B–B(dmab) and (pin)B–B(dbab) could be activated by σ-bond metathesis with IPrCu(I)–OtBu complexes (IPr ¼ 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (Scheme 43). The selectivity of the BdB bond cleavage reaction in favor of the formation of

HN H N

O B O

[Ir(μ-Cl)(cod)]2 phosphine

O O

B

B

NH

B N H

R

R

Scheme 41 Activation of unsymmetrical diboron (pin)B–B(dan) with Ir(I) complex and further regioselective diboration.

78


Me N

O B

O Ph3P Pt Ph3P

+

B

O

N Me

Ph3P Pt Ph3P

B O B

NMe

MeN

O

Bn N

O B

+

B

O

Ph3P Ph3P

N Bn

Ph3P Pt Ph3P

Pt

B O B

NBn

BnN

Scheme 42 Activation of unsymmetrical diboron (pin)B–B(dmab) and (pin)B–B(dbab) with [Pt(η2-CH2]CH2)(PPh3)2].

Dipp Me N

O B

N Me

Cu

OtBu

Cu

N

Dipp B O

N

N Bn

OtBu

O

Dipp

N Dipp

Bn N

N Cu OtBu

+

B

N Me

Dipp

O B

O

B

N

Dipp

Bn N

Me N

N

+

B

O

Dipp

N

Cu N

B N Bn

Dipp

Scheme 43 Activation of unsymmetrical diboron (pin)B–B(dmab) and (pin)B–B(dbab) with [IDippCu(I)–OtBu] complexes.

diaminoboryl complexes may be explained by the higher Lewis acidity of the Bpin moiety, which favored the formation of pinB–OtBu and the diaminoboryl Cu(I) complexes. In this context, an internally activated, sp2–sp3-hybridized diboron compound, PDIPA diboron (pinacolato diisopropanolaminato diboron) prepared by Santos et al., has shown to easily interact with CuCl salts to favor the B–B cleavage and form the corresponding Cu-Bpin species (Scheme 44) which were very active in the copper-catalyzed, β-borylation of α,β-unsaturated conjugated compounds152,153 and allenoates.154

79


O

O B O

B

N H

+

CuCl

O

O

O B O

Cu

+

B

N

+

HCl

O

Scheme 44 Activation of unsymmetrical diboron PDIPA diboron with CuCl.

4. SUMMARY AND OUTLOOK Symmetrical and unsymmetrical diboron compounds have become useful reagents for organic synthesis.155–165 The mode of activation depends on the nature of the diboron compound and the inherent Lewis acid properties. From tetrahalide diboron compounds to tetra(alkoxy)diboron compounds, the reactivity diminishes significantly at the same time that stability of the resulting organoboron compounds increases. Taking advantage of this trend, chemists chose to use tetra(alkoxy)diboron reagents in organic synthesis and in particular to be added to unsaturated hydrocarbons. To accomplish the activation of the BdB bond, an extra component is required, which in most of the cases is based on transition metal complexes in low oxidative states.166 In principle, the metal complexes modified or not by ligands interact with the diboron to promote the B–B cleavage through oxidative addition or σ-bond metathesis. Sometimes, the unsaturated substrate interacts first with the metal complex and the new metal species reacts further with the diboron reagents. Even the heterogeneous activation of the diboron reagents can be exerted by nanoparticles, with the concomitant recovery and reuse of the active sites. It seems that all the efforts to activate tetra(alkoxy)diboron compounds are justified since the model Cl2B–BCl2 reagent, which develops borylation without metal activation, is too difficult to handle. However, the progress in this field has opened a new window toward the activation of tetra(alkoxy) diboron compounds in a metal-free context by the formation of Lewis acid–base adducts167–175 and their addition to unsaturated substrates can be performed with total chemo-, regio-, and stereoselectivity. Therefore, this historical prospection toward the activation of diboron has moved from direct reaction of Cl2B–BCl2 with alkenes to the current methodology that circumvent the activation of tetra(alkoxy)diboron reagents with simple LBs. But metal activation has been and still is a

80


guarantee of precise activation and delivery of the boryl moieties. The need for simple but efficient methodologies is the driving force of this exciting field, in which the boron atom becomes the real protagonist.

ACKNOWLEDGMENTS We would like to thank our respective research groups for assistance with this manuscript, as well as the Natural Sciences and Engineering Research Council of Canada, Mount Allison University, the Spanish Ministerio de Economia y Competitividad (MINECO)-project CTQ2013-43395P.

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135. Braunschweig H, Lutz M, Radacki K. Synthesis of ansa-[2]boracyclopentadienylcycloheptatrienylchromium and its reaction to the ansa-platinumbis(boryl) complex by oxidative addition of the boron-boron bond. Angew Chem Int Ed Engl. 2005;44:5647–5651. 136. Braunschweig H, Kupfer T, Lutz M, Radacki K, Seeler F, Sigritz R. Metal-mediated diboration of alkynes with [2]bormetalloarenophanes under stoichiometric, homogeneous, and heterogeneous conditions. Angew Chem Int Ed Engl. 2006;45:8048–8051. 137. Braunschweig H, Fuß M, Mohapatra SK, et al. Synthesis and reactivity of boron-, silicon, and tin-bridged ansa-cyclopentadienyl-cycloheptatrienyl titanium complexes (troticenophanes). Chem Eur J. 2010;16:11732–11743. 138. Braunschweig H, Koster M. Diborane(4)yl and bridged borylene complexes from 1,2dipyrrolidino- and 1,2-dipiperidinodiborane(4) derivatives. J Organomet Chem. 1999;588:231–234. 139. Wagner A, Kaifer E, Himmel H-J. Diborane(4)-metal bonding: between hydrogen bridges and frustrated oxidative addition. Chem Commun. 2012;48:5277–5279. 140. Wagner A, Kaifer E, Himmel H-J. Bonding in diborane-metal complexes: a quantumchemical and experimental study of complexes featuring early and late transition metals. Chem Eur J. 2013;19:7395–7409. 141. Ramı´rez J, Sanau´ M, Fernańdez E. Gold(0) nanoparticles for selective catalytic diboration. Angew Chem Int Ed Engl. 2008;47:5194–5197. 142. Tamura M, Fujihara H. Chiral bisphosphine BINAP-stabilized gold and palladium nanoparticles with small size and their palladium nanoparticle-catalyzed asymmetric reaction. J Am Chem Soc. 2003;125:15742–15743. 143. Grirrance A, Corma A, Garcia H. Stereoselective single (copper) or double (platinum) boronation of alkynes catalyzed by magnesia-supported copper oxide or platinum nanoparticles. Chem Eur J. 2011;17:2467–2478. 144. Chen Q, Zhao J, Ishikawa N, Yamamoto Y, Jin T. Remarkable catalytic property of nanoporous gold on activation of diborons for direct diboration of alkynes. Org Lett. 2013;15:5766–5769. 145. Yan G, Jiang Y, Kuang CH, et al. Nano-Fe2O3-catalyzed direct borylation of arenes. Chem Commun. 2010;46:3170–3171. 146. Iwadate N, Suginome M. Differentially protected diboron for regioselective diboration of alkynes: internal-selective cross-coupling of 1-alkene-1,2-diboronic acid derivatives. J Am Chem Soc. 2010;132:2548–2549. 147. Noguchi H, Shioda T, Chou CM, Suginome M. Differentially protected benzenediboronic acids: divalent cross-coupling modules for the efficient synthesis of boron-substituted oligoarenes. Org Lett. 2008;10:377–380. 148. Noguchi H, Hojo K, Suginome M. Boron-masking strategy for the selective synthesis of oligoarenes via iterative SuzukiMiyaura coupling. J Am Chem Soc. 2007;129:758–759. 149. Iwadate N, Suginome M. Synthesis of masked haloareneboronic acids via iridiumcatalyzed aromatic C–H borylation with 1,8-naphthalenediaminatoborane (danBH). J Organomet Chem. 2009;694:1713–1717. 150. Iwadate N, Suginome M. Synthesis of B-protected β-styrylboronic acids via iridiumcatalyzed hydroboration of alkynes with 1,8-naphthalenediaminatoborane leading to iterative synthesis of oligo(phenylenevinylene)s. Org Lett. 2009;11:1899–1902. 151. Borner C, Kleeberg Ch. Selective synthesis of unsymmetrical diboryl PtII and diaminoboryl CuI complexes by B–B activation of unsymmetrical diboranes(4) {pinB–B[(NR)2C6H4]}. Eur J Inorg Chem. 2014;15:2486–2489. 152. Gao M, Thorpe SB, Santos WL. sp2-sp3 Hybridized mixed diboron: synthesis, characterization, and copper-catalyzed β-boration of α,β-unsaturated conjugated compounds. Org Lett. 2009;11:3478–3481.

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153. Gao M, Thorpe SB, Kleeberg Ch, Slebodnick C, Marder TB, Santos WL. Structure and reactivity of a preactivated sp2-sp3 diboron reagent: catalytic regioselective boration of α,β-unsaturated conjugated compounds. J Org Chem. 2011;76:3997–4007. 154. Thorpe SB, Guo X, Santos WL. Regio- and stereoselective copper-catalyzed β-borylation of allenoates by a preactivated diboron. Chem Commun. 2011;47:424–426. 155. Hall DG. Boronic Acids—Preparation, Applications in Organic Synthesis and Medicine. Weinheim: Wiley-VCH; 2005. 156. Mkhalid IAI, Barnard JH, Marder TB, Murphy JM, Hartwig JF. C-H activation for the construction of C-B bonds. Chem Rev. 2010;110:890–931. 157. Marder TB, Norman NC. Transition metal catalyzed diboration. Top Catal. 1998;5:63–73. 158. Ishiyama T, Miyaura N. Metal-catalyzed reactions of diborons for synthesis of organoboron compounds. Chem Rec. 2004;3:271–280. 159. Takaya J, Iwasawa N. Catalytic, direct synthesis of bis(boronate)compounds. ACS Catal. 2002;2:1993–2006. 160. Schiffner JA, M€ uther K, Oestreich M. Enantioselective conjugate borylation. Angew Chem Int Ed Engl. 2010;49:1194–1196. 161. Hartmann E, Vyas DJ, Oestreich M. Enantioselective formal hydration of α,β-unsaturated acceptors: asymmetric conjugate addition of silicon and boron nucleophiles. Chem Commun. 2011;47:7917–7930. 162. Lillo V, Bonet A, Fernańdez E. Asymmetric induction on β-boration of α,β-unsaturated compounds: an inexpensive approach. Dalton Trans. 2009;2899–2908. 163. Dang L, Lin Z, Marder TB. Boryl ligands and their roles in metal-catalysed borylation reactions. Chem Commun. 2009;3987–3995. 164. Mantilli L, Mazet C. Copper-catalyzed asymmetric β-boration of α,β-unsaturated carbonyl derivatives. ChemCatChem. 2010;2:501–504. 165. Calow ADJ, Whiting A. Catalytic methodologies for the β-boration of conjugated electron deficient alkenes. Org Biomol Chem. 2012;29:5485–5497. 166. Beletskaya I, Moberg C. Elementelement additions to unsaturated carboncarbon bonds catalyzed by transition metal complexes. Chem Rev. 2006;106:2320–2354. 167. Lee K, Zhugralin AR, Hoveyda AH. Efficient CB bond formation promoted by N-heterocyclic carbenes: synthesis of tertiary and quaternary B-substituted carbons through metal-free catalytic boron conjugate additions to cyclic and acyclic α,β-unsaturated carbonyls. J Am Chem Soc. 2009;131:7253–7255. 168. Bonet A, Gulya´s H, Fernańdez E. Metal-free catalytic boration at the β-position of α,β-unsaturated compounds: a challenging asymmetric induction. Angew Chem Int Ed Engl. 2010;49:5130–5134. 169. Bonet A, Pubill-Ulldemolins C, Bo C, Gulya´s H, Fernańdez E. Transition-metal-free diboration reaction by activation of diboron compounds with simple Lewis bases. Angew Chem Int Ed Engl. 2011;50:7158–7161. 170. Ibrahem I, Breistein P, Co´rdova A. One-pot three-component highly selective synthesis of homoallylboronates by using metal-free catalysis. Chem Eur J. 2012;18:5175–5179. 171. Wu H, Radomkit S, O’Brien JM, Hoveyda AH. Metal-free catalytic enantioselective C–B bond formation: (pinacolato)boron conjugate additions to α,β-unsaturated ketones, esters, Weinreb amides, and aldehydes promoted by chiral N-heterocyclic carbenes. J Am Chem Soc. 2012;134:8277–8285. 172. Sole C, Gulya´s H, Fernańdez E. Asymmetric synthesis of α-amino boronate esters via organocatalytic pinacolboryl addition to tosylaldimines. Chem Commun. 2012;48:3769–3771.


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173. Kleeberg C, Crawford AG, Batsanov AS, et al. Spectroscopic and structural characterization of the CyNHC adduct of B2pin2 in solution and in the solid state. J Org Chem. 2012;77:785–789. 174. Cid J, Carbo´ JJ, Fernańdez E. A clear-cut example of selective Bpin-Bdan activation and precise Bdan transfer on boron conjugate addition. Chem Eur J. 2014;20:3616–3620. 175. Braunschweig H, Damme A, Dewhurst RD, et al. Quaternizing diboranes(4): highly divergent outcomes and an inorganic Wagner-Meerwein rearrangement. J Am Chem Soc. 2013;135:8702–8707.

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CHAPTER THREE

Catalytic Oxidation of Alcohols: Recent Advances Maximilian N. Kopylovicha,*, Ana P.C. Ribeiroa, Elisabete C.B.A. Alegriaa,b, Nuno M.R. Martinsa, Luísa M.D.R.S. Martinsa,b, Armando J.L. Pombeiroa,*

a Centro de Quı´mica Estrutural, Instituto Superior Tećnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal b Chemical Engineering Department, ISEL, R. Conselheiro Emı´dio Navarro, Lisboa, Portugal *Corresponding authors: e-mail address: [email protected]; pombeiro@tecnico. ulisboa.pt

Contents 1. Introduction 2. Aerobic and Peroxidative Oxidations 2.1 Metal Catalysts 2.2 Organocatalysts, Organic Radicals, and Other Additives 2.3 Prospective Substrates and Oxidation Agents 3. Acceptorless Dehydrogenative Oxidations 4. Oxidative Desymmetrizations 5. Cascade and Sequential Reactions 6. Conversion of Renewable Sources and Hydrogen Production 6.1 Transformation of Renewable Materials into Added-Value Compounds 6.2 Alcohol Oxidation for Hydrogen Storage and Production 7. Irradiation-Promoted Oxidations 7.1 Photocatalytic Oxidations 7.2 MW-Promoted Oxidations 7.3 Others 8. Catalysts Recyclization 8.1 Heterogeneous Solid Oxides, Alloys, and Related Materials 8.2 Supported Catalysts 8.3 Nano, Dispersed and Micellar Catalysts 8.4 ILs and Related Systems with Phase Division 8.5 Other Directions Acknowledgments References

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ABBREVIATIONS [C2mim] 1-ethyl-3-methylimidazolium [C4mim] 1-butyl-3-methylimidazolium [C4py] 1-butyl-pyridine [C6mim] 1-hexyl-3-methylimidazolium [C8mim] 1-octyl-3-methylimidazolium 1-Me-AZADO 1-methyl-2-azaadamantane N-oxyl 2IBAcid 2-iodobenzoic acid ABNO 9-azabiciclo[3.3.1]nonane N-oxyl Aliquat N-methyl-N,N-dioctyloctan-1-ammonium chloride AZADO 2-azaadamantane N-oxyl Bmim 1-buthyl-3-methylimidazolium BOX bis(oxazoline) bpyO bis(oxazoline)α,α0 -bipyridonate Cp cyclopentadienyl Cp* pentamethylcyclopentadienyl DESs deep eutective solvents DIAD diisopropyl azodicarboxylate DKR dynamic kinetic resolution DPIO 4,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl Emim 1-ethyl-3-methylimidazolium FDCA 2,5-furandicarboxylic acid HFCA 5-hydroxymethyl-2-furancarboxylic acid HMB hexamethylbenzene HMF 5-hydroxymethylfurfural Hmim 3-methylimidazolium IBA iodosobenzoic acid IBX o-iodoxybenzoic acid IBXF o-iodoxybenzoic acid with a fluorous tag IL ionic liquid (room-temperature) ketoABNO 2,2,6,6-tetramethylpiperidine-1-oxyl LED light emitting diode MOF metal-organic framework MW microwave NAD nicotinamide adenine dinucleotide NBS N-bromosuccinimide NHC N-heterocyclic carbene NHPI N-hydroxyphthalimide Nor-AZADO 2-azanoradamantane N-oxyl NP nanoparticle NT nanotube NTf2 bis(trifluoromethylsulfonyl)imide OKR oxidative kinetic resolution Oxone potassium peroxomonosulfate KHSO5 PINO phthalimide-N-oxyl SPB surface plasmon band TBAB tetrabutylammonium bromide

Catalytic Oxidation of Alcohols

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TBHP tert-butyl hydroperoxide (tBuOOH) TBN tert-butyl nitrite TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl radical TOF turnover frequency TON turnover number

1. INTRODUCTION The oxidation of alcohols to carbonyl-containing compounds1,2 or their full oxidations3,4 are among the central reactions in organic chemistry5,6 and are of interest for the development of environmentally benign processes,7,8 production of new materials9,10 and energy sources.11,12 Due to their pivotal role in industrial fields and expected further applications,13 these reactions continue to attract a great attention, disclosing new catalysts,14,15 substrates, oxidants with peculiar features, and applications.1 Concerning the oxidants, stoichiometric oxidations with transitionmetal compounds or sulfoxides are still in common use, despite the formation of a large amount of undesirable products.1 The most used oxidants include small organic molecule-based reagents, e.g., Dess-Martin periodinane, Swern, Moffatt, Corey-Kim oxidants, SO3/pyridine, some of them being moisture-sensitive and expensive (e.g., N,N0 -dicyclohexylcarbodiimide, oxalyl chloride), or metal-based systems (such as Jones, Collins, Oppenauer reagents, pyridinium chlorochromate (PCC), pyridinium dichromate, barium permanganate, manganese dioxide, ruthenium tetroxide, silver carbonate).1 A recent environmental compatibility and sustainability approach leads toward aerobic oxidations with transition-metal catalysts (based on Pd, Ru, Fe, Cu, Pt, Au, Ir, Rh, etc.,) and dioxygen or hydrogen peroxide as oxidants.1–14 The use of molecular oxygen as a stoichiometric reoxidant in combination with a catalytic metal has practical advantages due to the favorable economics associated with O2 and the formation of environmentally benign by-products (water and hydrogen peroxide). Advances on the development of new methodologies, oxidation agents, catalysts and applications have been regularly surveyed,1–13 and the field continues to be one of the most extensively and actively investigated areas of current organic synthesis. Newly developed green oxidations of alcohols usually involve active and selective recyclable catalysts that ideally should

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work with dioxygen, air, or other cheap oxidants, not leaving aside toxic or wasteful by-products. However, despite some remarkable advances, only few of the known methods are capable of offering an economic and practical oxidation toward a particular industrially important transformation. Many of the found catalytic systems suffer from high reagent cost, instability, employment of hazardous metals or oxidants, harsh reaction conditions, operational complexity, functional group incompatibility, or production of unprocessable wastes.1 Thus, there is a continuing demand for new catalytic systems that could overcome such challenges. Moreover, other perspectives for alcohol oxidation have been tested, including atom-efficient transformations (e.g., direct synthesis of esters), hydrogen transfer and production, oxidation of natural substrates, such as cellulose, cascade and sequential reactions, etc. The achievements in the alcohol oxidations until 2010 have been covered in several books, book chapters, and reviews,1–15 and thus the current work focuses on the most recent advances in the 2010–2014 period with some notable examples back to 2005.

2. AEROBIC AND PEROXIDATIVE OXIDATIONS Aerobic and peroxidative oxidations of alcohols, in particular of benzylic alcohols, are typical model reactions due to their importance and generality; inexpensive O2, H2O2 or tert-butyl hydroperoxide (TBHP) oxidants, and simple procedures are usually involved.1–15 In this section, an overview of some interesting catalytic systems, which were lately introduced into the field of alcohol oxidation, is presented. This concerns mainly homogeneous systems, since recent advances on heterogeneous catalysts are included in Section 8. Moreover, a glance at new substrates and oxidants which could successfully be used in a near future and make a difference in terms of efficiency, selectivity, economy and/or sustainability of the processes, is also presented.

2.1 Metal Catalysts Historically, Pt, Pd, Ru, Ir, and Rh complexes were among the most effective catalysts for alcohol oxidation. The series was expanded to 3d metals, e.g., V16 and Cu/TEMPO systems (TEMPO ¼ 2,2,6,6tetramethylpiperidine-1-oxyl radical) which have been reinvented and developed,17,18 and now includes representatives of most of the groups


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and subgroups of transition and even nontransition metals.19 In contrast to those of noble metals, the newly introduced catalysts based on abundant 3d and related metals typically operate by redox mechanisms that usually involve one-electron (radical) processes.20 During the last decade, the search for new and effective catalysts for alcohol oxidation has mainly concentrated on finding cheaper and more effective metal–ligand combinations,21 achieving regio- and enantioselective reactions,22 and explaining mechanistic details of action of known catalytic systems.23 In addition, recent reports24,25 showed that there is a continuous interest concerning the structural details of the catalysts. For instance, the role of nuclearity in multinuclear copper(II) complexes26,27 was recently discussed.28 When using dioxygen, a challenge to overcome concerns the fact that it is a four-electron oxidant, while the aerobic oxidation of alcohols to carbonyl compounds involves two electrons. Apart from that, partially reduced oxygen species are usually more reactive than O2 itself. Hence, the introduction of special “oxidation buffer agents” which can balance the specific energetic requirements of the substrates with the possibilities of the oxidant is an important task, and complex, metallorganic, or organocatalysts can play the role of such agents. Generally, the effective catalytic systems contain an organic component, as a ligand in a coordination compound or an additive, but sometimes simple salts, such as Mn(II) acetate,29 can be efficient catalysts. Organic ligands in complexes can play different roles: adjust electronic and steric properties, provide the required solubility or arrangement of central metal ions or protect them from overoxidation or reduction. Thus, a systematic study of the catalytic activity of palladium complexes with commercially available pyridine-containing ligands30 found the conditions where precipitation of Pd black does not occur. Similarly, tertiary phosphine oxides (O]PR3) can be used as ligands for Pd catalysts.31 Recently, the importance of the trinuclear Pd3O2 intermediate [(LPdII)3(μ3–O)2]2+ (L ¼ 2,9-dimethylphenanthroline) in Pd-assisted catalysis was unveiled.32 This trinuclear compound is a product of oxygen activation by reduced palladium species and is an important intermediate in the aerobic oxidation of alcohols.32 The introduction of new ligands is an important task in the development of new catalytic systems. For instance, some polymers can be used as macroligands to host metal ions. The polymer ligands can not only provide the catalyst reutilization, but also stabilize the central metal ions and prevent their aggregation (e.g., precipitation of palladium black, if a Pd(II) catalytic system is applied).33 This approach is rather attractive and combines the

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Scheme 1 Synthesis of Pd(II) complexes with poly(l-lactide) and poly(caprolactone) macroligands.33

advantages of homogeneous and heterogeneous catalysts. It involves coordination of Pd(II) by 4-pyridinemethylene-end-capped poly(l-lactide) and poly(caprolactone) (Scheme 1). The polymer-anchored catalysts are soluble under the applied catalytic conditions, but upon addition of n-pentane or methanol the polymer-anchored catalyst precipitates, and thus can be easily separated from the reaction mixture. These catalysts are effective in the oxidation of several primary and secondary alcohols with O2.33 In a related work, a design of enzyme-inspired star block-copolymers with branched topologies and protein-like tertiary or quaternary structures was performed.34 These polymers incorporate hydrophilic, superhydrophobic, and polydentate metal-binding sites and self-assemble in water, their mode of assembly being controlled by the composition of the polymer. An important feature of the star block-copolymers is that they incorporate perfluorocarbons and, due to that, their emulsions in water can attract and preconcentrate O2 in the vicinity of the active metal site. Addition of Cu(II) and TEMPO leads to an effective catalytic system for oxidation of alcohols to aldehydes in water.34 A series of tetradentate pyridyl-imine terminated Schiff-bases, bis(pyridyl-imine) terminated siloxane and other related polymers, can be used as ligands to host copper(II) ions.35 These CuBr2/polyL/TEMPO catalytic systems (polyL stands for polydimethylsiloxane derived pyridyl-imine terminated ligand) are effective for aerobic oxidations of primary and secondary alcohols under aqueous conditions. Chiral N,O-ligands, e.g., inexpensive L-proline, can also be used to prepare copper catalysts that are particularly effective for the oxidation of sterically hindered, allylic or heterocyclic alcohols such as 1-(3-pyridyl)ethanol, 1-(2-furfuryl)ethanol,


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2-thienyl, 2-furyl and 3-pyridyl methanol.36 Related mono- and dicopper(II) aminopolyalcoholates were easily prepared by self-assembly and also studied.37 The selectivity parameters for oxidative transformations were measured and discussed, supporting free-radical mechanisms. Copper complexes with hydrazone ligands have been extensively studied during the last decade as oxidation catalysts38 and their family continues to grow. For example, an easy to synthesize and to handle trinuclear dihydrazone copper(II) complex [Cu3(L)(μ2–Cl)2(H2O)6] can be used as a reusable (up to eight runs) catalyst for the selective oxidation of a wide variety of alcohols, not being deactivated by N/S-heteroatom-containing substrates.39 Copper-containing metal-organic frameworks (MOFs) based on 5-(4pyridyl)tetrazole building blocks, easily prepared in situ by 1,3-dipolar cycloaddition between 4-cyanopyridine and azide in the presence of copper(II) chloride, were successfully applied40 as precatalysts for the low power (10 W) microwave (MW)-assisted peroxidative oxidation of secondary alcohols leading to the corresponding ketones with yields up to 86% and turnover frequencies (TOFs) up to 430 h1 after 1 h, in the absence of any added solvent or additive. Zr(IV)-based robust MOFs with open Fe- and Cr-monocatecholato metal sites on the structure of the organic linkers were prepared by postsynthetic metal exchange,41 an approach that allows good control over the number of metal-binding sites, and can be used as a facile and efficient way to obtain MOFs that cannot be directly synthesized under solvothermal conditions. The Cr-metalated MOFs are efficient, versatile, and reusable heterogeneous catalysts for the oxidation of alcohols to ketones with TBHP or H2O2 as oxidants. Biomimetic Cu(II) and Fe(II) complexes with bis- and tris-pyridyl amino and imino thioether ligands and vacant (or potentially so) coordination positions (Fig. 1)42 are active as catalyst precursors for the solvent- and halogen-free MW-assisted oxidation of 1-phenylethanol by TBHP, in the presence of pyridazine or other N-based additives. Maximum TOF of 5220 h1 (corresponding to 87% yield) was achieved just after 5 min of reaction time under the low power MW irradiation. The same authors reported43 the catalytic activity of related copper, iron, and vanadium systems with mixed-N,S pyridine thioether ligands. The Cu and Fe complexes proved to be useful catalysts in various MW-assisted alcohol oxidations with TBHP, at 80 °C. Thus, S-containing ligands can also be used to create effective catalyst precursors. Another green and easy to prepare iron-based catalyst, [Fe(BPA)2] (OTf )2, with the commercially available bis(picolyl)amine (BPA) ligand

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Figure 1 Iron and copper complexes with bis- and tris-pyridyl amino and imino thioether ligands.42,43

Figure 2 Bis(picolyl)amine 8-hydroxyquinolinate (HQL).

(BPA),

thymine-1-acetate

(THA),

and

(Fig. 2), chemoselectively oxidizes a variety of secondary alcohols in the presence of primary ones into the corresponding hydroxy ketones within 15 min at room temperature with 3 mol% catalyst loading and H2O2 as oxidant.44 The complex can also be generated in situ and operates similarly to the preformed one. In situ generated iron chloride complexes with thymineacetate or 6-(N-phenylbenzimidazoyl)-2-pyridinecarboxylic acid ligands (Fig. 2) have been also recently synthesized and proved to be selective and convenient catalysts for the oxidation of benzylic and allylic alcohols.45 They can be applied to sensitive compounds like perillyl alcohol, geraniol, or carveol, while diols can be oxidized in good yields without oxidative cleavage of products. Mechanistic investigations reveal that thymine-acetate possesses organocatalytic activity for the oxidation of alcohols. Aerobic alcohol oxidations with vanadium catalysts continue to widen their substrate scope and applications.16,46 The recently studied mechanism of the intramolecular oxidation of benzyl alcoholate ligands in 8-hydroxyquinolinato(L) vanadium(V) complexes of the type [LV(O) (OR)] resembles those proposed for certain metalloenzyme-catalyzed


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Scheme 2 Key steps in base-assisted dehydrogenation for [LV(O)(OR)] complexes.20

oxidations and involves unusual ligand exchange and intermolecular deprotonation at the benzylic position (Scheme 2).20 This biomimetic pathway differs from the previously identified hydride-transfer and radical mechanisms for transition-metal-mediated alcohol oxidations. As a result, new ways to enhance the activity and selectivity of vanadium catalysts were proposed. They include the control of the outer coordination sphere and application of ligands with appropriately positioned pendant bases to serve as proton shuttles. Related V complexes show activity toward the oxidative decomposition of pinacol with CdC bond cleavage and aerobic oxidation of 4-methoxybenzylalcohol and other lignin model compounds.47 Other oxidovanadium(V) complexes with cis-2,6-bis-(methanolate)-piperidine ligands of the type depicted on Scheme 3 were applied as catalysts to convert prochiral alkenols into 2-(tetrahydrofuran-2-yl)-2-propanols, 2-(tetrahydropyran-2-yl)-2-propanols, oxepan-3-ols and epoxides, upon oxidative alkenol cyclization with TBHP as oxidant (Scheme 3).48 These catalysts are rather stable and possess improved chemoselectivity, e.g., epoxidation of geraniol occurs enantioselectively. It was ruled out the vanadium(V) tert-butyl peroxy complex formation is a key step to activate peroxides. Silver N-heterocyclic carbene (NHC) catalysts (Scheme 4) can be applied not only for the selective oxidation of alcohols to aldehydes or carboxylic acids but also for further tandem one-pot synthesis of imines

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Scheme 3 Example of cis-2,6-bis-(methanolate)-piperidine ligands (A) and oxidation of alkenols by TBHP, catalyzed by the piperidine-derived vanadium complexes (B).48

Scheme 4 Example of a Ag(NHC) catalyst (A) and oxidation of alcohols to aldehydes or carboxylic acids and tandem one-pot synthesis of imines catalyzed by them (B).49

Figure 3 Example of a ruthenium(III) catalyst for the aerobic oxidative dehydrogenation of benzyl alcohols.51

(see Section 5).49 Rhodium porphyrin complexes have been successfully applied as catalysts for the selective oxidation of functionalized alcohols, since they tolerate a variety of functional groups, such as methoxy, C]C, and thiofuran moieties.50 The proposed catalyst is robust and does not degrade under the studied conditions (1 atm O2, 80 °C, 7 h). A porphyrin rhodium(III) methoxide complex was identified as a key intermediate in the proposed mechanism. The cyclometalated complex bearing phenylpyridine [RuCl(ppy)(tpy)][PF6] (ppy ¼ 2-phenylpyridine; tpy ¼ 2,20 :60 ,200 -terpyridine) (Fig. 3) is an example of ruthenium(III) catalysts


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Figure 4 Ionic catalyst containing a Ru(III)-complex cation and a α-Keggin-type phosphotungstate anion.52

for the aerobic oxidative dehydrogenation of benzyl alcohols to benzaldehydes.51 The complex was also applied as catalyst for the one-pot synthesis of benzonitriles from benzyl alcohol with ammonia. Another approach to develop new catalysts is the combination of different metals and even different types of complexes in one system, e.g., an ionic compound containing a Ru(III)-complex cation and a α-Keggin-type phosphotungstate anion (Fig. 4).52 This compound is robust because the phosphotungstate [PW12O40]3 anion, although exhibiting a negligible contribution to the activation of benzyl alcohol, significantly stabilizes the structure. On the other hand, due to the high polarity and the ionic nature of the complex, ionic liquids (ILs) can be used as solvents. The catalyst was applied as an efficient catalyst for aerobic oxidations of alcohols, free of base and nitroxyl radical, and can be reused at least 5 without significant loss of activity.52 Other heteronuclear complexes are effective for the solvent-free peroxidative (with H2O2) oxidation of primary and secondary alcohols, e.g., a trinuclear complex with a dicopper(II)–monozinc(II) center.53 Finally, it is noteworthy that the alcohol oxidation reaction can be used for the direct one-pot synthesis of coordination compounds. The application of such a technique allows to generate in situ aldehydes, ketones, and other carbonylic derivatives, which are not available commercially, are unstable or cannot be prepared and isolated by conventional methods. As a result, new interesting coordination compounds can be prepared with ligands which are not attainable by usual synthetic methods.54

2.2 Organocatalysts, Organic Radicals, and Other Additives In spite of their high activity, catalytic systems that employ transition metals exhibit a number of disadvantages. For instance, substrates with a chelating ability can bind to the metal and hamper the reaction. Moreover, the presence of the metal can show some environmental impact. Therefore, new transition-metal-free systems have been searched for.7,13,15 One of such systems mimics the Anelli–Montanari protocol (see below) and employs an oxoammonium salt that carries out substrate oxidation while NO2

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(generated in situ from nitric acid, nitrates, nitrites, or hydroxylamine) regenerates the salt.55 It is possible to operate aerobic NOx systems under halide free conditions; however, participation of halides (bromine, hypobromous acid, nitrosyl bromide, or nitrosyl chloride) as active co-oxidants can potentially widen the scope of such reactions. In addition, an efficient transition-metal-free catalytic system mediated by N-bromosuccinimide (NBS) for the aerobic oxidation of various aromatic alcohols, under mild conditions, was recently reported.56 For instance, benzyl alcohol is oxidized to benzaldehyde with 99% conversion (94.5% selectivity) by the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone–NaNO2–NBS system under 0.3 MPa of O2 for 2 h at 90 °C. The NH4NO3/TEMPO/H+ catalytic system was reported57 as efficient, under mild aerobic conditions, for the chemoselective oxidation of a comprehensive range of alcohols, including those bearing oxidizable heteroatoms (S, N, O), alkyl-, cycloalkyl-, and allyl-type substituted substrates. Very recently, tetra-n-butylammonium bromide was successfully applied58 as simple but efficient organocatalyst for the peroxidative (with TBHP) oxidation of a variety of functionalized benzylic/allylic alcohols under mild conditions. It shows excellent selectivity for secondary benzylic alcohols over aliphatic alcohols. The analog tetra-n-butylammonium iodide was employed for the alfa-oxyacylation of ketones by benzylic alcohols leading to alfa-acyloxyketones, with TBHP, affording moderate to good yields.59 1,2-Di(1-naphthyl)-1,2-ethanediamine efficiently catalyzes the oxidation of alcohols by using TBHP as oxidant. Secondary benzyl alcohols are oxidized in almost quantitative yields, and the catalyst displays a high activity toward hindered cycloaliphatic secondary alcohols.60 Quinine-derived urea has been identified as a highly efficient organocatalyst for the enantioselective oxidation of 1,2-diols using bromination reagents as the oxidants, at ambient temperature, to yield a wide range of α-hydroxy ketones in good yield (up to 94%) and excellent enantioselectivity (up to 95% ee).61 Nitroxyl radicals (Fig. 5), such as 9-azabiciclo[3.3.1]nonane N-oxyl (ABNO), 9-azabiciclo[3.3.1]non-3-one N-oxyl (ketoABNO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-2-azaadamantane N-oxyl (1-Me-AZADO) and 2-azanoradamantane N-oxyl (Nor-AZADO), and especially TEMPO, are widely used promoters for the aerobic oxidation of alcohols to the corresponding carbonyl compounds due to their high efficacy and selectivity.1–23 TEMPO is applied62 in industrial processes using aerobic or Anelli–Montanari1,63 type (neither aerobic nor peroxidative) conditions (Scheme 5): the oxoammonium salt of TEMPO carries out


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Figure 5 Structures of the nitroxyl radicals (from left to right) 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-2-azaadamantane N-oxyl (1-Me-AZADO), 2-azanoradamantane N-oxyl (NorAZADO), 9-azabiciclo[3.3.1]nonane N-oxyl (ABNO), and 9-azabiciclo[3.3.1]non-3-one N-oxyl (ketoABNO).

Scheme 5 Anelli–Montanari's oxidation of alcohols.63

the alcohol oxidation in the presence of an excess of hypochlorite (which may lead to undesirable chlorinated by-products). In contrast, few applications of nitroxyl radicals as catalysts for the peroxidative oxidation of alcohols were reported, despite its enhancing effect on the alcohols conversion to the respective ketones.38f,40,64 Compared to TEMPO, the less hindered AZADO and ABNO radicals exhibit significantly enhanced reactivity toward a wide range of alcohols, including structurally hindered secondary alcohols that TEMPO fails to efficiently oxidize due to the steric congestion near its active center.65,66

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O

O

N

O

NHPI

N

OH

O

O

PINO

Figure 6 N-hydroxyphtalimide (NHPI) and phthalimide-N-oxyl nitroxyl (PINO) radical.

Moreover, the sterically unhindered Nor-AZADO is more catalytically active than AZADO, 1-Me-AZADO, ABNO, and TEMPO in the aerobic oxidation of alcohols to their corresponding carbonyl compounds.65 Other related to TEMPO additives have been used, e.g., N-hydroxyphthalimide (NHPI) which generates the nitroxyl radical phthalimide-N-oxyl (PINO, Fig. 6).66 The PINO radical and analogs have been utilized for a range of aerobic oxidation reactions. PINO has been shown to be more reactive than TEMPO, but it is not stable and is usually formed from NHPI in situ by means of an initiator. Usually the stable nitroxyl radicals alone cannot directly catalyze the oxidation of alcohols with dioxygen or peroxide, so they rely on the assistance of various cocatalysts that play an important role in activating the oxidation agent. The most used cocatalysts are first row transition-metal complexes where Cu compounds with various N-donor ligands account for the prime ones.67 In many instances this combination serves as some kind of model to compare catalytic properties of copper compounds. For example, the performances of two asymmetric tetranuclear (with the {Cu4(μ–O)2(μ3– O)2N4O4} core) and dinuclear (with the {Cu2(μ–O)2N2O2} core) copper(II) complexes were compared in the catalytic TEMPO-mediated aerobic oxidation of benzylic alcohols.28 In spite of their similarity, the complexes perform differently: the tetranuclear copper(II) (R) complex is highly active leading to yields up to 99% and TONs up to 770, while the (S,R)-2 dinuclear complex is not so efficient under the same conditions. However, no solid explanation of the activity differences was proposed. Nevertheless, almost all of the nitroxyl radicals are quite inefficient for the oxidation of aliphatic and secondary alcohols, and require an additional base for the oxidation to proceed. One of the reasons for the poor performance toward secondary alcohols is believed to be the steric hindrance (as secondary alcohols are bulkier) and the involvement of a bimolecular reaction between the Cu-alkoxide and TEMPO on the final step.55 This can be overcome by switching from TEMPO to the less sterically hindered nitroxyl radicals ABNO68 or AZADO.69 Thus, Cu(I)/ABNO systems effectively


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oxidize a wide range of secondary alcohols, including substrates with bulky groups close to the alcohol group, and still be effective for primary alcohols. With the AZADO catalyst system, substrates containing primary and secondary alcohols can also be oxidized in good to excellent yields.69 In contrast to the Cu/TEMPO combination, Fe(III)/TEMPO systems readily catalyze the aerobic oxidation of secondary alcohols and do not usually require any base or less sterically hindered ligands.70 The best activity was observed with weakly coordinating solvents (e.g., dichloroethane) unlike Cu/TEMPO systems, which normally are more active in acetonitrile. The performance and substrate scope (primary and secondary allylic, benzylic, or inactivated aliphatic alcohols) of Fe–TEMPO catalyst systems is improved by the presence of NaCl.70e The use of cobalt and manganese cocatalysts with TEMPO and its derivatives has been known for some time.71 The utilization of nitrate salts suggests that the oxoammonium salt oxidizes the substrates. There are also reports with heterogeneous Cu/Mn oxide cocatalysts; in these cases low loadings of TEMPO were used while the heterogeneous catalyst can be recycled. Cobalt can be used as a sole effective metallic component within a Co(NO3)2/dimethylglyoxime/TEMPO system, with only 1 mol% catalyst loadings.71 The polyoxometalate H5PV2Mo10O40 oxidizes TEMPO to form the oxoammonium salt, which then electroxidizes alcohols to their corresponding carbonyl compounds.72 Despite the fact that this system was used industrially (by DSM),62 it has received little research interest. Recently, functionalized TEMPO was immobilized with an IL on silicacoated magnetic nanoparticles (NPs).71d They catalyze the aerobic oxidation of a range of alcohols and the catalyst can be separated with an external magnet and recycled 10 without a significant loss of activity. The first example of vanadium as the sole metallic component in TEMPO-catalyzed aerobic alcohol oxidation in acetonitrile was recently reported.73 However, the catalyst did not perform well with secondary aliphatic alcohols, even with extended reaction times. The level of sophistication of stable radicals may increase in the near future as it is becoming clear that the performance of catalysts can be improved by tuning both steric and electronic effects of these radicals.74 Another successful approach is the use of heterogenized radicals. Thus, a free-radical porous coordination polymer [Cu(DPIO)2(SiF6)] [DPIO ¼ 4,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl] (Fig. 7), possesses one-dimensional channels with incorporated nitroxyl catalytic sites.

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Figure 7 4,7-Bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl (DPIO).

When dioxygen or air is used as oxidant, this polymer acts as an efficient, recyclable, and widely applicable catalyst for selective oxidation of various alcohols to the corresponding aldehydes or ketones.75 Fullerene has been employed successfully to anchor TEMPO moieties and used as an organocatalyst for the aerobic oxidation of primary and secondary alcohols in the presence of tert-butyl nitrite (TBN) as cocatalyst. The reaction showed a general applicability to various alcohols, and the catalyst was recovered easily and could be recycled for at least seven cycles with no loss in catalytic activity.76 In a rare example of Fe/TEMPO catalyst systems, Fe(NO3)3 or NaNO2 were not used; instead, the FeCl36H2O/TEMPO combination was coupled with a silica support.77 In a related study, TEMPO was covalently bound to silica and combined with FeCl3 6H2O/NaNO2.78 The TEMPO immobilization was performed via reductive amination of 4-oxo-TEMPO with amine-functionalized mesoporous silica SBA-15.79 A very efficient oxidation with a loading of 0.01 mol% of the expensive TEMPO radical, with 8 mol% FeCl3 6H2O and 10 mol% NaNO2, in toluene and O2 (1 atm) at 25 °C, was achieved. The iron salt and heterogeneous TEMPO could be recycled at least for five cycles. A continuous flow approach employing a microreactor with a Fe–TEMPO system comprised of a heterogeneous iron oxide NP catalyst stabilized on a mesoporous aluminosilicate support, was reported.80 A 42% yield of benzaldehyde is achieved in a single pass of the reactor, but the reaction conditions were rather harsh (120 °C and 35 bar of pure oxygen).

2.3 Prospective Substrates and Oxidation Agents Finding or developing new starting materials for the aerobic and peroxidative oxidation of alcohols is an important task, namely by widening the range of substrates that are of significance in fine chemical synthesis. For instance, since aldehydes are produced by the oxidation of alcohols, while aldehydes by themselves can participate in further reactions, aminoxidation of alcohols to nitriles and amide synthesis can be directly achieved using Mn catalysts for both the transformations in one-pot (Scheme 6).81


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Scheme 6 Proposed reaction pathway for the ammoxidation of alcohols over MnO2.81

Scheme 7 Oxidation of indole carbinols using the Fe(NO3)39H2O/TEMPO catalytic system.82

Indole derivatives with carbonyl units (Scheme 7) have been found in natural products, possess versatile bioactivity and are important intermediates in many organic syntheses. The introduction of the carbonyl moieties to indoles significantly enhances their reactivity and can be achieved by oxidation of indole carbinols with Fe(NO3)39H2O/TEMPO/NaCl at room temperature and atmospheric pressure of dioxygen, using toluene as a solvent.82 NaCl is an important accelerator for the oxidation, reducing the reaction time from 15 to 2 h; however, the role of Cl is not quite clear. Recently, there has been a growing interest in new processes to derive chemicals and fuels from renewable carbon feedstocks, e.g., lignocellulosic biomass (see below). Hence, the study of homogeneous oxovanadium and copper catalysts toward aerobic oxidation of lignin model compounds is of great potential.83 In this transformation, the vanadium catalyst affords primarily ketone products, while the copper catalytic system leads to the products of CdC bond cleavage reactions, thus reflecting different mechanisms of oxidation. Glycerol is another substrate of importance in the biomass conversion, and it can be transformed to the added-value dihydroxyacetone with a supported palladium catalyst.84 A related approach is the added-value modification of nature-derived materials. For instance, pullulan, a polysaccharide extracted from the fermentation medium of the Aureobasidium pullulans bacteria, can be functionalized by the introduction of various groups, such as carboxylic ones, what significantly influences its hydrophilicity, etc.85

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Vanadium was found to be active in the direct transformation of 4-pentenols to 3-acyloxy-γ-butyrolactones, using TBHP as oxidant (Scheme 8).86 In this interesting oxidative conversion, stereocenters of the tetrahydrofuran moiety retain their configuration. Allylic alcohols can be also oxidized to stereodefined α,β-unsaturated aldehydes/ ketones with the retention of the C]C double-bond configuration, using Fe(NO3)39H2O/TEMPO/NaCl, under atmospheric pressure of oxygen at room temperature.87 The same catalyst was found to be effective in the conversion of homopropargylic alcohols to the corresponding homopropargylic ketones, which can be further isomerized to 1,2-allenic ketones.88 Iron catalysts are also effective in the oxidative tandem assembly of 3-(2-oxoethyl) indolin-2-ones from N-arylacrylamides and alcohols through the respective 1,2-difunctionalization of the C]C double bond in N-arylacrylamides (Scheme 9).89 Until very recently, it was very difficult to prepare carbonyl compounds from alcohols with strong electron-withdrawing groups adjacent to the RCHOH moiety. For instance, 2,2,2-trifluoroethanol is often used as a solvent in oxidation reactions since it has been considered inert to the oxidation. However, the aerobic oxidation of “inert” perfluoro-substituted alcohols to their corresponding carbonyl derivatives has been recently achieved using Pt(II) complexes with dipyrido[3,2-a:20,30-c]-phenazine ligands (Scheme 10).90 Thus, in the presence of H2SO4 and O2 oxidant, trifluoroethanol was successfully oxidized to trifluoroethyl trifluoracetate with >98% selectivity.

Scheme 8 Oxidation of 4-pentenols to 3-acyloxy-γ-butyrolactones.86

Scheme 9 Synthesis of 3-(2-oxoethyl)indolin-2-ones from N-arylacrylamides and alcohols.89


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Scheme 10 Aerobic oxidation of perfluoro-substituted alcohols.90

Scheme 11 Oxidation of imidazole diol to give imidazole-4,5-dicarbaldehyde.91

A bimetallic Pt/Bi/C catalyst was used for the oxidation of imidazole diol to give imidazole-4,5-dicarbaldehyde, a key step in the synthesis of pro-drugs of hepatitis C virus replicase inhibitors (Scheme 11).91 Introduction of new oxidative agents is also an important task since “classical” oxidants, such as pyridinium chromates, are mostly not ecological, produce high amounts of toxic agents and frequently are not enough selective. The oxidations with eco-friendly air, oxygen, and peroxides, in spite of being very attractive in theory and in small scale, encounter a number of difficulties when being scaled-up. In fact, oxidations in flammable organic solvents will virtually eliminate such oxidants due to risk of inflammation and other hazards related to high oxygen pressures. Apart from that, in many instances these attractive oxidants do not operate or the reaction goes through an undesired way, e.g., overoxidation or production of a number of by-products. Hence, the search for new effective oxidants is continuously growing. Rather selective and mild oxidizing agents, namely hypervalent halogen compounds or their precursors (e.g., iodic acid, o-iodoxybenzoic acid (IBX), NaBrO2, NaBrO3, etc.), have been widely introduced into the field of alcohol oxidation during last few years.92 IBX, the less hazardous 2-iodosobenzoic acid (IBA) and even the commercially available 2-iodobenzoic acid (2IBAcid) (Fig. 8) in the presence of Oxone (potassium peroxomonosulfate KHSO5), an environmentally acceptable reagent, as a co-oxidant, were shown to be effective for the oxidation of many primary and secondary alcohols in user- and eco-friendly solvent mixtures.93 In this process, the reduced form of IBX, namely IBA is used in catalytic amounts and is reoxidized with Oxone, in aqueous media, thus providing an attractive green protocol.

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However, the application of IBX as oxidant can lead to some unpredictable results, e.g., unexpected dehomologation of primary alcohols to one-carbon shorter carboxylic acids (Scheme 12).94 In this case, the combination of IBX and molecular iodine affords a different type of active hypervalent iodine species, which was isolated and shown to be crucial for the reaction. It should be noted that usually the dehomologation involves complicated multistep procedures, whereas in the described oxidative strategy it proceeds smoothly in one step. Another useful direct oxidation of secondary alcohols was performed using performic acid within just 15 min.95 The modification of IBX with a fluorous tag (IBXF, Fig. 9) allows its easy recovery since insoluble fluorous IBA can be separated from the reaction mixture by simple filtration, and be reused without significant loss of its activity.96 Further, IBXF can be easily generated in situ from cheaper and readily available Oxone. Other modifications of the oxidizing component are being introduced. For instance, a combination of NH2OHHCl and NaIO4 was recently proposed for the selective and mild oxidation of alcohols to the corresponding carbonyl compounds at room temperature.97 Concerning particular substrates, a wide range of β-hydroxyketones was

Figure 8 o-Iodoxybenzoic acid (IBX), 2-iodosobenzoic acid (IBA), and 2-iodobenzoic acid (2IBAcid).

Scheme 12 Oxidative dehomologation of primary alcohols to one-carbon shorter carboxylic acids.94

Figure 9 Modified IBX with a fluorous substituent for its easy recovery.96


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selectively oxidized in quantitative yields to β-diketones with IBX being a superior oxidant for this transformation.98 The ability to perform oxidations without generating species harmful for potential intermediates of further transformations is important to perform multistep synthesis, such as the domino reactions described below (Section 5). In this respect, the use of aryl halides as readily available, stable and cheap oxidants (hydride acceptors) is a powerful option due to the production of inert, dehalogenated aryl by-products in anaerobic conditions. Commercially available Pd and Ni complexes with NHC ligands were found to be active in a temperature-controlled domino oxidation/R-ketone arylation with aryl halide.99 tert-Butylnitrite (t-BuONO) was recently introduced as a convenient and easy-removable oxidant for an environmentally benign conversion of primary and secondary benzylic alcohols to ketones and aldehydes, which can be readily isolated by simple evaporation of the reaction mixtures since t-BuONO decomposes giving only volatile side products.100 The oxidation requires neither metal-based reagents nor organic catalysts and presumably involves a nitrosyl exchange and a subsequent thermal decomposition of benzylic nitrites. t-BuONO can potentially be recovered, since several alkyl nitrites are known to be industrially produced from the corresponding alcohols and gaseous NO under an O2 atmosphere. A related approach was realized for the simple, high-yield conversion of various achiral and chiral alcohols to carbonyl compounds using TEMPO or AZADO in conjunction with BF3OEt2 or LiBF4 as precatalysts and t-BuONO as oxidant.101 A NO+/NOpair was used for mild anaerobic nitroxide reoxidation, which allowed the oxidation of enantiomerically pure substrates without racemization. K3[Fe(CN)6] was applied as a secondary oxidant in a chemoselective osmium(VI)-catalyzed oxidation of benzylic, allylic, and propargylic alcohols.102 An uncommon oxidation agent, diisopropyl azodicarboxylate (DIAD), can be used as an effective terminal oxidant103; in this case 1,2-diols were oxidized to hydroxyl ketones or diketones depending on the amount of DIAD used. Diaziridinone (Scheme 13) as oxidant allows the reactions to

Scheme 13 Diaziridinone as oxidant for acid- or base-sensitive substrates.104

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be performed in neutral conditions and makes it compatible with acid- or base-sensitive substrates.104 As a result, various acyclic and cyclic secondary benzylic alcohols with alkenyl, alkynyl, thioether, silyl ether, amide, carbamate, ketal, ester, and heterocyclic moieties were effectively oxidized to ketones in 73–99% yields, while no racemization of stereocenters occurred during the oxidation. A sure way of avoiding the toxic chemicals is the use of electric current as oxidizing agent. Thus, an electrochemical process for selective oxidation of 1,2-diols to the corresponding α-hydroxyketones in water using [Me2SnCl2] catalyst, KBr, and platinum electrodes has been introduced.105 The “Br+” ions, generated at the anode, are oxidants, while OH ions, electro-generated at the cathode, play the role of a base. However, in this synthetic strategy the toxicity of organotin catalysts is a drawback. This issue was addressed in another study,106 where methylboronic acid [MeB(OH)2] was used as a safe alternative. In this case, the selective oxidation of 1,2-diols to the corresponding α-hydroxy ketones in aqueous medium most probably occurs through the formation of boronate esters.

3. ACCEPTORLESS DEHYDROGENATIVE OXIDATIONS The aerobic and peroxidative oxidations, described in the previous sections, in spite of being very attractive in many aspects, can produce a considerable amount of by-products. Other problems concern the overoxidation and risk of explosion due to the coexistence of oxygen and organic reactants or solvents. In this respect, the oxidant- and acceptor-free dehydrogenation (Scheme 14) is an alternative environmentally friendly route for the conversion of alcohols into aldehydes or ketones, since gaseous H2 can be easily separated from the reaction mixture. Moreover, this strategy is much more attractive from the atom efficiency viewpoint and can provide a promising route for H2 synthesis and storage.107 Thus, some advances in the oxidant- or acceptor-free oxidations based on the catalytic dehydrogenation of alcohols accompanied by the evolution of hydrogen gas will be described in this section. Some new attractive processes were introduced recently in terms of efficiency and substrates, and examples are described below.

Scheme 14 Oxidant- and acceptor-free dehydrogenation of alcohols.


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To date, the homogeneous catalytic systems for the dehydrogenative oxidation of alcohols are mainly based on ruthenium,107,108 iridium,109 and, in much lesser extent, other transition-metal107 complexes and metal-organic compounds. For instance, water-soluble Cp*Ir (Cp* ¼ pentamethylcyclopentadienyl) complexes with α-hydroxypyridine109 or bipyridonate109 ligands have shown a high catalytic activity in dehydrogenative oxidation of a wide variety of primary and secondary alcohols and reversible dehydrogenation–hydrogenation between 2-propanol and acetone. The Ir catalysts are reusable109 and can accelerate the selective dehydrogenation of biologically important and complex molecules, e.g., β-estradiol to give estrone (Scheme 15).109 Production of hydrogen gas can be also achieved with the Ir catalysts from 2-propanol, thus providing a prototype for a hydrogen storage system, based on the interconversion between 2-propanol and acetone (Scheme 16).109 The reversible transformation with hydrogen storage and evolution was repeated several times without loss of the catalytic activity. A ligand-promoted mechanism of dehydrogenation was proposed,

Scheme 15 Selective dehydrogenation of β-estradiol to estrone.109

Scheme 16 Interconversion between 2-propanol and acetone.109

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acting the ligand as a proton acceptor in the activation step and as a proton donor in the dehydrogenation step, thus playing a dual role in cooperative catalysis.109 A detailed discussion of the mechanism of the pH-dependent alcohol dehydrogenation in aqueous solution catalyzed by related [C,N] or [C,C] cyclometalated Cp*Ir complexes was recently reported,110 generally supporting the mechanistic suggestions and demonstrating a significant dependence of the studied reaction system on pH. Structurally related PCP-pincer Ir complexes (Fig. 10) were synthesized by straightforward [4+2] cycloaddition and employed as catalysts in the acceptorless dehydrogenation of alcohols.109 Such complexes can be easily modified with a functional sidearm that is capable of interacting with the catalytic site, thus making them suitable candidates for catalytic studies involving ligand–metal cooperation. The H2 formation involves an intramolecular cooperation between the structurally remote functionality and the metal center.109 A theoretical study on the mechanism of the acceptorless alcohol dehydrogenation mediated by the iridium catalyst [Cp*Ir(bpyO)] (bpyO ¼ α,α0 bipyridonate) suggests that the metal–ligand cooperative work involves aromatization/dearomatization of the bpyO ligand.111 On the basis of those results, the new ruthenium catalyst [(HMB)Ru(bpyO)(H2O)] (HMB ¼ hexamethylbenzene) was designed (Fig. 11).111 Another ruthenium catalyst formed from the Ru-hydride precursor [RuH2(PPh3)3CO] and pincer PNP-ligands effectively promotes the dehydrogenative oxidation

Figure 10 PCP-pincer Ir complex.109

Figure 11 [(HMB)Ru(bpyO)(H2O)] ruthenium catalyst.111


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of alcohols with TOFs as high as 1.4 104 h1 after 20 min, at moderate temperatures (99% ee of hydroxyl ketones and 50% ee of lactones.143 These catalysts can be also applied for an efficient oxidative kinetic resolution of racemic secondary alcohols affording R enantiomers with >99% ee and with 46–50% yields.135

Scheme 27 Stereospecific synthesis of (S)-erythrulose.140


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Scheme 28 Enantiospecific preparation of lactones by oxidative lactonization of primary meso-diols.142

Scheme 29 Chemoselective oxidation of a secondary alcohol moiety with Ru catalysts.144

Chemoselective oxidation of a secondary alcohol moiety can be also performed with ruthenium catalysts with phenylindenyl ligand.144 The selective oxidation of 1-phenylethanol to acetophenone from a mixture of phenylethanol isomers, without oxidizing the other isomer, can then be achieved (Scheme 29). In general, only the secondary alcohol moieties are oxidized and the catalyst can be used for the chemical separation of isomers or specific oxidation of highly functionalized molecules. The OKR of unactivated racemic alcohols with dioxygen of air as the hydrogen acceptor was effectively performed at room temperature with [(aqua)Ru(salen)] complexes as catalysts.145

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Scheme 30 Conversion of 2-diols to the corresponding chiral α-ketoalcohols.146

The use of cheaper, more available, and recyclable catalysts is a common aim in the catalytic studies. Under this perspective, many recent publications on the oxidative asymmetric resolution of alcohols involve 3d and other abundant metals. Thus, in situ combination of copper(II) triflate and (R,R)-Ph-BOX [BOX ¼ bis(oxazoline)] with NBS as an oxidant allowed to achieve a desymmetrization of 2-diols to afford the corresponding chiral α-ketoalcohols (Scheme 30).146 An air-stable, well-defined (NHC)–Ni0 complex is another effective 3d-metal catalyst for the mild anaerobic catalytic oxidation of secondary alcohols with such functionalities as ether, tertiary amine, and alkene, using nonanhydrous, -degassed 2,4-dichlorotoluene as both oxidant and solvent.147 The use of inexpensive, stable, and ease to handle chlorinated solvent as an oxidant makes this catalytic system attractive for multistep synthesis and scaling-up; moreover, potentially harmful and/or difficult to remove species are less likely to be formed. Additionally, primary benzylic and alkylic alcohols are unreactive with this catalytic system and can be recovered. Jacobsen’s chiral Mn(III)-salen complexes constitute another example of cheap and available catalysts for the enantioselective oxidation of racemic secondary alcohols148; in this case, sodium hypochlorite (NaClO) was used as oxidant. Related macrocyclic chiral Mn(III) salen complexes were applied for the OKR of secondary alcohols with diacetoxyiodobenzene [PhI (OAc)2] and NBS co-oxidants, in a biphasic dichloromethane-water solvent mixture149; the catalyst can be easily recycled up to 7 without losing its performance. Transition metals can be eliminated from the catalytic systems. Thus, a quinine-derived urea organocatalyst is effective in the enantioselective oxidation of a wide range of diaryl-substituted meso-1,2-diols using bromination reagents as oxidants (Scheme 31).61 The method is simple, operates at ambient temperature and utilizes available reagents to yield α-hydroxy ketones in good yields (up to 94%) and enantioselectivities (up to 95% ee).


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Scheme 31 Enantioselective oxidation of diaryl-substituted meso-1,2-diols.61

Moreover, both chemo- and enantioselectivities can synergistically contribute to the specific oxidation of racemic diols bearing different substituents, affording a hydroxy ketone as the sole product.61

5. CASCADE AND SEQUENTIAL REACTIONS Some products of partial oxidation of alcohols, in particular aldehydes, are widely used as starting materials in many organic transformations. Hence, they potentially can react in situ with other components of the reaction mixture, giving, e.g., alkenes, imines, or α-functionalized carbonyl compounds. Selectivity issues will arise, but a proper choice of reaction conditions, catalysts, and other additives would hopefully provide good yields and selectivities. Sometimes the products of partial oxidation of alcohols can be isolated from the reaction mixture after the oxidation step and transformed further using the same catalyst.150 In this case, one deals with a sequential transformation; an example is given by the conversion of aromatic alcohols to the corresponding aldehydes using molecular oxygen and a copper– TEMPO catalytic system,38e where the formed aldehydes can be isolated and transformed further with the same copper catalyst to nitroalcohols upon the nitroladol (Henry) reaction (Scheme 32). If further transformations of the products of partial oxidation of alcohols occur in one-pot, the term tandem or cascade reactions is generally used.151 Hydrogen atoms attributed to the starting alcohols can be combined with an inorganic oxidant,152 leave the reaction environment, e.g., as H2,12 or be transferred to another substrate, forming an added-value product. In the last case, cheap and easy to operate alcohols are usually used as sacrificial reducing agents. At the same time, in many instances the hydrogen atoms can

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Scheme 32 Sequential transformation of alcohols to aldehydes and further to nitroalcohols.38e O Catalyst R

OH

1/2

R

R

O

+ H2

Scheme 33 Dehydrogenative coupling of alcohols to esters.

151

interact with an intermediate.151 Formally, a net oxidation of alcohols in this case will not occur, but only a proton transfer. However, if one considers the mechanism of such transformations, an oxidation (dehydrogenation) is an essential rate-determining step. Hence, appropriate catalysts and specific conditions of the alcohol oxidation (dehydrogenation) can serve as a starting point in the search of new catalytic systems for the sequential or tandem reactions of this type. As mentioned above, hydrogen atoms, removed from the alcohol substrate, can return to form the product; however, if the final hydrogenation step could not occur, a product that is more oxidized than the starting material is obtained. The formation of esters from alcohols and of amides from alcohols and amines concern the most representative and studied reactions of this type. In these cases, aldehydes, formed on the first oxidation stage from alcohols, undergo Tishchenko- and Cannizzaro-type reactions, where esters or carboxylates and alcohols are formed upon fusion or disproportionation of aldehydes, respectively. Thus, the dehydrogenative coupling of alcohols to esters with evolution of H2 (Scheme 33) is one of possible effective variants of the tandem reactions with net oxidation of alcohols.151 In contrast to the normal esterification of an acid and alcohol, in which an equilibrium mixture is formed, the evolved hydrogen, which is valuable by itself, can shift the equilibrium to completion. Generally, the direct catalytic transformation of alcohols to esters, without the use of the corresponding acid or acid derivative, is a very attractive approach. Ru(II) hydride complexes based on electron-rich PNP and PNN ligands of the type depicted on Fig. 13 (which can undergo aromatization/ dearomatization steps) efficiently and selectively catalyze the acceptorless dehydrogenation of primary alcohols to esters and H2 with high TONs


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Figure 13 Ru(II) catalysts for acceptorless dehydrogenation of alcohols to esters.132

under relatively mild conditions.132 This provides a convenient method for the synthesis of esters in view of its high efficiency, simplicity, and facile product isolation. Thus, the reaction gave up to 99% of ester yield after 6 h at 115 °C, if started from 1-hexanol. Concerning the mechanism, it was demonstrated that hemiacetal formation from the aldehyde and alcohol followed by its dehydrogenation is more likely to occur than a Tischenkotype reaction involving the aldehyde.132 Heterogeneous reusable catalysts, e.g., supported platinum-based ones, can be utilized for the acceptor-free dehydrogenative coupling of alcohols to esters under additive-free and solvent-free conditions at 180 °C, with isolated yields within the 53–91% range.153 The activity depends on the support material and on the loaded transition metal. Thus, the SnO2 support contains Lewis acid sites that activate carbonyl groups of adsorbed aldehyde intermediates, while the Pt/SnO2 combination possesses the best promoting activity among the studied transition metals, e.g., Ir, Re, Ru, Rh, Pd, Ag, Co, Ni, and Cu loaded on SnO2. Direct amide bond formation is a rather important transformation since the amide functionality is a widely spread unit in synthetic intermediates, pharmaceuticals, polymers, and natural products. Hence, its simple formation from cheap and convenient starting materials, such as alcohols, is of interest. Generally, the amide bond construction is similar to the ester formation, but the competing N-alkylation process significantly complicates the picture (Scheme 34). Thus, the search for effective and selective catalysts is of a clear need. Magnetic Fe3O4@EDTA–Cu(II) NPs can be such catalysts if benzyl alcohols and amine hydrochloride salts are used as substrates with TBHP as an oxidant. The corresponding amides are formed, while the catalyst can be easily recovered by magnetic forces and reused several times without loss of activity.154 To improve selectivity in the oxidation-addition cascades, the catalysts and reducing agents should be able to maintain the catalytic cycle and respond to the polarity differences between different radicals. Using this approach, synthesis of side-chain-extended tetrahydrofurans from alkenols and acceptor-substituted alkenes was achieved.155 Co-1,3-diketones were

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Scheme 34 Competing processes in the dehydrogenative formation of amides from alcohols and amines.154

used as catalysts to activate molecular oxygen for the oxidative cyclization (Scheme 35); a sequence of polar and free-radical reactions is believed to occur in this case. Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be applied for the related hydrogen-transfer reactions, thus leading to new added-value compounds.8 The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. Apart from the described examples, other interesting alcohol transformations can occur where different reactions are combined to give new and sometimes unpredictable result. For instance, oxidation of one substrate and reduction of another one can lead to the same product, thus favoring


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Scheme 35 Alkenol oxidation—olefin addition cascade catalyzed by Co complexes.155

Scheme 36 Oxidation of methanol to formate salts.152

sustainability and profit, if the product is of a significant added value. The process where methanol is effectively transformed into formate salts (Scheme 36)152 can serve as an example of a reaction of this type. In this case, catalytic methanol dehydrogenation is combined with bicarbonate hydrogenation giving high TONs (>18,000), TOFs (>1300 h1) and yields (>90%) of formate salt. The bicarbonate behaves as a very convenient hydrogen acceptor since the same product (formate) is formed from bicarbonate hydrogenation and methanol dehydrogenation, while utilization of hazardous gases and chemicals was avoided. The obtained formate salts are essential chemicals with a variety of uses. The current industrial production of formates involves absorption of hazardous, flammable, and difficult to transport carbon monoxide under high pressure in solid sodium hydroxide at 160 °C. A related direction concerns the elaboration of coupled systems, where one substrate is oxidized into an added-value product, while another one is reduced, also giving a product with added value. As an example, coupled systems for selective oxidation of aromatic alcohols to aldehydes and reduction of nitrobenzene into aniline156 can be mentioned. This oxidation– reduction coupling was realized using a cadmium-based composite material as a photocatalyst under visible-light illumination, giving, in one-pot, 45% and 26% yields of benzaldehyde and aniline, respectively.

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6. CONVERSION OF RENEWABLE SOURCES AND HYDROGEN PRODUCTION 6.1 Transformation of Renewable Materials into Added-Value Compounds Many renewable raw materials contain hydroxyl groups connected to carbon atom and hence can be considered as alcohols. One of the most abundant renewable raw materials is lignocellulosic biomass, which is composed of carbohydrate polymers such as cellulose or hemicellulose, and an aromatic polymer lignin. The lignocellulose is mainly used in construction materials, as firewood and for production of biofuels, such as biodiesel, ethanol, and hydrogen, which are sustainable alternatives to fossil fuels with reduced CO2 emission. However, modification and effective conversion of biomass to fine chemicals with an added value is an active field of modern research. For instance, partial oxidation of cellulose157 can lead to significant change in its properties, e.g., ability to absorb water, what is important for many applications. Generally, the conversion of components of lignocellulose into value-added chemicals is a complex process with many parallel reactions, low yields and sometimes unpredictable results. One of the current directions is the direct conversion of biomass to formic or acetic acids.158 In this process, remarkable total yields up to 80% were achieved.159 Vanadium-substituted phosphomolybdic acids (H3+nPVnMo12nO40) were used as catalysts, while molecular oxygen was employed as an oxidant.159 The catalysts could be reusable at least 3 without significant loss of activity. Both formic and acetic acids are of wide use in chemical, pharmaceutical, and agricultural industries; formic acid is also recently evaluated as a perspective carrier to store or generate H2.160,161 Other important added-value products, such as levulinic acid,162,163 sorbital,164 ethylene glycol,165 5-hydroxymethylfurfuran,155 lactic,166 glycolic,167 and gluconic168 acids can also be prepared by selective conversion of cellulose and biomass-derived carbohydrates. Different conditions were proposed to optimize the yields and selectivities; the use of supercritical conditions or various catalysts and additives are among the most used procedures to increase effectiveness of these conversions. For instance, Keggintype heteropolyacid catalysts H5PV2Mo10O40 can effectively accelerate the conversion of mono-, olygo-, and polysaccharides to formic acid with good yields.152 Similar phosphomolybdic acid H3PMo12O40 was used as a


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bifunctional catalyst to accelerate both the hydrolysis of cellulose and the subsequent oxidation reactions to give glycolic acid with molecular oxygen in a water medium.167 Conversion of substrates, readily available from natural sources, e.g., glycerol (a cheap by-product of biodiesel production), furans, or carbohydrates, is another direction which is under active exploration.84,169 The liquid phase oxidation of glycerol with bimetallic Au/Pd and Au/Pt catalysts supported over MgO leads to an enhanced glycerol conversion and increased selectivity toward oxidized C3 products under mild conditions and without the addition of a base.170 Another tested bimetallic catalyst allows proceeding with a selective oxidation of biomass alcohols to the corresponding aldehydes by a visible-light-driven synergistic photoelectrochemical (PEC) catalysis system with Au/CeO2–TiO2 nanotubes (NTs) as photocathodes and using mild conditions.171 The obtained conversion of benzyl alcohols was up to 98% while the selectivity toward benzaldehyde was >99%. Along with conventional catalysts, thermostable enzymes are prospective agents to convert glycerol into materials with high added values, e.g., dihydroxyacetone, a synthetic precursor and sunless tanning agent. Using glycerol as a second substrate, (R)-1,2-propanediol can be also produced from hydroxyacetone in a one-enzyme bioelectrocatalytic reactor.172 In this study, NAD-dependent Thermotoga maritime glycerol dehydrogenase (TmGlyDH) was employed as a main catalytic agent, while the NAD(H)-cofactor can be immobilized and regenerated electrochemically. It was also demonstrated that TmGlyDH can be a useful catalyst for producing optically active products, e.g., for resolving a racemic mixture of 1,2propanediol leaving the (S)-enantiomer aside. Other important and available derivatives of biomass are sugars, and their conversion and application are also of a high recent interest.163,164 For instance, novel magnetically separable carbonaceous nanohybrids were recently prepared from porous starch.173 These porous polysaccharidederived materials are highly magnetic up to 450 °C and thus can be used as recyclable catalysts, e.g., oxidations of benzyl alcohols and xylose dehydration to furfural. Gold-based materials were shown to be more active than other metal catalysts in the first step of the 5-hydroxymethylfurfural (HMF) oxidation, leading to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) very quickly, even though they showed less activity for the subsequent conversion of HFCA to 2,5-furandicarboxylic acid (FDCA).174 The HMF oxidation over

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Au and Pt catalysts in the presence of high amounts of NaOH was recently investigated with the use of isotopically labeled dioxygen and water. The source of inserted oxygen was shown to be water rather than oxygen. Unfortunately, in all the studied gold-based samples, process efficiency and catalyst stability were rather low. However, the modification of Au-based catalysts with Pt or Pd metal produced stable and recyclable catalysts.175 Bimetallic Au8Pd2 species supported over active carbon have the highest activity and stability for the production of FDCA.

6.2 Alcohol Oxidation for Hydrogen Storage and Production The reserves of fossil fuels are rapidly depleting, while the pollution caused by their intensive exploitation constitutes another challenge to overcome. Among the alternative and clean fuels, hydrogen is one of the most promising ones since it can be produced from renewable resources, has a high energy density, zero carbon emission, etc. However, other physical properties of H2 make its handling a rather difficult task. Therefore, easy-to-handle hydrogen-containing sources should be introduced, for instance, alcohols which can be used to store and transport H2. Thus, methanol contains 12.6% hydrogen, but the hydrogen production from alcohols generally involves rather costly oxidative steam reforming at high temperatures (over 200 °C) and pressures (25–50 bar) and with noble metal catalysts to obtain high purity hydrogen. Furthermore, these methods usually generate greenhouse gases, while the evolved CO itself may result in poisoning the involved catalysts. Alcohol dehydrogenation in the presence of a hydrogen acceptor (e.g., O2, H2O2, acetone) concerns another perspective, but the application of the hydrogen acceptor contradicts the atom economy and hence efficient acceptorless alcohol-dehydrogenation protocols are of clear need. Many molecular catalysts have been applied for the hydrogen production from alcohols,12,176 ruthenium and rhodium ones being particularly effective in catalytic acceptorless dehydrogenations.107 Thus, an efficient lowtemperature aqueous-phase methanol dehydrogenation process, which is facilitated by ruthenium complexes with pincer-type ligands, e.g., [RuHCl(CO)(HN(C2H4PiPr2)2)], has been described.177 Hydrogen generation by this method proceeds at 65–95 °C and ambient pressure with catalyst TOFs up to 4700 h1 and TONs above 350,000; furthermore, no base is needed. It was demonstrated for the first time that it is possible to efficiently dehydrogenate the thermodynamically less-favorable primary


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aliphatic alcohols below 100 °C. This eventually could make the use of methanol as a practical hydrogen carrier a feasible task. Concerning heterogeneous catalytic systems, Rh is an active metal for the ethanol steam reforming to produce H2.178,179 In the reforming the supports also play an important role regarding the activity, selectivity and stability of the catalyst, and MgAl2O4 was found to be an appropriate support for the Rh catalysts.180 This Rh/MgAl2O4 system is believed to be a bifunctional catalyst,177 whereas the activation of ethanol takes place both on the metal particle and on the support basic and acidic sites leading to the formation of intermediate compounds (Scheme 37). Ethylene can also be formed giving a lower hydrogen production [Scheme 37 (3)] due to deactivation of the acidic sites with coke formation on the support.177 Primary alcohols, like ethanol, can be obtained from biomass fermentation, but their further transformations toward other carriers with higher energy density are still under development. However, a simple but effective non-catalytic way for the production of high purity hydrogen from primary alcohols under basic conditions has been recently reported.181 At this reaction, one mole of ethanol reacts with one mole of a base (e.g., NaOH, Scheme 38) giving two moles of H2 and one mole of sodium acetate. One of the main advantages of this approach is that no catalyst is required, and no environmentally harmful gas, such as CO or CO2, is produced in the process. The authors also report that temperature is a key factor affecting the rate of gas generation: as long as the temperature is higher than 120 °C, hydrogen and carboxylate are produced. If methanol is used as a starting material, the corresponding formate salts can be produced apart from H2. Since formates, in their turn, have been evaluated as potent carriers to store and produce hydrogen,160,161 additionally giving valuable oxalates or carbonates, the overall process has a high

Scheme 37 Ethanol steam reforming main reaction (1) and intermediate ethanol dehydrogenation (2) and undesirable ethanol dehydration (3).

Scheme 38 Production of hydrogen from ethanol.181

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Scheme 39 Proposed177 mechanism for H2 generation with ethanol as substrate.

potential to become a large-scale industrial process. The water tolerance of the reaction was also tested, and it was found that high water content inhibits the hydrogen production. In spite of that, the normal bioethanol can be used to generate high purity hydrogen at a relatively low temperature; although the rate of the reaction might be lowered, a moderate extension of the reaction time would overcome this obstacle. The proposed177 mechanism of this transformation (Scheme 39), as supported by GC/MS and isotope labeling studies, involves removal of the hydroxide proton of ethanol by a base, thus giving a molecule of water and ethanolate (Step 1). This anion rapidly reacts with water to give H2, acetaldehyde, and hydroxide ion (Step 2). Reaction of the hydroxide ion with the carbonyl carbon of acetaldehyde (via nucleophilic addition) forms alkoxide which is then deprotonated and gives a dianionic Cannizzaro intermediate, that subsequently reacts with another molecule of acetaldehyde to give the final product (sodium acetate) and a molecule of ethanol (Step 3).

7. IRRADIATION-PROMOTED OXIDATIONS Acceleration and promotion of chemical reactions by irradiation with radiowaves of various frequency ranges is an established field with multiple applications in laboratory practice and in industry. However, new and interesting results in this topic continue to appear. The range of radiowave frequencies is expanding and now includes virtually all the spectrum starting from gamma-rays and down to the low-energy MW irradiation. Although there is some consensus on the key steps of photochemical reactions promoted by short-waves, e.g., UV and visible-light irradiations, it is still debatable the mechanism by which low-energy irradiation, such as MWs, influence the reaction kinetics.182 In this section, we shall not discuss the


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mechanisms of irradiation action (there are plenty of books and reviews on that matter, e.g., see Refs. 182c,d and references therein), but focus on some recent applications of irradiation in alcohol oxidation.

7.1 Photocatalytic Oxidations Photochemical reactions induced by ultraviolet and visible-light irradiations mainly involve transformations of molecules by direct absorption of light. In these cases, the energy of the UV/vis photon is significantly higher than the energy of Brownian motion and is generally high enough to directly cleave molecular bonds. Application of photocatalytic and photoelectrocatalytic methods for partial oxidations of both aliphatic and aromatic alcohols to give aldehydes and other related products, mainly using titanium oxide as catalyst, have been reported by several groups.183,184 Due to its direct and in many cases specific action toward certain chemical bonds, many irradiation-induced processes are more sustainable, consume less energy, generate less by-products, or can be conducted using greener solvents than analogous conventional reactions. On the other hand, sometimes the application of irradiation is crucial to perform a specific reaction, which cannot be conducted with a reasonable rate or would not occur at all, if the irradiation was not applied. For instance, piperonal, a compound of great importance for cosmetics, agrochemical, and pharmaceutical chemistries, is traditionally synthesized by isomerization and subsequent oxidation of safrole, an ingredient of some rather expensive and rear essential oils. Other proposed routes involve harmful reagents or environmentally unsafe heavy metals, while selective photocatalytic oxidation (Scheme 40) allows to use piperonyl alcohol, a common chemical which is approximately 500 less expensive than piperonal, as a starting material, under organic-free aqueous conditions, using UV irradiation and cheap commercial TiO2 as a photocatalyst.184 The photocatalytic oxidation of aromatic alcohols to aldehydes in water with rutile and anatase TiO2 NPs under UV light irradiation was studied.185,186 For example, photooxidation of four-substituted aromatic alcohols to the corresponding aldehydes, over rutile TiO2 NPs, showed

Scheme 40 Photocatalytic synthesis of piperonal from piperonyl alcohol.184

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45–74% of selectivity.185 The effect of surface and physical properties of TiO2 NPs on the selective photocatalytic activity under UV light irradiation was also investigated. Photocatalytic selective oxidation of ethanol to acetaldedyde in a fluidized bed photoreactor was studied on new structured photocatalysts based on direct supporting the VOx/TiO2 on the surface of commercial ZnS-based phosphors.187 Oxidation of benzylalcohol to benzaldehyde with yields and selectivity values higher than 40% and 80%, respectively, was reported using ferric ions as homogeneous catalysts and oxygen as an oxidant under UV-solar simulated radiation. To avoid occurrence of side reactions, in consequence of generated undesired reactive OH radicals188 due to the possible Fe(III) aquo-complexes photolysis, reactions were carried out at pH 0.5.189 Selective oxidation of benzyl alcohol to benzaldehyde and reduction of nitrobenzene into aniline, under visible-light illumination, was also reported using CdS/g-C3N4 composite as a photocatalyst.156 The conversion of the alcohol into the aldehyde was achieved by direct holes oxidation, and the reduction of nitrobenzene into aniline by direct electrons reduction. The CdS/g-C3N4 photocatalyst exhibits enhanced photocatalytic activity and excellent photostability relatively to single g-C3N4 and CdS, with an optimum percentage of CdS of ca. 10 wt.%. Under irradiation of visible light (420 nm) for 4 h under N2 purge conditions, the yields of benzaldehyde and aniline are 44.6% and 26.0%, respectively.156 Recently, the first example of a MOF as a promising visiblelight photocatalyst toward the selective oxidation of alcohols to their corresponding aldehydes was reported. The Zr-based MOFs, Zr-benzenedicarboxylate (UiO-66) and its derivative Zr-2-NH2-benzenedicarboxylate (UiO-66(NH2)) were prepared via a solvothermal method and successfully applied to photocatalytic reactions.190 A reaction mechanism, involving the production of charge carriers and photogenerated electrons and holes, was proposed. Oxygen reacts with an electron to form O2 , while the photogenerated holes can directly oxidize the organic reactive substrates to carbonium ions. The formed superoxide radicals further react with the carbocations, which leads to the final products.187 A recent development in this field concerns the construction of a photoreactor with membrane separation, namely pervaporation, in order to prevent overoxidation of the aldehyde. A significant rate acceleration was observed in the copper-catalyzed aerobic photooxidation of sugars using a Cu–TEMPO catalytic system (Scheme 41).191 It was suggested that the transformation of the


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Scheme 41 Synthesis of tetrahydroazapanes through light-activated Cu/TEMPOcatalyzed aerobic alcohol oxidation.191

Scheme 42 Plasmon-mediated oxidation of sec-phenethyl (R ¼ CH3) and benzyl (R ¼ H) alcohols in the presence of supported AuNP.192

Cu(II)–TEMPO intermediate is induced photochemically. Curiously, this reaction is highly selective for primary alcohols, and hence tetrahydroxyazepanes can be easily synthesized via specific oxidation of a benzyl glucoside, followed by reductive amination and nucleophilic ring expansion (Scheme 41). Recent technological advances and price drop in laser and light emitting diode (LED) production allow extensive studies on how an irradiation of a certain wavelength influences a specific chemical reaction. Thus, a comparative study of laser, LED, and MW irradiations in both sec-phenethyl and benzyl alcohol oxidations to acetophenone and benzaldehyde, respectively, was performed.192 Excitation with monochromatic 530 nm LEDs (Scheme 42) gave yields as good or better than the corresponding laser and MW techniques, with a maximum conversion of 95% after 20 min. Hence, LEDs can provide a new economical and power-efficient alternative

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Scheme 43 Possible pathways in the plasmon excitation of supported metallic nanoparticles.192

light source for plasmon-mediated reactions, where Au metal nanoparticles (AuNPs), with a strong absorption of the surface plasmon band (SPB) within the visible region, favor ejection of electrons and induce a variety of photoinitiated electron transfer pathways (Scheme 43). A highly efficient and selective catalytic oxidation of biomass alcohols to the corresponding aldehydes was shown to be driven by a synergistic action of visible-light and electrochemical catalysis employing Au/CeO2–TiO2 NTs as photocathodes, under mild conditions.171 At the bias potential of 0.8 V and under visible-light irradiation for 8 h, the conversion of benzyl alcohol was 98% with the selectivity toward the benzaldehyde formation being >99%.

7.2 MW-Promoted Oxidations The use of MW irradiation with a lower energy than UV–vis light has attracted much attention in synthetic organic chemistry due to the improvements on efficiency, rates of reactions, and energy consumption, as well as on selectivities, what has positioned this technology as a useful alternative energy source in organic synthesis, with an environmentally friendly nature. The MW-enhanced chemistry is based on the efficiency of the interaction of molecules with electromagnetic waves generated by a “microwave dielectric effect”. This process mainly depends on the ability of a specific


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mixture (substrates, catalyst, and solvents) to absorb MW energy and convert it into heat. Polar molecules have good potential to absorb MWs and convert them to heat energy, thus accelerating the reactions compared with the conventional heating. The ability of a specific material (e.g., a substrate or a solvent) to convert electromagnetic energy into heat is known as loss tangent, tan δ. A reaction medium with a high tan δ is required for efficient absorption and consequently rapid heating. Despite alcohols and most of organic compounds have a considerably lower dielectric constant compared to water, they heat much rapidly in a MW field on account of their high tan δ. Furthermore, polar components (such as ILs) can be added to increase the absorbance of a reaction medium. However, the nature of the MW effect is still debatable and in some cases it has been proved that it concerns a heating effect.182 External infrared (IR) temperature controllers mainly used in MW-assisted homogeneous reactions do not accurately monitor the sample temperature and usually tend to understate its value.182 There are many examples of the successful application of MW-assisted chemistry to organic synthesis; these include the use of benign reaction media, solvent-free conditions, and application of solid supported and reusable catalysts. Over the past few years, it was demonstrated that many transition-metal-catalyzed bond transformations can be significantly enhanced by employing MW heating under sealed-vessel conditions, in most cases without requiring an inert atmosphere. Recently, a MW-promoted procedure for one-pot, two-step conversion of aryl alcohols to aryl fluorides via aryl nonafluorobutylsulfonates (ArONf ) was reported (Scheme 44).193 Moderate to good yields were achieved by this MW-assisted palladium-catalyzed fluorination sequence. The in situ conversion of aryl alcohols to aryl nonaflates using CsF as base, followed by the MW heated (180 °C) fluorination, catalyzed by [Pd2(dba)3] (dba ¼ dibenzylideneacetone) and t-BuBrettPhos [2-(di-tert-butylphosphino)20 ,40 ,60 -triisopropyl-3,6-dimethoxy-1,10 -biphenyl], allowed full conversion after 30–60 min reaction.191

Scheme 44 MW-assisted fluorination of aryl triflates.191

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Rhodium- and ruthenium-catalyzed hydrogen-transfer type oxidation of secondary alcohols (e.g., 5-tetradecanol, cyclododecanol, cyclooctanol, 4-t-butylcyclohexanol, or 1-p-tolyl-1-hexanol) lead, in moderately to excellent yields, to the corresponding ketones by employing MW heating at 140 °C for 15 min, using 2 equiv. of methyl acrylate as hydrogen acceptor and 5 mol% of [RhCl(CO)(PPh3)2] as catalyst, in a water/N,N-DMF solvent mixture (Scheme 45).194 No conversion was observed in the absence of MW irradiation. Primary alcohols, such as n-heptanol, n-tridecanol, or 1,7-heptanediol, were oxidized using a 2.5 mol% of [RuCl2(PPh3)2] and 2 equiv. of methyl vinyl ketone under solvent-free conditions and MW heating at 120 °C for 15 min (Scheme 46).194 The accelerating effect of MW irradiation in the synthesis of ketones from secondary alcohols with TBHP as oxidant was also reported.26,38,40,64 In fact, in the presence of the dicopper(II) [Cu2(Hedea)2(N3)2](0.25H2O) (Hedea ¼ N-ethyldiethanolamine) complex with the {Cu2(μ–O)2} diethanolaminate core, the oxidation of 1-phenylethanol is dramatically accelerated when the reaction mixture is subject to MW irradiation, achieving a very high yield of acetophenone (91%) after 15 min of reaction, in contrast with the 51% acetophenone formed after 30 min when using conventional heating.26 Several recent examples use copper(II) or Cu(II)/TEMPO catalytic systems,38 namely alkoxy-1,3,5-triazapentadien(e/ato) copper(II) complexes (yields up to 97% and TONs up to 485 after 60 min, and TOFs of up to 1170 h1 after 10 min reaction)195 or bis- and tris-pyridyl amino and imino thioether Cu and Fe complexes, with a maximum yield of acetophenone of 99% after 30 min at 80 °C. The maximum TOF of 5220 h1 (corresponding to 87% yield) was achieved just after 5 min of reaction time under the low MW power of 10 W.42,43

Scheme 45 MW-assisted oxidation of secondary alcohols with a Rh(I) catalyst and methyl acrylate.194

Scheme 46 MW-assisted oxidation of primary alcohols with a Ru(II) catalyst and methyl vinyl ketone.194


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In a related study, the same group reported a series of mixed-ligand dinuclear manganese(II) Schiff base complexes as catalysts for the MW-assisted oxidation of alcohols (Scheme 47).64 Acetophenone yield of 81% is obtained using a maximum of 0.4% molar ratio of [Mn(H2L)– (py)2]2(NO3)22CH3OH (H2L ¼ hydrazone Schiff base) catalyst relative to the substrate (1-phenylethanol) in the presence of TEMPO and in aqueous basic solution, under mild conditions.64b A recent publication has described the efficient ruthenium-catalyzed C-3 reductive alkylation of 4-hydroxycoumarin by dehydrogenative oxidation of benzylic alcohols.196 The optimization of reaction parameters, such as type of catalyst, type of solvent, activation method, reaction time, temperature, and base, was performed (Scheme 48). Under optimized conditions, using [RuCl2(PPh3)3] as catalyst in tert-amyl alcohol under MW irradiation at 140 °C for 2 h, afforded a satisfying selectivity/conversion. MW irradiation also permitted a shorter reaction time for the selective solvent-free oxidation of primary, secondary, allylic, and benzylic alcohols with pyridinum sulfonate chlorochromate and pyridinum sulfonate fluorochromate as oxidizing agents, compared with the use of solvent. For example, cholest-5-en-3-ol acetate and cholest-5-en-3-ol benzoate were chemoselectively oxidized at position 7 in solvent-free conditions with 82% and 87% conversion, respectively.197 A CrO3-catalyzed oxidation of homopropargyl alcohols with TBHP under MW irradiation was found to be an efficient and rapid alternative

Scheme 47 MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone with a Mn catalyst.64

Scheme 48 MW-assisted selective C-3 alkylation of 4-hydroxycoumarin with benzyl alcohol.196

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for the preparation of 1,2-allenic ketones. The optimal conditions were achieved when a solution of 1-phenylbut-3-yn-1-ol in CH2Cl2 was irradiated with MW at 40 °C for 0.2 h in the presence of 5 mol% of CrO3 as catalyst and 3 equiv. of TBHP as oxidant.198 Tetrabutylammonium decatungstate(VI) was reported to possess a catalytic activity in the oxidation of selected alcohols with hydrogen peroxide as oxidant, in 1,2-dichloroethane/water or acetonitrile/water solvent systems. A pronounced accelerating effect on the reaction rate was observed when a MW conditions were used.199 A highly active (NHC)-Pd catalytic system can be applied for anaerobic oxidation of secondary alcohols at very mild temperatures. This procedure allows assessing one-pot domino procedures for the synthesis of R-arylated ketones from secondary aryl alcohols with very good yields.200 Recently, a successful use of MW heating for the synthesis of [Pd(acac)Cl(NHC)] and [PdCl2(3-Cl-pyridine)(NHC)] complexes was reported.201 This protocol affords the desired compounds in yields comparable to those obtained using conventional heating, but drastically reduces the reaction times. A similar protocol was thereafter applied to the synthesis of a series of [Ni(Cp)Cl(NHC)] complexes (Scheme 49).202 These complexes where applied in the catalytic anaerobic oxidation of alcohols using 2,4chlorotoluene as solvent and oxidant.202 In recent years, the use of a room-temperature IL in MW-assisted synthesis, as a (co-) solvent and/or (co-)catalyst, is becoming an increasingly exploited area since the ionic nature of ILs allows them to absorb MW energy very efficiently. Besides that, ILs exhibit several inherent benefits, such as low vapor pressures, high thermal and chemical stability, and nonflammability. The role played by the combination of an IL and MW was explored in the activation of H2O2 in [hmim]Br (hmim ¼ 3-methylimidazolium) used as catalyst and solvent, under MW irradiation, for the metal-free chemoselective oxidation of various alcohols into the corresponding carbonyl compounds.203 In addition, a new metal-free methodology for the synthesis of anthraquinone has been reported.203

Scheme 49 MW-assisted synthesis of [Ni(Cp)Cl(NHC)] complexes, catalysts for alcohol oxidation.202


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A Co(II) complex supported on SBA-15 has been employed as a highly active and reusable catalyst in the selective oxidation of various alcohols, in which the improved rates of reactions (from hours to minutes), yields and even selectivities in some cases, illustrate the usefulness of MW protocols as alternative methodologies in organic synthesis.204 A MW-assisted solvent-free peroxidative oxidation of benzyl alcohol to benzaldehyde catalyzed by magnetic Ni-doped MgFe2O4 NPs was also successfully performed.205 The catalyst is reusable and eco-friendly, is applied in a small amount and the reaction time is short.205

7.3 Others Application of ultrasounds is among the new alternative techniques which can accelerate heterogeneous reactions by increasing the surface (dispersity) of the reagents, while in homogeneous systems ultrasounds assist the even heat distribution in a reactor.206 Thus, the application of ultrasounds or of MW irradiation may substantially shorten the reaction time in oxidations of alcohols with PCC, from hours to minutes.207 It is believed that the ultrasound produces erosion on the PCC surface and therefore accelerates its interaction with the organic substrates. The involvement of ultrasonic irradiation in a drastic reduction on the amount of PCC used was also indicated.207 An ongoing approach on the development of new environmentally friendly protocols involves the combination of MW and ultrasonic irradiation in ILs. In this context, MW and ultrasound activation methods have been used in the oxidation of five- to eight-membered cyclanols in the presence of H2O2/H2WO4 and several hydrophobic ILs as cocatalysts.208 Quantitative oxidation of cyclohexanol, after only 30 min at 90 °C, was achieved in the presence of [Aliquat][NTf2] (Aliquat ¼ N-methyl-N,Ndioctyloctan-1-ammonium; Ntf2 ¼ bis(trifluoromethylsulfonyl)imide), prepared from Aliquat 336 (Scheme 50) which is well known as a phase transfer

Scheme 50 Aliquat 336 (A) and [Aliquat][NTf2] (B).

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Scheme 51 Ultrasound- and microwave-assisted one-pot oxidation of cyclohexanol to ε-caprolactone in [bmim][BF4] ionic liquid.209

agent. The aliphatic cation of Aliquat is more efficient than the cyclic cations of other tested ILs.208 Another example of application of MW and ultrasound methods is the one-pot, tandem oxidation of cyclic alcohols to their respective lactones using KHSO5 (potassium peroxy-monosulfate) as oxidant and an IL as a solvent (Scheme 51).209 Ultrasound and MW irradiation reduced the reaction time for the cyclohexanol oxidation by Oxone®, catalyzed by a TEMPO nitroxyl radical, in the presence of tetrabutylammonium bromide (TBAB) in [bmim][BF4] (bmim ¼ 1-butyl-3-methylimidazolium), from 8, using normal heating, to 5 and 0.5 h, respectively, with similar yields of ca. 80%. A new class of ILs with peroxymonosulphate anions was also synthesized and employed in the model oxidation.209 As illustrated above, the activation of substrates, intermediates, and other components of reaction mixtures by using irradiation of different kinds can efficiently influence the reaction kinetics in the oxidation of alcohols. Further application of such techniques should widen the spectrum of used substrates and obtained products in this field.

8. CATALYSTS RECYCLIZATION Many homogeneous catalytic systems, in spite of being very active and interesting from a fundamental point of view, frequently cannot lead to a practical solution of technological problems due to their high cost, instability, and difficulty of isolation and recyclization. Hence, efforts have been devoted to obtain recyclable catalytic systems, which, even if being less active than state-of-art homogeneous representatives in a single use, can be reused many times. Several methods can be used to achieve recyclable catalytic systems, such as the following ones: (i) utilization of heterogeneous solid catalytic materials; (ii) formation of dispersed nano-, sol-gel, and micellar systems; (iii) phase division, where a homogeneous catalyst and substrate are usually well soluble in one solvent, while the product is soluble in another solvent,


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which in its turn is unmixable with the first one. Other methods that generally lead to such approaches have also been developed, e.g., immobilization of intrinsically homogeneous catalysts on a solid support without significant loss of their activity. In this case (supported catalysis), advantages of homogeneous catalysts (e.g., enantioselectivity) can be combined with easy recovery and reutilization. Another idea concerns the use of physical forces, e.g., magnetic fields, to remove a catalyst from the reaction mixture. Some advances in the development of these and related ideas on catalysts recyclization and improvement of the overall activity of catalytic systems for alcohol oxidation are discussed below.

8.1 Heterogeneous Solid Oxides, Alloys, and Related Materials Many classical heterogeneous catalysts, e.g., oxides and related compounds with incorporated ruthenium, gold, palladium, or platinum were found to be effective for the aerobic oxidation of alcohols.2 Silver210 and cobalt211 were also included in this catalytic family and have raised expectations regarding the availability of the catalysts. Benzyl alcohol is a typical model substrate to test a chosen heterogeneous catalyst and the reaction conditions. Apart form the formation of benzaldehyde, disproportionation, and dehydration can occur to give toluene or dibenzyl ether (Scheme 52).212,213 Therefore, selective, active, and recyclable heterogeneous catalysts are highly sought after. In this respect, AuPd alloys supported on activated carbon, as well as the monometallic Pd and Au were tested for alcohol oxidation in the presence of O2.212 The AuPd alloy possesses a higher catalytic activity than the monometallic Pd (TOF of 38 or 54 h1 for Pd or AuPd alloy, respectively) maintaining a high

Scheme 52 Reaction scheme for benzyl alcohol oxidation.212

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selectivity (>94%) toward benzaldehyde. An unexpected result was that, under the same conditions, Au was inactive. The better activity was explained by the smaller interatomic distances in AuPd which leads to a better contact between the reagents and the catalyst. A direction in the development of heterogeneous catalysts for alcohol oxidation concerns the synthesis of composite materials, such as Fe(III) substituted Keggin-type clusters dispersed in amorphous silica matrix (PWFe/SiO2).214 This composite was tested as a catalyst in the oxidation of alcohols into aldehydes using H2O2 as a “green” oxidant. Under mild reaction conditions, the catalyst showed a high activity and selectivity, with good yields for all the tested substrates. The good catalyst activity was attributed to the large surface area owing to its micro and mesoporous structures and strong acidity of the polyoxometalate component. The stability and reusability of the catalyst was also quite good; moreover, the catalyst preparation is simple and direct from cheap starting materials. An example of a flow chemistry process is the Oppenauer oxidation of secondary benzylic alcohols using partially hydrated zirconia and various carbonyl compounds as oxidants (Scheme 53).215 The authors applied this procedure to electron-rich and electron-deficient substrates, with improvement in temperature (as low as 40 °C) and an easy reaction workup. The reuse of the catalyst was performed several times, without loss in catalytic efficiency. Cerium(IV) oxide-based heterogeneous catalysts are of interest in oxidation owing to the unique redox properties of cerium, i.e., if the crystallite size of ceria decreases, an increase in the oxygen vacancy defect concentration occurs leading to attractive catalytic properties.216 On the other hand, due to peculiar catalytic activity of gold, its combination with other materials is also appealing, especially when the combination promotes the

Scheme 53 Example of the flow Oppenauer oxidation of secondary benzylic alcohols.215


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development of eco-friendly processes. Thus, the composite Au/CeO2 material was prepared by two distinct methods: homogeneous deposition–precipitation (ACH) and direct anionic exchange (ACD).216 The efficiency of the two processes of catalyst fabrication was evaluated using the aerobic oxidation of benzyl alcohol under solvent- and base-free reaction conditions. The ACH catalyst exhibited a higher alcohol conversion (64.5%) than the ACD catalyst, (53.8%), with comparable selectivities. This difference, according to the authors, is due to presence of smaller sized gold NPs and the higher number of oxygen vacancy sites on the ceria surface: the ACH sample shows surface oxygen vacancies, whereas no oxygen vacancies were found in the ACD sample. The conversion of benzyl alcohol increased with reaction time, while the selectivity on benzaldehyde diminished with an overproduction of benzyl benzoate. The recyclability of both catalysts decreased after repeated use, which might be due to their structural changes. The related Au/CeO2–Al2O3 catalyst was reported to be effective for the heterogeneous oxidation of 1-tetradecanol, used as a model to test the catalytic potential of the material toward other fatty alcohols.217 All the reactions were performed at 80–120 °C with molecular oxygen at atmospheric pressure and no added base. The highest conversion was 38%, while the reaction selectivity was up to 70% for tetradecanoic acid and up to 80% for tetradecanal. Hydrotalcites have also attracted much attention as useful precursors for the development of new environmentally friendly catalysts. Thus, Pt/Au alloy NPs, supported over hydrotalcites and with soluble starch as a green reducing and stabilizing agent, are catalysts for the selective aerobic oxidation of glycerol and 1,2-propanediol.218 The reaction conditions were mild; the oxidation was being performed in aqueous solution with no base added and using molecular oxygen. The authors tested the individual metals as catalysts and concluded that the bimetallic catalyst exhibits some synergetic properties. The high activity and selectivity of these bimetallic catalysts suggest that Pt atoms gain more electrons than Au atoms in PtxAuy–starch/ hydrotalcites as a result of the alterations of geometry and due to two types of electron transfers: (i) from the starch ligand to both Au and Pt atoms and (ii) from Au to Pt atoms.218 A series of cobalt-doped vanadium phosphorus oxide catalysts was prepared using a classical organic method, followed by calcination, and tested for the oxidation of benzyl alcohol with TBHP as an oxidant.219 The catalytic activity increases with the temperature growth up to 68% at 90 °C, while selectivity toward benzaldehyde varies in different solvents and reaches

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Scheme 54 Oxidation of benzyl alcohol with cobalt-doped vanadium phosphorus oxide catalysts.219

100% following the trend: acetonitrile > chloroform > toluene > dioxane. The effect of the competitive adsorption between the solvents and benzyl alcohol was discussed. The proposed mechanism (Scheme 54) involves two active phases, where a suitable V5+/V4+ balance is required; the presence of Co increases the average oxidation number of vanadium creating higher amount of V5+ species, which are essential for the reversible V4+/V5+ redox mechanism. A porphyrin-containing cellulose derivative, namely hematin-appended 6-aminocellulose, performed well as a catalyst for the oxidation of guaiacol and synapyl alcohol.220 The catalytic material is insoluble in most alcohols and can be considered as a heterogeneous biomimetic catalyst. The high oxidation activity and stability of the catalyst might be due to the cellulose backbone that inhibits the self-aggregation of the hematin moieties. To probe the potential of the cellulose backbone as a chiral catalyst, oxidation of sinapyl alcohol was performed, but the material showed no chiral behavior.

8.2 Supported Catalysts Supported catalysts can be considered as heterogeneous ones which, however, exploit some features of homogeneous catalytic systems, e.g., selectivity and activity. Thus, vanadium-substituted phosphotungstic acid immobilized on amine-functionalized MCM-41 exhibited high activity and selectivity in the oxidation of aromatic alcohols to the corresponding aldehydes with H2O2, even after five cycles.221 It should be mentioned that not only catalytic systems as a whole, but also their components, in particular the most expensive and unstable ones, can be immobilized and reused. For instance, fullerene has been employed as a molecular support for TEMPO


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and this combination was further applied for the oxidation of primary and secondary alcohols using the Anelli protocol.76 In another study,222 MCM-41 was used as a support, to anchor undecatungstophosphate. The thus prepared bifunctional catalyst was tested for oxidation, as well as esterification, of benzyl alcohols. Kinetic studies revealed that the reactions follow first order kinetic patterns, and the low values of activation energy for esterification and oxidation are indicative that the reaction rate is governed by a chemical step. A curious feature of the supported tungstophosphate is the drastic change in selectivity of the reaction with time and increasing temperature. Thus, 100% selectivity toward benzaldehyde was achieved for 2 h but, after 24 h, the selectivity shifted toward benzoic acid. When the reaction temperature increases, the conversion of the alcohol also grows, as well as the selectivity toward benzoic acid. This overoxidation of benzaldehyde to benzoic acid might be due to the high acidity of the catalyst or an effect of the support. The effect of a support can provide a key to achieve a high catalytic efficiency: for instance, TONs up to 63,000, with selectivities of 99% and conversions between 71% and 99% were reached when the oxidation of activated, nonactivated, and heterocyclic alcohols were studied with 1 atm of molecular oxygen and a zirconia supported ruthenium catalyst.223 Such high TONs were attributed to the properties of ZrO2 surface, in particular hydroxyl groups and coordinatively unsaturated Lewis acidic-basic Zr4+ and O2 pairs. Developments in support materials, which can facilitate electron transfer, increase long-term stability of the catalysts, etc., are highly desirable. With Pd dispersed over mesoporous SiO2, the oxidation of crotyl and cinnamyl alcohols (Scheme 55) can be achieved with high efficiency.224,225

Scheme 55 Cinnamyl alcohol oxidation.224

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The generation of atomically dispersed Pd2+ surface species at low palladium loadings promotes the high activity in the oxidation of allylic alcohols. The hierarchically ordered nanoporous Pd/SBA-15 was shown to be a key factor to obtain aldehydes upon oxidation of sterically hindered allylic alcohols, such as phytol and farnesol. The results show how important is the capability of support materials to stabilize the metal oxide and to provide a specific pore size thus promoting the mass-transfer. A robust matrix material can also be employed as a support to accommodate active metals, metal oxides, polymers, etc. Thus, three-dimensional graphene-based frameworks were used to support copper phthalocyanines and show improved thermal and chemical stability combined with a competitive overall cost and availability.226 This catalyst was successfully tested for the selective aerobic oxidation of alcohols to the corresponding carbonyl compounds. The high catalytic activity of the material was related to π–π interactions between benzene moieties of reactants and graphene that favor the interaction of the reagents and catalytic sites. On the other hand, the presence of basic sites on the support also helps to improve the selectivity. The use of more active and stable reducible oxides, like TiO2, has some advantages over the nonreducible ones, such as SiO2 and Al2O3. Thus, mesoporous titanium dioxide (anatase) was applied as a support for the deposition of gold NPs and the thus prepared material was used for the vapor phase oxidation of benzyl alcohol.227 The activity of the catalyst decreased in terms of TOFs against metal loading, what is probably due to agglomeration of gold NPs and hence lower number of available active metal sites.

8.3 Nano, Dispersed and Micellar Catalysts One of the ways to enhance the activity of heterogeneous catalytic systems consists in providing a high surface/volume ratio, i.e., its dispersion down to nano-scale. Thus, nanoporous stainless steel (NPSS) electrode materials with copper and a film of palladium were fabricated and it was found that their porosity and the presence of Cu improve the long-term stability of the Pd film on the surface.228 The presence of Cu has a significant effect on the catalytic activity, the reaction kinetics, and poisoning tolerance of the NPSS/Cu/Pd electrode. The electrode was tested for the electrooxidation of glycerol, with comparable results of those of palladium-carbon catalysts, possibly due to the large electrochemically active surface area. Palladium clusters can be encapsulated in a microporous silica shell; the thus prepared heterogeneous catalyst was tested for the solvent-free aerobic


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oxidation of various hydrocarbons and alcohols and showed a high activity, with TOF values of up to 54,740 h1.229 The activity and selectivity depend on the sizes of pores and substrates. The recyclability was also tested and reached 20 without any loss of activity for benzyl alcohol oxidation; the corresponding overall TONs were estimated to be ca. 280,000. Palladium NPs supported on carbon NTs functionalized with various organosilane modifiers have been tested for the selective aerobic oxidation of benzyl alcohol and quasi-TOFs based on the active surface area as high as 288,755 h1 were obtained.230 The high selectivity toward benzaldehyde was ascribed to the low surface hydrogen concentration leading to diminished formation of toluene, and to the low surface basicity that hampers the disproportionation of benzaldehyde to form benzoic acid. In addition, the basic catalyst support facilitates small and highly dispersed Pd NPs with narrow size distribution, what favors the high activity of this catalyst even after five consecutive runs. Another related catalytic system, composed of conjugated microporous polymers, with encapsulated palladium NPs with 1.6–3.5 nm in size, showed a high catalytic activity for the benzyl alcohol oxidation to benzaldehyde with conversions and yields up to 74%.231 Gold-containing poly(urea-formaldehyde) microparticles were prepared by in situ polymerization using a series of stabilization agents and tested for the selective oxidation of glycerol.232 The glycerol conversion and the glyceric acid selectivity have opposite behaviors when the size of gold particle decreases. If the surface of stabilizer is hydrophobic, then the selectivity to C3 products in the resulting catalysts is enhanced. If stabilizers with hydrophilic surfaces are applied, the formation of C–C bond cleavage products is preferable. Besides oxidants, solvent, and other components of the reaction mixture, the environmental compatibility of the catalyst support also deserves to be addressed. Novel biocompatible thiol-functionalized fructose-derived nanoporous carbon support produced by hydrothermal carbonization can significantly diminish the environmental impact.233 This porous carbon material with supported gold NPs was highly active in selective aerobic oxidation of several alcohols to the corresponding aldehydes and ketones. The catalyst was easily recovered and reused 6 without leaching of metals or loss of activity. The proposed mechanism for the catalytic oxidation of alcohols involves the base promoted deprotonation of alcohol to form alkoxide on the Au surfaces. Then gold catalyzes the β-hydrogen elimination to produce the corresponding aldehyde, along with the formation of O2 and H2O.233

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Another remarkable direction concerning the promotion of the catalyst active area and its recyclability consists in the use of micelle catalytic systems. They can combine the advantages of homogeneous and heterogeneous catalyzes, because, on one hand, the catalyst and substrate are soluble in one solvent, while the product possesses a high solubility in another one. The transfer of the product to another phase shifts the equilibrium, while the catalyst is responsible for the kinetics. Usually micellar structures are used, where catalysts are confined within the small droplets of one solvent, separated from another one and commonly stabilized by surfactants. Thus, enzyme-inspired star block-copolymers with limited branching were tested in catalytic systems for the oxidation of alcohols in water, in particular for a Cu/TEMPO-catalyzed alcohol oxidation reaction34 90% conversion of benzyl alcohol to benzaldehyde was obtained after 44 h of reaction. The fact that the polymers are able to preconcentrate molecular oxygen is of particular significance for further developments.

8.4 ILs and Related Systems with Phase Division The use of ILs234 and supercritical fluids235 in catalytic oxidation has been regarded as a new possibility for catalyst recycling and enhancing the product yield and selectivity. For instance, recycling of expensive TEMPO is not a trivial task due to the homogeneous character of most of the TEMPO catalytic systems. Moreover, in some cases there are also drawbacks due to overoxidation that lower conversion and selectivity. The replacement of organic solvents by ILs can provide an effective strategy to avoid such problems.236 Thus, a vanadium-based catalyst, TEMPO and sulfonic acid cocatalysts were grafted on the [C4py][BF4] IL and used for oxidation of alcohols with H2O2 as oxidant, exhibiting good activity and recyclability.237 In another example, TEMPO was incorporated into supported [C6mim] [BF4] IL (Fig. 14A) and exhibited high activity for alcohol oxidation using bis(acetoxy)iodobenzene (BAIB) as the terminal oxidant.236 The catalyst can

Figure 14 Strategies for immobilizing TEMPO on ILs. [C4mim][BF4] supported TEMPO (A) and IL@SBA-15-TEMPO (B).79


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be recycled together with the IL without loss of the efficiency for several cycles. TEMPO can be also grafted on SBA-15 solid support be combined with 1-methyl-3-butylimidazolium ([C4mim][Br]) IL thus forming the IL@SBA-15-TEMPO catalytic system (Fig. 14B).79 This system possesses improved selectivity and good recyclability for the oxidation of alcohols to aldehydes and ketones with TBN as an oxidant in AcOH. Deep eutective solvents (DESs) are a novel class of ILs that are generally obtained by the interfusion of quaternary ammonium salts and hydrogen bond donors (e.g., amides, amines, alcohols, and carboxylic acids). Their ionic nature and relatively high polarity provide good solubility for many ionic species, such as metal salts. They also have other advantages over common ILs, such as the simple and easy preparation as pure phases from cheap and easily available components or high chemical stability toward atmospheric moisture and temperature. These novel DES were used to incorporate Fe(NO3)3 9H2O in TEMPO; the DES–TEMPO/Fe(NO3)3 system showed good performances in the selective oxidation of various alcohols to the corresponding aldehydes and ketones, using molecular oxygen as an oxidant and under mild and solvent-free conditions.238 As expected, the DES was easily recovered and recycled up to 5 without significant loss of catalytic activity. Glycols constitute other media which have been widely used in organic transformations as environmentally benign solvents and soluble supports for liquid phase synthesis. ILs and polyethylene glycols can also be combined in the solvent–free aerobic oxidation of alcohols to give an excellent catalytic effect and easy catalyst recovery.239 The bifunctionalized combined IL-glycol PEG1000 catalytic system ([Imim-PEG1000-TEMPO][CuCl2]) shows catalytic properties similar to those of its nonsupported counterpart in terms of yields as high as 95% with 100% conversion and selectivity toward ketone. Moreover, ([Imim-PEG1000-TEMPO][CuCl2]) could be recycled and reused without significant loss of catalytic activity after five runs. The combination of several of the above described approaches can be also used. Thus, precious metal catalysts can be supported on nanomaterials and combined with ILs.240 The supported gold NPs on graphene oxide (GO) with an ionic liquid framework (Au@GO-IL) has been shown to be a highly active, and leaching tests, such as hot filtration test and AAS analysis, indicate that the catalytic reaction is mainly heterogeneous in nature. The reusability of this catalyst was tested for 5 without a significant decrease in its catalytic activity.240 Also using gold NPs, but performing oxidation under MW

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irradiation, 2-hydroxybenzyl alcohol was converted successfully to 2-hydroxybenzaldehyde in the presence of 3-chloroperoxybenzoic acid and hydrogen peroxide in methanol.241 Pd NPs are commonly used in oxidations of alcohols and can be incorporated into an IL.242 The morphology of the particles can be suited to different applications, e.g., flower-like particles, due to their concave tetrahedral subunits, exhibited a high electrocatalytic activity toward ethanol and methanol oxidation compared with that of the commercial Pd black catalyst.242 A Pd complex containing triphenylphosphine and a Schiff base catalyst was used243 for the study of the solvent effect in carbonylation of primary and secondary alcohols to aldehydes and ketones, in the presence of NaOCl as an oxidant. By kinetic study of different proportions between the imidazolium-based IL ([C2mim][PF6]), it was shown that the acceleration of the reaction depends on the mixing proportion and that the best ratio was 1:1. Another interesting aspect of using ILs instead of molecular solvents is the possibility of bypassing steps. Thus, in a propane oxidation using rhodium (palladium)–copper–chloride catalytic systems immobilized in ILs, propane is oxidized to acetone, bypassing the isopropanol formation step.244 Methane was also studied and is oxidized under more severe conditions than propane, giving methyl trifluoroacetate as the main product. Several hydrophobic methylimidazolium-based ILs were studied in the oxidation of cyclohexanol to cyclohexanone with H2O2 and WO3 as a catalyst.245 In the biphasic cyclohexanol-ILs system, 1-octyl-3methylimidazolium chloride ([C8mim]Cl) IL was found to effectively promote cyclohexanol oxidation to 100% conversion of cyclohexanol and 100% selectivity to cyclohexanone, what is accounted for by the biphasic character of the system. The oxidation of cyclohexanol occurs in aqueous phase containing H2O2 and the catalyst, while the produced cyclohexanone is transferred to the organic phase, minimizing its further oxidation. Higher concentrations of [C8mim]Cl favor the oxidation possibly by stabilizing reaction intermediates in the catalytic process.245 A combination of a tungsten species (in particular, tungstic acid H2WO4) and an IL allowed to obtain very good results in the oxidation of five- to eight-membered cyclanols, using aqueous H2O2 MW or ultrasound activation with the ammonium-based IL Aliquat 336.208 The oxidation reaction was studied in several ILs and no effect of the aromaticity of the IL cation was observed. However, the anion of the IL seems to be important and the yields increased with its size. This can be a key factor for the choice of ILs.


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The use of lanthanides with ILs for oxidation reactions is still scarce, but a hydrogen peroxide–urea adduct and catalytic (CF3SO3)3La in an IL was recently used for the oxidation of a secondary alcohol to ketone.246 A number of 1,2-diols, α-hydroxyketones and other aromatic, and aliphatic secondary alcohols have been successfully oxidized to the corresponding ketones with yields from 74% up to 92% and reaction times between 0.5 and 3 h. Imidazolium-based ILs were used as nonconventional media in alcohol dehydrogenase (ADH)-catalyzed reactions in enzymatic catalysis.247 When containing up to 50% of the IL, the overall conversion could be improved in some cases, while the stereoselectivity of the enzyme remained unaltered.247 Besides enzymatic catalysis, the development and use of green and efficient methods to transform lignocellulosic biomass (along with cellulose and hemicellulose) into fuels and high value-added chemicals is another appealing area. Thus, one-pot sequential oxidation and aldol condensation reactions of veratryl alcohol in the basic ionic liquid (BIL) 1-butyl-3-methylimidazolium 5-nitrobenzimidazolide, which acted as the solvent and basic additive, was studied.248 The effects of different factors, such as the type of catalyst, reaction time, reaction temperature, and the amount of BIL, on the oxidation reaction were investigated. It was shown that the catalytic performance of individual Ru@ZIF-8 (zeolitic imidazolate framework8) or CuO was very poor for the oxidation of veratryl alcohol to veratryl aldehyde. Interestingly, Ru@ZIF-8 + CuO was very efficient for the oxidation reaction and a high yield of veratryl aldehyde could be obtained, indicating the synergistic effect of the two catalysts in the BIL. The veratryl aldehyde generated by the oxidation of veratryl alcohol could react directly with acetone to form, in high yield, 3,4-dimethoxybenzylideneacetone by aldol condensation reaction catalyzed by the BIL.

8.5 Other Directions Supercritical CO2 is the most used supercritical fluid due to its favorable characteristics. Its low toxicity, relatively low critical temperature and stability allow most compounds to be extracted with little damage. In addition, the solubility of many extracted compounds in CO2 varies with pressure, allowing selective extractions. These possibilities are particularly useful in the reactions involving gaseous reagents such as oxidation with O2. Concerning the selective aerobic oxidation of alcohols, CO2 can dry wet material thus allowing to achieve a high selectivity to aldehyde by suppressing the

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formation of carboxylic acid via the favored hydration of aldehyde.249 In another work250 it was reported that the conversion of benzyl alcohol to benzaldehyde in CO2 increased 50% for a pressure increase of 7% because the substrate and products are distributed differently in the organic and supercritical phases, thus affecting both the reaction rate and selectivity. A promising perspective for catalyst recyclization concerns the application of magnetically recoverable catalysts that can be readily collected by magnetic attraction without the use of traditional isolation methods, such as filtration, extraction or centrifugation. For instance, magnetic CoFe2O4 NPs can efficiently catalyze the oxidation of alcohols to the corresponding carbonyl products and then be easily recovered with assistance of a magnetic field and reused several times.251 If the catalytically active particles are nonmagnetic, they can be immobilized on the surface of a magnetic carrier, e.g., Fe2O3 or Fe3O4, be applied for the oxidation of various alcohols, and then be recovered by application of a magnetic field. Thus, superparamagnetic Fe3O4@EDTA–Cu(II) NPs were readily prepared and identified as an effective catalyst for the tandem transformation of benzyl alcohols and amine hydrochloride salts into the corresponding amides with TBHP as an oxidant.154 After completion of the reaction, the catalyst can be removed from the reaction vessel by assistance of an external magnet and reused at least 6 without significant loss of its activity. Bifunctional bimetallic alloys in which both catalytic and magnetic functions are simultaneously provided can be applied. For example, Co-Pd bimetallic alloy NP catalysts were prepared and employed for the aerobic oxidation of a variety of alcohols in water.252 The catalysts then were magnetically recovered and reused for further oxidation. Leaching of Co and Pd was in orders of only 103 mol% and 106 mol%, respectively. The proposed explanation for the low leaching involves an electrolytic “protection” of the more expensive Pd component: Eox of Pd(0)/Pd(II) (0.95 V) is higher than Eox of Co(0)/Co(II) (0.28 V). Hence, Co(0) is oxidized by Pd(II) to give Co(II), while Pd(0) aggregates with initial NPs and remains within the catalyst. To conclude, heterogeneous catalysis with a wide range of possible supports is rather appealing for industrial applications. The easy recovery and recycling ability are two of the most desirable features. Although some concerns still remain regarding their limited catalytic activity and deactivation,253 many of the recently introduced protocols for the alcohol oxidation involve supported transitional metal catalysts.174 The pursuit of oxidation systems without such limitations and which are readily accessible, stable, inexpensive, environmentally acceptable, and can promote selective oxidation under mild reaction conditions is still under way.


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ACKNOWLEDGMENTS This work has been partially supported by the Foundation for Science and Technology (FCT), Portugal (FCT Doctoral Program CATSUS, UID/QUI/00100/ 2013 and “Investigador 2013” [IF/01270/2013/CP1163/CT0007] programs). N. M. R. M., A. P. C. R., and M. N. K. express gratitude to FCT for doctorate, post-doc fellowships and working contract, respectively.

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CHAPTER FOUR

Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of Michael Limbach* CaRLa (Catalysis Research Laboratory), Heidelberg, Germany BASF SE, Synthesis & Homogeneous Catalysis, Ludwigshafen, Germany *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 1.1 Thermodynamics and Rationale 1.2 State-of-the-Art Functionalization of Alkenes and Alkynes with CO2 2. Shedding Light into the Dark: Lactone Formation 2.1 Lactone Formation: Mechanistic Intermezzo 3. Lactone Cleavage and Final Ligand Exchange 3.1 Lactone Cleavage with Auxiliaries to Force a β-H Elimination 3.2 Mechanistic Course of the β-H Elimination from Neutral Nickelalactones 3.3 Lactone Cleavage with Brønsted Bases 3.4 Acrylate/Ethylene Exchange 4. Catalytic Reactions 5. Catalysts Gone Astray 6. Conclusions and Outlook Acknowledgments References

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1. INTRODUCTION 1.1 Thermodynamics and Rationale The exploitation of carbon dioxide (CO2) for the production of world-scale chemicals, such as formic acid,1b has industrial potential, as CO2 is a cheap and abundantly available C1 building block.2 Nevertheless, only a few reactions and catalysts enable the straightforward catalytic functionalization of industrially viable starting materials, such as alkanes and alkenes with CO2, to industrially relevant target molecules. Advances in Organometallic Chemistry, Volume 63 ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.03.001

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Among those, there is a small set of “dream reactions,”3 i.e., economically highly attractive transformations, which do not yet exist due to major technological or scientific hurdles (Fig. 1): the direct carboxylation of alkanes, 0 such as methane to acetic acid ΔGR ¼ + 55:0kJ=mol , the twofold carbox 0 ylation of butane to adipic acid ΔGR ¼ + 77:4kJ=mol , or the direct synthesis of acrylic acid and its derivatives from alkenes and CO2 0 ΔGR ¼ + 42:7kJ=mol . In the latter transformation, addition of auxiliaries like alkyl halides (e.g., RX ¼ MeI) would lead to alkyl acrylates, but does 0 not alter the unfavorable thermodynamic balance ΔGR ¼ + 21:0kJ=mol . All those transformations are highly endergonic and do not occur spontaneously. In order to shift the thermodynamic equilibria to the product side, the free acids have to be removed from the reaction with a base via their salts, i.e., acetates, adipates, or acrylates. It has to be pointed out that—albeit now the reaction becomes feasible from a thermodynamic point of view 0 (ΔGR ¼ 56:2 kJ=mol for sodium acrylate)—an economic penalty comes now into play: seldom enough are the salts, the desired commercial products but the acids. Salt cleavage adds often significant costs and by-products. For the hydrogenation of CO2 to formates, innovative base-recycling concepts have been developed1 and might prove useful for other acid/base pairs, too.

Figure 1 Dream reactions in the context of CO2 functionalization and their thermodynamic feasibility. A dream reaction is an economically highly attractive transformation, which is currently unfeasible due to a major scientific and/or technological challenge.

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Acrylates and their downstream products are manufactured globally on a multimillion ton level as they are ubiquitous in daily life as hygiene products, coatings, adhesives, and food preservatives. Sodium acrylate is the most important industrial acrylic acid salt and is used as monomer for superabsorbent polymers. The combined thermodynamic need and market demand for the salt instead of the free acid, made sodium acrylate an ideal target molecule for our investigation.

1.2 State-of-the-Art Functionalization of Alkenes and Alkynes with CO2 The hydrocarboxylation of activated unsaturated hydrocarbons with CO24 is an established methodology in organic chemistry to yield either α,β- (for alkynes),5 β,γ-unsaturated α-branched (for allenes and 1,3 alkadienes),6 or α-branched carboxylic acid derivatives (for styrenes),7 but requires the stoichiometric use of reductants (i.e., AlEt3, hydrosilanes, Et2Zn, RMgX) or directing groups in the substrate (Fig. 2A).7c The catalytic carboxylation of unsaturated hydrocarbons with CO2 to α,β-unsaturated carboxylates (i.e., acrylates) has been a topic of academic as well as industrial research for three decades, since seminal work of Hoberg8 and Yamamoto9 in the early 1980s. Although Hoberg et al. reported the catalytic nickel-catalyzed reaction of alkenes and isocyanates (isoelectronic to CO2) to acrylamides,10 there has been no catalyst for the direct carboxylation of CO2 and alkenes, neither based on nickel nor based on other metals from the nickel (Pd, Pt)11–20 or iron triad,21 titanium,22 molybdenum,23 tungsten,24 zirconium,25 or rhodium.26 Basic obstacles for a catalytic transformation remained: (a) the ender 0 gonic nature of the overall reaction ΔGR ¼ + 42:7 kJ=mol , (b) the high activation barrier for the proposed β-hydride elimination from a nickelalactone or other Hoberg-type complexes (ΔG ¼ 164 kJ/mol),27,28 and (c) the limitation to a small set of ligands paired with unproductively low reaction temperatures down to 70 °C (Fig. 2B and C).29 Apart from the mentioned Hoberg complexes, the mechanistic course of the reaction has been purely speculative and was poorly supported with experimental findings. This is why despite of the maturity of the technology toward formiates,1 the development toward acrylates has been remaining in its infancies over several decades.

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Figure 2 State-of-the art hydrocarboxylation of alkenes/alkynes with CO2 (A), an early mechanistic proposal for the reductive carboxylation of ethylene (B), and recent kinetic considerations in a putative catalytic cycle (C).

2. SHEDDING LIGHT INTO THE DARK: LACTONE FORMATION 2.1 Lactone Formation: Mechanistic Intermezzo Hoberg et al. and others30–33 have revealed metallalactones, in particular nickelalactones (Hoberg complexes),8 to be stable and isolable intermediates of the potential catalytic coupling of ethylene and CO2 (Fig. 2B). The reaction has been described with numerous ligands,34 solvents, and substrates (alkenes, alkadienes, alkynes, allenes)35 and the experimental efforts have early on been supported by quantum mechanical studies: for a monodentate DBU-model ligand (DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene),27


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bipyridine (bipy),28 and recently for chelating bisphosphines, phosphites, or bisamines.36 It is widely accepted that the reaction starts from an ethylene complex, since it is found to be more stable in calculations than the metallalactone and its formation prior to lactone formation has been observed experimentally (Fig. 3). For the more oxophilic molybdenum, lactone formation occurs via prior formation of a stable CO2–ethylene complex by ligand exchange and by subsequent coupling of CO2 and ethylene.37,38 Lactones of other metals than nickel resulting from the direct coupling of CO2, and alkenes, alkynes, alkadienes, or allenes have only been described for an allene (a single palladalactone),39 nevertheless, as a substance class they are known.12,14 We have focused our experimental activities on bidentate ligands, such as bisphosphines. In a systematic study of the oxidative coupling of ethylene and CO2 with Ni(COD)2 as a Ni(0)-precursor and bidentate phosphines bearing either phenyl- or tert-butyl substituents at the phosphorus atoms and differing in the length of the carbon bridge –CH2(CH2)n– in the ligand backbone (n ¼ 0–2), we observed the following trends (Fig. 3A): (1) Sterically, nondemanding bidentate ligands as bis(diphenylphosphino) methane (dppm) or certain monodentate ligands form aggregated complexes (e.g., dimers, trimers), as already discussed by Langer and Walther et al.,40–44 and (2) the higher homologues of dppm, such as 1,2-bis (diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane

Figure 3 Experimental findings (A) and competing mechanisms for lactone formation for the dtbpe-ligand (B).

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(dppp), rapidly form the corresponding known tetracoordinate Ni(0) species Ni(dppe)245 and Ni(dppp)2 instead of the expected lactones [Ni ((CH2)2CO2)(dppe)] and [Ni((CH2)2CO2)(dppp)].46 This does not at all mean that those ligands do not form catalytically active species (as has been shown later), but for our first systematic studies they turned out to be “difficult” for the mentioned reasons. We rationalized that this coordinative saturation of the metal might be avoided by switching to more bulky tert-butyl substituents at the phosphorus atoms (density functional theory (DFT) calculations predicted the formation of tetracoordinated species with such ligands to be clearly endergonic ΔG > 50 kJ/mol). Indeed, ligands such as di-tert-butylphosphinoethane (dtbpe) and di-tert-butylphosphinopropane (dtbpp) resulted in immediate formation of the corresponding ethylene complexes, which formed in the presence of CO2, the desired nickelalactones. Only [Ni((CH2)2CO2)(dtbpe)] was stable enough to be isolated and stored as a solid for several months without decomposition and external gas pressure. Other lactones (dtbpm, dtbpp) easily expelled CO2 to revert to the initial ethylene complexes. Lactone formation in tetrahydrofuran (THF) was slow with those ligands, but significantly accelerated in chlorobenzene. For nickel(0) complexes with bidentate phosphine ligands, we have identified two mechanistic borderline cases for lactone formation: an “inner-sphere mechanism” and an “outer-sphere mechanism” (Fig. 3B)36:a In general, in the gas phase, the “inner-sphere” mechanism is favored, but this changes for typical organic solvents such as THF. The “outer-sphere” mechanism is significantly more favorable for sterically hindered ligands, and the activation barriers for ligands such as bipyridine (bipy) become essentially identical for both mechanisms. More electron-rich ligands lead, in general, to lower barriers for both mechanisms. However, while steric hindrance does not affect the “outer sphere” mechanism, it leads to much higher barriers for the “inner sphere” mechanism. Although it is in general not completely clear, which mechanism is favored for what reason, probably things are most easily understood for the homologous series dtbpm, dtbpe, and dtbpp: increasing the bite angle leads to larger steric bulk and the “inner-sphere” barrier systematically increases while the “outer sphere” barrier is much less affected. For the inner-sphere mechanism, the insertion is believed to occur in one step, where the CO2 molecule concertedly coordinates to nickel and inserts into a NidC bond (G{ ¼ 124 kJ/mol). This is comparable to the mechanistic course proposed by Buntine et al. for the reaction with DBU as a ligand, where two DBU ligands according to calculations stay coordinated.27


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Depending on the ligand and the level of theory, we found a weak associative precoordination of CO2, which, however, certainly does not indicate a stable compound of any significance. The attack of CO2 on the coordinated ethylene in the “outer sphere” without precoordination of CO2 (G{ ¼ 110 kJ/mol) is perhaps most easily understood as the reverse reaction of the decarboxylation of the β-H-agostic, formally zwitterionic intermediate B1 (see Fig. 3B). A similar mechanism for attack of CO2 at amide and hydroxide groups of nickel compounds with subsequent insertion of CO2 into the NidN/NidO bonds via zwitterionic intermediates has been computed recently.47 Carboxylation of Ni(II) and Pd(II) allyl complexes has also been proposed to proceed via zwitterionic intermediates resulting from the attack of CO2 at the terminal carbon of the η1-coordinated allyl group.48–53 B1 is isoelectronic to the thermodynamic insertion product in cationic olefin polymerization such as in the Brookhart systems,54 and is fairly stable, if solvation is taken into account. This means that a second, lower barrier Gs{ ¼ 82kJ=mol needs to be overcome for recoordination of the carboxylate unit to nickel. For other ligands, such as dmpm, dmpe, bipy, and dtbpm, B1 is not stable and the “outer-sphere” transition state TS–B1–C1 leads directly to C1 (see Fig. 4). A third mechanistic option with precoordination of CO2 to a vacant coordination side created by the dissociation of one arm of the bidentate ligand seems to be unlikely (transition state for dtbpm ΔΔG{ ¼ 15 kJ/mol higher in free energy than that of the “inner-sphere mechanism”).36a Alternatively, the role of a η2 side-on bound Ni–CO2 complex55 as entry into a potential catalytic cycle has been debated previously (Fig. 5). Similarly

Figure 4 Structures of the transition states involved in lactone formation in solution. From left to right: TS–B1–C1, TS–A–B1, and TS–A–C1. Bond lengths in pm. Hydrogen atoms on the dtbpe-ligand are omitted for clarity.

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Figure 5 The elusive side-on bound Ni–CO2 complex is—at its best—innocent.

to Aresta et al.28 and different from Dedieu et al.,56 we conclude that an η2bound CO2 complex of the catalytically active [Ni(dtbpe)] fragment is at best innocent. Based on the kinetic barriers to form a lactone either from an alkene or CO2 complex, the alkene precursor is clearly preferred (190.6 vs. 108.8 kJ/mol).

3. LACTONE CLEAVAGE AND FINAL LIGAND EXCHANGE 3.1 Lactone Cleavage with Auxiliaries to Force a β-H Elimination Nickelalactones do not easily convert into other catalytically active species in the sense of a β-H elimination. Therefore, a variety of stoichiometric auxiliaries have been investigated not only such as electrophiles (i.e., alkyl halides31–33,57–59 or protons58,60), Lewis acids,29,58,59,61 a combination of both62 but also physical measures such as heat60,63–65 or even ultrasound (cf. Fig. 6D–G).60 All attempts in common is that the fate of the organometallic species involved in the reaction was ill-defined and so far no catalytic reaction has been reported for this route to overcome both of the abovementioned fundamental thermodynamic and kinetic hurdles (see Figs. 1 and 2C). The relevance and interplay of various catalytically active species in the sense of the elusive β-H elimination from nickelalactones as discussed in literature depend clearly on ligand, substrate, and reaction


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Figure 6 Manifold attempts to cleave nickelalactones at certain (A) and/or uncertain fate of the organometallic species (B).

conditions66: In dimethylformamide (DMF), according to Walther and coworkers, 1,10 -bis(diphenylphosphino)ferrocenyl (dppf ) or 1,10 -bis (diisopropylphosphino)ferrocenyl (dippf )-ligated lactones decomposed at temperatures >40 °C,67 whereas temperatures as high as 140 °C are needed to extrude CO2 from a bipyridine-ligated nickelalactone in the solid state.33 No direct interconversion of nickelalactones to their corresponding π-complexes and vice versa has been reported so far and only Hoberg found indirect evidence for a thermally induced β-H elimination by isolation of cinnamic acid after acidolysis of a crude reaction mixture (styrene, CO2, Ni(COD)2, DBU, THF, >85 °C, 24 h, cf. Fig. 6G).68 A formal β0 -H elimination was proposed based on the isolation of β,γ-unsaturated carboxylic acids in good yields upon cleavage of P,N-ligated lactones (i.e., 2-[2-(dicyclohexylphosphino)ethyl]-pyridine as ligand) with nonaqueous HCl in Et2O at room temperature.58,59 Such lactones even released α,βunsaturated carboxylic acids upon exposure of the crude reaction mixture to the Lewis acidic BeCl2 prior to acidolysis58,29b. Hoberg rationalized that partial ligand decoordination by protonation of the P,N-ligand’s pyridine-N would lead to a 14-electron species, which is prone to hydride elimination.

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A similarly induced β-H elimination might explain Hoberg’s experimentally observed BeCl2-mediated ring contraction between at the time postulated 5- and 4-membered nickelalactones with the same P,N-ligand (Fig. 6).58,59 Transient hydridic species have been observed by Herrmann et al. by 1H NMR in the course of the cleavage of a TMEDA-ligated nickelalactone (TMEDA, N,N,N 0 ,N 0 -tetramethylethylenediamine) with electrophiles57b, and point clearly to a β-H elimination. Moreover, Bernskoetter and coworkers found experimental evidence for a Lewis acid-mediated β-H elimination as they observed an equilibrium between a neutral dppf-ligated 4-membered nickelalactone and its 5-membered isomer (Fig. 6C).69 A similar equilibrium was observed between bis-(dicyclohexylphosphino)ethane (dcpe)-ligated 4- and 5-membered lactones generated from the corresponding neutral species and NaBAr3 F .57 Further, indirect evidence for the β-H elimination31,62 from a nickelalactone and a subsequent transfer of the hydrogen from the metal to the oxygen of the acrylate moiety have been reported by Bernskoetter et al. (Fig. 6B): they observed the isomerization of a 5-membered neutral dcpe-ligated lactone at 55 °C within 1 day.63 We have observed a similar equilibration for a dtbpe-ligated nickelalactone but only after activation with MeOTf; finally, the methyl cation was bound to the oxygen (Fig. 6A).57 It should be mentioned that Yamamoto et al. have reported as early as in 1987 the ring contraction of a 6-membered dcpeligated lactone to its β-branched 5-membered isomer.70 This ring contraction is expected to occur via β-H elimination, rotation of the acrylate moiety and reinsertion, as we confirmed by calculations for the methyl case.57 So far, it is not clear if the lack of reactivity of neutral nickelalactones and the activation by Lewis acids is solely a kinetic or thermodynamic effect or if both apply. Cationic lactones were accessible by protonation of an acrylic acid π-complex (dtbpe-ligand, H(Et2O)2BArF3 ),57 as was a η2-acrylate complex from a 4-membered nickelalactone (dcpe ligand, 2 d, room temperature) by deprotonation with the sterically hindered phosphazene base tertbutyliminotri(pyrrolidino)phosphorane (BTPP, cf. Fig. 6C).67 However, catalysis was not demonstrated in any of the mentioned cases. This might also be a consequence of the incompatibility of strong bases such as BTPP or Lewis acids and CO2 under reaction conditions.

3.2 Mechanistic Course of the β-H Elimination from Neutral Nickelalactones We have identified three pathways for the β-H elimination of neutral dtbpeligated nickelalactones that connect 5-membered (C1) and 4-membered


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(C2) species (cf. Fig. 7). One involves decoordination of the oxygen of the lactone to B1 followed by rotation of the acrylate moiety (TS–B1–C2: Gss{ ¼ 165kJ=mol), and is very similar to that described for methylated nickelalactones.57 This path corresponds to the reaction responsible for branching in olefin polymerization54,71 and is strongly disfavored if solvation effects are not taken into account. The other possibility is a path where the oxygen stays coordinated and the trigonal bipyramidal intermediate B2 is formed. The hydride in B2 is trans to the oxygen. From this intermediate, both carbons can insert into the NidH bond, to give either C2 or B1, where the latter barrier is higher (TS–B1–B2: G{ ¼ 113 kJ/mol). The third path involves formation of the κ1-O-coordinated hydride complex B3. Again, the acrylate can insert into the NidH bond to give either C2 or B1, the barrier now being slightly higher for C2 (TS–B3–C2: G{ ¼ 109 kJ/mol). Overall, the rearrangements, where oxygen stays coordinated to nickel (via either intermediate B2 or B3), are similar in activation barrier and are clearly preferred over the direct, zwitterionic pathway. Both

Figure 7 Mechanistic course of lactone interconversion and spontaneous rearrangement of nickelalactones.

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of these processes are computed to have activation barriers which are slightly too high to be accessible at room temperature. Finally, the question arises if and how an acrylic acid π-complex can be formed. Obviously, β-H elimination with ring contraction is not productive since the hydrogen is still bound to a carbon atom. Possibly, the investigated mechanisms are understood best by studying the reverse reaction, starting from the acrylic acid π-complex C3 (see Fig. 7). The π-bound acrylic acid can protonate the nickel atom to generate (intermediate) β-H-agostic complexes that rearrange to the 5- (TS–B1–C3: G{ ¼ 131 kJ/mol) and 4-membered (TS–C2–C3–I: Gs{ ¼ 138kJ=mol) lactones. Furthermore, acrylic acid can directly protonate its own carbon atoms, leading again— via β-H-agostic intermediates—to the 4- and 5-membered nickelalactones. This is not unreasonable for protonation of the β-carbon (leading to the 4-membered ring C2), as the transition state for proton transfer is a 5-membered ring (TS–C2–C3–II: G{ ¼ 154 kJ/mol). The third variant for hydrogen transfer to the oxygen is described best as an internal deprotonation of intermediate B3 where the carboxylate deprotonates the hydride to give the highly unstable intermediate B4. This is similar to the mechanism proposed for complexes bearing one DBU ligand.27 The transition state TS–B4–B5 (G{ ¼ 133 kJ/mol) refers to a rotation of the κ1-O-coordinated acrylic acid moiety to give the η2-C,Ocoordinated acrylic acid complex B5, which easily rearranges to the favored η2-C,C binding mode. In conclusion, β-H elimination giving rise to equilibria between neutral 4- and 5-membered lactones C1 and C2 might be feasible at room temperature. But the investigated pathways do not lead to the coordinated acrylic acid complex nor at elevated temperatures. According to these results, the lack of reactivity between acrylic acid complexes and lactones observed for bidentate nickel complexes, which in literature, has been attributed to a high kinetic barrier for β-H elimination and is more precisely due to a missing low energy path for a H-transfer from either the carbon or the nickel atom to oxygen.

3.3 Lactone Cleavage with Brønsted Bases The idea to cleave the kinetically inert nickelalactones with a base to make the overall reaction exergonic is easy and appealing: the reaction free energy in THF is around ΔG ¼ 21 kJ/mol. This means if a base is used as cleaving auxiliary, for quantitative deprotonation (ΔG 10 kJ/mol), the base


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should have a conjugate acid that is less acidic than acrylic acid by at least 6 units on the pKa scale. In water, this would mean that typical amine bases are just capable of making the overall reaction quantitative.72 Taking into account, concentration effects (high concentrations of base, ethylene and CO2, low concentration of acrylate) will make the reaction even more exergonic. In aprotic solvents, where acrylic acid is much less acidic relative to neutral bases, the situation becomes more difficult: In DMSO, protonated DBU is only about ΔpKa ¼ 1.6 less acidic than acetic acid (in water, the difference, ΔpKa, between acetic acid and acrylic acid is 0.5).69,73 The strongest neutral base that has been tested for the deprotonation of nickelalactones was BTPP (see above), the corresponding acid of which should then be approximately 5–6 pKa units less acidic then acetic acid in DMSO.70 Even though among the manifold mechanistic scenarios considered for the nickelacycle to nickel acrylate transformation, Buntine et al.’s DFT investigations27 discard pathways via deprotonation at the nickelalactone’s C1 and C2 carbons by a base-like DBU, experimentally in our hands strong anionic alkali metal bases such as alkoxides (NaOtBu) or hexamethyldisilylamides (NaHMDS) converted [Ni((CH2)2CO2)(dtbpe)] in PhCl at room temperature and within minutes in 90% yield to [Ni(η2-CH2]CHCO2Na)(dtbpe)] (cf. Fig. 8A). Not only is the fundamental thermodynamic limitation overcome by switching to Na-acrylate (ΔG ¼ 59 kJ/mol) but also is the kinetic barrier for metallacycle cleavage significantly reduced (98 kJ/mol). A similar reaction with NaOMe or aq. NaOH required prolonged reaction time and elevated temperature, while less basic reagents like NaOPh and others did not lead to conversion of lactone. This reaction is most likely affected by abstraction of one of the lactone’s fairly acidic α-protons next to the carbonyl group by a base. Interestingly, the cation was found to play a crucial role in this transformation: when instead of NaOMe the quarternary ammonium salt [NBu4OMe] was used, [Ni((CH2)2CO2)(dtbpe)] was not converted even after a significantly longer reaction time of 72 h (see Fig. 8A). Addition of an alternative sodium source to [NBu4OMe], such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate ðNaBAr3 F Þ, led again to a fast lactone cleavage to [Ni(η2-CH2] CHCO2Na)(dtbpe)] (47% yield in 72 h). The Lewis acidity and coordination ability of the sodium cation seem to be necessary to stabilize the carboxylate which is formed during the course of the elimination reaction. Significant activity of neutral bases was only observed in the case of the phosphazene superbase BTPP (pKa ¼ 28.35)70 in combination with NaBAr3 F as external Na+ source (40% yield after 72 h).

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Figure 8 Productive lactone cleavage with Brønstedt bases (A) and/or additives and final ligand exchange (B).

Calculations were carried out with the COSMO-RS model to account for the solvent effects of PhCl: for very reactive species like alkoxide monomers or dissociated Na+ and RO ions, deprotonation next to the lactone carbonyl group is clearly feasible. The formation of such ions, however, is endergonic by more than 100 kJ/mol, and cubane-like alkoxide tetramers40 have a computed free binding energy of 90 kJ/mol per monomer. We therefore conclude that (a) the base (both the alkali metal and the alkoxide) plays an important role in all lactone cleavage reactions studied so far, that (b) energy barriers are lower than those for β-hydride elimination mechanisms and for nickelalactone formation, and thus that (c) lactone formation and deprotonation in α-position to the lactone carbonyl group appear to be the most viable route from CO2 and ethylene to coordinated acrylate.

3.4 Acrylate/Ethylene Exchange The final mandatory step of the desired catalytic cycle is a ligand exchange reaction of in situ formed [Ni(η2-CH2]CHCO2Na)(dtbpe)] with ethylene to liberate Na-acrylate and to deliver [Ni(η2-C2H4)(dtbpe)] to reinitiate the cycle (Fig. 8B). Indeed, [Ni(η2-CH2]CHCO2H)(dtbpe)], a dimeric


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π-complex with η2-coordinated acrylic acid, failed to undergo an exchange reaction even at a pressure of as high as 30 bar ethylene, whereas the π-complex with η2-coordinated deprotonated acrylic acid [Ni(η2-CH2] CHCO2Na)(dtbpe)] gave by 1H NMR spectroscopy, an almost equimolar amount of Na-acrylate at only 8 bar of ethylene.74 Quantum mechanics (QM) calculations indicate that the reactivity difference between both π-complexes toward ethylene is likely a consequence of the endergonic character of this reaction in the case of [Ni(η2-CH2] CHCO2H)(dtbpe)] (ΔG ¼ 24 kJ/mol), due to a stronger stabilization by metal to ligand back donation. In contrast, the acrylate for ethylene exchange is exergonic (ΔG ¼ 12 kJ/mol) for the ionic complex [Ni(η2CH2]CHCO2Na)(dtbpe)] with computed barriers of 79 and 78 kJ/mol (vs. 88 and 99 kJ/mol for the protonated complex). In fact, dtbpe turned out to be a ligand enabling the isolation of all relevant intermediates and the study of all elementary reactions in combination with NaOtBu as suitable base, and this was the key to puzzle together the first catalytic cycle.

4. CATALYTIC REACTIONS Up to date, there are only three reports on catalytic systems for the carboxylation of alkenes with CO2. Two of three originate from our lab, among them the first catalytic system at all early in 2012. It comprises a two-stage setup: lactone formation in a CO2-rich regime and lactone cleavage in a CO2-poor environment (see Fig. 9).71 The separation into two mechanistic half-cycles was necessary as the alkoxides used for lactone cleavage irreversibly form fairly stable carbonic acid half-esters with CO275: while the conversion of [Ni(η2-C2H4)(dtbpe)] into nickelalactone [Ni((CH2)2CO2)(dtbpe)] proceeds quickly at a fairly high pressure of CO2 (40 bar), the transformation of [Ni((CH2)2CO2)(dtbpe)] into [Ni(η2CH2]CHCO2Na)(dtbpe)] and the final ligand exchange reaction of [Ni(η2-CH2]CHCO2Na)(dtbpe)] with ethylene to Na-acrylate were performed in its absence. Following this procedure in consecutive cycles, we were able to obtain a yield of 1,020% in Na-acrylate, which represents 10 catalytic turnovers (TON ¼ 10). This was the first example of a clearly catalytic reaction for this chemistry at all. The Na-acrylate formed in the reaction was finally separated by simple aqueous extraction and its identity was proven by HPLC-MS and NMR spectroscopy. Noteworthy, the only organic product found in the organic and aqueous phase was the targeted sodium acrylate. Even though the catalytic system is not yet efficient by

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Figure 9 The first catalytic cycle at all. The incompatibility of NaOtBu and CO2 required the separation of CO2-rich lactone formation and CO2-poor lactone cleavage.

industry standards (TOF ¼ 1/d), we consider it to be a very good starting point for further process optimization. A catalytic cycle was enabled as (a) we targeted the exergonic reaction to an acrylic acid salt instead the highly endergonic formation of acrylic acid itself (ΔG ¼ 2136a vs. 59 kJ/mol36a in THF) and (b) overcame the kinetically unfeasible β-H elimination from the nickelalactone (ΔG{ ¼ 103 kJ/mol for dtbpe)71 by its deprotonation in α-position with activation barriers that are feasible at room temperature. Recently, a catalytic cycle with a “hard” Lewis acid as cleavage agent in combination with an amine base to trap excess acid liberated from the anion of the Lewis acid has been disclosed by Vogt et al. and a TON of up to 21 was demonstrated (Fig. 10).76 Based on the seminal findings of Rieger et al., the authors assumed that a “hard” Lewis acid would compete with the carboxy group in a nickelalactone for binding the metal and thus induce a β-H elimination reaction (cf. Fig. 6E and F).31 This is especially true for the combination of the “hard” lithium cation and the soft iodide anion. Accordingly, without additionally added base such as triethylamine (50 equiv.), a significant amount of Li-propionate and Ni(II)I2 formed for a dppe-ligated lactone (Fig. 10A). Crucial for a catalytic reaction with this ligand was the addition of overstoichiometric amounts of Zn (25 equiv.) to reduce Ni(II)


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Figure 10 One-pot catalytic cycles leading to acrylates from CO2 and ethylene.

to Ni(0). This turns out to be a principal limitation to the reaction and enters the door to multiple catalyst deactivation pathways.57 Fine-tuning of ligand as well as CO2, and ethylene pressure yielded a TON of up to 21 within 3 days (dcpp, 50 equiv. Et3N, 100 equiv. Zn, PhCl, 50 °C). The addition of Zn was not mandatory for certain ligands (e.g., for dcpp, dcpb, dcpe), but the highest possible TON of 21 was only obtained with a reductant due to the already mentioned catalyst deactivation.57 Simultaneously, we revisited our approach for the two-stage reaction to find a base strong enough for the deprotonation of the Hoberg complexes but not so nucleophilic that it would deactivate under CO2 pressure: we found the surprisingly clean reaction of a dtbpe-ligated lactone to an acrylate π-complex in the presence of an excess (10 equiv.) of sodium 2-fluorophenoxide, whereas in the presence of only 2 equiv. of this base quantitative decarboxylation to the ethylene complex was the predominant reaction (see Figs. 8A and 10B). In contrast to alcoholates (which form carbonic acid half-esters),72 or strong N-bases such as DBU (which form carbaminates),77 the less nucleophilic phenoxide anion enables a reaction under CO2 pressure. Based on this finding, we found a robust nickel catalyst and reaction setup for the one-pot, direct carboxylation of activated alkenes, such as ethylene, styrenes, and 1,3-dienes with CO2 (Fig. 10B).78 The α,β-unsaturated carboxylic acid salts are formed in high yields and selectivity

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Figure 11 Scope and limitations of base (A) and substrate (B) for the catalytic one-pot carboxylation of olefins with CO2.

for the linear product (Fig. 11). Whereas monodentate phosphines (i.e., PPh3, PtBu3) and small bite-angle ligands (i.e., dtbpm, dppm) failed in the reaction with sodium 3-fluorophenoxide (50 equiv.) under CO2 and ethylene pressure (10 and 5 bar), the electron-rich P-stereogenic bisphosphine ligands, (S)-BINAPINE,79 (S,S,R,R)-TangPhos,80 (R,R,S,S)-DuanPhos,81 and (R,R)-BenzP*,82 gave high TONs up to 16. A polar, aprotic solvents like ethers (THF, dioxane) or even toluene led to higher reactivity; in chlorobenzene, oxidative addition to form Ni(II) turned out to be a limiting side reaction.66 Even though BenzP* was not the most active ligand at 80 °C, it turned out to be robust at elevated temperatures (TON 10 at 80 °C vs. 35 at 100 °C with sodium 2-fluorophenoxide). At 120 °C, the reductive decarboxylation of the nickelalactone was pronounced and the reaction became sluggish.63,71 The electronic and steric influence of substituents on the phenoxide’s core was crucial for reactivity (Fig. 11A): At 80 °C, sodium phenoxide (50 equiv. with respect to Ni(COD)2) as well as its derivatives bearing substituents with +I effect in ortho-position (i.e., sodium 2-methyl- and 2,6-dimethyl-phenoxide) gave comparably low TONs of ca. 4. In a series of fluorophenoxides (substituent with –I effect), the meta- or even better ortho-substituted derivative led to an increase in TON but not the parasubstituted one (TON 8, 10, and 2, respectively). This trend holds true for DuanPhos (TON 16 and 21, respectively), albeit at a lower level.


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The activity of the phenoxide seems to correlate with its pKa value in H2O with a maximum for sodium 2-fluorophenoxide (pKa 8.82).83 Obviously, a base successful in the reaction is sufficiently Brønstedt basic to deprotonate the nickelalactone, and of reasonably poor nucleophilicity to avoid its neutralization by reacting with CO2. Addition of finely powdered zinc (100 equiv.) had a beneficial effect on the reaction with BenzP* (TON 69 vs. 39). A huge excess of base (300 equiv.) increased TON further (107 vs. 69 for BenzP*) but was not pronounced for all of the ligands. Among a series of activated alkenes, i.e., styrenes, Michael acceptors, and 1,3-butadienes, CO2 was only incorporated once into the product (Fig. 11B). (E)-configured starting alkenes yielded exclusively (E,E)configured products. Thus, 1,3-butadiene and (E)-piperylene gave exclusively the linear monocarboxylated α,β,γ,δ-unsaturated carboxylic acids salts (TON 116 and 90, respectively). One or more so two alkyl substituents on the double bonds significantly reduced reactivity (cyclohexadiene, isoprene, 2,3-dimethylbutadiene, TONs from 7 to 64). In case of isoprene, where there are two terminal double bonds for a potential reaction with CO2, both products were formed in roughly equimolar amounts (TONs 7 and 9). The reaction of styrene and CO2 under optimized reaction conditions yielded only sodium (E)-cinnamate (TON 12).64 Electron-donating or -withdrawing groups in 4-position of the styrene frame (i.e., R ¼ OMe or CF3) led to a reactivity drop represented by a TON < 10. Terminal, unactivated alkenes or internal ones did not react to give the corresponding acrylates, most likely due to their lower tendency to form the initial BenzP*–Ni(0)–alkene complex (e.g., cyclopentene, norbornene, (Z)-3hexene), or the corresponding lactones (e.g., propylene, 1-hexene). Apart from ethylene (high TON of 107), the reaction also tolerates different functional groups. This is especially true for 1,3-butadienes: while n-butylvinylether gave only traces of sodium 3-methoxy-2-propenoate, the corresponding (E)-1-methoxy-1,3-butadiene yielded sodium (E,E)5-methoxy-2,4-pentadienoate with a reasonably high TON of 99. Similarly, an (E,Z)-mixture of methyl 2,4-pentadienoate yielded a mixture of muconic acid isomers with an overall TON of 75. 2-Vinylpyridine was the only substrate which led to the salt of an α-branched α,β-unsaturated acid, albeit the TON was low (TON 2). Out of the three different double bonds of the natural product myrcene, only the least substituted one was transformed with very high (E)-selectivity to the corresponding linear α,β-unsaturated carboxylic acid derivative (TON 11).

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5. CATALYSTS GONE ASTRAY The organometallic chemistry of nickel is “rich,” i.e., besides the outlined mechanistic course via catalytically active intermediates, there are manifold detours, dead-ends, decomposition pathways, and whole avenue of side reactions—depending on reaction conditions (temperature, pressure), solubility, ligands, catalyst precursors, and many more.28,65 For instance, do N-ligands like 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)8,84 and especially bipyridines34a yield metallalactones of poor solubility in THF at low reaction rates. They also seem to facilitate the disproportionation of CO2. Bidentate P-ligands, such as 1,2-bis (dicyclohexylphosphino)ethane (dcpe), bear a convenient 31P NMR spectroscopic handle for mechanistic studies and nickelalactones derived from those ligands show a much higher solubility in common solvents. But already Hoberg et al. reported that bidentate phosphine ligands, such as dcpe at Ni(0) complexes, require special attention on the stoichiometry of CO2 to avoid disproportionation of CO2 to the corresponding carbonato- and biscarbonyl complex.35f,85,86 According to our mechanistic understanding, none of those acts as catalytically active species without stoichiometric addition of a reductant (Fig. 12). We have already pointed out that uncrowded, good donor ligands such as dppe form tetracoordinated Ni(0) complexes such as Ni(dppe)2 in a fast reaction instead of the catalytically active ethylene complex. Small biteangle ligands such as dppm lead to binuclear tetracoordinated Ni(0) complexes. But even if the ethylene complexes form, instead of reacting with

Figure 12 Catalysts gone astray.


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CO2 they can incorporate further alkene molecules to yield 7- instead of 5-membered lactones,87 and finally pentenoates instead of acrylates. The 4-membered nickelalactones which originate via isomerization from their 5-membered congeners over time incorporate a second molecule of CO2 and thus lead to metallasuccinates,29 and incorporation of additional alkene and CO2 leads to long-chain dicarboxylic acids.88 The isolatable and wellcharacterized Ni(II)CO2 complex [Ni((CH2)2CO2)(dtbpe)] forms at temperatures above 55 °C in reasonable yield (50% after 6 h), besides significant amounts of the ligand oxidation products dtbpe-dioxide and its monoxide as already reported by Hillhouse et al.89 The alcohols liberated from the alkoxides during nickelalactone deprotonation can principally add to the various Ni(0) species present and from Ni(II)-bisalkoxide complexes.

6. CONCLUSIONS AND OUTLOOK After more than 30 years of intensive research efforts in industry and academia and limited progress in the field, in the last 4 years, various catalysts emerged for the reaction of alkenes and CO2 to acrylates. Even though for the moment published data are only available for nickel, we are confident that further catalysts based on other metals will develop. Key to the development of the first catalyst was an intensive mechanistic understanding, both experimentally and theoretically. We do not want to go that far to state that the development of a catalyst for this important transformation has been delayed for so many decades as most studies have focused on phenomenological observations, i.e., the presence of product. This is per se not a bad strategy for the optimization of existing reactions, but it was not helpful for the targeted development of an absolutely new reaction—despite of the remarkable chemical intuition of our congeners in this field. There is still a long way to go for an industrial application of the new methodology. Especially, space–time yield and catalyst efficiency have to be improved. Questions like separation of the catalytically active species from the product and regeneration remain major topics for future research as well as the search for other catalytically active metals—the dream has just started.

ACKNOWLEDGMENTS M.L. works at CaRLa of Heidelberg University, being cofinanced by Heidelberg University, the state of Baden-W€ urttemberg and BASF SE. Support from BMBF (grant 01RC1015A) is gratefully acknowledged. I want to thank multitude of postdoctoral fellows, Ph.D. students, and cooperation partners at CaRLa who have been working over the years on this

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challenging reaction. Their names are acknowledged in the corresponding references. With their intellectual sharpness, passion, enthusiasm, and resilience paired with hard word they have made our joint dream come true.

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73. Schwesinger R, Schlemper H, Hasenfratz C, et al. Extremely strong, uncharged auxiliary bases; monomeric and polymer-supported polyaminophosphazenes (P2–P5). Liebigs Ann. 1996;1996:1055–1081. 74. Lejkowski ML, Lindner R, Kageyama T, et al. The first catalytic synthesis of an acrylate from CO2 and an alkene—a rational approach. Chem Eur J. 2012;18:14017–14025. €ber Halbester der Kohlensaüre. 75. Behrend W, Gattow G, Dra¨ger M. Untersuchungen u Z Anorg Allg Chem. 1973;397:237–246. 76. Hendriksen C, Pidko EA, Yang G, Scha¨ffner B, Vogt D. Catalytic formation of acrylate from carbon dioxide and ethene. Chem Eur J. 2014;20:12037–12040. 77. Villiers C, Dognon JP, Pollet R, Thue´ry P, Ephritikhine M. An isolated CO2 adduct of a nitrogen base: crystal and electronic structures. Angew Chem. 2010;20:3543–3546, Angew Chem Int Ed Engl. 2010;49:3465–3468. 78. Huguet N, Jevtovikj I, Gordillo A, et al. Nickel-catalyzed direct carboxylation of olefins with CO2: one-pot synthesis of α, β-unsaturated carboxylic acid salts. Chem Eur J. 2014;20:16858–16862. 79. Tang W, Wang W, Zhang X. A bisphosphepine ligand with stereogenic phosphorus centers for the practical synthesis of β-aryl- β-amino acids by asymmetric hydrogenation. Angew Chem. 2003;115:3633–3635, Angew Chem Int Ed Engl. 2003;42:3509–3511. 80. Tang W, Zhang X. A chiral 1,2-bisphospholane ligand with a novel structural motif: applications in highly enantioselective Rh-catalyzed hydrogenations. Angew Chem. 2002;114:1682–1684, Angew Chem Int Ed Engl. 2002;41:1612–1614. 81. Liu D, Zhang X. Practical P-chiral phosphane ligand for Rh-catalyzed asymmetric hydrogenation. Eur J Org Chem. 2005;646–649. 82. Imamoto T, Tamura K, Zhang Z, et al. Rigid P-chiral phosphine ligands with tertbutylmethylphosphino groups for rhodium-catalyzed asymmetric hydrogenation of functionalized alkenes. J Am Chem Soc. 2012;134:1754–1769. 83. Liptak MD, Gross KC, Seybold PG, Feldgus S, Shields GC. Absolute pKa determinations for substituted phenols. J Am Chem Soc. 2002;124:6421–6427. 84. Hoberg H, Schaefer D, Burkhart G. Oxanickelacyclopenten-Derivate, ein neuer Typ vielseitig verwendbarer Synthone. J Organomet Chem. 1982;228:C21–C24. 85. Mastrorilli P, Moro G, Nobile CF. Carbon dioxide-transition metal complexes IV. New Ni(0)-CO2 complexes with chelating diphosphines: influence of P-NiP angle on complex stabilities. Inorg Chim Acta. 1992;192:189–193. 86. Burkhart G, Hoberg H. Oxanickelacyclopentene derivatives from nickel(0), carbon dioxide, and alkynes. Angew Chem. 1982;94:75, Angew Chem Int Ed Engl. 1982;21:147–152. upfung von Alkenen mit CO2 87. DBU, cf.Hoberg H, Peres Y, Milchereit A. C-C-Verkn€ an Nickel(0): n-Pentesaüren aus Ethen. J Organomet Chem. 1986;307:C41–C43. pentafluoropyridine, cf. Hoberg H, Ba¨rhausen D. Nickel(0)-induzierte CC-Verkn€ upfung von CO2 mit 1,3-Butadien zu linearen C13-Saüren. J Organomet Chem. 1989;379: C7–C11. 88. For 1,3-butadiene: Hoberg H, Peres Y, Milchereit A, Gross S. Nickel(0)-induzierte CC-Verkn€ upfung zwischen CO2 und 1,3-butadien zu C9-mono- oder C18Di-carbonsaüren. J Organomet Chem. 1988;345:C17–C19. Hoberg H, Gross S, Milchereit A. Nickel(0)-catalyzed production of a functionalized cyclopentanecarboxylic acid from 1,3-butadiene and CO2. Angew Chem. 1987; 99:567–569. Angew Chem Int Ed Engl. 1987;26:571–572. for 1,3,7-octatriene: Behr A, Kanne U. Nickel complex induced C-C-linkage of carbon dioxide with trienes. J Organomet Chem. 1986;317:C41–C44. 89. Anderson JS, Iluc VM, Hillhouse GL. Reactions of CO2 and CS2 with 1,2-bis(di-tertbutylphosphino)ethane complexes of nickel(0) and nickel(I). Inorg Chem. 2010;49:10203–10207.

CHAPTER FIVE

Poly-NHC Complexes of Transition Metals: Recent Applications and New Trends Andrea Biffis*, Marco Baron, Cristina Tubaro Dipartimento di Scienze Chimiche, Universita` di Padova, Padova, Italy *Corresponding author: e-mail address: [email protected]

Contents 1. Introduction 2. Novel Poly-NHC Ligands 2.1 Chiral Poly-NHC Ligands 2.2 Pincer and Pseudopincer Poly-NHC Ligands 2.3 “Janus-Type” Poly-NHC Ligands 2.4 Poly-NHC Ligands Containing Abnormal, Remote, or Mesoionic Carbenes 2.5 Miscellaneous Poly-NHC Ligands 3. Novel Poly-NHC Metal Complexes 3.1 Metal Complexes with the Metal Center in Unusual Oxidation State 3.2 Polynuclear Metal Complexes with Original Architectures 3.3 Heteropolymetallic Complexes 4. Applications 4.1 Catalysis 4.2 Photophysics 4.3 Medicinal Chemistry 5. Conclusions and Outlook References

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1. INTRODUCTION The success enjoyed in the last 20 years by N-heterocyclic carbenes (NHCs) as ligands toward transition metal centers1 has fuelled from the very beginning the parallel development of a great number of polytopic ligands featuring more than one NHC moiety (poly-NHCs). Such polydentate ligands raised the interest of the scientific community for several reasons. First of all, use of poly-NHCs as chelating ligands should obviously impart Advances in Organometallic Chemistry, Volume 63 ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.02.002

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greater stability to the resulting metal complex. Furthermore, use of polyNHCs as bridging ligands should provide an excellent scaffold for the preparation of polynuclear homo- and heterometallic complexes, thanks to the robustness of the carbene–metal interaction. Finally, in more recent times, the disclosure of NHCs characterized by widely different steric and electronic features, due to the nature of the substituents, of the heterocycle or of the carbene itself (e.g., the so-called abnormal, remote, or mesoionic carbenes2) has paved the way to the preparation of heteroleptic poly-NHCs bearing carbene moieties with different characteristics, which should enable the achievement of a much broader diversity in the properties of the corresponding mono- and polymetallic complexes. The field of poly-NHC ligands was extensively reviewed in 2009,3 and since then, only a few personal accounts or reviews dedicated to some peculiar aspects of the chemistry of such ligands and their metal complexes have appeared in the literature, several of them only in Chinese language.4 Consequently, this chapter will focus on developments in the chemistry of polyNHC ligands having been disclosed in the last 5 years. Our aim is to provide a comprehensive but critical review of the field, by highlighting the most novel and actual aspects of this chemistry. In this regard, we have chosen dividing the chapter in three parts. The first part will deal with novel poly-NHC ligand systems, featuring original properties in terms of sterics, electronics, chirality, and denticity. The second part will deal with novel poly-NHC metal complexes, in which the novelty may originate from less common metals and oxidation states, unusual stoichiometries, particular structural or conformational properties, higher nuclearities, or the presence of metal centers of different nature. The third and final part will deal with applications, starting with the more common use of these complexes as catalysts but extending toward other areas in which poly-NHC metal complexes are being increasingly employed, such as photophysics or medicinal chemistry. The field is on the move; hence, peculiar attention will be given to the most recent, innovative, promising applications in these various fields.

2. NOVEL POLY-NHC LIGANDS Lots of previously unreported poly-NHC ligands have been disclosed in the literature during the last 5 years. However, most of the investigations can be categorized according to a small number of target compound classes, namely (i) chiral poly-NHC ligands; (ii) pincer or pseudopincer poly-NHC

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ligands; (iii) poly-NHC ligands featuring electronic communications between the carbene moieties (“Janus-type” ligands); and (iv) poly-NHC ligands featuring abnormal, remote, or mesoionic carbene moieties. These compound classes will be treated in detail in the following.

2.1 Chiral Poly-NHC Ligands Research on chiral NHCs has flourished in the course of the last 20 years paralleling the success of NHCs as ligands for catalytically active metal complexes and as organocatalysts in their own right.5 Several chiral poly-NHCs have been also proposed and prepared, but success in their application as ligands for enantioselective metal catalysts has been up to now moderate, although in the course of the last few years remarkable degrees of asymmetric induction have been achieved in a few instances. The topic has been reviewed by Shi.6 Being flat, the core N-heterocyclic structure of an NHC is intrinsically nonchiral; hence, the chirality has to be introduced by a proper choice of substituents. In doing so, care should be taken to reduce the rotational freedom of such substituents, which would otherwise make the chiral space around the metal center quite ill-defined and consequently lead to low enantioselectivity. In this connection, use of chelating poly-NHC ligands could be in principle advantageous, as chelation obviously enhances the rigidity of the ligand. Chelation is however per se not sufficient to achieve high degrees of asymmetric induction, and indeed, the majority of the reported chiral poly-NHC ligands appear still too flexible to impart good enantioselectivity to the catalyst. Various ligand architectures have been proposed, in which the chiral element is almost invariably contained in the bridge between the carbene units. The most relevant ligand types that have been employed in the course of the last 5 years are summarized in Fig. 1. Ligands of type 1 and 2 were originally proposed in the first 2000s by the groups of Shi7 and of Douthwaite,8 respectively, and some of them have recently allowed to obtain notable degrees of asymmetric induction. Indeed, ligands of type 1 are arguably the most successful chiral poly-NHC ligands reported to date, as their complexes with rhodium(III) and palladium(II) have been tested with good results in several reactions: in particular, use of palladium(II) complexes with ligands of type 1 imparted very high enantioselectivities (up to 99% ee) to the addition of arylboronic acid or β-ketoesters to enones or N-protected imines9 and to the oxidative kinetic

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Figure 1 Recently developed chiral poly-NHC ligand architectures.

resolution of alcohols.10 The success of these ligands is probably to be ascribed to the rigidity of the backbone, which allows to properly define the chiral space around the metal center. Ligands of type 2 were instead recently rediscovered to show that with the proper wingtip substituents, they were able to afford enantioselectivities in Pd-catalyzed, asymmetric Suzuki–Miyaura couplings up to 64%.11 Ligands of type 3 were originally proposed from 2007 by the group of Veige.12 Complexes of Pd(II), Ir(I), and Rh(I) were successfully synthesized, which however invariably displayed low degrees of enantioselectivity when employed as catalysts, e.g., in enantioselective olefin hydrogenation or hydroformylation. The reason seemed to be the flexibility of such ligands; indeed, in 2010, Veige demonstrated that enantioselectivities could be substantially improved by connecting the wingtip R substituents of 3-type ligands through a bridge, thus forming a chiral cyclophane ligand; the increased rigidity imparted by the bridge allowed to reach enantioselectivities up to 50% with Pd centers in the asymmetric 1,4conjugate addition of phenylboronic acids to 2-cyclohexen-1-one.13


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Interestingly, Veige also investigated extensively on the influence of the nature of the N-heterocycle and of the wingtip substituent on the asymmetric induction. Whereas the steric and electronic properties of the wingtip substituent, or even the presence of chiral centers on it, had very little effect on enatioselectivity, a far more important effect was observed by employing benzimidazolylidene ligand moieties instead of simple imidazolylidene.14 The reason seems to be again steric rather than electronic: the presence of the fused phenyl ring makes the ligand more rigid and consequently allows a better definition of the chiral space around the metal. Ligand rigidity seems however not to be a critical parameter in all instances for achieving good enantioselectivities. For example, a ligand of type 4 was reported in 2010 to impart excellent degrees of enantioselectivity (up to 99%) in the hydrogenation of (E)-diethyl 2-benzylidenesuccinate catalyzed by Rh(I), Pd(II), or Au(I) centers; the ee values were similar to those obtained using the structurally related chiral diphosphine DIOP.15 Chiral acyclic dicarbene ligands have been quite intensively investigated as well. Such diaminocarbenes are prepared by a completely different route compared to carbenes based on N-heterocyclic scaffolds: whereas the preparation of the latter most usually involves deprotonation of azolium salt precursors, the preparation of the former is generally accomplished by reaction of two metal-coordinated isocyanide ligands with a diamine linker. Using this approach, chelating ligands of type 5 were prepared and studied by Slaughter from 2007 onward;16 the ligands provided enantioselectivities up to 59% in Pd-catalyzed asymmetric aza-Claisen reactions. More success in this respect has been obtained with dinuclear Au complexes bearing ligands of type 6, prepared in the very same way. The peculiarity of these ligands is the presence of the pyridyl group as wingtip substituent, which engages in hydrogen bonding with the NdH bond on the other nitrogen adjacent to the carbene, thus forming a quasi-cyclic structure; for this reason, these ligands have been termed “hydrogen-bonded heterocyclic carbenes”. Gold complexes of these ligands were initially prepared by the group of Espinet17 and tested as catalyst in the cyclopropanation of styrene with propargyl pivalate and in the intramolecular hydroalkoxylation of allenes, but enantioselectivities were very modest. Slightly modified ligands of the same family were however shortly thereafter employed with success by the group of Toste18 as catalysts for a dynamic kinetic asymmetric transformation of propargyl esters, leading to enantioselectivities up to 99%.

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More recently, other chiral poly-NHC ligand structures have been preliminarily proposed and evaluated. Aizpurua et al.19 have demonstrated that axial chirality can be imparted to mesoionic 4,40 -bis(1,2,3-triazole)dicarbenes of type 7, which were disclosed shortly before by the group of Bertrand.20 Complete stereoinduction has been achieved upon dimetallation of the corresponding bistriazolium carbene precursors with Ag2O in the presence of a chiral substituent at the N atoms adjacent to the carbene carbons. The group of Dervisi studied dinuclear complexes of Pt(II), Ag(I), and Au(I) with chiral dicarbene ligands of type 8 prepared from dehydrohexitol; complexes of Ag(I) and Au(I) turned out to be emissive, as several other recently reported polynuclear complexes of group 11 metals with NHCs (see Section 4.2), and this brought the authors to investigate their chiroptical properties, performing the first investigation of this kind on NHC metal complexes.21 Chiral poly-NHC ligands stemming from derivatization with NHC groups of chiral metal complexes, such as ligands of type 9 reported in Fig. 1, have been preliminary investigated. The imidazolium-derivatized salen ligand was first loaded with Pd(II) or Ni(II) centers coordinated to the salen moiety; subsequently, Pd(II) dicarbene complexes were generated either upon direct reaction between Pd(OAc)2 and the pendant imidazolium groups or upon the silver route (intermediate generation of silver(I)-NHC complexes upon reaction with Ag2O and subsequent transmetallation to Pd(II)). Complexes of this kind may become interesting candidates for bimetallic catalysis, but, on the other hand, the inherent flexibility of the complex should make the transmission of the chiral information from the salen ligand to the dicarbene complex quite problematic. Indeed, first catalytic tests performed in the 1,4-addition of oxindoles to 2-nitrostyrene showed no degree of asymmetric induction with these complexes.22 Finally, ligands of type 10 were originally developed, in which the chiral information is provided by a benzyl carbon adjacent to the NHC groups. This kind of structure can be potentially very extensively varied by changing the substituents at the chiral carbon; however, care should be taken in the preparation of the corresponding metal complexes, since benzylic CdH groups are relatively acidic and can become deprotonated under the conditions employed for carbene formation, thereupon potentially losing the chiral information contained in them. Molybdenum and palladium complexes with these ligands have been reported, but no application of these complexes in, e.g., asymmetric catalysis was described.23


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2.2 Pincer and Pseudopincer Poly-NHC Ligands Pincer ligands containing more than one NHC moiety have been very intensively investigated during the past two decades, to the point that a dedicated review on this subject appeared already in 2007.24 In the course of the last 5 years, research on this particular class of ligands has concentrated, on the one hand, on ligands capable of readily switching from “pincer” to “pseudopincer” upon detaching the central coordinating moiety from the metal center and, on the other hand, on anionic pincer-type ligands. In the latter case, the negative charge, which is generally located on the central moiety of the ligand connecting the two NHC units, serves to increase the reactivity of the complex, rendering, for example, the metal center more electron rich, and/or to balance the positive charge on the metal, yielding less-charged complexes more amenable to vaporization or to dispersion in media of low polarity such as organic solvents/organic matrices, a feature potentially useful for applications in materials science and/or medicinal chemistry. Examples of the most interesting recently developed ligand structures are reported in Fig. 2. For example, several pincer ligands with a central amido moiety, such as those of type 11, have been developed by several groups and their complexes with metal centers such as palladium(II), platinum(II), and rhodium(I) have been characterized.25 The aim was on the one hand to synthesize robust, electron-rich organometallic compounds which easily undergo oxidative addition at the metal center and, on the other hand, to develop bifunctional catalysts in which the central amido moiety can cooperatively act with the metal center in a catalytic event, for example, by accepting/releasing protons

Figure 2 Recently developed poly-NHC pincer and pseudopincer ligand architectures.

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in a transfer hydrogenation process. Whereas the first feature was indeed obtained with rhodium(I) centers, bifunctional catalysis with ligands of this kind is actually yet to be demonstrated. Very recently, similar ligands bearing two 1,2,3-triazol-5-ylidene as mesoionic carbene moieties have been prepared by the group of Bertrand; unusually stable dicarbene complexes of copper(II) could be prepared with this ligand.26 Another type of potential pincer ligand with an anionic central moiety has been proposed by Poethig and Strassner.27 The ligands have the structure displayed in 12 and feature an anionic triazinone central moiety. Such moiety is actually a very weak electron donor; hence, these ligands have not been employed up to now to prepare pincer complexes, but rather dinuclear complexes of silver(I) and gold(I), in which the central triazinone moiety remains uncoordinated. The resulting complexes are emissive in the solid state and are neutral, which facilitates their dispersion into, e.g., polymer films for photophysical applications. The group of Huynh has extensively investigated on pincer and pseudopincer complexes bearing a thioether group as central coordinating moiety (see structure 13). In these complexes, the thioether ligand is labile and can be conveniently removed from the coordination sphere of the complex by oxidation to sulfoxide.28 More recently, Huynh has found out that even without oxidation of the thioether moiety, there exists an equilibrium in complexes of this ligand with palladium(II) between the true pincer form and a pseudopincer coordination isomer with uncoordinated sulfur; the position of the equilibrium appears to be determined by the electronic properties of the employed NHC groups.29 Along the same line, the group of Biffis, building on previous results by Hahn et al.,30 recently reported macrocyclic pincer-type ligands (structure 14) which are forced by the small size of the macrocycle to coordinate to d8 metal centers (palladium(II), platinum(II)) with the two NHC moieties in cis fashion and consequently with the pyridyl group outside the coordination sphere of the metal.31 Only upon oxidation of the metal to the +IV oxidation state, the pyridine group can enter the metal coordination sphere. Such a behavior may have notable consequences on the stabilization of organometallic complexes with the metal in high oxidation state, such as, for example, those that are involved in several catalytic cycles which are known to exploit the Pd(II)/Pd(IV) catalytic manifold.32 Pincer-type ligands with more complex central moieties have also been prepared. For example, ligands with a central 1,8-naphthyridine moiety


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(structure 15) have been originally prepared by Chen et al.33 and subsequently investigated also by the group of Liu.34 Similar to the case of the ligands with the triazinone bridge, the complexes reported up to now with ligands of type 15 do not always have the central naphthyridine moiety involved in coordination to the metal center(s); the actual involvement depends on the number and nature of the metal centers and can give rise to fairly complicated polynuclear complexes. Finally, very recently pincer-type ligands featuring two NR,NH carbene moieties (structure 16) have been reported for the first time.35 Carbene ligands with one or even two NH groups adjacent to the carbene have long been known and can be synthesized in a variety of ways.36 Nevertheless, until recently, they were treated as curiosities rather than ligands with potential applications, in view of their putative instability, when coordinated to metal centers, toward the migration of the metal from the carbene carbon to the unfunctionalized nitrogen. Several reports appeared in the recent literature have however demonstrated that the stability of such compounds can be remarkable, particularly when the NH-bearing carbene is part of a chelating bidentate ligand, and exploitation of the NH moiety in, e.g., bifunctional catalysis has been envisaged.37 Furthermore, deprotonation of the NH group is easy and liberates a coordination site for additional metal centers: indeed, Flowers and Cossairt used35 this possibility for the preparation of polynuclear metal complexes with ligand 16, featuring a ruthenium(II) center coordinated to the carbene and phosphine moieties and an iron(II) or cobalt(II) center coordinated to the heterocyclic nitrogens (Fig. 3). Many more pincer-type ligands have been developed in the last years featuring abnormal, remote, or mesoionic NHCs, which will form the object of Section 2.4.

Figure 3 Formation of bimetallic complexes from poly-NHC metal complexes bearing free NH moieties.

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2.3 “Janus-Type” Poly-NHC Ligands In general, Janus-type carbenes are classified as ditopic ligands with two linearly opposed N-heterocyclic carbenes and have been named after the Roman god Janus, who is depicted in art with two fused faces, one looking forward and the other backward. The increasing interest toward these ligands lies basically in their structure, which prevents chelation toward a single metal center but indeed favors the bridging coordination, thus affording essentially bimetallic complexes or even NHC-based organometallic polymers. The possibility to prepare homo- or heterobimetallic complexes has steered the application of these compounds toward catalytic tandem or cascade reactions, assisted by a presumed cooperation between the two metal centers. The main classes of dicarbenes of this type are summarized in Fig. 4. The simplest Janus-type di-NHC is the 1,2,4-triazol-3,5-diylidene (Fig. 4, ditz) introduced by Bertrand and coworkers,38 but whose coordination chemistry has been exploited successively by several groups. In particular, Peris and coworkers have synthesized a number of homo-bimetallic Rh(I)/Rh(I),39 Ir(I)/Ir(I),39 Ir(III)/Ir(III),40 Ir(I)Ir(III),40 Ru(II)/Ru(II),41 and Pd(II)/Pd(II)42 as well as heterobimetallic Rh(I)/Ir(III),40 Rh(I)/ Ir(I),39 Ir(III)/Ru(II),43 Ir(III)/Pd(II),44 Ir(III)/Pt(II),45 Ir(III)/Au(I),46 and Ru(II)/Pd(II)47 complexes and have demonstrated that the catalytic performances displayed by the bimetallic species are somewhat superior to the monometallic analogues (for a more detailed description of the complexes and of the investigated catalytic reactions, see Sections 3.3 and 4.1). In recent years, dinuclear complexes of palladium(II) with bridging ditz ligand have been successfully synthesized also by Huynh et al. and used as catalysts in C5-arylation of 1-methylimidazole.48 In all the cited cases, it was argued that the increased catalytic efficiency of the bimetallic systems, compared to the analogue mononuclear complexes, was due to the short intramolecular distance between the two metal centers (ca. 6 A˚); this might suggest a cooperation between the two metals, favored by the presence of electronic communication via the delocalized electron density on the azole ring. Driven by these results, other ligands have been successively developed, bearing a π-delocalized aromatic linker between the two carbenes (17–19) and whose main interesting feature relies on the possibility to enable electronic communication between the two carbene moieties, and as a consequence between the two coordinated metal centers. Ligand 17 has been extensively studied by Bielawski and coworkers, who isolated the Ir(I)/Ir(I), Rh(I)/Rh(I), and Ag(I)/Ag(I) bimetallic

Figure 4 Examples of “Janus-type” di-NHC ligands.

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complexes.49 The pyrene- and pyracene-based ligands 18 and 19 have been proposed by the group of Peris, who also synthesized some bimetallic Ru(II)/Ru(II), Pd(II)/Pd(II), Ir(I)/Ir(I), and Rh(I)/Rh(I) complexes and used them in selected catalytic transformations.50 In this case, the improved catalytic performances of the bimetallic systems compared to the monometallic ones have been rationalized in terms of higher local concentration of metal in the bimetallic complexes and of affinity of the di-NHC core for aromatic substrates via π-stacking. The presence of an interaction between the two metal centers in bimetallic complexes with ligands 17–19 has been mainly assessed through cyclic voltammetry measurements (especially differential pulse voltammetry, DPV): generally, the communication is weak so that they belong to class II systems according to Robin and Day classification.51 Peris and Gusev have further supported this experimental evidence by mean of the Tolman electronic parameter (TEP), determined via DFT calculations. In particular, they found that the interaction between the two metal centers depends only on the σ-delocalized electron density, while the π contribution is negligible; this is not surprising if one considers that also in the MdNHC bond, the σ character is predominant. The strength of the communication between the two metals depends therefore only on the spatial separation between them ˚ .52 This has been demand becomes negligible for distances exceeding 10 A onstrated by comparing the features of ligands 24 and 25,53 which do not present any bridging π-delocalized electron density, with those of systems 17–19. Ligands 17 and 20–22 allow to isolate not only bimetallic complexes49,54,55 but also self-assembled materials,56 and in addition, they have been recently employed for the generation of stable main-chain NHC-based organometallic polymers, via reaction of the diazolium salt with, for example, Pd(OAc)2. The main characteristics and applications of these metallopolymers have been reviewed by Bielawski and coworkers a couple of years ago.57 For example, a palladium polymer with ligand 17 (with R ¼ d(CH2CH2O)3CH3) has been successfully used as heterogeneous catalyst in Suzuki–Miyaura coupling reaction in water, with high activity and almost complete recyclability (see Section 4.1).58 The redox-active quinone core in ligand 21 can be used to modulate the electron donation to the metal fragment: in fact, in its oxidized form, the ligand has a minor donor ability than in its reduced form.57 This opens the possibility to obtain materials, whose properties can be changed by varying the ligand oxidation state.


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Similar results might be obtained also with ligand 23, in which the two benzimidazol-2-ylidene units are each linked to a Cp ring of a ferrocene bridging group59; despite the relatively flexible structure, this ligand adopts a Janus-type arrangement when coordinated to Ir(I), probably favored by π-stacking of the benzimidazole aromatic ring. Electrochemical and TEP measurements proved that the iridium centers and the ferrocene units are coupled and that the electron-donating ability of the dicarbene ligand can be tuned by changing the oxidation state of iron in the ferrocenyl unit. In 2014, the first two examples of mono- and dianionic Janus-type dicarbenes were isolated and characterized. The group of Chiu proposed the spiroborate-linked di-benzimidazol-2-ylidene ligand 26 and its rhodium(I) complex.60 TEP measurements demonstrated that the electrondonating ability of this anionic ligand is intermediate between neutral and anionic NHC, probably because the negative charge of the borate bridge is redistributed over two carbene moieties. The dianionic ligand 27 combines two facially opposed maloNHCs and allows the isolation of neutral zwitterionic bimetallic gold(I) and silver(I) complexes, in which the negative charge is localized in the malonate backbone of the ligand, in the outer-coordination sphere of the metal.61 The examples briefly aforementioned regard only Janus-type di-NHCs; similar tri-NHCs are rather rare and usually referred as “Cerberus-type” NHCs (Fig. 5), in this case named after Cerberus, the three-headed mythological guardian of the underworld. The first to be described was ligand 28, reported by Bielawski.62 Despite the interesting approach and the possibility to use this ligand as building block in organometallic networks, only one report appeared regarding its coordination chemistry (with R ¼ H) to transition metals, like Pd(II) and Au(I).63 The obtained complexes are in fact rather unstable and decomposed rapidly in refluxing acetonitrile.

Figure 5 Examples of “Cerberus-type” tri-NHC ligands.

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The analogous Pd(II) or Au(I) complexes bearing the planar triphenylene-based ligand 29 are instead very stable and their catalytic performances in Suzuki–Miyaura reaction and α-arylation of propiophenone (for the Pd(II) complex) as well as in hydroamination of phenylacetylene (for the Au(I) complex) are superior to those of similar mononuclear complexes with the benzimidazol-2-ylidene ligand; this difference has been ascribed to the topological planarity of the triphenylene-based ligand, which might assist the interaction between the aromatic substrates and the catalyst via π-stacking.63 Peris has in fact established in the case of the rhodium(I) complex with ligand 29 that the communication between the metal centers is inefficient, despite the π-delocalized connecting system64; this result is in good agreement with the same conclusion reached for the Janus-type di-NHCs.52 The three carbene units are practically decoupled also in the trimetallic rhodium(I) system with ligand 30, as expected considering the nonaromatic nature of the triquinacene core.65 The analyzed tri-NHCs ligands are interesting not only for a possible improvement in the catalytic efficiency of the derived trimetallic systems but also for applications in material science; for example, the trimetallic gold(I) complex with ligand 29 could be employed in the development of main-chain organometallic microporous polymers.66

2.4 Poly-NHC Ligands Containing Abnormal, Remote, or Mesoionic Carbenes The statements abnormal and mesoionic carbenes (aNHCs and MICs) are synonyms and are referred to free carbenes for which it is not possible to draw a structure resonance formula without formal charges (Fig. 6). Differently, remote carbenes (rNHCs) are carbenes without any heteroatom in the α and α0 positions of the carbene center. These three types of ligands are characterized by a different structure with respect to the one usually owned by NHCs. In the case of normal carbenes (nNHCs), the carbene donor is stabilized by the presence in the α and α0 positions of two heteroatoms. To emphasize this difference, aNHCs, MICs, and rNHCs are also called

Figure 6 General structure of normal, abnormal or mesoionic, and remote carbenes.


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nonclassical carbenes. Nonclassical carbenes have gained attention in the literature more recently than the normal analogues. The first report on this topic was published by Araki et al.,67 but the field started to be systematically investigated in 2001 by the group of Crabtree, with particular reference to the chemistry of imidazol-4-ylidene ligands.68,69 Moreover, a further upgrade to this field was given in 2008 by Albrecht who reported the employment of 1,2,3-triazol-4-ylidene ligands.70 The properties and reactivity of nonclassical carbenes are actually studied by a multiplicity of research groups, and different authors have already reviewed this topic in the last years, among them in particular Albrecht and Crabtree in 2013 and Schubert in 2014.2,71–73 Consequently, only general principles and more recent developments concerning poly-NHC ligands based on these moieties are reported here (Fig. 7). A common feature of abnormal/

Figure 7 Recently developed poly-aNHC ligands.

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mesoionic and remote carbenes is that they are stronger electron-donor ligands than nNHCs.2b This is a consequence of the different structure of these ligands; in particular, nNHCs have two heteroatoms, usually nitrogen, bound to the carbene center, that inductively reduce the electron donor ability of the carbene. Otherwise in the structure of aNHCs/MICs, only one heteroatom is bound to the carbene, while for rNHCs no heteroatoms are bound to the carbene center. Consequently, the electron donor properties of the last two classes of ligands are enhanced compared to nNHCs. This is a very important feature of nonclassical carbenes that has driven them to become a potentially important tool in the development of novel homogeneous catalysts.2b The higher donor properties of aNHCs/MICs have been verified with different and orthogonal techniques, both experimental and theoretical.2,71–73 Although classical and nonclassical free carbenes present some differences, when they are coordinated in the corresponding complexes, they behave in a similar way, and for this reason, they can be considered part of the same class of ligands.2a Recently, Albrecht and coworkers have reported the employment of the di-aNHC ligand 31 to promote the oxidative addition of chlorine and bromine to the corresponding platinum(II) complex.74 The presence of the di-aNHC chelate ligand in the coordination sphere of the platinum(II) center leads to a smooth oxidative addition to the corresponding platinum(IV) stable complexes. In this case, the enhanced electron donor ability of imidazol-4-ylidene ligands is a useful tool to stabilize platinum in its highest oxidation state, although it must be said that also nNHC ligands are able to stabilize platinum(IV) centers (see also Section 3.1).31,75 Sarkar et al. reported a dinuclear Ir(III) complex with the di-MIC ligand 32 and they observed coordination of the two carbene moieties but also a double orthometalation of the para-phenylene bridging group.76 In this case, the di-MIC ligand coordinates two different metal centers in a bridged fashion, promoting the double orthometalation of the central phenyl ring. The obtained dinuclear complex was tested as catalyst in the transfer hydrogenation reaction using benzaldehyde and acetophenone as benchmark substrates. The complex with di-MIC 32 converts the substrates in the corresponding alcohols faster than similar mononuclear MIC iridium(III) complexes in the same reaction conditions. The authors propose indeed a cooperative effect between the two iridium(III) centers, although the reaction mechanism is still under investigation. Another example of the employment of di-MIC ligands has been published by Sankaraman and coworkers. They have recently reported in two different contributions the synthesis of a


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palladium(II) complex with a chelating C2-symmetrical di-MIC ligand bearing a 1,10 -biphenyl linker (33).77,78 In particular, they have used two different protocols to synthesize the palladium(II) complex, respectively, upon transmetalation of the di-MIC ligand from the corresponding silver(I) complex77 and upon direct metalation of the ligand precursor by using palladium(II) acetate as metal precursor;78 particularly interesting are the silver-free synthetic conditions that are very mild and free from byproducts. In fact, in this case, the complex was obtained by simply mixing the ligand precursor and 1.2 equiv. of palladium acetate in dichloromethane at room temperature for 14 h.78 In different studies, Mayer and Bernard have investigated the regioselectivity of the metalation of some diimidazolium salt with anionic coordinating bridges (precursors of ligands 34 and 35).79,80 Trying to obtain some palladium(II) dicarbene complexes, they found in fact that the C2 (normal) or C4 (abnormal) carbene formation may both take place. In particular, the group of Mayer has reported some experiments that may account for the selectivity between the normal and abnormal mode of binding in the case of palladium(II) centers.79 Differently from iridium and osmium polyhydride compounds, where the imidazolium counteranion seems to play an important role in favoring the C2dH bond cleavage or the C4dH oxidative addition, the counteranion effect is strongly reduced in the case of group 10 metals.2a The discriminating element in this case is rather the presence of an external base: when the reaction is carried out without a base, the abnormal dicarbene complex is formed, whereas in the presence of an excess of base (such as NH4OAc) the normal di(carbene) palladium(II) complex is obtained. By tuning the reaction conditions, the authors report the synthesis of the two normal and abnormal regioisomers in good yields.79 Finally, also complexes with tripodal nonclassic carbene ligands have been prepared. Sarkar and coworkers have recently reported the synthesis of a trinuclear palladium(II) complex with a tris-mesoionic donor based on a phenylene moiety as bridging group (36).81 They fully characterized the tri-MIC palladium(II) complex and they also investigated its electrochemical properties. Looking at the reduction potential of the Pd(II)/ Pd(0) redox couple, they interestingly found that the reduction of the three palladium(II) centers take place at different potentials, and this may suggest a strong electrochemical coupling among them. The same effect has not been observed for the corresponding tri-nNHC complex.81 Smith et al. have described the synthesis of a tri-MIC ligand with a borate unit as bridging group (37).82 In this case, the tridentate ligand coordinates a single metal center, nickel(II) or manganese(I).

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An interesting and specific characteristic of poly-NHC ligands is the possibility of combining different carbene donors to form the so-called hetero-poly-NHC ligands. As already reported by Huynh and coworkers, the combination of two different donors did not lead always to ligands with intermediate properties with respect to the corresponding homoleptic compounds. In fact, enhanced properties (e.g., catalytic properties) may be recorded in the corresponding complexes, which have been interpreted in terms of an “electronic asymmetry”.83 The most recent examples on this topic are reported in Fig. 8. Peris et al. in this frame have prepared a series of imidazolylidene– pyridylidene mixed ligands and synthesized the corresponding rhodium(III) and iridium(III) complexes. In the adopted synthetic conditions, the imidazolylidene ligand is always bound at the C2 position to the metal center, although the pyridylidene ligand may coordinate the metal center in different ways: normally (38a) and remotely (38b and 38c).84,85 On the basis of the structural analysis of the complexes, the authors were also able to establish the relative trans influences of the pyridylidene and imidazolylidene. The pyridylidene in the ligands 38b and 38c provides similar trans influence, larger than the one provided by the imidazolylidene. Working on the size of the wingtip substituents of a methylene-bridged diimidazolium salt, Bera and coworkers were capable to synthesize a ruthenium(II) complex with the mixed normal/abnormal di-NHC ligand 39.86 The increasing size of the wingtip group, from n-butyl to mesityl, influences dramatically the coordination properties of the bidentate ligand. In fact, starting with the less bulky n-butyl substituent, the mixed C2/C4 di-NHC complex is obtained, while with the mesityl group the classical C2/C2 di-NHC complex is formed. Remarkably also the oxidation state

Figure 8 Recent examples on “hetero-poly-NHC” ligands.


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and the coordination geometry of the ruthenium complexes are different in the two cases. With ligand 39, an octahedral ruthenium(II) complex with two nNHC/aNHC dicarbene ligands and two solvent molecules mutually in cis is isolated, while in the case of the C2/C2 dicarbene an octahedral ruthenium(III) complex with two dicarbene ligands and two chlorides mutually in trans is formed. Another recent example was reported by Huynh et al., who synthesized a series of propylene bridged heterodicarbene ligands 40; the corresponding palladium(II) complexes were tested as catalysts in the direct arylation of pentafluorobenzene with 4-chlorobromobenzene.87 The presence of two different carbene donors allows the very fine tuning of the catalyst stereoelectronic properties. The benefits provided by the presence of a mixed donor ligand were investigated also by Elsevier and coworkers.88 They prepared 1,2,3-triazolylidene–imidazolylidene ligands of type 41 and studied their coordination to rhodium(I) and iridium(I) metal centers. The obtained complexes were tested in the transfer hydrogenation of several aldehydes and ketones. The heteroditopic complexes are able to reduce different substrates in good yields.88 Finally, Pidko et al. reported the synthesis of the CNC pincer ligands 42 derived from a lutidine-bridged diimidazolium salt and synthesized catalytically active ruthenium(II) complexes.89 Starting from the same ligand precursor with bulky mesityl wingtip groups, the authors prepared both a normal dicarbene or a mixed normal/ abnormal dicarbene ruthenium(II) complex by simply adding or not to the reaction LiBr. When LiBr is added to the reaction mixture, the normal dicarbene complex is obtained; on the contrary in the absence of LiBr, the mixed di-NHC complex is isolated. The influence of the counteranion on the coordination mode of NHC was described also for iridium and osmium NHC complexes and the results obtained by Pidko are in line with those already reported.68,69 Also in this case, the possibility of selecting the NHC binding mode of the CNC pincer ligand provides another useful tool for tuning the properties of this type of complexes that were successfully employed in the catalytic ester hydrogenation to alcohols (see also Section 4.1).89

2.5 Miscellaneous Poly-NHC Ligands Some additional reports on novel and/or interesting types of poly-NHC ligand architectures have appeared in the recent literature (Fig. 9). Dicarbenes featuring a direct NdN bond between the heterocyclic rings have been the subject of further investigation following the initial reports by

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Figure 9 Miscellaneous recently developed poly-NHC ligand architectures.

Peris et al. on the di-1,2,4-triazol-5-ylidene ligand 43.90 Such ligands share the same topology of ubiquitous 2,20 -bipiridines and their congeners and are therefore of interest despite their decreased electron-donating ability due to the presence of the NdN connection between the rings. Recent work on this topic has been carried out by the group of Bertrand, who first introduced analogous mesoionic di-1,2,3-triazol-5-ylidene ligands (see structure 7). The group of Kunz instead disclosed the phenanthrolineanalogous ligands of type 44; interestingly, such ligands appear to be much more stretchable than phenanthrolines and beside forming mononuclear chelate complexes are also able to act as bridging ligands forming dinuclear complexes, especially with group 11 metals in the +I oxidation state.91 Finally, very recently, the group of Hahn devised for the first time a strategy for the preparation of nonsymmetrical ligands of type 45, in which one 1,2,4-triazolylidene moiety was directly linked to an imidazolylidene or benzimidazolylidene one.92 Anionic polydentate ligands comprising an NHC moiety have been very intensively investigated as well, in particular as ligands toward early transition metals, lanthanoids and actinoids93; much rarer are however examples of polydentate ligands of this kind bearing more than one NHC group, mostly confined to monoanionic pincer-type ligands of the type discussed in Section 2.2. In 2009, the group of Yagyu and Jitsukawa first reported on tetradentate, dianionic dicarbene ligands of type 46 and its Mn(II) complex.94 More recently, the groups of Strassner95 and of Che96 independently reported coordination of the same ligand with platinum(II), yielding stable, neutral complexes with square-planar coordination geometry exhibiting good solubility in apolar media and excellent luminescence properties (see also Section 4.2).

3. NOVEL POLY-NHC METAL COMPLEXES In the course of the last 5 years, an enormous number of new metal complexes of poly-NHCs has been disclosed in the literature, which sum


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up to all those reported in the two decades before. A comprehensive illustration of these new achievements would obviously extend well beyond the length of a review article. Furthermore, several recent reviews have already exhaustively covered the coordination chemistry of NHC ligands, including poly-NHCs, with numerous metal centers, including group 1 to group 7 metals,97 iron,98 palladium,99 and group 12 metals.100 Consequently, in the context of this review, we will limit our discussion on novel complexes of poly-NHC ligands to what we believe to be the most actual research trends in this specific topic, namely the preparation of polyNHC metal complexes with the metal center in unusual oxidation states, the construction of organometallic supramolecular structures upon coordination of poly-NHCs to metal centers, and finally the preparation of heteropolymetallic complexes. The discussion will be particularly focussed on group 8–11 metals, which are not extensively covered by the recent reviews listed above. Several other poly-NHC complexes of transition metals will be discussed in the context of their applications in the next section of the review.

3.1 Metal Complexes with the Metal Center in Unusual Oxidation State Very intensive research has been carried out in the last years on the preparation of NHC metal complexes of middle to late transition metals in high oxidation states, due to the recognition of the peculiar stabilizing power of these ligands. NHCs are in fact neutral, strongly sigma-donating ligands which in contrast to, e.g., trialkylphosphanes are capable of efficiently interacting also with hard metal centers; use of chelating poly-NHCs provides a further mean to stabilize complexes of metal centers with high coordination numbers. Particularly successful has been the research activity concerning nitride complexes of group 7 and 8 metals, which led to the unprecedented isolation and characterization of a four-coordinate nitride complexes of iron(V) with a tripodal tricarbene-borate ligand 47 (Fig. 10) upon oxidation of the corresponding iron(IV) complex.101 Similarly, use of a tripodal tricarbene ligand featuring a central amino group allowed to prepare manganese nitrido complexes with the metal in the oxidation states III, IV, or V (complexes 48–50); in this case, use of the tricarbene ligand notably helps in stabilizing manganese nitrido complexes with the metal in intermediate oxidation states (III, IV), which were previously detected but not thoroughly characterized.102

Figure 10 Examples of poly-NHC complexes with the metal center in unusual oxidation state.


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Chelating dicarbene ligands were found very useful for stabilizing technetium(V) dioxo species, forming complexes of structure 51, which turned out to be stable to water, a potentially very useful feature for the application of these compounds as radiopharmaceuticals.103 Several investigations have been conducted on complexes of group 10 metals, particularly palladium and platinum, in their oxidation state IV, in view of the assumed key role of these complexes in CdH activation processes. Di- and tetracarbene complexes of platinum(IV) with chelating dicarbene ligands were prepared by Strassner in 2010 upon oxidation of the corresponding platinum(II) complexes with bromine or iodobenzene dichloride.75 The platinum(IV) complexes turned out to be stable and could be structurally characterized: the halide ligands in the case of the tetracarbene complexes were found to invariably occupy axial positions deriving from formal trans addition to the complex. Similarly, oxidation of a CNC pincer complex of platinum(II) to platinum(IV) (complex 52) was reported by the group of Limbach in 2011: interestingly, whereas with iodine as oxidant a stable platinum(IV) complex could be obtained and structurally characterized, when methyl iodide was utilized, the reaction required heating to 100 °C and the presence of an equilibrium was evident between the platinum(IV) and the platinum(II) species.104 In 2014, the group of Biffis reported on the oxidation with bromine of platinum(II) complexes with the ligand 14.31 Stable platinum(IV) complexes were obtained, in which spectroscopic evidence for the coordination of the bridging pyridyl group was provided; unfortunately, structural characterization of the compounds proved unfeasible, and the corresponding reaction with palladium(II) complexes yielded only the starting material. Indeed, the preparation of polyNHC complexes of palladium(IV) proved much more challenging than the preparation of platinum(IV) compounds. Eventually, palladium(IV) tetrachloride complexes with chelating dicarbene ligands were accessed upon oxidation with chlorine or iodobenzene dichloride of the corresponding palladium(II) dichloride complexes.105 As expected, such compounds can act as powerful stoichiometric chlorinating agents toward alkanes, alkenes, and arenes and the process has been also made modestly catalytic by taking advantage of the enhanced reactivity of cationic complexes of the same kind in which a chloride ligand has been replaced by a pyridine group, and by using the palladium(IV) tetrachloride complex as sacrificial oxidant.106 Poly-NHC metal complexes with the metal in low oxidation state are also of interest. Obviously, the presence of several NHC ligands destabilizes these oxidation states in the absence of other π-acceptor ligands (e.g., CO);

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hence, the resulting complexes can be expected to be powerful reducing agents and/or to easily undergo oxidative addition processes. Indeed, in 2009, the group of Spicer and Murphy reported on the preparation of a cobalt(II) compound with a macrocyclic tetracarbene ligand (“crown carbene,” structure 53) previously reported by the same authors.107 The cobalt(II) complex could be reduced to the corresponding cobalt(I) compound by electrochemical or chemical reduction with sodium amalgam. The resulting complex could efficiently effect the monoelectronic reduction of aryl iodides, bromides, and even chlorides, and in the case of aryl iodides and bromides, it could be even employed catalytically in an electrochemical reduction process. Interestingly, the rigidity of the crown carbene macroligand prevents further oxidation of cobalt to the +3 oxidation state, which should be even more favored due to the electron-donating power of the carbene ligands, but which requires extensive rearrangement in order to achieve the preferred octahedral coordination geometry for cobalt(III). The same group later reported on a nickel(II) complex with the same ligand, which could be reduced with sodium amalgam to the corresponding putative nickel(0) complex; however, the authors demonstrated that the resulting compound should be better interpreted as a nickel(II) complex with a dianionic tetracarbene ligand, which is noninnocent in the reduction process. Also in this case, the resulting complex is a powerful two-electron reducing agent effecting reductive cleavage of arenesulfonamides and epoxides as well as Birch reductions of aromatic rings with high selectivity.108 Finally, very recently, nickel(0)109,110 and platinum(0) complexes110 with chelating dicarbene ligand were reported for the first time. Although several zero-valent group 10 metal complexes with one or two NHC ligands have been described in the past, complexes with chelating dicarbene ligands were known only for palladium. Such compounds are expected to exhibit high reactivity toward oxidative addition, and indeed, the nickel(0) complexes irreversibly activated upon coordination the CdC bond of benzonitrile producing a nickel(II) compound with coordinated phenyl and cyanide.110

3.2 Polynuclear Metal Complexes with Original Architectures The use of poly-NHC allows to easily obtain polymetallic systems; mainly di-111–116 and tri-nuclear30,117–119 complexes have been reported, although some examples of higher nuclearity species have been also presented.4b,120


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The interest toward these complexes is related to their catalytic activity (see Section 4.1), luminescence properties (see Section 4.2), and biomedical applications (see Section 4.3) and also to their interesting supramolecular structural features. Depending on the nature of the poly-NHC ligand and of the employed metal center, several types of polynuclear architectures (organometallic macrocycles, metal cages, including helical ones, cylinders, etc.) can in fact be generated. Furthermore, an even greater number of polynuclear structures have been produced with mono- and poly-NHC ligands functionalized at the wingtip substituents or at the bridge between the NHC units with other donor groups (like pyridine, pyrimidine, naphthyridine, alkoxy groups, etc.), which coordinate to metal centers, thus generating very complex architectures. A detailed discussion of all these examples goes beyond the scope of this contribution, as it was also the specific topic of other very recent reviews,4,121 so only a few highlights from the last 5 years will be presented herein, concerning in particular polynuclear structures which exhibit peculiar properties (Fig. 11). Additional examples of complexes of this kind will be provided in Section 4. The group of Liu reported in 2011 the preparation of an organometallic cyclophane made out of dinuclear Ag(I) complexes with bridging di-NHC ligand comprising an aryl group in the bridge (structures 54 and 55). The resulting 30-membered macrocycles were able to act as receptors for p-phenylenediamine molecules, as it was demonstrated with fluorescence and UV–vis spectroscopic titrations. This contribution highlighted the advantage of using the formation of poly-NHC metal complexes for the ring closing of functional organometallic macrocycles with molecular recognition properties.122 The same group extended shortly thereafter their investigations to dinuclear complexes of other metal centers (Ni(II), Hg(II)) and to di-NHC ligands featuring shorter quinoxaline bridges. The resulting 16-membered macrocycles were found to be efficient chemosensors for Cu2+.123 Finally, Xu and coworkers also reported the synthesis of the dinuclear Ag(I) and Hg(II) complexes, 56 and 57, respectively, with two bridging di-NHC ligands, which display sensing behavior.124 These systems present in fact fluorescence properties, and because of the peculiar structure of the bridge between the NHC units, they resemble calixarenes of different sizes, thus suggesting their potential application in host–guest recognition. These complexes quenched in fact their luminescence properties upon interaction with organic molecules, like, for example, diphenylacetylene, (E)-azobenzene, and fullerenes (C60 or C70).

Figure 11 Examples of organometallic cyclophanes characterized by sensing behavior.


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Several prominent examples of polynuclear poly-NHC metal complexes with original architectures were produced by the group of Hahn.121 To name but a few, in 2011, the group reported on the preparation of highly unusual di-NHC-based “molecular squares” (structure 58, Fig. 12): tetranuclear Ir(III) complexes with bridging Janus-type dicarbene ligands were obtained from a tetranuclear complex precursor upon cyclization of coordinated diisocyanides.125 More recently, the same group presented the dinuclear Ag(I) and Au(I) complexes 59 with bridging di-NHC ligands having a trans-stilbene bridge. The interesting feature in these complexes is that upon coordination to the metal centers, the trans-stilbene bridges are brought in close proximity to the point that 2 + 2 cycloaddition of the double bonds can be triggered photochemically to yield the cyclobutanebridged dinuclear tetracarbene complexes 60.111 The promotion of reactions between functional groups of poly-NHC ligands upon complex formation, highlighted herein, is a novel possibility of application of these complexes which merits further attention.

3.3 Heteropolymetallic Complexes In recent years, there has been an increasing interest toward the synthesis of heteropolymetallic systems supported by poly-NHC ligands.4c,126 The

Figure 12 Examples of polynuclear metal complexes with original architectures.

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interest toward this type of complexes resides in the possibility to conjugate the properties of two or more metal centers, which could act in a synergistic manner, for example, by catalyzing two different organic transformations consecutively or by mutually influencing their electronic properties, promoting catalysis or enabling new electronic transitions in the heteropolymetallic complex leading to strong luminescence properties (see Sections 4.1 and 4.2). The first examples of complexes of this kind with poly-NHC ligands were reported from 2007 onwards by the groups of Peris,39,40 Braunstein,127 and Cowie.128 In particular, Cowie in 2009 described in detail the possible synthetic pathways for the preparation of heterobimetallic Rh(I)/Ir(I), Pd(II)/Rh(I), and Pd(II)/Ir(I) systems starting from a diimidazolium salt having an alkyl bridge between the two heterocyclic rings (Fig. 13). Generally, the most straightforward procedure involves the stepwise incorporation of the two different metal centers; however, the optimization of the experimental conditions is quite tricky, due to the difficult choice of the base to deprotonate selectively only one imidazolium ring and to the tendency of the di(N-heterocyclic carbene) ligand to adopt a chelating coordination rather than a bridging one. For example, Rh(I)/Ir(I) complexes can be synthesized by using the internal-base method: upon reaction of the diimidazolium salt with the dinuclear precursor [Rh(OAc) (COD)]2 (in di-NHC/Rh 1/1 ratio), the coordination of only one imidazol-2-ylidene ring is afforded, while the second imidazolium moiety remains pendant. The latter can be finally deprotonated in a similar manner, by addition of the iridium species [Ir(OAc)(COD)]2, thus obtaining the desired heterobimetallic complex. This same approach has been successfully

Figure 13 Stepwise synthesis of heterobimetallic complexes bearing an alkyl-bridged di-NHC ligand.


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extended also to the synthesis of Rh(I)/Ir(I) complexes with a di-(mesoionic carbene) ligand129 and to mixed normal/mesoionic carbene ligands.130 The best way to limit or prevent the tendency to chelation toward a single metal center is the use of ligands similar to “Janus-type” poly-NHCs (see Section 2.3), in which the carbene units point to opposite directions. In this regard, several reports deal with the ligand 1,2,4-triazol-3,5-diylidene (ditz, Fig. 14) already described in Section 2.3.39,40,43–47 Hahn and Peris have recently reviewed all the heterodimetallic complexes with this ligand, focusing in particular on the synthetic aspects and on the homogeneous catalytic applications.126 In fact, the most interesting feature of these complexes is the proximity of the two metal centers, which could favor, by a synergistic behavior, tandem or cascade catalytic processes. Similar to the alkyl-bridged di-NHCs, also the bimetallic complexes with ditz ligand are synthesized by a sequential deprotonation/coordination of the two carbene fragments. Some details on the steric and geometric features of this specific ligand have been already discussed more in detail in Section 2.3: in particular, cyclic voltammetry measurements and DFT calculations allow to conclude that the two metals are weakly coupled (class II of Day Robinson), via the π-electron density of ditz. Other polymetallic complexes have been synthesized, exploiting the same approach, i.e., preventing the chelation of the poly-NHC ligand (Fig. 15), by using, for example, di-NHC with a p-phenylene bridge (complex 60),131 Y-shape tri-NHC ligands (complex of type 61),132 and tri-NHCs with 1,3,5- or 1,2,4-trisubstituted phenylene-bridging group (complexes 62 and 63).133,134

Figure 14 Heterobimetallic complexes with ditz ligand.

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Figure 15 Examples of poly-NHC heteropolymetallic complexes.

4. APPLICATIONS Investigations on the chemistry of poly-NHC metal complexes have of course not been limited to the preparation of complexes with novel features and properties, as extensively outlined above, but have also heavily involved their applications. Catalysis has long been the main field in which the use of these complexes was envisaged, but in more recent years, other applications of poly-NHC metal complexes have been identified and pursued with a notable degree of success. Besides catalysis, today there are at least two other main application areas of these complexes, namely their use as emissive moieties in the development of photo- or electroluminescent devices, and as bioactive compounds, typically with antimicrobial or antitumoral properties; the following sections will extensively cover the recent advances in these fields.


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4.1 Catalysis Catalysis has been the first and most important field of application of polyNHC metal complexes. Actually, already the first report on a reaction catalyzed by NHC metal complexes, published by the group of Herrmann in 1995,135 involved inter alia a Pd complex with a chelating dicarbene ligand. From then on, an enormous number of investigations were carried out on the catalytic potential of poly-NHC metal complexes, powered by considerations similar to those that led to the development of polyphosphane ligands in parallel with monodentate analogues. Coordination of more than one NHC unit to a metal center in a chelate fashion is namely expected to increase the stability of the catalyst while promoting at the same time the reactivity of the metal center through the increased electron donation brought about by the carbene ligands. Furthermore, use of poly-NHC ligands paves the way to the development of polynuclear homo- or multimetallic complexes as catalysts, in which two or even more metal centers, brought in close proximity by coordination to the same ligand or even interacting electronically through it, bring about a catalytic process by acting together in a cooperative or tandem fashion (see also Sections 2.3 and 3.3). Although most reported NHC-based metal complex catalysts are based on mono-NHC ligands, some notable success has been recorded also with poly-NHC metal complexes, thanks to their special features listed above. The topic will be covered in the following subsections according to the kind of catalyzed reaction. Reference will be made whenever appropriate to enantioselective variants of the reactions catalyzed by metal complexes with chiral poly-NHC ligands, which have been discussed in Section 2.1. 4.1.1 Cross-Coupling Reactions Cross-coupling reactions were the first reactions in which poly-NHC metal complexes (and NHC metal complexes altogether) were successfully employed as catalysts. Considering also the popularity gained by such reactions over the last 20 years, further corroborated by the award of the Nobel Prize in Chemistry for their discovery in 2010,136 it cannot be deemed surprising that cross-coupling reactions have become the favorite tool to probe the catalytic efficiency of poly-NHC metal complexes, especially palladium(II) complexes. However, in spite of the great number of complexes of this kind that have been evaluated, little advance has been made in the discovery of more efficient catalytic systems, as poly-NHC complexes

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display in most cases lower activity in cross-coupling reactions than monoNHC analogues.137 Indeed, in order to achieve high product yields, polyNHC metal complexes generally have to be employed under comparatively drastic reaction conditions (in particular at temperatures higher than 100 °C) and deliver high product yields only with the most reactive substrate classes (in particular aryl iodides and bromides). It is well known that with these substrates and reaction conditions, “ligandless” palladium species in low concentration can also efficiently act as catalysts for many of these reactions138; hence in these cases, it becomes difficult to assess whether the observed catalytic efficiency is truly that of the poly-NHC palladium complex or rather of decomposition products thereof. Indeed, in several instances, it has been demonstrated that Pd complexes originally employed as catalysts merely act as a reservoir of ligand-free, catalytically competent Pd species that are slowly released into solution, and that it is such slow but continuous release that allows at the very end to reach higher catalytic efficiencies compared, e.g., to simple palladium(II) salts at similar initial concentration, which are more readily deactivated upon formation of Pd black.139 Consequently, we will not attempt in this section to exhaustively summarize the research activity in this field over the last 5 years, but we will rather concentrate on the examples, in which the role of the poly-NHC ligand can be best ascertained. The majority of studies in this field have addressed the Suzuki reaction (Eq. 1), which is arguably the most successful and popular cross-coupling methodology to date, thanks to the availability and ease of handling of the reactants, to the economical significance of the biaryl reaction products, and to the efficiency exhibited by several catalytic systems, particularly by Pd-containing catalysts:

ð1Þ

The most outstanding among the recently proposed poly-NHC-based catalytic systems for this reaction was reported by the group of Karimi and Akhavan in 2009–2011 and consisted in differently functionalized organometallic polymers of palladium(II) with the Janus-type ligand 17.58 The different functionalization influenced the dispersion of the polymers in water and, consequently, their catalytic efficiency in this solvent. The catalytic activity of these systems was very high, and in best cases, it allowed the


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reaction to proceed at room temperature in reasonable reaction time and at low catalyst loading (0.05 mol%) even in the case of unreactive aryl chlorides. Unfortunately, the actual nature of the catalytically competent species remained elusive and could not be unequivocally determined in spite of several efforts. In 2009, the group of Smith reported on a “canopied” di-NHC ligand acting as a trans-chelating ligand toward palladium(II) (Fig. 16, structure 64), which exhibited good catalytic activity in the Suzuki reaction of chlorobenzene with several arylboronic acids in 1,4-dioxane at 100 °C: the activity turned out to be superior to that of diphosphane or diphosphite ligands based on the same ligand scaffold.140 Similarly, in 2011, the group of Luo reported on a cis-chelated palladium(II) complex of biphenyl-linked bis(imidazol-2-ylidene) (structure 65) which was also able to convert several aryl chlorides through Suzuki reactions in dioxane at 100 °C; use of a strong base like KOtBu was however needed for efficient reaction.141 No explanation for the apparently superior catalytic efficiency exhibited by these complexes compared to other poly-NHC palladium(II) complexes was provided. In 2012, Micksch and Strassner carried out a systematic study on several chelating dicarbene Pd complexes as precatalysts for the Suzuki reaction.142 The dicarbene ligands all possessed a methylene bridge between the NHC and aryl wingtip substituents at the NHC nitrogen atoms carrying sterically and electronically different groups. The study was conducted with aryl bromide substrates under mild conditions (temperature 40 °C), which consequently allowed to draw conclusions on the effect of the nature of the ligand on the reaction outcome. In particular, it turned out that more strongly electron-donating groups on the aryl wingtip substituents favored the reaction, whereas their steric bulk hindered it. Several poly-NHC Pd catalysts have been devised for carrying out Suzuki reactions in water. Water solubility of the complexes is ensured by the introduction, on the wingtip substituents or on the bridge between the two carbene units, of hydrophilic groups (e.g., sulfonate groups, carboxylate groups, PEG chains, etc.). Catalytic activities were however moderate (with the exception of the water-soluble polymeric system by Karimi and Akhavan mentioned above),58 and generally, only aryl bromide substrates could be employed for the reaction, even at 100 °C. Good results were obtained by the group of Peris with the bis-sulfonated CNC pincer catalyst 66, though a bis-NHC palladium(II) complex described in the same work performed even better: activated, electron-poor aryl chlorides could also be reacted with good conversions, albeit temperature had to be further raised beyond the boiling point of water (110 °C).143

Figure 16 Palladium(II) complexes used in cross-coupling reactions.


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In recent years, extensive investigations have been performed on the use of nickel rather than palladium complexes for catalyzing the Suzuki reaction,144 and indeed, several recent studies have also concerned the catalytic properties of poly-NHC complexes of nickel(II), particularly with pincer-type ligands; best results were obtained with CNC complex 67, which exhibited good catalytic activity at 100 °C even with aryl chlorides as well as with aryl tosylates and mesylates. Triphenylphosphane was however required as a cocatalyst.145 Finally, an asymmetric variant of the Suzuki reaction was developed using as catalyst a poly-NHC metal complex with ligand 2 (see also Section 2.1). The complex exhibited a rather low catalytic activity toward aryl iodides and bromides as substrates. However, chiral binaphthyls were prepared with ees up to 70%.11 The Heck reaction is another CdC coupling reaction that has been very intensively employed to test the reactivity of Pd-based catalysts (Eq. 2): ð2Þ The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane ligands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often limited to the reaction of aryl iodide or electronpoor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T > 150 °C with tetraalkylammonium salts as promoters).146 An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68.48b In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters; the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive.147 Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions.

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The Sonogashira reaction is another reaction which is notoriously not very efficiently catalyzed by Pd-NHC species (Eq. 3): ð3Þ Nevertheless, the group of Peris reported in 2009 that dinuclear Pd complexes with the Janus-type ditz ligand were quite effective in the sequential Sonogashira/hydroxyalkoxylation coupling leading to benzofurans (Fig. 17), with a catalytic efficiency comparable to that of mono-NHC analogues. The reaction worked well with 2-iodobenzyl alcohol, whereas only moderate yields could be obtained with the bromo derivative and the chloro derivative was quite unreactive.42b In 2013, Huynh and Lee reported on Pd complexes of type 70 with triazol-2-ylidene-based CNC ligands, exhibiting good catalytic activity in the Sonogashira reaction of aryl bromides in the absence of copper cocatalysts (which usually accelerate the reaction but also make it air sensitive and less selective) as well as of excess amine base.148 Similarly, in 2014, the group of Biffis reported on the catalytic efficiency in the copper- and amine-free Sonogashira reaction of Pd complexes with the macrocyclic ligand 14. In this case, the structure of the macrocyclic ligand forces the two carbene units in a cis-coordination to the metal; consequently, the central pyridyl group remains outside the coordination sphere but close to the palladium center and can potentially act as a base facilitating what is believed to be the critical step of the reaction (i.e., the switch from π-coordinated alkyne to σ-coordinated alkynyl with concomitant deprotonation).149 Also in this case, the reaction worked well with aryl bromides and iodides.31 Di- and trinuclear silver(I) and copper(I) complexes with poly-NHC ligands have also been described as more economical catalysts for the Sonogashira reaction, but in this case, the reactivities of the complexes were found to be rather low, so that only aryl iodides could be employed as substrates.150

Figure 17 Sequential Sonogashira/hydroxyalkoxylation coupling leading to benzofurans, catalyzed by dinuclear Pd(II) complexes.


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Reports on poly-NHC complexes as catalysts for the Kumada coupling (Eq. 4) have been quite rare, despite the successful results often achieved with mono-NHC metal catalysts for this reaction: ð4Þ Examples deserving mention especially concern complexes of chelating di-NHC ligands with metals different from palladium, in particular nickel(II)151 and iron(II).152 In both cases, good catalytic efficiencies were recorded, the complexes being able to activate also electron-rich aryl chlorides at room temperature. However, complexes with mono-NHC ligands of the same metals are known to exhibit even superior catalytic efficiencies. Complexes of iron(II) with chelating dicarbene ligands have been recently employed also to catalyze the homocoupling of Grignard reagents, though in this case catalytic efficiencies were only marginally higher than simple iron salts.153 An unusual CdC cross-coupling reaction is the formal acylation of aryl halides with aldehydes yielding aryl ketones. Efficient catalysis of this reaction was originally reported in 2008 by the group of Xiao using in situ formed Pd/dppp complexes as catalysts.154 Recently, the group of Peris has investigated on several dinuclear Pd complexes with poly-NHC ligands, including Janus-type, pyracene-linked di-imidazolylidenes 19 and phenylene/biphenylene-bridged di-imidazolylidenes, as catalysts for this reaction. Catalytic efficiencies with different aryl halides were comparable to the original Pd/dppp system, though no substrate scope was investigated with respect to the aldehyde.50b,155 Mechanistically related to this reaction is the more popular α-arylation of ketones, which was briefly investigated by Peris using trinuclear palladium(II) complexes with triscarbene ligands based on triphenylene or triptycene scaffolds. Catalytic results in the α-arylation of propiophenone with aryl bromides were slightly better with these trinuclear complexes than with mononuclear mono-NHC analogues, thus proving the catalytic benefit deriving from the trinuclear structure.63 CdN coupling reactions are among the catalytic processes in which NHC-Pd complexes have been most conveniently employed (the so-called Hartwig–Buchwald coupling, Eq. 5); however, to the best of our knowledge, the application of poly-NHC-Pd complexes as catalysts for this reaction has not led to significant results up to now: ð5Þ

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Figure 18 Poly-NHC-Cu complexes employed as catalysts in CdN couplings.

On the other hand, poly-NHC-Cu complexes (Fig. 18) have been employed as catalysts with some degree of success. Trinuclear copper(I) complexes with tris-NHC ligands of structure 71 were namely disclosed by the group of Biffis as powerful catalysts for Ullmann-type couplings of azoles with aryl iodides, bromides, and even activated chlorides.150b,156 Such results were confirmed by subsequent investigations from the group of Whittlesey on a trinuclear copper(I) complex with a tripodal triscarbene ligand featuring a different stoichiometry (structure 72).157 The catalytic performance of these and other NHC-copper complexes is however generally worse than that of Cu complexes with chelating ligands with nitrogen or oxygen atoms, which according to the current state of the art represent the most effective ligands for copper in these catalytic processes.158 Finally, CdN bond formation has been also accomplished through aerobic oxidative amination of arylboronic acids (Evans–Chan–Lam coupling) under poly-NHC-copper(II) catalysis. In particular, azoles and aromatic amines were successfully coupled with arylboronic acid using catalyst 73, with catalytic efficiencies comparable to those of other previously reported copper(II)-based catalytic systems for this reaction.159 4.1.2 CdH Activation and Functionalization CdH activation/functionalization reactions represent one of the classes of chemical transformations that have been most intensively studied in recent years. Poly-NHC metal complexes have a long-standing tradition of involvement as catalysts in these processes, which can be tracked back to their stability under the relatively harsh reaction conditions that are often needed to drive such reactions to completion in a reasonable time. For example, the group of Strassner reported already in 2002 that palladium(II) complexes with chelating dicarbene ligands can catalyze the


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CdH activation of methane in trifluoroacetic acid under comparatively mild conditions (20–30 bar CH4, 80–100 °C) oxidizing it to methyl trifluoroacetate using potassium persulfate as oxidant with TONs up to 30.160 More recently, this reaction protocol has been extended to the 2-trifluoroacetoxylation of propane161 and to the trifluoroacetoxylation of linear alkyl trifluoroacetates with moderate selectivity for the omega-1 position.162 Very recently, Strassner was able to show that using the same complexes as catalysts, it was possible to functionalize the secondary CdH bond of propane also employing dioxygen as the terminal oxidant; key to success was the use of NaVO3 as cocatalyst, which enabled the coupling of the CdH bond activation process with aerobic cooxidation through a bromide/bromine redox pair.163 Another CdH bond functionalization reaction that has been intensively investigated using complexes with poly-NHC ligands is the so-called Fujiwara hydroarylation, namely the intermolecular addition of an aryl CdH bond across the triple bond of an alkyne, generally producing the thermodynamically less stable cis-arylalkene (Fig. 19).164 In 2008, the group of Biffis reported that palladium(II) and platinum(II) complexes with chelating dicarbene ligands could be conveniently employed as catalysts for this reaction using one order of magnitude lower of palladium compared to simple Pd(OAc)2 employed by Fujiwara.165 The Pd catalyst in this reaction is believed to electrophilically activate the alkyne upon coordination for subsequent Friedel–Crafts-type attack on the arene; consequently, the metal remains in the +2 oxidation state throughout the catalytic cycle and the dicarbene ligand serves mainly to stabilize the catalyst. Shortly thereafter, it was shown that when using silver salts as cocatalyst to remove halide ligands from the coordination sphere of the Pd complex, the reaction could be conveniently run at room temperature, and that upon proper choice of the acid promoter, the reaction could be driven toward the product deriving from formal insertion of either one or two alkyne molecules into the CdH bond of the arene.166 Finally, the reaction could be run more efficiently by using ionic liquids as reaction medium, which

Figure 19 Fujiwara hydroarylation of alkynes.

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allowed increasing the activity of the catalyst and conveniently recovering and recycling it by extraction of the reaction products with an apolar organic solvent.167 Similar results in the catalysis of the same reaction were more recently reported by the group of Peris employing mono- and dinuclear complexes of palladium(II) with the Y-shaped tris-NHC ligand of type 61.132b Direct arylations make out another very popular class of Pd-catalyzed CdH functionalization reactions, which are however based on the palladium(0)/palladium(II) catalytic manifold. Consequently, poly-NHC palladium catalysts, which generally proved to be not very efficient for cross-coupling reactions based on the same manifold, met with little success also in the case of these catalytic processes. An example was provided by the € group of Ozdemir, who employed palladium(II) complexes with butylenelinked, chelating dicarbene ligands as catalysts for the direct 2-arylation of benzothiazole: aryl bromides could be conveniently employed as substrates, though at a rather high reaction temperature (130 °C), and long reaction times were needed to achieve good yields.168 Similarly, Guo and Huynh employed several dinuclear Pd complexes with the Janus-type ditz ligand as catalysts for the 5-arylation of N-methylimidazole: also in this case, aryl bromides could be employed as substrates, albeit at high temperature (140 °C) and with moderate yields, whereas only very low yields could be obtained with 4-chloroacetophenone.48a Direct arylation can be very efficiently catalyzed also by other metal centers besides palladium(II), in particular rhodium(III) and ruthenium(II). In € 2009, the groups of Ozdemir and Bruneau jointly reported on the catalytic activity of ruthenium(II) compounds with o-xylylene-bridged, chelating dicarbene ligands for the direct arylation of 2-phenylpyridine. Good catalytic efficiencies were observed even with aryl chlorides as substrates, so that the process resulted in the predominant formation of bis-o-arylated products (Fig. 20).169 Comparable catalytic efficiencies were more recently obtained by the group of Peris using a dinuclear ruthenium(II) complex with a Janus-type ligand 18 based on pyrene.50d The dinuclear complex displayed good catalytic efficiency also in the hydroarylation of terminal olefins using the same substrate; however, Peris also noticed that the catalytic efficiencies of the dinuclear dicarbene complex in these reactions were only marginally higher than those of a related mono-NHC Ru complex and very similar to simple [RuCl2(p-cymene)]2. In 2011, the group of Peris also reported on watersoluble iridium(III) complexes with chelating dicarbene ligands which were


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Figure 20 Direct arylation of 2-phenylpyridine catalyzed by Ru(II) complexes.

Figure 21 Alkane dehydrogenation catalyzed by dicarbene CCC pincer Ir(III) complexes.

able to effect selective hydrogen/deuterium exchange through cleavage and formation of CdH bonds at the o-positions of 2-phenylpyridine as well as at selected positions of other arylated and vinylated N-heterocycles. Reactions were carried out in D2O with 5 mol% catalyst and resulted in very high yields, though the reaction conditions were relatively harsh (120 °C).170 Another very popular class of CdH activation reactions is alkane dehydrogenation. Use of robust PCP pincer complexes of iridium(III) has led in the recent past to the development of very efficient catalytic systems for this reaction.171 By analogy with these successful complexes, in 2010 the group of Chianese proposed dicarbene CCC pincer iridium(III) complexes 75 as catalysts for alkane dehydrogenation (Fig. 21).172 With cycloalkanes as standard substrates, up to about 102 turnovers were obtained for acceptorless dehydrogenation to cyclic monoolefins, whereas in the presence of hydrogen acceptors the reaction was more sluggish, achieving at the best 10 turnovers. These results are encouraging but far below those previously obtained with PCP pincer complexes of iridium(III).

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A similar complex bearing a “normal” and an “abnormal” coordinated carbene moiety was reported by Zuo and Braunstein; its catalytic activity in the transfer dehydrogenation of cyclooctene was however markedly lower than that of the complexes by Chianese.173 Remarkably, CCC pincer complexes of iridium(III) developed by Chianese and by the group of Herrmann and K€ uhn174 turned out to be active catalysts also for another CdH functionalization reaction, namely the borylation of arenes. Activities were however low compared to iridium(III) complexes with other ligands which were extensively optimized over the years.175

4.1.3 Hydrogenations/Transfer Hydrogenations Reports on hydrogenation reactions catalyzed by NHC metal complexes are scarce, which is related to the fact that when an NHC ligand and a hydride are coordinated to the same metal center, reductive elimination of the corresponding azolium ion may become a favored decomposition pathway for the complex. Indeed, for example in 2011, the group of Albrecht reported on complexes of palladium(II) with abnormal, chelating dicarbene ligands as catalysts for olefin hydrogenation. They found that the onset of catalytic activity coincided with decomposition of the complex to form Pd colloids which were the catalytically competent species; corresponding palladium(II) complexes with conventional chelating dicarbene ligands were instead stable under the reaction conditions, but they also turned out to be inactive for the hydrogenation reaction.176 On the other hand, palladium(II), rhodium(I), or dinuclear gold(I) complexes with the chiral dicarbene ligand 4 exhibited low activity but very high enantioselectivity (up to 99% ee) in the asymmetric hydrogenation of prochiral olefins, the dinuclear gold(I) complexes being the most efficient catalyst (see also Section 2.1).15 Palladium complexes with conventional chelating dicarbene ligands have been employed also by the group of Elsevier as catalysts for the stereoselective semihydrogenation of alkynes, but the resulting catalytic activities were very low compared to related mono-NHC complexes.177 Efficient hydrogenation of alkenes, including generally unreactive hindered, unfunctionalized alkenes, has been instead accomplished by the group of Chirik using as catalyst an iron(0) complex with a CNC pincer-type ligand (Fig. 22); catalytic efficiencies were markedly higher than with related complexes bearing diiminopyridine pincer-type ligands, which was attributed to the increased electron donation to the iron center provided by the carbene moieties.178


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Figure 22 Hydrogenation of alkenes using as catalyst an iron(0) complex with a CNC pincer-type ligand.

Success has been met also with the hydrogenation of CO2 to formate, which was investigated by the group of Peris with Cp*iridium(III) or (η6-arene)ruthenium(II) complexes bearing chelating dicarbene ligands; the recorded catalytic efficiencies were comparable to those of the best catalysts reported to date for this reaction, and it was even possible to run the reaction as a transfer hydrogenation using iPrOH as hydrogen source, though the catalytic efficiencies were in this case lower.179 Very recently, ruthenium(II) complexes of CNC pincer ligands of type 42 were employed as catalysts for the hydrogenation of esters to alcohols with good results. The catalysts remained stable in spite of the relatively harsh reaction conditions employed (50 bar H2, 50 °C, 10 mol% KOMe as promoter) and displayed higher catalytic activity than related PNP, PNN, and CNN pincer complexes of ruthenium, though the activities were lower than state-of-theart homogeneous ruthenium catalysts for this reaction.89 Numerous examples are available in the literature for the application of poly-NHC metal complexes in transfer hydrogenation reactions. Examples of the transfer hydrogenation of CO2 with iPrOH have been mentioned above, but in general, the reports concern the more conventional transfer hydrogenation of ketones. Transfer hydrogenations have become a standard tool to probe the catalytic potential of ruthenium, rhodium, and iridium complexes, much the same role played by cross-coupling reactions for palladium or nickel complexes. As in the case of cross-couplings, though, investigations in transfer hydrogenations with poly-NHC metal complexes have invariably disclosed levels of catalytic efficiency which are markedly lower than those of the currently best catalysts for these reactions, which are usually based on other ligand systems. Therefore, they will not be presented in detail in this review. More complex “hydrogen-borrowing” processes than simple transfer hydrogenations have been much more rarely

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investigated with poly-NHC metal complexes. One example of catalysis of this kind of reactions using a CCC pincer palladium(II) complex (preformed or synthesized in situ) was reported by Kose and Saito in 2010.180 The target reaction was the “cross coupling” of a secondary and a primary alcohol (the latter used in excess) through a reaction sequence presumably involving alcohol dehydrogenation to carbonyl compounds, aldol condensation, dehydration, and hydrogenation (Fig. 23). It is quite unusual for Pd complexes to catalyze reactions of this kind, but nevertheless, preliminary mechanistic investigations by the authors seem to confirm the active role of Pd in the hydrogen transfer steps of the reaction. Another example was reported in 2012 by the group of Liu.181 Dinuclear iridium(I) complexes with a bridging saturated dicarbene ligand turned out to be efficient catalysts for the N,N0 -dialkylation of phenylenediamines with various alcohols (Fig. 24). Interestingly, with simple [Ir(COD)Cl]2 as catalyst, the intermediate imino–amino compound was predominantly formed. The authors claim the much higher selectivity for the diamino product

Figure 23 “Cross coupling” of a secondary and a primary alcohol catalyzed by Pd(II) di-NHC complexes.

Figure 24 N,N0 -Dialkylation of phenylenediamines with alcohols catalyzed by a dinuclear Ir(I) complex.


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exhibited by their catalyst to be the result of a cooperative mechanism between the two metal centers, though comparison with a mono-NHC Ir complex would be important in order to assess the role of cooperativity. 4.1.4 Hydrosilylations Hydrosilylations is another class of reactions where NHC metal complexes, particularly Pt complexes, have demonstrated great utility in the past. In the course of the last 5 years, quite a number of poly-NHC metal complexes have been investigated as catalysts for this kind of reactions as well. For example, the group of Hor in 2009 investigated on the catalytic efficiency of several mono- and dicarbene platinum(II) complexes, among them a complex with a chelating, methylene-bridged diimidazol-2,20 -diylidene ligand. The catalyst 76 exhibited good activity in the hydrosilylation of phenylacetylene or trimethylsilylacetylene with different silanes, producing comparable amounts of the alpha and the E-beta hydrosilylation product (Fig. 25). Monocarbene complexes performed better with phenylacetylene, whereas dicarbene complexes were more effective with trimethylsilylacetylene.182 In 2010, the group of Chen investigated on polydentate dicarbene and bis-dicarbene complexes of platinum(II) bearing pyridyl or picolyl groups as catalysts for the hydrosilylation of arylacetylenes. Catalytic efficiencies and reaction selectivities favoring the E-beta product were good, particularly with the chelating dicarbene catalyst 77, but a related monocarbene complex performed significantly better.183 Shortly thereafter the group of Peris reported on platinum(II) complexes with several chelating ditriazol5-ylidenes as catalysts for the same reaction. In this case, though, the

Figure 25 Hydrosilylation of phenylacetylene or trimethylsilylacetylene with different silanes, catalyzed by di-NHC Pt(II) complexes.

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performance of the catalysts was less satisfactory, either in terms of catalytic activity or in terms of selectivity toward the hydrosilylation products.184 A systematic study on a small library of platinum(II) complexes with chelating dicarbene ligands bearing different wingtip substituents as catalyst for the hydrosilylation of alkenes was reported by the group of Strassner in 2011. As in the case of the Suzuki reaction (see above), best results were obtained with electron-donating, not too sterically demanding wingtip substituents. Catalytic efficiencies were found to be comparable to the conventional Karstedt catalyst for this reaction, and superior to a previously reported and highly active mono-NHC complex. The dicarbene complex did not decompose to platinum black during operation, which is an additional positive feature of the system.185 Finally, poly-NHC complexes of other metals have occasionally been employed as catalysts for hydrosilylations as well. The group of Hollis reported in 2012 on dinuclear rhodium(I) complexes with a bridging dicarbene ligand (structure 78, Fig. 26) as catalysts for the hydrosilylation of phenylacetylene with dimethylphenylsilane.186 Results were comparable to those previously obtained with platinum dicarbene complexes, which is in contrast to previous reports on Rh-catalyzed hydrosilylations, in which the Z-beta product is predominantly formed without production of the alpha isomer. Additionally, the group of Glorius reported on iron(II) complexes with diamine-bridged, chelating dicarbene ligands (structure 79, Fig. 27) exhibiting moderate activity in preliminary catalytic tests on the hydrosilylation of acetophenone with diphenylsilane, which leads after hydrolysis to the corresponding alcohol. Interestingly, catalytic activity was markedly enhanced when MeLi was added to replace the chloride

Figure 26 Hydrosilylation of phenylacetylene with dimethylphenylsilane catalyzed by the dinuclear Rh(I) complex 78.


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Figure 27 Hydrosilylation of acetophenone, followed by hydrolysis to the corresponding alcohol; reaction catalyzed by the Fe(II) complex 79.

ligands with methyl groups. More work will be needed here to fully assess the potential of these catalysts in hydrosilylation reactions.187 It may be useful remarking that the asymmetric variant of this reaction has been extensively studied by the group of Shi using as catalysts rhodium(III) complexes with axially chiral, chelating di-NHC ligands of type 1 (see also Section 2.1). Moderate activities but very high enantioselectivities (up to 98% ee) were recorded with several ketones and β-ketoesters.6,7 4.1.5 Miscellaneous Reactions Poly-NHC metal complexes have been occasionally employed as catalysts also in other reactions. Among them, several reports are related to polymerization processes. For example, complexes of CCC pincer ligands with lanthanides were successfully utilized as catalysts in the selective cis-1,4polymerization of isoprene using trialkylaluminum compounds and Ph3CB(C6F5)4 as activators; the performance of the catalysts was comparable with that of related complexes with NCN pincer ligands described by the same authors.188 Nickel complexes with chelating dicarbene ligands were employed as catalysts in the vinyl polymerization of norbornene with MAO as activator; good catalytic efficiencies were recorded, but a related bis-carbene Ni complex performed similar to the chelating dicarbene complexes, which highlights the fact that the use of poly-NHC ligand systems is probably unnecessary in this reaction.189 A dinuclear ruthenium(II)-NHC complex (structure 80, Fig. 28) has been reported to efficiently catalyze the oxidation of stilbenes and other disubstituted olefins conjugated to an aromatic system to the corresponding

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alpha-diketones using t-butyl hydroperoxide (TBHP) as oxidant; a cooperative mechanism involving the two ruthenium centers has been suggested to explain the high reactivity of the complex but not demonstrated.190 Ruthenium(II) complexes with poly-NHC ligands have been also employed as catalysts for metathesis processes. However, in spite of the success enjoyed by mono-NHC Ru complexes as catalysts for this reaction class, use of poly-NHC Ru complexes invariably led up to now to low catalytic efficiencies, much lower than those routinely observed with benchmark ruthenium(II) metathesis catalysts.191 Chelating dicarbene palladium(II) complexes have been found to catalyze the Nazarov cyclization of divinyl-beta-ketoesters to the corresponding cyclopentenones (Fig. 29). The reactivity of the complexes was generally

Figure 28 Oxidation of stilbenes and other disubstituted conjugated olefins to the corresponding alpha-diketones, catalyzed by a dinuclear Ru(II) complex.

Figure 29 Nazarov cyclization of divinyl-beta-ketoesters to the corresponding cyclopentenones, catalyzed by di-NHC Pd(II) complexes.


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found to be lower than that of simple silver(I) salts as catalysts, but in specific cases, the utilization of the di-NHC-Pd complexes led to higher selectivity for a single diastereoisomer.192 Well-defined η6-arene iron(0) complexes with chelating dicarbene ligands were recently disclosed by the group of Driess. Apart from representing the first example of a genuine η6-arene iron(0) species, these complexes were also found in preliminary experiments to be efficient catalysts for the reduction of organic amides to amines using diphenylsilane as reducing agent; activities were comparable or even superior to benchmark iron-based catalysts for this reaction.193 An iron(II) complex with a macrocyclic tetracarbene ligand (Fig. 30) was synthesized by Cramer and Jenkins and found to exhibit excellent catalytic efficiency in the aziridination of alkyl-substituted olefin with electron-rich arylazides in moderate to good yields; remarkably, even very unreactive tetrasubstituted olefins could be converted to some extent with this catalytic system.194 Several metal complexes with CCC pincer-type dicarbene ligands have been investigated over the years by the group of Hollis as catalysts for the intramolecular hydroamination/cyclization of unactivated alkenylamines. Initial studies concerned rhodium(III) and iridium(III) complexes of type 75,195 but later investigations were extended to complexes of the same ligands with group 4 metals such as zirconium,196 hafnium,197 and titanium.198 Catalytic efficiencies were found to be markedly dependent on the nature of the metal and of the anionic halide coligands. Best results were obtained with zirconium and iodide ligands, though even with this combination the catalytic activity remained rather low compared to complexes of noble metal catalysts, so that long reaction times and high temperatures

Figure 30 Iron(II) complex with a macrocyclic tetracarbene ligand used as catalyst in the aziridination of olefins.

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(160 °C) were generally required. Very recently, the group of Peris investigated on the hydroamination of phenylacetylene with arylamines using trinuclear gold(I) complexes with triscarbene ligands based on triphenylene or triptycene scaffolds. Catalytic results with the triphenylene-based ligand proved slightly better than with mononuclear mono-NHC analogues, thus proving the catalytic benefit derived from the trinuclear structure.63 Cp*-iridium(III) complexes with chelating dicarbene ligands have been successfully employed as water oxidation catalysts, both in the presence of a sacrificial oxidant (Ce(IV) or NaIO4) or under photoactivated conditions with Ru(bpy)2+ 3 as photosensitizer and sodium persulfate as sacrificial acceptor of electrons; catalytic efficiencies were comparable with the most efficient iridium-based catalysts reported to date.199 Finally, it is worth mentioning that nickel or palladium complexes with poly-NHC ligands have been also employed as electrocatalysts for CO2 reduction (Fig. 31). Although the reaction rates are modest, the catalysts, particularly the Pd ones, appear stable against dimerization pathways ultimately leading to catalyst decomposition; hence, they hold promise for this reaction provided their activity can be improved.200 4.1.6 Bimetallic Catalysis At the end of this section, a special mention should be made of the intensive work performed by Peris on the development of poly-NHC-based heterobimetallic complexes. In these complexes, the two metal centers are capable of catalyzing two mechanistically distinct reactions on the same substrates, thereupon enabling overall tandem synthetic process. Several examples have been provided, including Ir/Rh, Ir/Pd, Ir/Pt, and Ru/Pd systems (see also Section 2.3), and several combinations of tandem processes with these metal centers as catalysts. This subject has been thoroughly reviewed in 2014 by Mata, Hahn, and Peris.126

Figure 31 Nickel and palladium complexes with poly-NHC ligands employed as electrocatalysts for CO2 reduction.


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4.2 Photophysics Luminescence is currently a research area attracting the attention of several organometallic chemists working with N-heterocyclic carbene ligands and the corresponding metal complexes, as demonstrated by the reviews that have recently appeared on this topic.201,202 The strength of the ligand field in NHC complexes is expected to be very high, given the great σ-donor ability of these ligands; it is believed that a possible direct consequence of this feature could be high energy emissions, which represent a fundamental requisite to obtain complexes emitting in the blue region, the most attractive color for the development of an efficient OLED device. Since the recent review by Visbal and Gimeno already covers the literature on luminescent late transition metal complexes bearing mono- and poly-N-heterocyclic carbene ligands starting from 2009,201 this part of the chapter will focus mainly on the articles appeared after that review and/or on those not included in it. The treated metal centers will be those usually studied in photophysics, hence the late transition metals belonging to the groups 7–11 of the periodic table. Regarding group 7, only rhenium(I) complexes bearing mono-NHC ligands have been synthesized and reported to be luminescent, and for this reason, they will not be further discussed in this review. In group 8 instead, examples of both ruthenium(II) and osmium(II) emissive complexes bearing poly-NHC are known. In general, ruthenium(II) complexes used for luminescent applications, like, for example, dye-sensitized solar cells, are Ru(II) polypyridine species, as [Ru(bpy)3]2+ or [Ru(tpy)2]2+ (bpy ¼ bipyridine, tpy ¼ 2,20 :60 ,200 terpyridine). The research activity in this field but using NHC ligands has been mainly devoted to the mere substitution of one or more bpy or tpy ligands with NHC ligands, functionalized with pyridine groups as substituents at the nitrogen atoms of the carbene heterocyclic ring. Focussing on di-NHCs, the majority of the examples present in the literature regard homo- or heteroleptic species (Fig. 32) in which one or both tpy ligands have been replaced by a di-NHC ligand bearing a pyridine (84, 86, 87, and 89), lutidine (85), or pyrazine (88) bridging group between the two carbene moieties.201 Recently, a similar approach was adopted for the synthesis of homoleptic Ru(II) complexes of type 90, with an asymmetric CNC pincer ligand (Fig. 33).203 Type 90 complexes in acetonitrile solution at room temperature emit at ca. 550–565 nm, upon excitation at their 1MLCT (around 390–400 nm) transition; these emissions correlate well with the adiabatic T1 ! S0 transition energies predicted by DFT calculations.

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Figure 32 Ruthenium(II) complexes with luminescence properties described and discussed in Ref. 201.

Figure 33 New examples of ruthenium(II) complexes characterized by luminescence properties.


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A series of cyclometalated Ru(II) complexes, 91 and 92, with a CCC pincer dicarbene ligand have been also proposed in this regard.204 However, only complex 91b in acetonitrile solution at room temperature shows an emission band at 808 nm, although with a very low quantum yield (Φem < 0.01%). The reports on emitting osmium(II) complexes are rather rare and regard complexes of type 93 (Fig. 34). An approach similar to the last one illustrated for the ruthenium(II) complexes has been extended also to the synthesis of emitting osmium(II) species. In particular, Xia and coworkers have studied the photophysical properties of the di-NHC CCC pincer-type complexes 94–96205; these complexes emit in the blue-green region (475–580 nm), reaching quantum yields up to 62% with complex 94b. The interesting results with this last complex led to the development of an OLED device, exhibiting an electroluminescence efficiency comparable to that of the standard iridium complex FIrpic (FIrpic ¼ {bis[2-(4,6-difluorophenyl)

Figure 34 Examples of osmium(II) complexes characterized by luminescence properties.

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pyridinato-C2,N](picolinato)iridium(III)}); in addition, the narrow emission band of the complex allows to predict a very high color purity of the device. Regarding group 9, all the examples involve poly-NHC iridium(III) complexes; two types of approaches in the use of di-NHC ligands in this context have been used so far. De Cola has synthesized homoleptic complexes of type 97 (Fig. 35) with a metalated CCC pincer di-NHC ligand; this species shows an emission quantum yield of 40% at 384 nm.206 Several other groups have instead used only one di-NHC coordinated as ancillary ligand in bis(o-metalated-2-phenylpyridine)iridium(III) complexes of type 98. In these cases, the transition associated to the emission is mainly influenced by the phenylpyridine ligand. A large number of examples in the field of luminescence properties regard the platinum(II) complexes with di-NHC ligands depicted in Fig. 36; emitting complexes with this metal can be square-planar homoleptic complexes as 99 with two chelating di-NHC ligand, heteroleptic complexes (100–104) bearing one di-NHC ligand, and complexes like 105 with a CCC pincer-type di-NHC ligand. Dinuclear complexes, like 106 and 107, have been also reported with the dicarbene ligand bridging the two metal centers. Recently, Venkatesan reported the platinum(II) complexes 108–110 (Fig. 37), whose structures are similar to complexes of type 100; the three complexes differ for the ancillary chelating dicarbene ligand and for the phenylacetylide coordinated moieties.207 All the complexes display a deep blue emission, in solution (dichloromethane at room temperature), in rigid matrix (2-methyltetrahydrofuran at 77 K), in solid state, and in PMMA film (doped with 10% of the platinum complex). The emission maxima are in the

Figure 35 Examples of luminescent iridium(III) complexes.

Figure 36 Platinum(II) complexes with luminescence properties described and discussed in ref. 201.

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Figure 37 New examples of platinum(II) complexes characterized by luminescence properties.

range of 430–450 nm and are not significantly influenced by the experimental conditions of the measurements; this emission band probably originates from a metal perturbed 3LLCT involving the π orbitals of the acetylide moiety. In particular, the emission quantum yields (Φem) in the solid state are inferior to 1%, possibly as a consequence of quenching due to aggregation and stacking, whereas in 10 wt% PMMA film they are in between 14% and 33%. Moreover, the Φem seems to be unaffected by the di-NHC ligand, while the best results are obtained with the coordination of p-OMe-phenylacetylide, the most electron donor alkenyl species. Together with platinum(II) complexes, also complexes of group 11 metals have been intensively studied for their luminescence properties, in particular dinuclear complexes of M(I) (M ¼ Cu, Ag, Au), in which one or two dicarbene ligands coordinate in a bridging fashion the two metal centers. The formula of this type of complexes is exemplified in Fig. 38. The group Y between the carbene moieties can be a simple alkyl bridge or an aromatic group, although more striking structures have been reported as well, like the chiral group in ligand 821 (Fig. 1) or the anionic group in ligand 12 (Fig. 2).27 In this type of complexes, the luminescent behavior is generally considered to be a consequence of the so-called metallophilic interaction. This interaction, also named “metallophilicity,” describes an intra- or intermolecular interaction between two centers with a d10 closed-shell electronic configuration. The low coordination number is an important prerequisite since it minimizes the steric repulsions between the ligands in the aggregates


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Figure 38 Dinuclear Cu(I), Ag(I), or Au(I) complexes with bridging di(N-heterocyclic carbene) ligands.

and, in this specific examples, the presence of one/two bridging di-NHC ligands should further favor the proximity of the metal centers and the onset of metallophilicity. The equilibrium distance separating two interacting centers is in fact lower than the sum of the van der Waals radii of two metal atoms. The energy associated to metallophilic interaction is weak and has been estimated between 7 and 12 kcal/mol. The metallophilic interaction is particularly notable for the heavier silver and especially gold centers, in which the spin–orbit coupling of the systems is enhanced, thus facilitating the access to triplet states via intersystem crossing. Furthermore, in the case of gold also relativistic effects should be taken into consideration, which perturb the molecular orbitals based on 5d and 6s/6p atomic orbitals, decreasing the energy gap between the HOMO and the LUMO, stabilizing at the same time the excited states, and consequently enhancing the probability of electronic transitions. Thus, it is not a coincidence that the number of the examples on luminescent complexes based on group 11 metal complexes increases down the group. Regarding copper, Tsubomura reported in 2009 the first example of a luminescent dinuclear copper complex, having two methylene-bridged diimidazol-2,20 -diylidene ligands;208 at the moment, this complex represents the only example of luminescent copper complex with a di-NHC ligand. Dinuclear silver complexes are instead much more represented, and some examples have been already mentioned and discussed in Section 3.2 (Fig. 11) for their chemosensing behavior;122–124 other species are reported in Fig. 39. In particular, Liu and coworkers synthesized complexes of type 111–113, which resulted to display a fluorescent emission around 350 nm, generally associated with metal perturbed intraligand processes; though the authors reported that the intensity of this band appears weaker than that of

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Figure 39 Some examples of luminescent silver(I) complexes.

the corresponding diimidazolium salt precursor, no studies on the efficiency of the emission, in terms of both quantum yield and lifetime of the excited state, were reported.115 Complex 114 displays a very broad band of fluorescent emission at 406 nm, which is however ascribed to the quinoline ring.209 A series of recent examples on photoluminescent dinuclear Au(I) complexes with bridging dicarbene ligands are shown in Fig. 40. Tubaro has synthesized a series of dinuclear gold(I) complexes of general formula [Au2(MeAz-Y-AzMe)2](PF6)2 and changed the nature of the bridge between the two carbene unit (Y ¼ (CH2)1–4, o-, m-, p-xylylene), as well as the type of heterocyclic ring (Az). The series of complexes with the propylene bridging group are luminescent in the solid state and, in particular, complex 115 displays an intense blue emission with an almost unitary quantum yield (Φem ¼ 96%). This emitting phenomenon could depend on the short Au–Au distance (3.272 A˚, in the solid state), which can be obtained by employing such a flexible bridge.210 However, by using other di-NHC ligands (complexes of type 116) with the propylene bridge, the same authors were not able to obtain emission efficiencies comparable to complex 115,


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Figure 40 Some examples of luminescent gold(I) complexes.

although in same cases they detected an even shorter Au–Au distance.211 In conclusion, with this class of complexes, the simple Au–Au distance is not the only factor that determines the photoluminescence properties of the complexes but also π–π stacking, cation/anion interactions, and the nature of substituents at the nitrogen atoms should be taken into account. In addition, Hemmert and coworkers investigated dinuclear gold(I) complexes bearing a di-NHC ligand with a propylene bridge between the carbene units and amido or alcoholic functions as nitrogen wingtip substituents (complexes 117 and 118). These complexes exhibit a blue emission, and the registered long emission lifetimes suggest a phosphorescent character for the emission.212 The interaction between the anions in solution and the dinuclear structure of the digold(I) complex can influence the emitting properties of the

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complex; this aspect has been recently investigated in depth by Barnard and Berners-Price, also by means of EXAFS analysis. They in fact observed that the emission at 396 nm characteristic of the complex 119, bearing a macrocyclic di-NHC ligand, might be quenched by addition of bromine anions to a dimethylsulfoxide solution of the complex; concomitantly a new emission band at 496 nm appears and has been ascribed to an association complex resulting from an interaction between the Br anions and the gold(I) centers.213 These results are interesting also for possible biomedical applications of the complexes, like, for example, as probes for halide anion concentration into living cells. In the described examples, the proximity between the two gold centers seems to be imposed by the presence of two di-NHC ligands bridging the two metals. Nevertheless, this is not a fundamental requirement: in fact, Vicente reported the first examples of dinuclear complexes in which the di-NHC ligand bridges two Au-alkynyl fragments.214 The emissions of complexes of type 120 have lifetimes in the order of μs and have been assigned to a 3[π ! π*] excited state centered on the alkynyl ligands (in a similar manner to the platinum complexes described previously) strongly metal perturbed via the aurophilic interaction. The examples of emitting gold(III) complexes bearing di-NHC ligands are much rarer if compared to gold(I). Complexes of type 121 have been reported (Fig. 41), in which the di-NHC is coordinated to two Au(III) complexes with a CNC pincer ligand. Also in this case, like in the previous one, the dicarbene ligand seems to act mainly as supporting ligand; in fact, the absorption and

Figure 41 Some examples of luminescent gold(III) complexes.


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emission properties (λem in the range of 470–560 nm and emission assigned to a metal perturbed 3[π ! π*] IL state) are very similar to the ones displayed by corresponding mononuclear complexes having a monodentate NHC ligand.215 Finally, in 2014 Che reported one of the first examples of gold(III) complexes with a cis-chelating di-NHC ligand. Complexes of type 122 are luminescent in solution at room temperature, and the emissions are ascribable to a metal perturbed 3[π ! π*] intraligand excited state of the employed cyclometalated ligand.216 The luminescent properties are not significantly influenced by the type of coordinated di-NHC ligand; in fact, the positions of the two emission bands are very similar (around 460 and 500 nm), and going from methylene to propylene as bridging group between the carbene units, only a slight increase in the quantum yield of emission [2% for CH2, 8% for (CH2)3] is observed; by contrast, the emission lifetimes became longer [14.7 μs for CH2, 46.4 μs for (CH2)3]. The whole of these data suggests that in the complexes with longer bridges between the NHC moieties, the distortions in the complexes due to the chelating coordination of the di-NHC are minimized and this results in a more efficient emission. Water-soluble variants of these species have been also synthesized by functionalizing with a sulfonate group the lateral chains of the nitrogen substituents (complexes 123–124); the photophysical properties of these complexes have been deeply investigated and the two observed emissions bands have been assigned to fluorescence (band at 450–470 nm) and to phosphorescence (band at higher λ); this particular emission behavior has been used for the ratiometric sensing of oxygen in aqueous solution; in fact, while the presence of oxygen quenched the phosphorescent emission, the fluorescence remains unaffected.

4.3 Medicinal Chemistry Among the different fields of applications characterizing the extremely versatile chemistry of poly-NHC complexes, bio-organometallic chemistry is one of the most recent and unexplored, and thereby it is a very appealing and fertile research topic.217–221 The first reports about the medicinal applications of metal complexes with NHC ligands have been published by Cetinkaya et al. in 1996–1997 on the antimicrobial activity of some Rh(I) and Ru(II) NHC complexes.222,223 After these pioneering works, few years have elapsed until the groups of Berners-Price (2004) and Youngs (2005) with their works, respectively, on the antitumor properties

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of Au(I)-NHC complexes and on the antimicrobial activity of Ag(I)-NHC complexes ignited a multiplicity of studies in this investigation field.224,225 Precisely, gold and silver are actually the most studied metal centers concerning the medicinal properties of the corresponding NHC complexes.226,227 Gold and silver have been used since many years in medicine. These two group 11 metals present a reduced toxicity compared to other metals used in medicinal chemistry such as platinum, and this limits the side effects of the corresponding metallodrugs. Silver NHC complexes are mainly studied as antimicrobials,228 as the consequence of their well-known anti-infective properties and also of the growing resistance of pathogens to the currently used antibodies that requires an unceasing development of novel medicines; silver complexes are also studied as antitumoral agents. After the observation that auranofin, an antiarthritic compound, also presents antitumor activity, gold complexes are mainly studied as potential antitumor agents.227 Gold NHC complexes present indeed a different mode of action against the neoplastic cells with respect to platinum-based drugs, and for this reason, they are considered one of the most important candidates for the production of novel chemotherapeutic medicines.229–232 In the context of metal complexes with poly-NHC ligands, which is the topic of this review, there is another reason leading poly-NHC silver and gold complexes to be extensively studied for biomedical applications. Poly-NHC ligands can in fact support metallophilic interactions present in the case of Ag(I) and Au(I) d10 metal centers.233 As outlined in the preceding section, metallophilic interactions are a crucial element of the chemistry of closed-shell metal center and the ligand ability in favoring or unfavoring this interaction can strongly influence the properties of the corresponding complexes. 4.3.1 Poly-NHC Gold Complexes Gold poly-NHC complexes are a promising class of antitumoral agents (Fig. 42). Different studies indicate that gold complexes act by inhibiting thioredoxin reductase and/or by targeting the mitochondria membrane, ultimately leading to mitochondria-induced apoptosis.229–232 More generally, it is possible to say that the target of gold-based drugs are proteins instead of nucleic acids or other biomolecules.227 Two different parameters are fundamental in the regulation of the biological activity of these complexes: the lipophilicity and the reactivity with thiol-containing biomolecules. The lipophilic/hydrophilic character of a complex indicates its attitude to permeate the mitochondrial membrane and also influences the protein binding


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Figure 42 Poly-NHC gold(I) and gold(III) complexes employed in medicinal chemistry.

ability.234,235 The reactivity with thiol-containing biomolecules, like, for example, reduced glutathione (GSH), is indicative of the stability of the complex in physiological conditions.236 An interesting properties of NHC-Au(I) complexes in general is that they often exhibit selective antiproliferative activity on cancer versus healthy cells lines. Remarkably, di-NHC-Au(I) complexes are more active than the corresponding monoNHC-Au(I) complexes.237

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Recently, Che and coworkers have reported in vivo anticancer activity of a dinuclear gold(I) complex (125a) with mixed bridging diphosphine and di-NHC ligands.238 This work represents the first example of a gold(I) complex able to inhibit cancer stem cell activity and in vivo angiogenesis in a tumor model. Complex 125a in fact shows favorable thiol reactivity; this means that it has an intermediate reactivity between the bis bridging diphosphine complex (125b) that is too reactive with thiols leading to a low selectivity between healthy and neoplastic cells and the homoleptic carbene complex (125c), that does not react with thiols, thus showing a lower antiproliferative activity. In their study, Che et al. found that complex 125 is a tight-binding inhibitor of TrxR and significantly inhibited tumor growth of HeLa xenografts and highly aggressive mouse B16-F10 melanoma with no observable side effects under in vivo conditions. In the same work, another important aspect is underlined about the employment of these types of gold(I) cationic complexes, namely the role of the counteranion associated to the cationic part. In fact, both the cation and the anion affect the solubility and hence the pharmacokinetics of the gold salts in the blood serum.238 The study reported by Che et al. is the only one done on gold complexes with poly-NHC ligands that presents in vivo experiments. However, there are several other studies on the in vitro antiproliferative activity of this class of compounds. Recently, Liu and coworkers have reported a very interesting study on the antitumor properties of two gold(I) complexes containing NHC ligands derived from cyclophanes (126 and 127).239 In this work, a detailed analysis of the mechanism of action of the complexes 126 and 127 was carried out, in particular using HeLa cells as model. Using confocal microscopy, it was found that in 127-treated cells, the gold complex was localized in the cytoplasm and in the mitochondria, rather than in other organelles. This suggests that Au(I)-NHC complexes induce cell death through the mitochondrial pathway and DNA may not be the target. Cell-cycle arrest ability test showed that the complexes do not induce modification in the cell-cycle distribution and this indicates that the complexes are not genotoxic. Differently, the ability of 126 and 127 in inducing cell death via the apoptotic way was demonstrated. In particular, their ability in the modification of the mitochondrial membrane potential was reported; this can lead to the release of mitochondrial material such as cytochrome c and apoptosis-induced factors. Further experiments evidenced that 126 and 127 are able to increase the intracellular ROS level and to induce the activation of caspase-3/7, which are two typical features of mitochondrial


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dysfunction and apoptosis. The regulation of the mitochondrial membrane permeability is a central aspect in the design of novel gold NHC complexes with antiproliferative activity. The hydrophilic/lipophilic character of a complex depends mainly on its molecular structure. This has stimulated the synthesis of complexes with differently functionalized ligands. For example, Veige et al. first report the study of the in vitro antiproliferative activity of a chiral Au(I)-NHC complex (128-(+/2)).240 Complex 128-(+/2) shows moderate cytotoxicity toward the cancerous cell lines with half inhibitory concentration values comparable with those of cis-platin. Although the authors were able to solve the racemic mixture 128-(+/2) into the enantiomerically pure 128-(2) and 128-(+), it was not possible from the experimental data to determine the enantioselective cytotoxicity. The mechanism of action of these complexes was also studied. Treating HeLa cells with 128-(+/2) for 24 h leads to DNA fragmentation, probably activated by CAD (caspase-activated DNase). This indicates that also in this case, the complex acts on the cancer cells by inducing apoptosis. Another different functionalization pathway was followed by the group of Tubaro. In particular, they used click chemistry to introduce different functional groups in the structure of bis(di-NHC) gold(I) and gold(III) complexes (129–132).241 The functionalization leads to complexes with higher antiproliferative activity than the pristine complexes 115 (Fig. 40) and 133. Moreover, the click functionalization also increases the selectivity of the complexes between neoplastic and healthy cells. As example, the IC50 value for complex 130 is 0.09 μM for MCF7 (breast carcinoma) cells, more than hundred times lower than for cis-platin (>10 μM), but 130 presents an IC50 value higher than 100 μM for healthy human epithelial cells. They also recognized a different stability and reactivity for gold(I) and gold(III) bis(di-NHC) complexes. In particular, gold(I) complexes are stable in relevant biological conditions and they show a very low reactivity versus biological thiols. On the contrary, gold(III) analogues are not stable in the same conditions, probably as a consequence of substitution reactions of the iodide ligands on the gold(III) centers by water or by other halides such as chlorides; furthermore, the complexes react very quickly with biological thiols in a redox process to afford a disulphide and the gold(I) analogues. Another example of the cytotoxic activity exhibited by gold(III) complexes with polycarbene ligands has been reported by Che and coworkers.242 In particular, in this case, a dicarbene ligand is used as bridging group between two cyclometalated gold(III) centers (134 and 135). In this case, the coordination environment of the gold(III) centers is different compared

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to complex 133 where the two carbene donors are mutually in trans. In complexes 134 and 135, the NHC ligands are in trans to a pyridine ligand that is the central binding site of a tridendate CNC pincer ligand. The different coordination set of the gold(III) centers leads to a different behavior of the complexes. In fact, 134 and 135 are good inhibitors of topoisomerase 1 (topo 1), an enzyme that unwinds chromosomal DNA and is an important cellular target for anticancer treatment. As a final comment, it is worth mentioning that poly-NHC gold complexes are studied also as anti-infectives. In this more recent field, some gold(I) and gold(III) complexes have shown in vitro antimalarial activity and ex vivo activity against keratin-associated eye infection.243,244 4.3.2 Poly-NHC Silver Complexes Silver(I) complexes with poly-NHC ligands have been also studied as anticancer agents. Several studies compare the properties of silver(I) complexes and those of the corresponding gold(I) complexes with the same ligands. These two metal centers have the same closed-shell electronic configuration (d10) and coordination geometry (linear). Moreover, silver(I) NHC complexes are often used as carbene transfer agent to obtain the corresponding gold(I) complexes. In general, silver(I) complexes show a lower antiproliferative activity with respect to the corresponding gold(I) complexes. This may be attributed to the lower stability of the silver(I) complexes in physiological conditions. For the same reason, also the selectivity of the silver(I) complexes for the cancerous cells versus the healthy cells is normally lower. This is true, for example, for complexes 136–138 that show a lower antiproliferative activity than the corresponding gold(I) analogues 126, 127, and 129 (Fig. 43).239,241 The mechanism of action of silver(I) NHC complexes as anticancer agents is still not well defined. However, there are some studies indicating that silver(I) NHC complexes with poly-NHC ligands show in vitro antiproliferative activity higher or at least comparable with cis-platin. For example, Willans and coworkers have reported the in vitro antiproliferative activity of a series of mono- and dinuclear silver(I) di-NHC complexes on MCF7 (human breast adenocarcinoma) and DLD1 (human colon adenocarcinoma).245 In this work, it is highlighted how dicarbene silver(I) complexes present higher antiproliferative activity than monocarbene complexes; in particular, complex (139) is the complex with the best performances. The higher activity of the di-NHC complexes is attributed by the authors to the higher stability of the complexes with these

Figure 43 Poly-NHC silver(I) complexes employed in medicinal chemistry.

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ligands. Also Haque et al. have reported a series of dinuclear di-NHC silver(I) complexes with high antiproliferative activity on HCT-116 (human colon cancer) and HL-60 (human promyelocytic leukemia) cell lines.246,247 In these works in particular, the role of the ligand is investigated: in fact, the dicarbene ligand may influence the activity of the corresponding dinuclear silver(I) complex by changing its lipophilic/hydrophilic character and also by supporting a metallophilic interaction. Argentophilic interactions, when present, seem to enhance the antiproliferative activity of the complexes, as in the case of complex 140.246 Silver(I) NHC complexes are also studied for their antimicrobial activity. Youngs and coworkers have recently reported an in vitro study on the activity of silver(I) NHC complexes against biosafety level 3 bacteria such as Burkholderia psuedomallei, Burkholderia mallei, Bacillus anthracis, MRSA, and Yersinia pestis.248 Different silver(I) NHC complexes have been analyzed; among them, the dinuclear dicarbene complex 141 shows a good behavior, and in particular it is more effective than the currently used clinical antibiotics ciprofloxacin and doxycycline in the treatment of biofilm organism of B. anthracis. Moreover, it must be remarked that possible antibiotic side effects like nephro- and ototoxicity are dramatically reduced with silver compounds. In a similar study, Hemmert and coworkers have studied the in vitro activity of different dinuclear di-NHC silver(I) complexes against Plasmodium falciparum, the parasite responsible of malaria.202 In their study, they identified two complexes, 142 and 143, characterized by a very low IC50 value, 1.2 and 1.5 μM respectively. Furthermore, no hemolytic properties have been shown by these two complexes, a problem that may affect this type of compounds. This case further underlines that the stabilizing effect of the dicarbene ligands and the possibility of supporting metallophilic interactions impart good properties to the complexes. 4.3.3 Other Poly-NHC Metal Complexes There are a small number of reports regarding the employment of polyNHC metal complexes in medicinal chemistry with metals different from gold and silver (Fig. 44). The most important study was reported in 2011 by Che and coworkers on the in vitro anticancer activity of cyclometalated Pt(II) dinuclear complexes with bridging dicarbene ligands (144 and 145).249 It is interesting to note that the mechanism of action of these Pt(II) complexes does not involve the interaction with DNA, as in the case of cis-platin. The presence of a CNN pincer ligand and of the poly-NHC ligand lends a high stability to


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Figure 44 Poly-NHC metal complexes employed in medicinal chemistry.

the Pt(II) centers that are stable in physiological conditions. Moreover, 144 and 145 are delocalized lipophilic cations. Inside the cell, these compounds are found to be mainly located in the cytoplasm, and they exercise their antiproliferative activity by inhibiting Survivin, a member of the inhibitors of apoptosis (IAP) protein family. In a more recent work, Dinda et al. have reported the synthesis of a Pt(II) and a Ru(II) complexes with a CNC pincer dicarbene ligands.250 Here, they have compared the antiproliferative activity of the complexes 146 and 147 on three different cancerous cell lines (A549, HCt116, and MCF7) and they found that the Ru(II) complex 147 presents IC50 values definitely lower than the Pt(II) complex 146 and also than cisplatin. The activity of ruthenium-based drugs is attributed to the ability of ruthenium to mimic iron in binding to biological molecules, such as human serum albumin and transferrin.217 Transferrin is the protein which supplies iron into cells, and transferrin receptors are overexpressed in cancer cells. Moreover, ruthenium-based drugs are in general much less toxic than platinum-based drugs and this is an added value in the field of medicinal chemistry. In the end, Ghosh and coworkers have studied the effect of the complexation by a tetradentate ligand with two carbene donors on the toxicity of Ni(II) compounds.251 Ni(II) ions are highly toxic for the

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organism, but the toxicity of this metal center is notably reduced by the complexation of the ligand designed in this work. This type of complexes like 148 may be used to develop resistance to nickel toxicity.

5. CONCLUSIONS AND OUTLOOK The chemistry of poly-NHC complexes of transition metals has developed considerably in the course of the last 5 years, and this development still continues today at a growing pace, as novel interesting aspects of this chemistry are progressively disclosed. The field has become strongly multidisciplinary, as widely different competencies are required in order to devise and synthesize novel poly-NHC ligands, to study their coordination chemistry and the properties of the corresponding metal complexes, and to investigate their applications. Particularly, the application arena of these complexes has considerably expanded from catalysis, where poly-NHC metal complexes have been successful but are still overshadowed by their mono-NHC analogues, to photophysics and medicinal chemistry, as outlined in the last section of this review. Success in these last fields has been notable, but further research is needed in order to capitalize on these still quite preliminary results. Furthermore, additional potential technological uses of these complexes are beginning to emerge, for example, as organometallic mesogens in the production of liquid crystals. In conclusion, research on poly-NHC complexes of transition metals is a very lively research field in which new exciting achievements are to be expected, particularly thanks to the collaborative efforts of illuminated chemists with different backgrounds.

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INDEX Note: Page numbers followed by “f ” indicate figures and “s” indicate schemes.

A Abnormal carbenes (aNHCs), 216–221, 217f π-Acceptor, 4–5 capabilities, 12 interaction, 3f Acceptorless dehydrogenative oxidations, 112–119, 112–113s, 114f, 115–117s, 117f, 119–120s Acetone, 113–114, 113s Acetophenone catalytic transfer hydrogenation of, 115–116, 115s deuteration of, 115–116, 115s hydrosilylation of, 248–249, 249f Acrolein, diboration of, 51s Acrylate and ethylene exchange, 188–189 formation, 183f Acrylic acid, 186–187, 190 π-complex, 184, 186 salt, 177 Additives, 101–106 Aerobic oxidation of alcohols, 94–112 metal catalysts, 94–101, 96s, 98f, 99–100s, 100–101f of perfluoro-substituted alcohols, 109s Ag(NHC) catalyst, 100s Alane (Al—H) bonds σ-complexes, 19–26, 19–20f, 20–22s, 24–26s, 26f interaction, 3–4 oxidative addition, 3–4 Alcohol to aldehydes, 125, 126s ammoxidation of, 106, 107s Anelli–Montanari’s oxidation of, 103s to carboxylic acids, 119, 120s catalysis, 245–247, 246f chemoselective oxidation of, 123, 123s cross coupling of, 245–246, 246f dehydrogenation of, 128

to esters, 126, 126s, 127f oxidant-and acceptor-free dehydrogenation, 112, 112s Alcohol–ketone interconversion, 114–115, 115s Alcohol oxidation, 93–94 benzyl, 145–146, 145s, 148s cinnamyl, 149–150, 149s for hydrogen storage and production, 132–134 Aldimines, diboration of, 48s Aliphatic PNP pincer ligands, 116 Aliquat, 143–144, 143s Alkane dehydrogenation, 243f Alkene-displacement approach, 24–25 Alkenes diboration of, 48–49 hydrogenation of, 244, 245f state-of-the-art functionalization of, 177, 178f β-Alkylated ketones, 116, 116s Alkynes Fujiwara hydroarylation of, 241–242, 241f state-of-the-art functionalization of, 177, 178f Alloys, 145–148 Allylic alcohols, 108 Amides, dehydrogenative formation of, 127, 128s Amine boranes dehydrogenation of catalytic, 13 secondary, 8s ligands, 8s Amine-substituted diboron compounds, metal activation, 70–73, 71–72s Aminoborane in B—H bonds activation, 15 dehydrogenation of, 17s monomer, 13 Ammonia, catalytic dehydrogenation of, 13

289

290 Ammoxidation, of alcohol, 106, 107s Anelli–Montanari protocol, 101–102, 148–149 Anelli–Montanari’s oxidation, of alcohols, 103s aNHCs. See Abnormal carbenes (aNHCs) Anionic borohydride-type ligands, 2–3 Anionic polydentate ligands, 222 Aromatic alcohols, photocatalytic oxidation of, 135–136 Aryl chlorides, 237 Aryl halides, formal acylation of, 239 Arylhydrazones synthesis, 118–119, 119s Aryl triflates, MW fluorination of, 139, 139s Atoms, overlaid positions of Mn, B, and H, 11f Aziridination, of olefins, 251, 251f

B B2(1,2-O2C2Me4) (B2pin2) activation, 44s, 49–50, 51s, 56s, 58–59s, 59, 62s, 64–65s, 68, 69s, 75–76s 1,2-diboration of allenes with, 55s Lewis acidity of, 43f Pt(0)-diboration of alkynes with, 45s synthetic applications of, 44s B2(1,2-O2C6H4)2 (B2cat2) activation, 52–54s, 53–54, 57s, 63–64s, 67–69s, 74 diboration of aldimines with, 48s diboration of olefins with, 47s Lewis acidity of, 43f oxidative addition, 53–54 Baeyer–Villiger reaction, 119 Basic ionic liquid (BIL), 155 Benzene activation, 21, 21s Benzyl alcohol oxidation, 136, 145–146, 145s, 148s Bidentate ligands, 179–180 Bimetallic catalysis, poly-NHCs metal complexes, 252, 252f Bimetallic complex formation, 211f Bimetallic Pt/Bi/C catalyst, 109 Biomimetic Cu(II) and Fe(II) complexes, 97 Biomimetic pathway, 98–99 Bis(picolyl)amine (BPA) ligand, 97–98, 98f Bis-aNHC chelate ligand, 217–218 Bisboryl Rh(III)-NHC complex, 66f

Index

Bis(boryl) complex, 46 Bis-MIC ligand, 218–219 σ-Bond metathesis barriers, 61f B2cat2 activation, 53s B2pin2 activation, 43–45, 44s, 58s oxidative addition, 54–55, 62s Borane (B—H) bonds activation, 2–3 in aminoborane complexes, 15 in group 9 borane complexes, 16f in group 8 complexes, 15f oxidative addition, 15 oxidative cleavage, 16–17 σ-complexes, 3f, 4–19 photolytic generation, 5s Borane ligand, 7–9 Boron–boron bonds, 39–40 dissociation energy, 43–45 Bridging ligands, 203–204 Brønsted bases, lactone cleavage with, 186–188, 188f Brookhart systems, 181 Brownian motion, 135

C Cannizzaro-type reactions, 126 Carbenes, Janus-type, 212 Carbon dioxide exploitation of, 175 state-of-the-art functionalization of alkenes/alkynes with, 177, 178f Carboxylation, of olefins, 192f Carboxylic acids, 119, 120s Cascade reactions, 125–129, 126s, 127f, 128–129s Catalysis alcohol, 245–247, 246f bimetallic, 252, 252f poly-NHCs metal complexes, 233–252 Catalysts, 194–195, 194f Ag(NHC), 100s bimetallic Pt/Bi/C, 109 dispersed, 150–152 heterogeneous, 145–146 ionic, 101f micellar, 150–152

291

Index

nano, 150–152 organo, 101–106 polymer-anchored, 95–96 recyclization, 144–156 ruthenium, 100f, 114–115, 114f silver N-heterocyclic carbene, 99–101 supported, 148–150 Catalytic carboxylation, of unsaturated hydrocarbons, 177 Catalytic dehydrogenation, of ammonia/ amine boranes, 13 Catalytic desymmetrization, 120–121 Catalytic reactions, 189–193, 190–192f Catalytic transfer hydrogenation, 115–116, 115s Cationic bisboryl Rh(III)-NHC complex formation, 66f Cationic κ1-H3BL complex formation, 7s Cationic lactones, 184 CCC pincer ligands, 249, 251–252, 255 C—C cross-coupling reaction, 239 Cerberus-type NHCs, 215, 215f C—H activation/functionalization reaction, poly-NHC metal complex, 209f, 240–244, 241f, 243f Chelating dicarbene ligands, 225 palladium(II) complexes, 250–251 Chelating ligands, 203–204, 207, 240 dicarbene, 225 trans-, 234–235 Chemoselective oxidation, of alcohol moiety, 123, 123s Chiral acyclic dicarbene ligands, 207 Chiral diboron compounds activation, 49s Chiral N-sulfonyldiamine ligands, 122 Chiral poly-NHC ligands, 205–208, 206f Chromium, borane σ-complexes, 5s Cinnamyl alcohol oxidation, 149–150, 149s Cis product, 40–41 Cl2B–BCl2 π-acidity of, 41–42 activation of, 39–42, 40s concerted interaction of, 41f with ferrocene, 41, 42s heterolytic cleavage of, 41s Lewis acidity of, 43f with olefins, 40s, 41f

C—N bond catalysts in, 240f formation, 240 CNC pincer complex oxidation, 225 CNC pincer-type ligand, 245, 245f Cobalt(II) complex, 116, 117f σ-Complexes alane, 19–26, 19–20f, 20–22s, 24–26s, 26f borane, 3f, 4–19 chromium, 5s photolytic generation, 5s tungsten, 5s E—H interaction in, 3f gallane, 26–31, 27s, 29–30s formation and dehydrogenation, 28, 28s manganese-mediated dehydrogenation, 28–29, 29s tetra-coordinate boranes, 5–9, 5–8s, 6f tri-coordinate boranes, 9–19, 10s, 11f, 12–13s, 14–16f, 17–18s, 18f Cross-coupling reactions, poly-NHCs metal complexes, 233–240, 236f Cyclometalated complex, 99–101

D Deep eutective solvents (DESs), 153 Dehomologation, oxidative, 110s Dehydrogenation alcohol, 128 alkane, 243f aminoborane complexes, 8s, 17–18, 17s β-estradiol to estrone, 113 ligand-promoted mechanism of, 113–114 manganese-mediated, 28–29, 29s mesitylborane, 17s oxidant-and acceptor-free, 112, 112s 2-pyridylmethanol, 117, 117s Dehydrogenative formation, of amides, 128s Dehydrogenative lactonization, of diols, 119, 119s Dehydrogenative oxidations, acceptorless, 112–119, 112–113s, 114f, 115–117s, 117f, 119–120s Density functional theory (DFT), 50 DESs. See Deep eutective solvents (DESs)

292 Desymmetrization catalytic, 120–121 oxidative, 120–125, 121–125s Deuteration, of acetophenone, 115–116, 115s DFT. See Density functional theory (DFT) Dialkoxy-diamino-diboron compounds, activation of, 77–78, 77–79s Dianionic ligand, 215 Diaziridinone, 111–112, 111s Diboration acrolein, 51s aldimines, 48s alkenes, 48–49 allenes with B2pin2, 55s Cl2B–BCl2, 40s methylacrylate, 51s olefins, 47s Diboron (pin)B–B(dan), activation of, 77, 77s Diboron (pin)B–B(dmab) and (pin)B–B (dbab), activation of, 77–78, 78s Diboron compounds activation, 42–76, 51s stability of, 42–43 Diboron PDIPA diboron, activation of, 78, 79s Diboron reagents, 73 activation of, 57–58 in copper borylation reactions, 60–62 gold(I) complexes to activate, 62 tetra(alkoxy), 62–63 unsymmetrical, 77 Dicarbene ligands, 225, 235 Janus-type, 229 Dihydrogen ligands, displacement, 12s Diisopropyl azodicarboxylate (DIAD), 111–112 Dimethyltitanocene reaction, 9 Di-MIC ligand, 218–219 Dinuclear Cu(I), Ag(I), or Au(I) complexes, luminescence properties, 258, 259f Dinuclear iridium(I) complexes, 246–247 Dinuclear ruthenium(II)-NHC complex, 249–250 Dinuclear silver complexes, 259–260 Diols, dehydrogenative lactonization of, 119, 119s

Index

Direct amide bond formation, 127 Direct arylation, of 2-phenylpyridine, 242, 243f Dispersed catalysts, 150–152 Divinyl-beta-ketoesters, Nazarov cyclization of, 250–251, 250f DKR. See Dynamic kinetic resolution (DKR) σ-Donor, 5–6, 12, 21–23 capabilities, 12, 29–30 interaction, 3f interactions in E—H, 3f phosphine ligands, 67 role, 6–7 Dream reactions, 176 in CO2 functionalization, 176f dtbpe-ligand hydrogen atoms on, 181f lactone formation, 179f Dynamic kinetic resolution (DKR), 120

E E—H bonds σ-complexes, interaction in, 3f in half-sandwich sigma complexes, 6f interaction of, 2 Enantioselective oxidation, 125s (S)-Erythrulose, stereospecific synthesis of, 121–122, 122s Esters formation, 126, 126s β-Estradiol, dehydrogenation of, 113 Ethanol mechanism for H2 generation with, 134s production from hydrogen, 133, 133s steam reforming reaction, 133, 133s

F Ferrocene, Cl2B–BCl2 with, 41, 42s Fujiwara hydroarylation, of alkynes, 241–242, 241f

G Gallanes bonds activation, 30, 30s σ-complexes, 26–31, 27s, 29–30s formation and dehydrogenation, 28, 28s

293

Index

manganese-mediated dehydrogenation, 28–29, 29s propensity of, 28 sensitivity of, 26–27 Glycerol, 107 Glycols, 153 Gold nanoparticles, 73, 73f, 153–154 NHC complexes, 263–265 poly(urea-formaldehyde) microparticles, 151 Gold complexes luminescence properties gold(I), 260–261, 261f gold(III), 262–263, 262f NHC, 263–264 Group 4 alane complex, 20f Group 13 elements, 2–4 Group 13 hydrides, 2 Group 8 metal complex, 69, 70s Group 9 metal complex, 8s, 62–68, 63s Group 10 metal complex, 45–57 Group 11 metal complex, 57–62 Guerbet reaction, 116, 116s

H Halide abstraction, 7s ligands, 225 metal activation of, 70–73, 71–72s Hartwig–Buchwald coupling, 239 Heck reaction, 237 β–H elimination, 182–184 from neutral nickelalactones, 184–186, 185f Heterobimetallic complexes with ditz ligand, 231, 231f synthesis, 230–231, 230f Heterogeneous catalysts, 127, 145–146 Heterogeneous catalytic systems, 133 Heterogeneous rhodium-on-carbon system, 118 Heterogeneous solid oxides, 145–148 Heteropolymetallic complexes, 229–231, 232f Hetero-poly-NHC ligands, 220, 220f Homogeneous catalytic systems, 113, 144, 148–149

Hydrides group 13, 2 metal-bound, 17–18 Hydroarylation, of alkynes, 241–242, 241f Hydrocarbons, unsaturated catalytic carboxylation of, 177 hydrocarboxylation of, 177 Hydrogenation alkenes, 244, 245f poly-NHCs metal complexes, 244–247, 245–246f Hydrogen-bonded heterocyclic carbenes, 207 Hydrogen-borrowing process, 245–246 Hydrogen production, 113–115, 132–134 alcohol oxidation for, 132–134, 133s Hydrosilylation acetophenone, 248–249, 249f phenylacetylene, 247–248, 247–248f poly-NHCs metal complexes, 247–249, 247–249f trimethylsilylacetylene, 247, 247f Hydrotalcites, 147 5-Hydroxymethylfurfural (HMF) oxidation, 131–132 8-Hydroxyquinolinate (HQL), 98f

I IBA. See 2-Iodosobenzoic acid (IBA) IBX. See O-Iodoxybenzoic acid (IBX) ILs. See Ionic liquids (ILs) Imidazole diol oxidation, 109s Imidazolium-based ILs, 154–155 Imidazolium-derivatized salen ligand, 208 Indole carbinols oxidation, 107, 107s Inner-sphere mechanism, 180–181 In situ BINAP-stabilized gold nanoparticles, 73, 73f In situ polymerization, 151 2-Iodosobenzoic acid (IBA), 109, 110f Ionic catalyst, 101f Ionic liquids (ILs), 101, 142, 144, 152–155 Iridium(III) complexes, luminescence properties, 256, 256f Iron complexes, 98f

294 Iron(II) complex, with macrocyclic tetracarbene ligand, 251, 251f Irradiation-promoted oxidations, 134–144

J Janus-type carbenes, 212 Janus-type dicarbene ligands, 229 Janus-type poly-NHC ligands, 212–216, 213f

K

κ2-aminoborane complex, 14f κ1-aminoborane complex formation, 15s

L Lactone cleavage with auxiliaries to force, 182–184 with Brønsted bases, 186–188, 188f formation, 119, 122, 123s, 178–182, 179f, 181f Lanthanides, 155 LBH3 ligands, 4–5 moiety, 4–5 Lewis acidity B2cat2, 43f B2pin2, 43f Cl2B–BCl2, 43f low, 43–45 Lewis bases (LBs), 43–45 adducts formation, 4–5 B2pin2 activation, 43–45, 44s Ligand-promoted mechanism, of dehydrogenation, 113–114 Ligands. See also Poly-NHCs ligands alkyl-bridged di-NHC, 230f, 231 anionic borohydride-type, 2–3 anionic polydentate, 222 bidentate, 179–180 bis-aNHC chelate, 217–218 bis-MIC, 218–219 bridging, 203–204 CCC pincer, 249, 251–252, 255 Cerberus-type tri-NHC, 215f chelating, 203–204 chelating dicarbene, 225 chiral acyclic dicarbene, 207 chiral N-sulfonyldiamine, 122

Index

dianionic, 215 dicarbene, 235 dihydrogen, displacement, 12s di-MIC, 218–219 dtbpe, 179f, 181f halide, 225 hetero-poly-NHC, 220, 220f imidazolium-derivatized salen, 208 Janus-type dicarbene, 229 Janus-type ditz, 242 macrocyclic tetracarbene, 251, 251f metal-coordinated isocyanide, 207 miscellaneous poly-NHC, 221–222, 222f mono-NHC, 239 organic, 95 pincer poly-NHC, 209–211 poly-aNHC, 217f on Pt(0) complex, 50s rigidity, 207 sigma-donating, 223 tri-NHCs, 216 Light emitting diode (LED), 137–138 Lignocellulose, 130 Luminescence properties, 253 dinuclear Cu(I), Ag(I), or Au(I) complexes, 258, 259f gold(I) complexes, 260–261, 261f gold(III) complexes, 262–263, 262f iridium(III) complexes, 256, 256f osmium(II) complexes, 255–256, 255f platinum(II) complexes, 256, 257–258f ruthenium(II) complexes, 253, 254f silver(I) complexes, 260f

M Macrocyclic tetracarbene ligand, 251, 251f iron(II) complex with, 251, 251f Macroligands, 96s Manganese-mediated dehydrogenation, 28–29, 29s Manganese σ-borane complex formation, 10–11, 11s Medicinal chemistry, poly-NHC gold complexes, 264–268, 265f metal complexes, 263–272, 271f silver complexes, 268–270, 269f Mesitylborane dehydrogenation, 17s Meso-diols, oxidative lactonization of, 122, 123s

295

Index

Mesoionic carbenes (MICs), 216–221 Metal activation amine-substituted diboron compounds, 70–73, 71–72s of halide, 70–73, 71–72s of tetra(alkoxy)diboron, 43–69, 47s Metal-bound hydride, 17–18 Metal catalysts, aerobic and peroxidative oxidations, 94–101, 96s, 98f, 99–100s, 100–101f Metal complexes in oxidation state, 223–226, 224f poly-NHCs, 204 novel, 222–231 polynuclear, 226–229, 229f Metal-coordinated isocyanide ligands, 207 Metallic nanoparticles, plasmon excitation of, 137–138, 138s Metallophilic interaction, 258–259 Metal–olefin complexes, 47–48 Metal-organic frameworks (MOFs), 97 Methanol, 133–134 oxidation of, 128–129, 129s Methylacrylate, diboration of, 51s Micellar catalysts, 150–152 Microwave (MW) application of, 144 assisted oxidation, 140, 140–141s, 144s assisted synthesis, 142, 142s fluorination of aryl triflates, 139, 139s irradiation, 138, 141–144 accelerating effect, 140 Microwave dielectric effect, 138–139 MICs. See Mesoionic carbenes (MICs) Mircowave-promoted oxidations, 138–143, 139–142s Miscellaneous poly-NHC ligands, 221–222, 222f Miscellaneous reactions, poly-NHCs metal complexes, 249–252, 250–251f MOFs. See Metal-organic frameworks (MOFs) Mono-NHC ligands, 239 MW. See Microwave (MW)

N Nano catalysts, 150–152 Nanocrystalline rhenium particles, 118

Nanoparticles (NPs) activation, 73–76, 73f, 75–76s gold, 153–154 Pd, 154 Nanoporous stainless steel (NPSS) electrode materials, 150 Nazarov cyclization, of divinyl-betaketoesters, 250–251, 250f Neutral nickelalactones, β-H elimination from, 184–186, 185f N-heterocyclic carbenes (NHCs), 203–204 Cerberus-type, 215, 215f moiety, 209, 222 N-heterocyclic structure, 205 poly, 203–204 stabilized alanes reaction, 24s N-hydroxyphthalimide (NHPI), 104–105, 104f Nickel, 180, 252f bidentate nickel, 186 organometallic chemistry of, 194 Nickelalactones, 183–184 β-H elimination from neutral, 184–186, 185f lactone interconversion and spontaneous rearrangement of, 185f Ni–CO2 complex, 181–182, 182f Nitroxyl radicals structure, 102–104, 103f N,N0 -dialkylation, of phenylenediamines, 246–247, 246f Noble metals, 116 Nonclassical carbenes, 216–218 Normal carbenes (nNHCs), 216–218 Novel poly-NHC ligands, 204–222 metal complexes, 222–231

O O-Iodoxybenzoic acid (IBX), 109–111, 110f Olefins, 250f aziridination of, 251, 251f with B2cat2, diboration, 47s Cl2B–BCl2 with, 40s, 41f one-pot carboxylation of, 192f Oppenauer oxidation, 146, 146s Oppenauer reaction, 120 Organic ligands, 95 Organic radicals, 101–106

296 Organoboron compounds, 41–43 Organocatalysts, 101–106 Organometallic cyclophane, 227, 228f Organotin compounds, 120–121 Osmium(II) complex, luminescence properties, 255–256, 255f Outer-sphere mechanism, 180–181 Oxidant-and acceptor-free dehydrogenation, 112, 112s Oxidation acceptorless dehydrogenative, 112–119, 112–113s, 114f, 115–117s, 117f, 119–120s agents, 95, 106–112, 107–111s, 110f alcohols, 93–94 benzyl alcohol to benzaldehyde, 136 cinnamyl alcohol, 149–150, 149s imidazole diol, 109s indole carbinols, 107, 107s methanol to formate salts, 128–129, 129s MW-promoted, 138–143, 139–142s Oppenauer, 146, 146s 4-pentenols, 108, 108s photocatalytic, 135–138, 135s, 137–138s stilbenes, 249–250, 250f Oxidation state metal complexes in, 223–226, 224f poly-NHCs metal complexes in, 225–226 Oxidative addition, 2–6, 11–12, 18s, 26s alane, 25 B2cat2, 53–54 σ-bond metathesis, 54–55 borane, 15, 18–19 B2pin2 activation, 43–45, 44s Ga—H, 30–31 reversible, 26s Oxidative dehomologation, 110s Oxidative desymmetrizations, 120–125, 121–125s Oxidative lactonization, of meso-diols, 122, 123s 3-(2-oxoethyl)indolin-2-ones synthesis, 108s Oxone®, 144

P Palladium, 252f catalysts, 234, 237 poly-NHC, 235

Index

catalytic activity of, 95 clusters, 150–151 NPs, 151, 154 Palladium(II) complexes, 244 chelating dicarbene, 250–251 in cross-coupling reactions, 233–240, 236f synthesis, 96s PCP-pincer Ir complexes, 114, 114f 4-Pentenols, oxidation of, 108, 108s Perfluoro-substituted alcohols, aerobic oxidation of, 109s Peroxidative oxidations, of alcohols, 94–112 metal catalysts, 94–101, 96s, 98f, 99–100s, 100–101f Phenylacetylene, hydrosilylation of, 247–248, 247–248f Phenylenediamines, N,N0 -dialkylation of, 246–247, 246f 2-Phenylpyridine, arylation of, 242, 243f Phosphine-free Pd complexes, 55s Photocatalytic oxidations, 135–138, 135s, 137–138s Photochemical reactions, 135 Photolytic generation, borane σ-complexes, 5s Photolytic ligand displacement, 22s Photophysics, poly-NHCs metal complexes, 253–263, 254–262f Pincer poly-NHC ligands, 209–211, 209f, 211f Plasmon excitation, of metallic nanoparticles, 137–138, 138s Plasmon-mediated oxidation, 137–138, 137s Platinum(II) complexes, 248 luminescence properties, 256, 257–258f PNP pincer ligands aliphatic, 116 cobalt(II) complex with, 117f Poly-aNHC ligands, 217f Poly(caprolactone) macroligands, 96s Poly(l-lactide) macroligands, 96s Polymer-anchored catalysts, 95–96 Polymer ligands, 95–96 Polymetallic complexes, 203–204, 231. See also Heteropolymetallic complexes Poly-NHCs, 203–204, 226–227, 235 Poly-NHCs ligands, 204–222, 229–231

297

Index

abnormal, remote/mesoionic carbenes, 216–221, 216–217f chiral, 205–208, 206f Janus-type, 212–216, 213f miscellaneous, 221–222, 222f novel, 204–222 pincer, 209–211, 209f, 211f pseudopincer, 209–211, 209f, 211f Poly-NHCs metal complexes, 204 application, 232–272 catalysis, 233–252 bimetallic, 252, 252f C—H activation/functionalization reactions, 209f, 240–244, 241f, 243f cross-coupling reactions, 233–240, 236f Cu complex, 240, 240f formation, 227 gold complex, medicinal chemistry, 264–268, 265f heteropolymetallic complex, 232f hydrogenations/transfer hydrogenations, 244–247, 245–246f hydrosilylations, 247–249, 247–249f medicinal chemistry, 263–272, 271f poly-NHC gold complex, 264–268, 265f poly-NHC silver complex, 268–270, 269f miscellaneous reactions, 249–252, 250–251f novel, 222–231 in oxidation state, 225–226 photophysics, 253–263, 254–262f polynuclear, 229 silver complexes, medicinal chemistry, 268–270, 269f Polynuclear metal complexes, 226–229, 229f Polynuclear poly-NHCs metal complexes, 229 2-Propanol and acetone, 113–114, 113s Pseudopincer poly-NHC ligands, 209–211, 209f, 211f Pt(0)-olefin complexes chiral diboron activation, 49s olefins diboration, 47s 2-Pyridylmethanol dehydrogenation of, 117, 117s oxidative addition of, 117

R Reaction mechanism, 136, 218–219 Recyclable catalytic systems, 144–145 Remote carbenes (rNHCs), 216–221 Renewable raw materials, transformation of, 130–132 Renewable sources, conversion of, 130–134 Rhenium(I) complexes, 253 Rhodium porphyrin complex, 99–101 rNHCs. See Remote carbenes (rNHCs) Robust matrix material, 150 Ruthenium catalyst, 100f, 114–115, 114f Ruthenium(II) complex, luminescence properties, 253, 254f

S Salt metathesis, 10–11, 11s, 21, 22s Sequential reactions, 125–129, 126s, 127f, 128–129s cascade and, 125–129, 126s, 127f, 128–129s Sigma-donating ligands, 223 Silver(I) complex, luminescence properties, 260f Silver NHC catalysts, 99–101 complex, 263–264 Sodium acrylate, 177 Sonogashira/hydroxyalkoxylation coupling, 238, 238f Sonogashira reaction, 238 Stable nitroxyl radicals, 104 Stable organic radicals, 105–106 Star block-copolymers, 96 Stilbenes, oxidation of, 249–250, 250f Substrates and oxidation agents, 106–112, 107–111s, 110f Supported catalysts, 148–150 Surface plasmon band (SPB), 137–138 Sustainability approach, 93 Suzuki reaction, 234–235, 237, 248

T Tandem reaction, 118–119, 125–126 TBHP. See Tert-butyl hydroperoxide (TBHP) TEMPO system, 102–105, 152–153, 152f

298 TEP. See Tolman electronic parameter (TEP) Tert-butyl hydroperoxide (TBHP), 97, 99, 141–142, 249–250 Tert-butyl nitrite (TBN), 106, 111 Tetra-coordinate boranes, σ-complexes featuring, 5–9, 5–8s, 6f Tetra(alkoxy)diboron, 42–43, 43f metal activation, 43–69, 47s group 8 metal complex, 69, 70s group 9 metal complex, 62–68, 63s group 10 metal complex, 45–57 group 11 metal complex, 57–62 Tetra(amino)diboron, 42–43 Tetrahydroazapanes synthesis, 136–137, 137s Thermodynamics, 175–177, 176f Thymine-1-acetate (THA), 98f Tishchenko-type reactions, 126 Tolman electronic parameter (TEP), 214–215 Trans-chelating ligand, 234–235 Transfer hydrogenations, poly-NHCs metal complexes, 244–247, 245–246f

Index

Transition metal complex, 25–26, 72–73, 77, 104, 120 diboration of alkenes, 62–63 diboron tetrafluoride activation, 70–71 electron-rich, 21 low valent metal, 43–45 Transition metals, 124–125 Transition states, structures of, 181f Tri-coordinate boranes, σ-complexes featuring, 9–19, 10s, 11f, 12–13s, 14–16f, 17–18s, 18f Trimethylsilylacetylene, hydrosilylation of, 247, 247f Tri-NHCs ligands, 216 Trinuclear compound, 95 Tris(alkoxy)borane formation, 43–45

V

Vacant π–orbital, 39–40 Vanadium, 108 Vanadium-substituted phosphomolybdic acids, 130