molecules Review
Hypervalent Iodine Reagents in High Valent Transition Metal Chemistry Felipe Cesar Sousa e Silva, Anthony F. Tierno and Sarah E. Wengryniuk * Department of Chemistry, Temple University, 1901 N. 13th St., Philadelphia, PA 19122, USA;
[email protected] (F.C.S.S.);
[email protected] (A.F.T.) * Correspondence:
[email protected]; Tel.: +1-215-204-0360 Academic Editor: Margaret A. Brimble Received: 29 March 2017; Accepted: 8 May 2017; Published: 12 May 2017
Abstract: Over the last 20 years, high valent metal complexes have evolved from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. Herein we will review the central role that hypervalent iodine reagents have played in this aspect, providing an ideal balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. Furthermore, these reagents are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. We will cover advancements in both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents. Keywords: hypervalent iodine; oxidation; oxidant; redox; high valent; high oxidation state; catalysis
1. Introduction Over the last 20 years, high valent metal complexes have transitioned from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. The choice of heteroatom can be critical depending on the application. For example, chloride ligands can aide in the stabilization and isolation of high valent complexes whereas acetate ligands are often more successful in catalytic manifolds. Hypervalent iodine reagents have seen wide application in this field as they are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. Furthermore, they provide an excellent balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. We will cover advancements in the use of hypervalent iodine reagents for both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents. Many of these areas have been covered in the context of more broad reviews and in those cases the discussion will not be comprehensive but focus on the key aspects and most relevant elements for this review. The discussion is organized broadly Molecules 2017, 22, 780; doi:10.3390/molecules22050780
www.mdpi.com/journal/molecules
Molecules 2017, 22, 780
2 of 54
Molecules 2017, 22, 780
2 of 54
by the metal center being oxidized, including palladium, platinum, gold, nickel, copper, and finally oxidized, including palladium, platinum, gold, nickel, copper, and finally isolated examples with isolated examples with other transition metals. Below a summary graphic has been provided that other transition metals. Below a summary graphic has been provided that includes oxidants common includes oxidants common to high valent transition metal chemistry that will be discussed in this to high valent transition metal chemistry that will be discussed in this review as well as common review as well as common nomenclature and how they will be presented in the text (Figure 1). nomenclature and how they will be presented in the text (Figure 1). Hypervalent Iodine Reagents - Featured in blue throughout text
Other Oxidants Commonly Utilized
General reactivity towards transition metals - 2-e – inner sphere oxidants (1-e – is possible but rare) - Transfer of oxidant ligands to metal center (carbon or heteroatom) - C–C or C–X reductive elimination pathways accessible - Re-establishment of active catalysts
General reactivity towards transition metals - 1e – or 2-e – inner sphere oxidants - Generation of electrophilic metal center - Can transfer functional groups to metal center (not always) Advantages - Powerful oxidants - Broad ligand scope for transference to metal center - Transfer of ligands resistant to reductive elimination
Advantages - Powerful oxidants - Inexpensive - Non-toxic - Environmentally benign - Rapid synthesis - Facile modulation of electronics and sterics
Disadvantage - Expensive - Harsh oxidantion conditions - Minimal modulation of electronic/steric effects
Disadvantages - Low atom economy - Generate organic byproducts (ex- phenyl iodine, iodobenzoic acid) - Limited by ligands available for transfer
Neutral, Acyclic λ3-Iodanes, Iodine(I) Reagents
O O I
R = Me; PhI(OAc) 2 phenyliodine(III)diacetate
R = CF3; PhI(OTFA) 2, PhI(OCOCF 3)2 phenyliodine(III) bis(trifluoroacetate)
R = Ph; PhI(O 2CPh) 2; PhI(OBz) 2 [bis-(benzoyloxy)iodo]benzene
R = tBu; PhI(O 2CtBu)2, PhI(OPiv) 2 phenyliodine(III)dipivalate
R O
R
O
Cl I Me
Me
One-Electron Oxidants
OAc I OAc
OH I OTs
Cl
phenyliodine(III)dichloride PhI(Cl) 2
Me
Hydroxo-phenyliodine(III)tosylate PhI(OH)OTs, Koser's reagent
H
O
CuCl 2
t-butylhydrogenperoxide tBuOOH, TBHP
O
O
N Cl
O
N-Chlorosuccinimide NCS
CH 3
IOAc Iodine monoacetate H 3C
Neutral, Cyclic, λ3-Iodanes O
TIPS
Me Me
Copper dichloride
N Br
I
N F
OTf S CF3
CH 3
1-Fluoro-2,4,6trimethylpyridinium triflate NFTPT
O
S - (trifluoromethyl)dibenzothiophenium triflate TDTT
N
1-[(triisopropylsilyl)ethynyl]-1,2benziodoxol-3(1H)-one;TIPS-EBX
(Poly)cationic, Acyclic λ3-Iodanes R I
N
2OTf
R = H [(Py) 2IPh]2OTf – N,N dipyridinium phenyliodine(III) triflate
R = CN, [(4CNPy) 2IPh]2OTf – N,N [4- cyano]dipyridinium phenyliodine(III) triflate R = NMe 2, [(DMAP) 2IPh]2OTf – N,N [4- dimethylamino]dipyridiniumphenyliodine(III) triflate
Iodonium salts R
BF 4
I+
R
SiH3
N 1-Chloromethyl-4-fluoro-1,4diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate), F-TEDA, SelectfluorR
PhO 2S PhO 2S
N F
N-fluorobis(phenylsulfonyl)imide NFSI
H 3C
I
Methyliodide, MeI PtCl 62Hexachloroplatinum(IV)
BF 4
R Diaryliodonium tetrafluoroborate [Ph 2I]BF 4
1,2 disilylbenzene
2KHSO5. KHSO 4.K 2SO4 OxoneR
I + C CR
SiH3
2BF4
F
N
O
Benzoquinone, BQ
OTf
Cl
R
O
N-Bromosuccinimide NBS
O
3,3 - Dimethyl-1-(trifluoromethyl)-1,2benziodoxole; Togni's reagent
Oxygen
Two-Electron Oxiants
[N-(phenylsulfonyl)imino]phenyliodinane PhINTs
I
O2
O
I NTs
Iodomesitylene diacetate MesI(OAc) 2
F 3C
O
O Alkynylliodonium tetrafluoroborate [PhICCR]BF 4
N Cl
O
N-Chlorosuccinimide NCS
Figure 1. Common oxidants utilized in high valent metal catalysis. Figure 1. Common oxidants utilized in high valent metal catalysis.
O
N Br
O
N-Bromosuccinimide NBS
Molecules 2017, 22, 780
3 of 54
2. Palladium Molecules 2017, 22, 780
3 of 54
2.1. Introduction
2. Palladium
Palladium has a long and storied history in transition metal catalysis, facilitating such iconic Introduction cross2.1. coupling reactions as the Heck, Suzuki-Miyara, Negishi, Buchwald-Hartwig, Sonagashira, and others. Its ubiquity stems not only fromin its excellent reactivity, also from a detailed Palladium has a long and storied history transition metal catalysis, but facilitating such iconic understanding of reactions its underlying mechanisms and Negishi, predictable reactivity, which facilitates cross coupling as the Heck, Suzuki-Miyara, Buchwald-Hartwig, Sonagashira, andnovel others. Its ubiquity stems not only from palladium its excellent reactivity, but alsoon from detailed understanding reaction development. For many years, catalysis relied theaPd(0)/Pd(II) redox couple, of itswork underlying mechanisms and shown predictable reactivity, facilitates novel reaction state however, in the 21st century has the power and which promise of the high oxidation development. For many years, palladium catalysis relied on the Pd(0)/Pd(II) redox couple, however, Pd(II)/Pd(IV) manifold, as well as the potential role of Pd(III) species in catalysis (Scheme 1). This work in the 21st century has shown the power and promise of the high oxidation state Pd(II)/Pd(IV) chemistry has enabled transformations previously inaccessible via traditional catalytic manifolds, manifold, as well as the potential role of Pd(III) species in catalysis (Scheme 1). This chemistry has most notably carbon-heteroatom bond forming reductive eliminations, the microscopic reverse of the enabled transformations previously inaccessible via traditional catalytic manifolds, most notably oxidative addition pathways commonly encountered in low valent palladium catalysis. In this context, carbon-heteroatom bond forming reductive eliminations, the microscopic reverse of the oxidative hypervalent reagents haveencountered emerged asinkey players, net two-electron oxidations addition iodine pathways commonly low valent facilitating palladium catalysis. In this context, at thehypervalent metal center accompanied transfer heteroatom most commonly iodine reagents haveby emerged as of keytheir players, facilitatingligands, net two-electron oxidations acetate at or chloride. Arguably the rapid advancements andheteroatom synthetic applications high valent palladium the metal center accompanied by transfer of their ligands, mostof commonly acetate or chloride. Arguably rapid advancements andobscure synthetic applications of with high other valentmetals. palladium chemistry have spurredthe investigations into more oxidation states As both chemistry have spurred investigations into more obscure oxidation states with other metals. As both synthetic applications and mechanistic details of this area have been comprehensively discussed in synthetic applications mechanistic of this area have comprehensively discussed in several excellent reviews and in recent years details [1–8], this section will been discuss the key role that hypervalent several excellent reviews in recent years [1–8], this section will discuss the key role that hypervalent iodine reagents have played in its development, with special attention paid to recent reports as well iodine reagents have played in its development, with special attention paid to recent reports as well as limitations of current methods that could be addressed through continued exploration of novel as limitations of current methods that could be addressed through continued exploration of novel oxidants. As the body of work in this area is extensive, this section will be organized based on the oxidants. As the body of work in this area is extensive, this section will be organized based on the type type of bond formation being targeted, C–X,C–N, C–N,and and C–C bonds, finally studies of bond formation being targeted,namely namely C–O, C–O, C–X, C–C bonds, andand finally studies focusing on Pd(III) species. focusing on Pd(III) species. Traditional: Pd(0)/Pd(II) Cross Coupling
Pd 0
Oxidative Addition R X
R Pd II X
Transmetallation [M] R1
R Pd II R1
Suzuki, Heck, Negishi, Buchwald-Hartwig, Stille, Sonagashira, etc.
R R1
C–C Reductive Elimination
Last 20 years: Pd(II)/Pd(IV) Manifolds: C–H Functionalization, Alkene Difunctionalization, Allylic Oxidation
R Pd II X
PdX 2
[O]
X
X R
Pd IV X X
R
X
Pd III Pd III R X
X
[O] = PhI(OR) 2, PhIX 2, [R 2I]X –, ArINTs, [(Py) 2IPh] 2X– NCS, NBS, NFSI, TTDT, others
C– X Reductive Elimination or S N2-Displacement R X
Scheme 1. Manifolds for palladium catalysis. Traditional methods via Pd(0)/Pd(II) and recent
Scheme 1. Manifolds for palladium catalysis. Traditional methods via Pd(0)/Pd(II) and recent advances advances in Pd(II)/Pd(IV) catalysis. in Pd(II)/Pd(IV) catalysis.
2.2. Palladium(IV)
2.2. Palladium(IV)
2.2.1. Introduction
2.2.1. Introduction
Canty reported the first X-ray structure of an alkyl Pd(IV) organometallic complex in 1986,
Canty reported the first X-rayof structure of to ana alkyl Pd(IV) organometallic complex in 1986, formed via the oxidative addition iodomethane dimethyl (bpy)Pd(II) complex (1, Scheme 2) [9]. formed via the oxidative of iodomethane to Pd(IV) a dimethyl (bpy)Pd(II) (1, oxidative Scheme 2) [9]. Canty’s work revealedaddition the octahedral geometry of species 1, as well complex as the clean addition/reductive reactivity and stability of thesespecies complexes, Canty’s work revealedelimination the octahedral geometry of Pd(IV) 1, aswhich well he ascomments the cleancould oxidative “suggest that development of a rich organometallic chemistry of palladium(IV) may be possible”. addition/reductive elimination reactivity and stability of these complexes, which he comments could This that report laid the foundation the rich chemistry, mechanistic and structuralmay understanding of This “suggest development of a richfororganometallic chemistry of palladium(IV) be possible”. this high oxidation state redox couple that has emerged over the last 20 years. report laid the foundation for the rich chemistry, mechanistic and structural understanding of this high oxidation state redox couple that has emerged over the last 20 years.
Molecules 2017, 22, 780
4 of 54
Molecules 2017, 22, 780
N N
Pd II
Me
MeI
Me Oxidation
1
N N
Me Pd IV I 2
Me
- (CH3)2
Me C–C Reductive Elimination
N N
Pd II
4 of 54
Me I
Scheme 2. Canty’s reportin resulting in X-ray the first X-ray crystallographic characterization a Pd(IV)alkyl species. Scheme 2. Canty’s report resulting the first crystallographic characterization of of a Pd(IV) alkyl species.
Pd(II)/Pd(IV) chemistry has become as a powerful synthetic manifold to facilitate transformations Pd(II)/Pd(IV) chemistry has become as a powerful synthetic manifold to facilitate transformations not accessible via traditional Pd(0)/Pd(II) catalysis (Scheme 1). This field has been pioneered by the not accessible via traditional Pd(0)/Pd(II) catalysis (Scheme 1). This field has been pioneered by Sanford the group in group the area C–H acetoxylation halogenation. There been several Sanford in theof area of C–H acetoxylation and and halogenation. There have have been several comprehensive reviews written topic in recent years readerthere is directed comprehensive reviews writtenon on this this topic in recent years and the and readerthe is directed for detailedthere for mechanistic discussion and an exhaustive report of applications [1,4,5]. Building from this work, detailed mechanistic discussion and an exhaustive report of applications [1,4,5]. Building from this Pd(IV) chemistry has been applied to the formation of a diverse array of C–N, C–O, C–X, and C–C work, Pd(IV) chemistry has been applied to the formation of a diverse array of C–N, C–O, C–X, and bond forming reactions. C–C bond forming reactions. 2.2.2. Carbon–Oxygen Bond Formation
2.2.2. Carbon–Oxygen Bond Formation There has been extensive work in the area of C–O bond formation via Pd(II)/Pd(IV) catalysis with hypervalent iodine reagents. This application is particularly well suited as the most common There has been extensive work in the area of C–O bond formation via Pd(II)/Pd(IV) catalysis hypervalent iodine oxidants, of the type PhI(OR)2 , transfer carboxylate ligands to the metal center that with hypervalent iodine reagents. This application is particularly well suited as the most common are then engaged in subsequent C–O bond forming reductive elimination. These carboxylate ligands hypervalent iodine oxidants, of the type and PhI(OR) 2, transfer carboxylate ligands to the metal center are also highly tunable, both sterically electronically, allowing for control of complex stability, pathway and selectivity. Approaches haveforming expandedreductive to include C–H functionalization, that are reaction then engaged in subsequent C–O bond elimination. Theseallylic carboxylate oxidation, as well as alkene difunctionalization. ligands are also highly tunable, both sterically and electronically, allowing for control of complex stability, 2.2.2.1. reaction pathway and selectivity. Approaches have expanded to include C–H functionalization, C–H Functionalization allylic oxidation, as well as alkene difunctionalization. The first report of Pd-catalyzed C–H acetoxylation using a hypervalent iodine oxidant was by Crabtree, who achieved the acetoxylation of benzene using Pd(OAc)2 with PhI(OAc)2 as the external 2.2.2.1. C–H Functionalization oxidant and –OAc source (Scheme 3) [10]. This built upon the work of Stock et al., which employed dichromate to perform an analogous transformation [11], however PhI(OAc)2 offered much higher The first report of Pd-catalyzed C–H acetoxylation using a hypervalent iodine oxidant was by selectivity and did not perform further product oxidations. In what has become a standard mechanistic Crabtree,proposal, who achieved acetoxylation of benzene using Pd(OAc) 2 with PhI(OAc)2 as the external Crabtree the proposed acetyl assisted C–H activation at Pd(II) (intermediate 3) would give oxidant and –OAc source (Scheme 3) [10]. This built upon the work of Stock et al., employed intermediate 4, which can then be diverted down one of two pathways. Along the desiredwhich pathway (PathtoB)perform 5 is then oxidized by PhI(OAc) with introduction of two acetyl groups. dichromate an analogous transformation [11],5 however PhI(OAc) 2 offered much higher 2 to Pd(IV) species Subsequent reductive elimination gives rise to acetoxylated product and regeneration of the Pd(OAc) . selectivity and did not perform further product oxidations. In what has become a 2standard Importantly, Crabtree noted that competitive formation of biphenyl (via Path A) is minimized with mechanistic proposal, Crabtree proposed acetyl assisted C–H activation at Pd(II) (intermediate 3) PhI(OAc)2 , indicating that oxidation to Pd(IV) in this system is significantly faster than a second C–H would give intermediate 4, which canalso then be diverted one two pathways. Along the desired activation step to give 6. It was found that in thedown absence of of oxidant, biphenyl was the only pathwayobservable (Path B) 5product, is thenthus oxidized bythat PhI(OAc) 2 to Pd(IV)4species 5 with introduction of two acetyl indicating Pd(II) intermediate will not undergo direct C–X reductive elimination, and emphasizing the significance of rise Pd(IV) in facilitating such transformations. groups. Subsequent reductive elimination gives to pathways acetoxylated product and regeneration of the this system displayed moderate activity formation and requiredof solvent quantities the Pd(OAc)While 2. Importantly, Crabtreeonly noted thatcatalytic competitive biphenyl (viaof Path A) is arene, it laid the foundation for the development of directed C–H acetoxylation, which has relied on minimized with PhI(OAc)2, indicating that oxidation to 2Pd(IV) in this system is significantly faster hypervalent iodine oxidants to efficiently acetoxylate C(sp )–H as well as C(sp3 )–H bonds.
than a second C–H activation step to give 6. It was also found that in the absence of oxidant, biphenyl was the only observable product, thus indicating that Pd(II) intermediate 4 will not undergo direct C–X reductive elimination, and emphasizing the significance of Pd(IV) pathways in facilitating such transformations. While this system displayed only moderate catalytic activity and required solvent quantities of the arene, it laid the foundation for the development of directed C–H acetoxylation, which has relied on hypervalent iodine oxidants to efficiently acetoxylate C(sp2)–H as well as C(sp3)– H bonds.
Molecules 2017, 22, 780
5 of 54
Molecules 2017, 22, 780
5 of 54 + metallic palladium
Molecules 2017, 22, 780
5 of 54 Pd II
+ metallic palladium C–C Reductive Elimination
6 Pd II
PhI(OAc) 2
Path A6 Ph–H minor
C–C Reductive Elimination PhI(OAc) 2 H
H
Pd(OAc) 2
H
O
Path A Ph–H Pd II(OAc) minor
Pd O H O O O 3 Pd O AssistedOC-H O Activation 3
Pd(OAc) 2
- AcOH - AcOH
4 Pd II(OAc) Path B major
PhI(OAc) 2 4
Path B PhI(OAc) 2 major OAc
Assisted C-H Activation C–X Reductive Elimination
PdIV OAc OAc OAc 5 PdIV OAc OAc
OAc C–X Reductive Elimination OAc
5 Scheme 3. Pd(II)/Pd(IV) catalyzed acetoxylation of benzene with use of PhI(OAc) 2 as external oxidant.
Scheme 3. Pd(II)/Pd(IV) catalyzed acetoxylation of benzene with use of PhI(OAc)2 as external oxidant.
The Sanford group’s contributions to this area beganwith in 2004 their2 as seminal on the Scheme 3. Pd(II)/Pd(IV) catalyzed acetoxylation of benzene use ofwith PhI(OAc) externalreport oxidant.
The Sanford group’s contributions to this area began in 2004 with their seminal report on the directed C–H acetoxylation of 2-phenylpyridine using Pd(OAc) 2/PhI(OAc) 2 (Scheme 4a) [12]. Since 2)–H The Sanford contributions thisinclude area began in 2004 with their seminal report on the then,C–H they have group’s extended this methodto to aPd(OAc) range of directing groups for C(sp directed acetoxylation of 2-phenylpyridine using /PhI(OAc) (Scheme 4a) [12]. Since 2 2 2 directed C–H acetoxylation of 2-phenylpyridine using Pd(OAc) 2/PhI(OAc) 2 (Scheme 4a) [12]. Since acetoxylation and a brief overview is included in Scheme 4b [13]. While other oxidants including then, they have extended this method to include a range of directing groups for C(sp )–H acetoxylation then, they have extended this used to include a range of 2 directing groups C(sp2)–H 2 and Oxone have been in this PhI(OAc) is by far including the most for common [2]. and and aMn(OAc) brief overview is included in method Scheme 4bchemistry, [13]. While other oxidants Mn(OAc) 2 acetoxylation and a brief overview is included Schemeof4bthis [13]. While other oxidants Sanford also demonstrated a polymer supportedinvariant chemistry, which allows including for facile Oxone have been used in this chemistry, PhI(OAc)2 is by far the most common [2]. Sanford also Mn(OAc)of 2 and Oxone have iodine been used in this chemistry, 2 is by far the most common [2]. recycling the hypervalent reagent, addressing the PhI(OAc) issue of iodobenzene byproducts produced demonstrated a polymer supported variant of this chemistry, which allows for facile recycling of Sanford also demonstrated a polymer supported variant of this chemistry, which allows for facile in these transformations (Scheme 4c) [14]. the hypervalent iodine reagent, addressing the issue of byproducts produced in these recycling of the hypervalent iodine reagent, addressing theiodobenzene issue of iodobenzene byproducts produced a. (Scheme 4c) b. transformations [14]. 4c) [14]. in these transformations (Scheme
R
Directed C(sp 2 )–H acetoxylation
Pd(OAc) 2, PhI(OAc) 2
N
a.
N
MeCN, 1002, °C Pd(OAc) R PhI(OAc) 2
N
MeCN, 100 °C
R N
Pd II N Pd II c.
L
[O]
L
c. N N
AcO
I I
N Ph N N 2 )–H acetoxylation Directed C(sp N
R
N
OAc N Ph N
R
R
[O]
AcO
OAc OAc
L L
b.
OAc OAc
n
n
OAc N N OAc Me
NOAc OAc N L PdIV OAcL N OAc L PdIV L OAc
R
O OAc N
R
O OAc
R N N
Pd(OAc) 2 AcOH, 100 °C Pd(OAc) 2
N OAc N
AcOH, 100 °C 1. Precipitate with CH3OH OAc 2. Regenerate with AcO2H
OAc OMe N OAc Me OMe N
R
AcO I AcO I n
n
1. Precipitate with CH3OH
Scheme 4. (a) First report on the directed C(sp2)–H acetoxylation of 2-phenylpyridine with 2. Regenerate with AcO2H Pd(OAc)2/PhI(OAc)2; (b) General scope of directing groups used for C(sp2)–H acetoxylation; 2)–H acetoxylation of 2-phenylpyridine with Scheme 4. (a) First report thedirected directed C(sp allows for facile oxidant (c) Polymer-supported λ3-iodane 2 )–Hrecycling. Scheme 4. (a) First report ononthe C(sp acetoxylation of 2-phenylpyridine with acetoxylation; Pd(OAc)2/PhI(OAc)2; (b) General scope of directing groups used for C(sp2)–H Pd(OAc)2 /PhI(OAc)2 ; (b) General scope of directing groups used for C(sp2 )–H acetoxylation; 3-iodane The acetoxylation of benzylic unactivated C(sp3)–H bonds can also be accomplished allowsand for facile oxidant recycling. (c) Polymer-supported 3 λboth
(c) Polymer-supported λ -iodane allows for facile oxidant recycling. with Pd(OAc)2 and PhI(OAc)2 (Scheme 5) [12,15,16]. In these reactions, sterics plays a large role in The acetoxylation of both benzylic and unactivated C(sp3)–H bonds can also be accomplished 3 )–H bonds with Pd(OAc)2 and PhI(OAc) 2 (Schemeand 5) [12,15,16]. In these sterics large role in The acetoxylation of both benzylic unactivated C(spreactions, canplays alsoabe accomplished
with Pd(OAc)2 and PhI(OAc)2 (Scheme 5) [12,15,16]. In these reactions, sterics plays a large role in
Molecules 2017, 22, 780
6 of 54
Molecules 2017, 22, 780
6 of 54
Moleculesregioselectivity, 2017, 22, 780 6 of 54 dictating with thethe lessless sterically hindered C–HC–H bond undergoing C–HC–H activation. While dictating regioselectivity, with sterically hindered bond undergoing activation. 3 3)–Hoften theWhile C(sp the )–H C(sp variant performs efficiently PhI(OAc) the oxidant, combinations variant often most performs most with efficiently with2 as PhI(OAc) 2 as the oxidant, dictating regioselectivity, with the less sterically hindered C–H bond undergoing C–H activation. of combinations Oxone/Mn(OAc) and peroxide oxidants have also been used of 3Oxone/Mn(OAc) 2, molecular oxygen, and peroxide oxidants have alsoeffectively been used[2]. 2 , molecular oxygen, While the C(sp )–H variant often performs most efficiently with PhI(OAc)2 as the oxidant, effectively [2]. Another interesting examplereagent used iodine(I) reagent IOAc, which was generated in situ Another interesting example used iodine(I) IOAc, which was generated in situ from PhI(OAc) combinations of Oxone/Mn(OAc)2, molecular oxygen, and peroxide oxidants have also been used 2 from PhI(OAc)22 and I2; was PhI(OAc) 2 alonein was ineffective and I2 ; PhI(OAc) alone ineffective this case [17].in this case [17]. effectively [2]. Another interesting example used iodine(I) reagent IOAc, which was generated in situ from PhI(OAc)2 and I2; PhI(OAc) 2 alone was ineffective in this case [17]. a. Benzylic C–H acetoxylation Pd(OAc) 2, X C–H acetoxylation a. Benzylic PhI(OAc) 2 R Pd(OAc) 2, AcOH/Ac2O, 100 °C X N PhI(OAc) 2 R H AcOH/Ac2O, 100 °C N b. Unactivated C(sp 3 )-H acetoxylation H MeO Pd(OAc) 2, b. Unactivated C(sp 3 )-H acetoxylation N PhI(OAc) 2 MeO Pd(OAc) 2, R H AcOH/Ac N 2O, 100 °C PhI(OAc) 2 R1 R
H
X R X N OAc N
R
OAc MeO MeO R R
N N
OAc R1 OAc
R = Me, R 1= Et PhI(OAc) 2 75% 1 45% Me, Et KR 2S=2O 8 R = PhI(OAc) 2 75% 45% K 2S2O8
AcOH/Ac2O, 100 °C Scheme 5. (a) Directed acetoxylation of benzylic C(sp3)–H (b) Directed acetoxylation of 1 R1 bonds; Scheme 5. (a) Directed Racetoxylation of benzylic C(sp3 )–H bonds; (b) Directed acetoxylation of 3)–H bonds. unactivated C(sp 3 )–H bonds. unactivated Scheme 5.C(sp (a) Directed acetoxylation of benzylic C(sp3)–H bonds; (b) Directed acetoxylation of
unactivated C(sp )–H bonds. The Sanford group has conducted extensive mechanistic studies on these processes, and the The Sanford has conducted extensive mechanistic studies on these processes, reader is directedgroup to some selected full reports for detailed discussion [2,18–20]. Their work hasand beenthe The Sanford group has conducted extensive mechanistic studies on these processes, and the reader is directed to some selected full reports discussion Their accompanied by that of Ritter and others, with afor keydetailed point being whether[2,18–20]. Pd(IV) species orwork Pd(III)has reader is directed to some selected full reports for detailed discussion [2,18–20]. Their work has been been accompanied by that of Ritter others, with athe keypossibility point being whether Pd(IV) species dimers are the active catalysts. Keyand reports detailing of Pd(III) intermediates are or accompanied bythe thatactive of Ritter and others, with a detailing key pointthe being whetherof Pd(IV) species or Pd(III)are Pd(III) dimers are catalysts. Key reports possibility Pd(III) intermediates discussed in more detail in Section 1.3. Sanford’s seminal report in this area outlines some of the key dimers are the active catalysts. Key reports detailing the possibility of Pd(III) intermediates are discussed morethe detail in Section Sanford’s seminal report in this outlines some of the key features in of both ligand scaffolds2.3. and hypervalent iodine oxidants thatarea allow for study of reactive discussed in more detail in Section 1.3. Sanford’s seminal report in this area outlines some of the key features both the ligand scaffolds and hypervalent iodine oxidants thatTwo allow forcyclometallated study of reactive Pd(IV)of intermediates as well as mechanistic elucidation (Scheme 6) [19]. rigid features of both the ligand scaffolds and hypervalent iodine oxidants that allow for study of reactive 2-phenylpyridine ligands incorporated to lend stability complexes, and Pd(IV) intermediates as wellwere as mechanistic elucidation (Schemeto6) the [19].resultant Two rigid cyclometallated Pd(IV) intermediates as well as mechanistic elucidation (Scheme 6) [19]. Two rigid cyclometallated suppress competing ligand and side reactions upon oxidation. Additionally, the acetate 2-phenylpyridine ligands wereexchange incorporated to lend stability to the resultant complexes, and suppress 2-phenylpyridine ligands were incorporated to lend stability to the resultant complexes, and ligands ofligand PhI(OAc) 2 were exchanged for aryl carboxylates that could be readily derivatized thus of competing exchange and side reactions upon oxidation. Additionally, the acetateand ligands suppress competing ligand exchange and side reactions upon oxidation. Additionally, the acetate used as facile handles to control the electronic parameters at the metal center. The Pd(IV) complex PhI(OAc) exchanged for aryl carboxylates that could be readily derivatized and thus (7) used ligands 2ofwere PhI(OAc) 2 were exchanged for aryl carboxylates that could be readily derivatized and thus obtained upon oxidation with PhI(CO2p–XAr) 2 where X=NO2 was able to characterized by X-ray as used facileashandles to control the electronic parameters at the metal center. The Pd(IV) complex facile handles to control the electronic parameters at the metal center. The Pd(IV) complex (7) (7) crystallography, revealing cis addition ofp–XAr) the two carboxylates ligands. Subsequent Hammett analysis obtained upon oxidation = NO able characterized X-ray 2 2p–XAr)22 where 2 was obtained upon oxidationwith withPhI(CO PhI(CO where X X=NO 2 was able toto characterized byby X-ray revealed a clear correlation between carboxylate electronics and the rate of reductive elimination, crystallography, revealing ciscisaddition ligands.Subsequent Subsequent Hammett analysis crystallography, revealing additionof ofthe the two two carboxylates carboxylates ligands. Hammett analysis indicating that the carboxylate acted in as a nucleophile in reductive elimination. revealed a clear correlation between carboxylate electronics and the rate of reductive elimination, revealed a clear correlation between carboxylate electronics and the rate of reductive elimination, indicating that the reductiveelimination. elimination. indicating that thecarboxylate carboxylateacted actedin inas asaa nucleophile nucleophile in reductive 3
N
Pd II
N
I
O O22C(p-XC C(p-XC66H H55))
R I X = H, OMe,OMe, OPh, F, Cl, Br, 2C(p-XC 6H 5) R Ac, CF3, CN, NO 2 R X = H, OMe, Me, OPh, F, Cl, Br, Hammett analysis shows correlation between carboxylate ligand Ac, CF 3, CN, NO 2 electronics and rate of reductive elimination N
Pd II
N
R
N
O 2C(p-XC 6H 5)
Hammett analysis shows correlation between carboxylate ligand electronics and rate of reductive elimination
Pd IV N O 2C(p-XC 6H 5) IV O Pd 2C(p-XC 6H 5) N7 O 2C(p-XC 6H 5) O 2C(p-XC C-O 6H 5) 7 Reductive Elimination C-O
N
Reductive Elimination
N (C 6H 5 p-X)CO 2 N Scheme 6. Seminal mechanistic investigation into Pd(IV)-mediated C–H acetoxylation. (C 6H 5 p-X)CO 2
6. Seminal mechanistic investigation into Pd(IV)-mediated C–H acetoxylation. 2)–H The Scheme YuScheme group reportedmechanistic an intramolecular C(spinto acetoxylation that also be rendered 6. Seminal investigation Pd(IV)-mediated C–Hcould acetoxylation. asymmetric using a Boc-Ile-OH chiral ligand (Scheme 27) [21]. This report was the first enantioselective The Yu group reported an intramolecular C(sp2)–H acetoxylation that could also be rendered application of Pd(II)/Pd(IV) and gave high yields enantioselectivities of benzofuranones. The Yu group reported catalysis an intramolecular C(sp )–Hand acetoxylation that could also be rendered asymmetric using a Boc-Ile-OH chiral ligand (Scheme 7) [21]. This report was the first enantioselective asymmetric using a Boc-Ile-OH chiral ligand (Scheme 7) [21]. This report was the first enantioselective application of Pd(II)/Pd(IV) catalysis and gave high yields and enantioselectivities of benzofuranones. application of Pd(II)/Pd(IV) catalysis and gave high yields and enantioselectivities of benzofuranones.
Molecules 2017, 22, 780 Molecules 2017, 22, 780
7 of 54 7 of 54
Molecules 2017, 22, 780
7 of 54
Ar Pd(OAc) 2, R Molecules 2017, 22, 780 7 of 54 O Boc-Ile-OH Ar Pd(OAc) 2, Ar R 1 1 O R R R Boc-Ile-OH PhI(OAc) , OHO ArO Pd(OAc) 22, Ar R RO R1 R1 KOAc, tBuOH O Boc-Ile-OH PhI(OAc) , up to 96% ee OH 2 O O R1 R1 KOAc, tBuOH PhI(OAc) OH 2, up tousing 96% ee 22)–H acetoxylation O Scheme 7. Enantioselective intramolecular C(sp chiral amino acid ligand. Scheme 7. Enantioselective intramolecular KOAc, C(sp )–H acetoxylation using chiral amino acid ligand. tBuOH up to 96% ee 2 Scheme 7. Enantioselective intramolecular C(sp )–H acetoxylation using chiral amino acid ligand. Ar
R
Sanford demonstrated an impressive example of non-directed C(sp2)–H acetoxylation through 2 )–H Scheme 7. Enantioselective intramolecular C(sp2)–H acetoxylation using chiral amino acid ligand.through Sanford demonstrated an impressive example of non-directed C(sp acetoxylation the addition of pyridine to enhance catalytic activity of the palladium catalyst (Scheme 8) [22]. In this 2 Sanford demonstrated an impressive example of non-directed C(sp )–H acetoxylation through thesystem, addition of pyridine to enhance catalytic activity of the palladium catalyst (Scheme 8) [22]. In this the ratio of [Pd]/pyridine proved critical as well as the selection of hypervalent iodine 2)–H (Scheme the addition pyridine to enhance catalyticexample activity of of non-directed the palladiumC(sp catalyst 8) [22]. In this Sanfordofdemonstrated an impressive acetoxylation through system, theSwitching ratio of to [Pd]/pyridine proved criticalMesI(OAc) as well as the PhI(OAc) selection2 of hypervalent iodine oxidant. thetomore sterically hindered improved bothiodine yield system, the ratio of [Pd]/pyridine proved critical as2 from the selection hypervalent the addition of pyridine enhance catalytic activityas of well the palladium catalystof (Scheme 8) [22]. In this oxidant. Switching to the more sterically hindered MesI(OAc) from PhI(OAc) improved both yield 2 2 and regioselectivity. oxidant. Switching to [Pd]/pyridine the more sterically hindered PhI(OAc)of 2 improved bothiodine yield system, the ratio of proved criticalMesI(OAc) as well as2 from the selection hypervalent and regioselectivity. and regioselectivity. oxidant. Switching to the 2 from PhI(OAc)2 improved both yield Cl more sterically hindered MesI(OAc) Cl Cl and regioselectivity. 2mol% [Pd] Cl Cl OAc Cl Cl
Cl Cl
Cl
ArI(OAc) 2, AcOH/Ac 2mol% [Pd] 2O 100 °C ArI(OAc) 2mol% [Pd] 2O 2, AcOH/Ac 100 °C ArI(OAc)2, AcOH/Ac2O [O] [Pd] 100 °C PhI(OAc) Pd(OAc) [O] 2 [Pd] 2
Cl
Cl Cl
MesI(OAc) 2 PhI(OAc) [O] 2 PhI(OAc) 2 MesI(OAc) PhI(OAc) 22 MesI(OAc) PhI(OAc) 222 MesI(OAc)
Pd(OAc) 2 Pd(OAc) [Pd] 2 Pd(OAc) 2 /pyr (1:0.9) Pd(OAc) Pd(OAc)22 Pd(OAc) (1:0.9) 2 /pyr Pd(OAc) Pd(OAc) 2 /pyr (1:0.9) 2
MesI(OAc) PhI(OAc) 22
Pd(OAc) Pd(OAc)22/pyr /pyr(1:0.9) (1:0.9)
Cl
+
Cl α
OAc OAc
α Yield α 8 Yield 8 8 Yield 59 88 64 59 8 64 59
+ +
Cl Cl
α : β
Cl β β
OAc OAc OAc
β 41 α :: 59 β 37 : 63 41α :: 59 β 29 :: 71 37 63 41 : 59 11 : 89 29 37 :: 71 63 11 29 :: 89 71
Scheme 8. Non-directed arene C–H acetoxylation. Effect of both catalyst and hypervalent iodine MesI(OAc) 2 Pd(OAc) 2 /pyr (1:0.9) 64 11 : 89 oxidant on reactivity. Scheme Non-directedarene areneC–H C–H acetoxylation. acetoxylation. Effect iodine Scheme 8. 8.Non-directed Effectofofboth bothcatalyst catalystand andhypervalent hypervalent iodine oxidant oxidant onon reactivity. Scheme 8. reactivity. Non-directed arene C–H acetoxylation. Effect of both catalyst and hypervalent iodine
Chen reported C(sp3)–H and C(sp2)–H alkoxylation using a picolinamide directing group oxidant on reactivity. (Scheme 9) [23]. The reaction is proposed proceed via displacement of acetate ligands by alkoxides 2)–H Chen reported C(sp3)–H and C(sp2to alkoxylation using a picolinamide directing group Chen reported C(sp3(8). )–H and C(sp )–H alkoxylation using a picolinamide directing group at a Pd(IV) intermediate The authors rule out an alternative S N 2-displacement of Pd(IV) by group ROH 2)–H (Scheme 9) [23]. The reaction is proposed proceed via displacement acetate ligands by alkoxides Chen reported C(sp3)–H and C(spto alkoxylation using a of picolinamide directing (Scheme 9) [23]. The reaction is proposed to proceed via displacement of acetate ligands by alkoxides since t-BuOH also participates to give C–OR bond formation. This reactivity is divergent from their at a Pd(IV) (8). The authors rule out an alternative SN2-displacement of Pd(IV) by ROH (Scheme 9) intermediate [23]. The reaction is proposed to proceed via displacement of acetate ligands by alkoxides at previous asince Pd(IV) intermediate (8).amination Theto authors rule out anformation. alternative SN reactivity 2-displacement of selective Pd(IV) by ROH reports on C–H using this same system, where 8 would undergo C–N t-BuOH also participates give C–OR bond This is divergent from their at a Pd(IV) intermediate (8). The authors rule out an alternative SN2-displacement of Pd(IV) by ROH since t-BuOH also participates to give C–OR bond formation. This is divergent from their reductive elimination in the absence ofusing an external nucleophile (for discussion bond formation, previous reports onparticipates C–H amination this same system, where 8reactivity wouldof undergo selective since t-BuOH also to give C–OR bond formation. This reactivity isC–N divergent from C–N their see Section 2.2.4.2, Scheme 27b). It was found that the use of other oxidants including AgOAc, Oxone, previous reports on C–H amination using this same system, where 8 would undergo selective C–N reductive elimination in theamination absence ofusing an external nucleophile (for discussion C–N bond formation, previous reports on C–H this same system, where 8 wouldofundergo selective C–N Ce(SO 4)elimination 2, K2S2O8, “F+in ” sources, as well as hypervalent iodine oxidants with other carboxylate ligands reductive the absence of an external nucleophile (for discussion of C–N bond formation, see Sectionelimination 2.2.4.2, Scheme It was that the use of other including Oxone, reductive in the27b). absence of found an external nucleophile (foroxidants discussion of C–N AgOAc, bond formation, 2)–H alkoxylation, either mono- or bisalkoxylation could be gave inferior conversions. In C(sp +” sources, seeall Section 2.2.4.2, Scheme 27b). It was found that the use of other oxidants including AgOAc, Oxone, Ce(SO 4 ) 2 , K 2 S 2 O 8 , “F as well as hypervalent iodine oxidants with other carboxylate ligands see Section 2.2.4.2, Scheme 27b). It was found that the use of other oxidants including AgOAc, Oxone, ++” achieved by altering the equivalents of PhI(OAc) 2. 2as Ce(SO ) , K S O , “F sources, as well hypervalent iodine oxidants with other carboxylate ligands all gave inferior conversions. In C(sp )–H alkoxylation, either monoor bisalkoxylation could be 4 2 4)2, 2K22S2O 8 8, “F ” sources, as well as hypervalent iodine oxidants with other carboxylate ligands Ce(SO
allachieved gave inferior conversions. InInC(sp either monomono-ororbisalkoxylation bisalkoxylationcould could by altering the equivalents of22)–H PhI(OAc) 2. all gave inferior conversions. C(sp )–H alkoxylation, alkoxylation, either bebe C(sp 3 )–H alkoxylation achieved byby altering the equivalents achieved altering the equivalentsofofPhI(OAc) PhI(OAc)22.. C(sp 3 )–H alkoxylation OAc OR N N O O C(sp 3 )–H alkoxylationPd(OAc) 2, N N OAc ORIV C–OR RE PhI(OAc) ROH IV 2 Pd Pd N N O O H HN O Pd(OAc) 2, RO HN O N N N N OAc OR ROH/xylenes C–OR RE OAc ORIV PhI(OAc) 2(1:4) ROH IV N 2 N 2 O O Pd Pd Pd(OAc) 2, H HN R O RO HN R O NN 8 NN C–OR RE ROH/xylenes PhI(OAc)(1:4) ROH R1 OAc OR 2 R1 Pd IV Pd IV H HN R 2 O RO HN R 2 O N N 8 ROH/xylenes (1:4) 1 OAc OR R1 C(sp 2R)–H alkoxylation R2 R2 8 R1 R1 Pd(OAc) 2, OMe O O O C(sp 2 )–H alkoxylation PhI(OAc) 2 2 N N N or Pd(OAc) 2, O O O R C(sp )–H alkoxylation R R OMe H H H PhI(OAc) 2 (1:4) N N N MeOH/xylenes OMe Pd(OAc) 2, OMeOMe N O N O N O or R R R H H H PhI(OAc) N N MeOH/xylenes2 (1:4) (1.5 N equiv.) PhI(OAc) (2.5 equiv.) PhI(OAc) 2N 2N N or OMe OMe R R R H H H N N N MeOH/xylenes (1:4) equiv.) equiv.) PhI(OAc) OMe OMe 2 (1.5 2 (2.5 Scheme 9. C(sp3)–H alkoxylation using picolinamide directing group PhI(OAc) via ligand exchange at Pd(IV). PhI(OAc) (1.5 equiv.)
PhI(OAc) (2.5 equiv.)
2 Scheme 9. C(sp3)–H alkoxylation using picolinamide2 directing group via ligand exchange at Pd(IV).
A similar transformation has also been reported by Rao using an 8-aminoquinoline directing group 3)–H alkoxylation using picolinamide directing group via ligand exchange at Pd(IV). 3 )–H Scheme 9. C(sp 9. C(sp alkoxylation using picolinamide directing group via ligand exchange at Pd(IV). 5-iodane Dess-Martin andScheme the unique choice of λ Periodinane (DMP, as the oxidant (Scheme 10)group [24]. A similar transformation has also been reported by Rao using an9)8-aminoquinoline directing and the uniquetransformation choice of λ5-iodane Dess-Martin Periodinane (DMP, as the oxidant (Scheme 10)group [24]. A similar has also been reported by Rao using an9)8-aminoquinoline directing 5 and the unique choice of λ -iodane Dess-Martin Periodinane (DMP, 9) as the oxidant (Scheme 10) [24].
Molecules 2017, 22, 780
8 of 54
A similar transformation has also been reported by Rao using an 8-aminoquinoline directing group 8 of 54 of 54 and the unique choice of λ5 -iodane Dess-Martin Periodinane (DMP, 9) as the oxidant (Scheme 10)8 [24]. Other oxidants including PhI(OAc) PhI(OAc)2 ,PhI(OTFA) PhI(OTFA)2,2K ,K Selectfluor all gave 8 , NaIO 4 , NaIO 3 , and Otheroxidants oxidantsincluding including 2S22S O28O , NaIO 4, NaIO 3, and Selectfluor all gave little Other PhI(OAc)22,,PhI(OTFA) 2, K2S2O8, NaIO4, NaIO3, and Selectfluor all gave little little or no conversion to desired products and competing functionalization of the 8-aminoquinoline or no no conversion conversion to to desired desired products products and and competing competing functionalization functionalization of of the the 8-aminoquinoline 8-aminoquinoline or directing group was also observed. The authors propose that DMP is not the terminal oxidant directing group was also observed. The authors propose that DMP is not the terminal oxidant but but directing group was also observed. The authors propose that DMP is not the terminal oxidant but 3 rather cyclic λλ33-iodane 10, formed in situ by attack of the alcohol on DMP, which then transfers the rather cyclic -iodane 10, formed in situ by attack of the alcohol on DMP, which then transfers the rather cyclic λ -iodane 10, formed in situ by attack of the alcohol on DMP, which then transfers the alkoxide to the palladium center upon oxidation. However, a similar ligand displacement at a Pd(IV) alkoxideto tothe thepalladium palladiumcenter centerupon uponoxidation. oxidation.However, However,aasimilar similarligand liganddisplacement displacementat ataaPd(IV) Pd(IV) alkoxide intermediate, analogous to Chen’s report, cannot be ruled out. intermediate, analogous to Chen’s report, cannot be ruled out. intermediate, analogous to Chen’s report, cannot be ruled out. Molecules 2017, 22, 780 Molecules 2017, 22, 780
R2 R2 R1 R1
O O
H H
N NH H
N N
Pd(OAc) Pd(OAc) 22 DMP/ROH, 100 °C DMP/ROH, 100 °C
R2 R2 R1 R1
OAc AcO OAc OAc AcO I OAc I ROH O ROH O
O O N NH H OR OR
N N
DMP (9) O DMP (9) O
OR OR I I O O 10 O 10 O
Scheme10. 10. C(sp C(sp333)–H alkoxylation of methylene positions using an 8-aminoquinoline 8-aminoquinoline directing directing group group Scheme Scheme 10. C(sp )–H alkoxylation of methylene positions using an 8-aminoquinoline directing group andDMP DMP asan an oxidant. and and DMP as as an oxidant. oxidant.
2.2.2.2. Alkene Alkene Difunctionalization, Difunctionalization, Allylic Allylic Oxidation Oxidation 2.2.2.2. Alkene Difunctionalization, Allylic 2.2.2.2. Oxidation Thedifunctionalization difunctionalization ofalkenes alkenes also possible employing Pd(II)/Pd(IV) catalysisand and hypervalent The difunctionalization of alkenes ispossible also possible employing Pd(II)/Pd(IV) catalysis and The of isisalso employing Pd(II)/Pd(IV) catalysis hypervalent iodine reagents. Through the traditional Pd(0)/Pd(II) catalysis, the Pd(II) intermediate (11) that arises hypervalent iodine reagents. Through the traditionalcatalysis, Pd(0)/Pd(II) catalysis, the Pd(II)(11) intermediate iodine reagents. Through the traditional Pd(0)/Pd(II) the Pd(II) intermediate that arises from initial heteropalladation undergoes rapid β-hydride elimination to regenerate an alkene alkene (11) that arises from initial heteropalladation undergoes rapid β-hydride elimination to regenerate from initial heteropalladation undergoes rapid β-hydride elimination to regenerate an (Scheme 11). By introducing an appropriate oxidant, 11 can instead be oxidized to a Pd(IV) species an alkene (Scheme 11). By introducing an appropriate oxidant, 11 can instead be oxidized to a Pd(IV) (Scheme 11). By introducing an appropriate oxidant, 11 can instead be oxidized to a Pd(IV) species (12), which which set up upisfor for subsequent reductive elimination. species (12),isis which setsubsequent up for subsequent reductive elimination. (12), set reductive elimination. X X
β-hydride β-hydride X X R R
11 11
R R [Pd II ] [Pd II ]
X1 X1 [Pd IV ] [Pd IV ] 12 X1 12 X1
X X
[O] [O]
R R
alkene products alkene products
X X
C–X RE C–X RE
R R
X1 X1
difunctionalized difunctionalized products products
Scheme 11. Alkene difunctionalization enabled via Pd(II)/Pd(IV) catalysis. Scheme 11. 11. Alkene Alkene difunctionalization difunctionalization enabled enabled via via Pd(II)/Pd(IV) Pd(II)/Pd(IV) catalysis. Scheme catalysis.
There have have been been several several reports reports on on 1,2-aminooxygenation 1,2-aminooxygenation of of alkenes alkenes via via this this approach approach that that There There have been several reports on 1,2-aminooxygenation of alkenes via this approach that utilize utilize phthalimide phthalimide as as the the nitrogen nitrogen source. source. Stahl Stahl reported reported the the use use of of allylic allylic ethers ethers in in the the utilize phthalimide as the aminoalkoxylation nitrogen source. Stahl reported alkenes the use of allylic ethers in the diastereoselective diastereoselective of terminal (Scheme 12a) [25]. Mechanistic studies diastereoselective aminoalkoxylation of terminal alkenes (Scheme 12a) [25]. Mechanistic studies aminoalkoxylation of terminal alkenes (Scheme 12a) [25]. Mechanistic studies revealed that an revealedthat thatan aninitial initial cisaminopalladation aminopalladation stepand and oxidation gavePd(IV) Pd(IV)intermediate intermediate 13,followed followed revealed cis step oxidation gave 13, initial cis aminopalladation step and oxidation gave Pd(IV) intermediate 13, followed by C–O bond byC–O C–Obond bondformation formationvia viaan anintermolecular intermolecularSSNN2-displacement 2-displacementby byacetate. acetate.Using Usingaasimilar similarapproach, approach, by formation via an intermolecular 2-displacement acetate. Using a similar approach, Sanford Sanford employed employed homoallylicSN alcohols in the the by diastereoselective formation of substituted substituted Sanford homoallylic alcohols in diastereoselective formation of employed homoallylic alcohols in 12b) the diastereoselective formation of substituted tetrahydrofuran rings tetrahydrofuran rings (Scheme [26]. Consistent with Stahl’s findings, Sanford reports cis tetrahydrofuran rings (Scheme 12b) [26]. Consistent with Stahl’s findings, Sanford reports aa cis (Scheme 12b) [26]. Consistent with Stahl’s findings, Sanford reports a cis aminopalladation/oxidation aminopalladation/oxidation sequence sequence however, however, in in this this case, case, the the presence presence of of the the homoallylic homoallylic alcohol alcohol aminopalladation/oxidation sequence however, in thiscoordination case, the presence of the homoallylic alcohol results in intramolecular results in intramolecular to give palladacycle 14. This leads to preferential direct C–O results in intramolecular coordination to give palladacycle 14. This leads to preferential direct C–O coordination to give palladacycle 14. This leads to preferential direct C–O bond forming reductive bond forming forming reductive reductive elimination eliminationrather rather than thanintermolecular intermolecularSSNN22 attack attack on on14. 14. Consistent Consistentwith with the the bond elimination rather thanofintermolecular SN 2 attack on 14. Consistent withsubstitution the necessary formation of necessary formation the six-membered palladacycle, additional on the alcohol necessary formation of the six-membered palladacycle, additional substitution on the alcohol the six-membered palladacycle, additional substitution on the alcohol backbone results in significantly backbone resultsin in significantly higheryields. yields. backbone results significantly higher higher yields.
Molecules 2017, 22, 780
9 of 54
Molecules 2017, 22, 780
9 of 54
a. Stahl (2006) PdCl 2(CH 3CN) 2, phthalimide
Molecules 2017, 22, RO780
NPhth OAc
RO
1 a.RStahl (2006) PhI(OAc) 2
RO
O via cis-aminopalladation/ Pd IV AcO oxidation
O
2
–OAc
–OAc R1oxidation 13
R1
AgBF4, R R= phthalimide Ar, 54-80% OH alkyl, H d7), which results in formation of a metal-metal single bond. Detailed mechanistic and electron (d8 -> d7 ), which results in formation of a metal-metal single bond. Detailed mechanistic and computational studies support that 82 is then able to undergo a concerted reductive elimination event computational studies support that 82 is then able to undergo a concerted reductive elimination event wherein both components of the new C–X bond arise from a single palladium center. A subsequent wherein both components of the new C–X bond arise from a single palladium center. A subsequent report from Ritter provides additional evidence that a Pd (III) dimer is the kinetically competent report from Ritter provides additional evidence that a Pd (III) dimer is the kinetically competent species in the acetoxylation of 2-phenylpyridine with Pd(OAc)2/PhI(OAc)2 [85]. species in the acetoxylation of 2-phenylpyridine with Pd(OAc)2 /PhI(OAc)2 [85].
Molecules 2017, 22, 780 Molecules 2017, 22, 780
26 of 54 26 of 54
PhICl 2
2
N N Pd(OAc)L
- 2L + 2L
PhI
N
Pd II O O
O Pd II O N 81
Bimetallic Oxidative Addition
Me Me N
Cl Pd III O O O Pd IIIO Cl
Fast Equilibrium
Cl N
Me Me N
82
Pd
O O
O Pd O L
Me Me
Cl
Bimetallic Reductive Elimination
HCl
Cl
H N
N
Scheme41. 41.First Firstevidence evidencefor for the the role role of of bimetallic Scheme bimetallicPd(III) Pd(III)dimers dimersinincatalysis. catalysis.
Together, these reports are the first to show the catalytic competence of Pd(III) dimers in C–X Together, reports thefuel first to show the catalytic competence of Pd(III) dimers in C–X bond formingthese reactions andare could further investigations in the area of bimetallic Pd(III) catalysis. bond forming reactions and could fuel further investigations in the area of bimetallic Pd(III) catalysis. Furthermore it supports the consideration of Pd(III) dimers along Pd(II)/Pd(IV) catalytic cycles Furthermore it supports iodine the consideration of Pd(III) dimers along Pd(II)/Pd(IV) catalytic cycles employing hypervalent oxidants. employing hypervalent iodine oxidants. 2.3.3. Conclusions 2.3.3. Conclusions High valent palladium chemistry represents one of the most well developed areas of high High valent palladium chemistry represents of theofmost well developed areas of high oxidation state metal catalysis. Advancements in theone formation a wide range of carbon-heteroatom oxidation state metalC–O, catalysis. the formation a wide rangehave of carbon-heteroatom bonds including C–X, Advancements C–N, as well asinC–C bonds via of this manifold been reported. bonds including C–X, C–N, as well as C–C viawidely this manifold been reported. PhI(OAc)2 PhI(OAc) 2 andC–O, diaryliodonium salts have beenbonds the most appliedhave hypervalent iodine reagents indiaryliodonium catalytic methodsalts development PhIClwidely 2 has been successfully used iodine for high valent in complex and have beenand the most applied hypervalent reagents catalytic isolation. Detailed mechanistic have revealed used clear for evidence for Pd(II)/Pd(IV) redox couples method development and PhICl2 studies has been successfully high valent complex isolation. Detailed in these processes, however the role Pd(III)-dimers along the catalytic cycles is in becoming more mechanistic studies have revealed clearofevidence for Pd(II)/Pd(IV) redox couples these processes, evident. Current limitations remain in the reductive elimination of challenging groups such as however the role of Pd(III)-dimers along the catalytic cycles is becoming more evident. Current fluoride remain or trifluoromethyl groups, and development of oxidants not possess competitive limitations in the reductive elimination of challenging groups that suchdo as fluoride or trifluoromethyl ligands for reductive elimination would significantly contribute to this area. groups, and development of oxidants that do not possess competitive ligands for reductive elimination
would significantly contribute to this area. 3. Platinum 3. Platinum Platinum has played a pivotal role in the evolution of oxidative couplings. Arguably, the “Shilov system” was has the first significant example intermolecular C–H functionalization, wherein the Platinum played a pivotal role ofinanthe evolution of oxidative couplings. Arguably, oxidation of alkanes to a mixture alcohols and alkyl chlorides was mediated by an aqueous solution the “Shilov system” was the first significant example of an intermolecular C–H functionalization, of [PtCl42−] and [PtCl62−] (Scheme 42a) [86]. Since this seminal discovery, extensive studies have been wherein the oxidation of alkanes to a mixture alcohols and alkyl chlorides was mediated by an aqueous conducted on the Pt(II)/Pt(IV) redox couple, as well as the potential formation of Pt(III) dimeric solution of [PtCl4 2− ] and [PtCl6 2− ] (Scheme 42a) [86]. Since this seminal discovery, extensive studies species. Oxidation of square planar Pt(II) to octahedral Pt(IV) is significantly more facile than have been conducted on the Pt(II)/Pt(IV) redox couple, as well as the potential formation of Pt(III) palladium (standard reduction potentials of [PtCl62−] and [PdCl62−] are +0.68 V and +1.29 V, dimeric species. Oxidation of square planar Pt(II) to octahedral Pt(IV) is significantly more facile respectively) [86], however, Pt(II)/Pt(IV) mediated oxidative couplings are comparatively rare. This 2− 2− ] are +0.68 V and +1.29 V, than palladium (standard higher reduction potentials of [PtCl 6 ] and 6 relative is due to the significantly barrier to reductive elimination for[PdCl Pt(IV) to Pd(IV) [87]. The respectively) [86], however, mediated oxidative couplings comparatively rare.for This enhanced stability of Pt(IV)Pt(II)/Pt(IV) complexes has been exploited in their use asare isolable model systems is due to theofsignificantly barrierredox to reductive for Pt(IV) relative to Pd(IV) [87]. the study more elusivehigher Pd(II)/Pd(IV) couples elimination [88]. Mechanistic insights provided by these The enhanced stability of Pt(IV) complexes has been exploited in their use as isolable model systems studies have been recently reviewed [88,89] and this section will cover recent advancements in Pt(IV) forchemistry the studyas ofthey morerelate elusive Pd(II)/Pd(IV) redox couples [88]. Mechanistic insights provided by these to hypervalent iodine reagents. studies have been recently reviewed [88,89] and this section will cover recent advancements in Pt(IV) chemistry as they relate to hypervalent iodine reagents.
Molecules 2017, 22, 780
27 of 54
Molecules 2017, 22, 780
27 of 54
Molecules 2017, 22, 780
27 of 54
a. Shilov (1976 ) Cl
a. Shilov (1976 ) H 2Pt IVCl6 H 2Pt IVCl6
a. TFA/H2O
H 3N
a. NH TFA/H b. 3 2O
HCl 3N
NH 4
Cl Cl Pt IV Cl Pt IV Cl
NH 4
b. NH 3 Cl Stoichiometric C-H activation at Pt(IV) Cl
Stoichiometric C-H activation at Pt(IV) b. Modern studies on Pt(IV) [O] = PhI(OAc) 2, PhICl 2, Ar2I[X], [(Py) 2IPh]2OTf – as astudies model system for Pd(IV) b. Use Modern on Pt(IV) [O] = PhI(OAc) 2, PhICl 2, Ar2I[X], [(Py) 2IPh]2OTf – R R Use as a model system for Pd(IV) - common in catalysis R R R - relatively unreactive R IV R R Pd PtIV - unstable - relatively stable common in to catalysis unreactive R - relatively R R RR --challenging isolate RR - often isolable IV IV Pd Pt - unstable - relatively stable R R R R R - challenging to isolate R - often isolable R R Development of Pt(II)/Pt(IV) C–H activation Development C–Hof Pt(II)/Pt(IV) C–H activation [O] activation C–H PtX 2 R Pt II X [O] –HX activation PtX 2 R Pt II X –HX C–X reductive elimination
X IV Pt X
X
R X Pt IV
X
R
X
C–X reductive elimination
has displayed divergent reactivity and selectivity to Pd(IV) has displayed divergent R X reactivity and selectivity to Pd(IV) R X
Scheme StoichiometricC–H C–Hfunctionalization functionalizationofofnaphthalene naphthaleneby byShilov; Shilov;(b) (b)General Generalscheme schemefor Scheme 42.42. (a)(a) Stoichiometric for modern applications Pt(II)/Pt(IV) redox couples with hypervalent iodine oxidants. Scheme 42. (a) Stoichiometric C–H functionalization of naphthalene by Shilov; (b) General scheme modern applications Pt(II)/Pt(IV) redox couples with hypervalent iodine oxidants. for modern applications Pt(II)/Pt(IV) redox couples with hypervalent iodine oxidants.
3.1. Complex Isolation 3.1. Complex Isolation 3.1. Complex Isolation Canty has conducted many of the seminal reports on the isolation of various Pt(IV) and Pd(IV) Canty has conducted many of the seminal reports on the isolation of various Pt(IV) and Pd(IV) complexes via conducted oxidation many with both and reports alkynyliodonium saltsof(Scheme 43) [63–66]. The Canty has of thearyl seminal on the isolation various Pt(IV) and Pd(IV) complexes via oxidation with both aryl and alkynyliodonium saltsa (Scheme 43) [63–66]. The iodonium iodonium salts were able to cleanly oxidize Pt(II) complexes with variety of ligand scaffolds, giving complexes via oxidation with both aryl and alkynyliodonium salts (Scheme 43) [63–66]. The salts were able cleanly oxidize Pt(II) complexes with awith variety of ligand scaffolds, giving rise rise to varying degrees of cis/trans isomers (83-cis 83-trans), at low temperature. Forscaffolds, characterization iodonium saltsto were able to cleanly oxidize Pt(II)or complexes a variety of ligand giving to purposes varying degrees of cis/trans isomers (83-cis or 83-trans), at low temperature. For characterization thesedegrees complexes were isomers then treated NaI resulting in triflate displacement, and a rise to varying of cis/trans (83-ciswith or 83-trans), at low temperature. For characterization purposes these were thenthen treated with NaI resulting triflate displacement, and a summary summary of complexes the complexes complexes synthesized via thiswith approach is in provided below displacement, (compounds 84–88). purposes these were treated NaI resulting in triflate and a of Throughout the complexes via this approach is approach provided 84–88). complexes Throughout reports, Canty notes the higher stabilityisbelow of the(compounds resultant summary of these thesynthesized complexes synthesized via this provided below platinum (compounds 84–88). these reports, Canty the higher stability ofhigher the resultant platinum complexes relative to palladium, relative to palladium, facilitating and structural characterization that was not possible with Throughout thesenotes reports, Cantyisolation notes the stability of the resultant platinum complexes facilitating and structural characterization thatcharacterization was notanalogous possible with the possible corresponding the corresponding palladium species. For a further discussion of the components relative toisolation palladium, facilitating isolation and structural thatpalladium was not with to studies, seea Section 2.2.5.1. palladium species. For furtherspecies. discussion the analogous components to Canty’s studies, theCanty’s corresponding palladium For aof further discussionpalladium of the analogous palladium components Canty’s2.2.5.1. studies, see Section 2.2.5.1. seetoSection
R R R R
[IAr 2]OTf or [IAr 2]OTf [PhIC orCR]OTf
PtII
–60 °C [PhIC CR]OTf
PtII
–60 °C
C CR
R 2R 2 = –Ph,
C CR
R 2IV Pt OTf PtIV 83-trans OTf 83-trans
N
PhIV Pt
N
Pt I IV N 84 R = Ph, Me I 84 R = Ph, Me
Pt I IV 85 I 85
N
PhIV Pt
R R2 or R R2
or
CR
Ph
Ph R R R R
R R R R
R 2 = –Ph,
Me Me Me Me
CR Pt IV Pt I IV 86 I 86
R Pt R IV OTf PtIV 83-cis OTf 83-cis
N N
R2 NaI 25 °C NaI 25 °C
R R R R
R 2IV Pt I IV Pt I
CO 2R NMe 2 CO 2R IV I PtNMe
2
NMe Pt2IV I 87 C NMe 2 CR 87 C CR
R R R2 or R R2
or
Pt R IV I IV Pt I
CR CR Me Pt IV Me Me Pt I IV Me 88 I 88
Me 2 P Me 2 P P Me 2 P Me 2
Scheme 43. Characterization of Pt(IV) upon oxidation with diaryl and alkynyliodonium salts. Scheme Characterizationof ofPt(IV) Pt(IV) upon upon oxidation oxidation with salts. Scheme 43.43.Characterization withdiaryl diaryland andalkynyliodonium alkynyliodonium salts.
In 2005, Sanford reported the oxidation of benzo[h]quinoline supported Pt(II)acac complex 89 2 in an reported effort to the gainoxidation mechanistic insights into the supported analogous Pt(II)acac Pd(II)/Pd(IV) system with In PhI(OAc) 2005, Sanford of benzo[h]quinoline complex 89 In 2005, Sanford reported the oxidation of benzo[h]quinoline supported Pt(II)acac complex 89 (Scheme 44) [90]. it was found that solvent had pronounced effect on product 2 in Interestingly, an effort to gain mechanistic insights into thea analogous Pd(II)/Pd(IV) system with PhI(OAc) with PhI(OAc)2 in an effort to gaindimer mechanistic insights while into the analogous Pd(II)/Pd(IV) system distribution. AcOH, Pt(III)–Pt(III) 90 wasthat obtained, of Pt(IV) alkoxides (91, 92) (Scheme 44)In [90]. Interestingly, it was found solvent hada mixture a pronounced effect on product (Scheme 44) [90]. Interestingly, it was found that solvent had a pronounced effect on product were obtained in the Pt(III)–Pt(III) alcoholic solvents, with dependent the steric bulkalkoxides of the alcohol. distribution. In AcOH, dimer 90 wasratios obtained, while a on mixture of Pt(IV) (91, 92) distribution. In AcOH, Pt(III)–Pt(III) dimer 90 ratios wasligands, obtained, a mixture of Pt(IV) alkoxides Alcoholic solvents are alcoholic not able tosolvents, serve as with bridging thuswhile resulting in preferential formation were obtained in the dependent on the steric bulk of the alcohol. (91,Alcoholic 92) were obtained solvents, ratios dependent onpreferential the steric formation bulk of the solvents are in notthe ablealcoholic to serve as bridgingwith ligands, thus resulting in
Molecules 2017, 22, 780
28 of 54
Molecules 2017, 22, 780
28 of 54
alcohol. Alcoholic solvents are not able to serve as bridging ligands, thus resulting in preferential of the monomeric species (91, 92) (91, and92) such were were hypothesized to be to formation of the monomeric species andcomplexes such complexes hypothesized to analogous be analogous intermediates in Pd(II)/Pd(IV) C–H oxygenations. In contrast, isolation of Pt(III)–Pt(III) 90 was to intermediates in Pd(II)/Pd(IV) C–H oxygenations. In contrast, isolation of Pt(III)–Pt(III) 90 was surprising asasthese these intermediates not invoked been invoked in Pd(II)/Pd(IV)-catalyzed surprising intermediates had had not been in Pd(II)/Pd(IV)-catalyzed processes,processes, however however subsequent studies from Ritter, and Sanford, others have theofviability of Pd(III) subsequent studies from Ritter, Sanford, othersand have shown theshown viability Pd(III) dimers in dimers in these processes (see Section 2.3.1.2). Only 0.5 equivalents of PhI(OAc) 2 were needed for these processes (see Section 2.3). Only 0.5 equivalents of PhI(OAc)2 were needed for formation of formation of either the monomeric or bimetallic providing evidenceproceed that both either the monomeric or bimetallic species, providing species, strong evidence thatstrong both pathways via pathways proceed via two-electron oxidation. two-electron oxidation. Me O N
PtII 89
O O
Me Me
0.5 equiv. PhI(OAc) 2
N N
AcOH
PtIII PtIII
O O O O
Me Me Me
O
0.5 equiv. PhI(OAc) 2 ROH
90 Me
Me OR N
PtIV OR 91
O O
Me Me
Me
O N
PtIV OAc 92
O OR
ROH
Ratio (91:92)
MeOH EtOH iPrOH
2.0:1.0 1.6:1.0 0.4:1.0
Scheme 44. 44. Benzo[h]quinoline Pt(II) oxidation oxidation studies. studies. Effect of solvent solvent on on oxidation oxidation product product ratios. ratios. Scheme Benzo[h]quinoline Pt(II) Effect of
A subsequent study by Sanford examined the oxidation of 2-phenylpyridine Pt(II) complex 93 with A subsequent study by Sanford examined the oxidation of 2-phenylpyridine Pt(II) complex 93 PhICl2, which led to a mixture of cis and trans Pt(IV) complexes (94-cis 94-trans, Scheme 45a) [91]. This with PhICl2 , which led to a mixture of cis and trans Pt(IV) complexes (94-cis 94-trans, Scheme 45a) [91]. was particularly interesting as the analogous Pd(II) complex has been found to give exclusive This was particularly interesting as the analogous Pd(II) complex has been found to give exclusive formation of the cis isomer (95) upon oxidation with PhICl2. Pt(II) complex 93 was also subject to a formation of the cis isomer (95) upon oxidation with PhICl2 . Pt(II) complex 93 was also subject to delicate interplay between Pt(IV) and Pt(III)–Pt(III) dimer formation upon oxidation, similar to their a delicate interplay between Pt(IV) and Pt(III)–Pt(III) dimer formation upon oxidation, similar to previous report [90], however in this case product ratios were contingent on choice of external their previous report [90], however in this case product ratios were contingent on choice of external oxidant. Whereas PhICl2 gave exclusively Pd(IV) monomers, altering the oxidant to NCS provided a oxidant. Whereas PhICl2 gave exclusively Pd(IV) monomers, altering the oxidant to NCS provided a Pt(III)–Pt(III) dimer as the major product (not shown). Rourke and co-workers showed that treatment Pt(III)–Pt(III) dimer as the major product (not shown). Rourke and co-workers showed that treatment of Pt(II) complex 96 with PhICl2 resulted in two-electron oxidation with concomitant C–H activation of Pt(II) complex 96 with PhICl2 resulted in two-electron oxidation with concomitant C–H activation to provide Pt(IV) dichloride 98 even at temperatures as low as −40 °C (Scheme 46b) [92]. This result to provide Pt(IV) dichloride 98 even at temperatures as low as −40 ◦ C (Scheme 46b) [92]. This is notable as previous studies employing other oxidants (peroxides and molecular oxygen) produced result is notable as previous studies employing other oxidants (peroxides and molecular oxygen) complex mixtures. It is proposed that oxidation proceeds via a five-coordinate, cationic Pt(IV) produced complex mixtures. It is proposed that oxidation proceeds via a five-coordinate, cationic intermediate (97) that is highly active towards arene functionalization. Pt(IV) intermediate (97) that is highly active towards arene functionalization. Building on Ritter’s use of poly(cationic) λ3-iodanes in high oxidation state nickel and Building on Ritter’s use of poly(cationic) λ3 -iodanes in high oxidation state nickel and palladium-mediated fluorination (see Sections 2.2.3.1 and 5.3), Dutton investigated the potential of palladium-mediated fluorination (see Section 2.2.3.1 and Section 5.3), Dutton investigated the potential poly(cationic) λ3-iodanes to access a range of dicationic Pd(IV) and Pt(IV) complexes through the of poly(cationic) λ3 -iodanes to access a range of dicationic Pd(IV) and Pt(IV) complexes through delivery of neutral heterocyclic ligands to the metal center (Scheme 46) [93]. They found that 2the delivery of neutral heterocyclic ligands to the metal center (Scheme 46) [93]. They found phenylpyridine Pt(II) complex 93 could be cleanly oxidized to Pt(IV) complex 100 with a DMAPthat 2-phenylpyridine Pt(II) complex 93 could be cleanly oxidized to Pt(IV) complex 100 with a derived poly(cationic) λ3-iodane 99 (Scheme 46a). However, oxidation of dimethyl Pt(II) 101 led to a DMAP-derived poly(cationic) λ3 -iodane 99 (Scheme 46a). However, oxidation of dimethyl Pt(II) 101 led less defined product distribution (Scheme 46b), possibly arising from oxidative disproportionation. to a less defined product distribution (Scheme 46b), possibly arising from oxidative disproportionation. This reactivity has been documented in similar Pt(II)/Pt(IV) redox couples, however the intermediacy This reactivity has been documented in similar Pt(II)/Pt(IV) redox couples, however the intermediacy of Pt(III) intermediates cannot be ruled out in this case [94]. These oxidations were also performed on of Pt(III) intermediates cannot be ruled out in this case [94]. These oxidations were also performed on the analogous palladium complexes, which were found to be too unstable for isolation and the analogous palladium complexes, which were found to be too unstable for isolation and underwent underwent rapid disproportionation. While the Pt(IV) species 100 and 102 were isolable, it is notable that they are considerably less stable than complexes possessing anionic chloride or acetate ligands, and similar stability trends were observed for the palladium complexes. While this is detrimental in
Molecules 2017, 22, 780
29 of 54
rapid disproportionation. While the Pt(IV) species 100 and 102 were isolable, it is notable that they are considerably less stable than complexes possessing anionic chloride or acetate ligands, and similar Molecules 2017,were 22, 780observed for the palladium complexes. While this is detrimental in the29 of 54 stability trends context of Molecules 2017, 22, 780 29 of 54 complex isolation, it could be adventitious for enhancing the reactivity of both high-oxidation state the context of complex isolation, it could be adventitious for enhancing the reactivity of both highpalladium andstate platinum species catalysis. the context of complex isolation, it couldspecies be adventitious for enhancing the reactivity of both highoxidation palladium and in platinum in catalysis. oxidation state palladium and platinum species in catalysis. a. Sanford (2008) a. Sanford (2008) N N
PtII PtII
N N
PhICl 2 PhICl 2
N N
93 93
Cl Cl Pt IV Pt IV N N
Cl Cl
Cl Cl IV Pt Pt IV Cl Cl
N N
94-cis (64%) 94-cis (64%)
Cl Cl Cl Pd IV Cl Pd IV N N
N N
N N
95 95
Pd(IV)- cis formed exclusively Pd(IV)cis formed exclusively
94-trans (29%) 94-trans (29%)
b. Rourke (2008) b. Rourke (2008) N N
F F
F F
Cl PtII Cl PtII N N
PhICl 2 PhICl 2
96 96
N N Cl PtIV Cl IV Pt Cl Cl N N
F F F
Arene C–H Activation Arene C–H Activation 84% 84%
N N Cl Pt IV Cl IV Pt Cl Cl N
F F F
N
97 97 intermediate proposed
98 98
proposed intermediate
Scheme (a) Oxidation of 2-phenylpryidine Pt(II) complex with PhICl 2. Divergent complex Scheme 45. (a)45. Oxidation of 2-phenylpryidine Pt(II) Pt(II) complex with PhICl . Divergent complex geometry Scheme (a)Pd(II); Oxidation of 2-phenylpryidine complex with 2PhICl 2. Divergent complex geometry45. from (b) Pt(II) oxidation with PhICl 2 resulting in arene C–H activation. from geometry Pd(II); (b)from Pt(II) oxidation with PhICl resulting in arene C–H activation. 2 Pd(II); (b) Pt(II) oxidation with PhICl2 resulting in arene C–H activation. a. a.
NMe 2 NMe 2
N N
Pt II Pt II
N N
+ +
93 93
N NI I N N
2 OTf – 2 OTf –
2 2
NMe 2 NMe 2
N N
- PhI - PhI
NMe 2 NMe 2 99 99
N N Pt IV Pt IV N N
NMe 2 NMe 2
N N
100 100
b. b.
NMe 2 NMe 2 N N
Pt II Pt II 101 101
Me Me Me Me
+ +
99 99
N N Me Pt IV Me Me IV Pt Me Me 102 Me 102
N N
N N
N N
PtII PtII 103 103
Pt II Pt II
N N Me Me
NMe 2 NMe 2 104 104
N N N N
NMe 2 NMe 2
2 2
NMe 2 NMe 2
Products Detected by Mass Products Detected Spectrometry by Mass Spectrometry
Scheme 46. Dicationic [PhI(4-DMAP)2][OTf] mediated oxidation of (a) 2-Phenylpyridine Pt(II) complex Scheme 46. Dicationic [PhI(4-DMAP)2][OTf] mediated oxidation of (a) 2-Phenylpyridine Pt(II) complex
93; (b) Pt(II) complex 101.2 ][OTf] mediated oxidation of (a) 2-Phenylpyridine Pt(II) complex Scheme 46. Dimethyl Dicationic [PhI(4-DMAP) 93; (b) Dimethyl Pt(II) complex 101. 93; (b) Dimethyl Pt(II) complex 101. 3.2. Catalytic Applications 3.2. Catalytic Applications While platinum is most commonly employed as a model system, recent advancements have shown 3.2. Catalytic Applications While platinum most commonly employed as a model recent advancements have shown its viability in catalyticismanifolds. Particularly interesting is the system, finding that platinum-catalyzed processes its viability in catalytic manifolds. Particularly interesting is the finding that platinum-catalyzed processes While platinum is most commonly employed as a model system, recent advancements have often display divergent reactivity and selectivity to those mediated by palladium. often display divergent reactivity and selectivity to those mediated by palladium. shown its Suna viability in catalytic manifolds. Particularly interesting is the finding that platinum-catalyzed reported that PtCl2 with PhI(OAc)2 cleanly acetoxylated the C-3 position on various indoles Suna reported that PtCl2 2was with PhI(OAc) 2 cleanly acetoxylated the C-3 position on various indoles processes often display divergent reactivity and selectivity those mediated palladium. (Scheme 47) [95]. Pd(OAc) also a competent catalyst,tohowever PtCl2 wasbyfound to be more (Scheme 47) [95]. Pd(OAc)2 was also a competent catalyst, however PtCl2 was found to be more
Molecules 2017, 22, 780
30 of 54
Suna reported that PtCl2 with PhI(OAc)2 cleanly acetoxylated the C-3 position on various Molecules 2017, 22, 780 30 of 54 indoles (Scheme 47) [95]. Pd(OAc)2 was also a competent catalyst, however PtCl2 was found to Molecules 2017, 22, 780 30 of 54 be more giving efficient, giving cleanerand reactions and higher isolated yields. Other oxidants efficient, cleaner reactions higher isolated yields. Other oxidants including K2S2O8,including m-CPBA, K S O , m-CPBA, t-BuOOH, and Cu(OAc) were all completely ineffective (0% conversion) and Mg 2 2 8 2 t-BuOOH, and Cu(OAc)2 were all completely ineffective (0% conversion) and Mg peroxyphthalate efficient, givinggave cleaner reactions and higher isolated yields. Other oxidantsoxidation including to K2the S2O8active , m-CPBA, peroxyphthalate a complex mixture of products. While not reported, Pt(IV) gave a complex mixture of products. While not reported, oxidation to the active Pt(IV) species would t-BuOOH, Cu(OAc) 2 were all completely ineffective (0% conversion) and Mg peroxyphthalate species wouldand likely proceed through similar intermediates shown likely proceed through intermediates to thosesimilar showntointhose Scheme 43. in Scheme 43. gave a complex mixture of products. While not reported, oxidation to the active Pt(IV) species would likely proceed through intermediates similar to those shown in Scheme 43. OAc R3 R3
N RN1
R2 R2
5 mol% PtCl 2, PhI(OAc) 2 5 mol% PtCl 2, AcOH PhI(OAc) 2 AcOH
R3 R3
R
R1 = CO 2R, Me, Ar R 2 = Me, Ar RR1 ==CO R, Me, Ar 3 Br,2 I, OMe, CN, NO 2 R 2 = Me, Ar R 3 = Br, I, OMe, CN, NO 2
OAc
R2
N R2 NR1 R
1 1 Scheme 47.PtCl PtCl22/PhI(OAc) /PhI(OAc)22 mediated Scheme 47. mediated C-3 C-3 acetoxylation acetoxylation of of indoles. indoles. Scheme 47. PtCl2/PhI(OAc)2 mediated C-3 acetoxylation of indoles.
In 2013, Sanford provided the first example of an intermolecular C(sp2)–H arylation enabled by In 2013, Sanford provided the first example of an intermolecular C(sp22 )–H arylation enabled by In 2013,manifold Sanford provided the first of an intermolecular C(spa)–H arylation enabled a Pt(II)/Pt(IV) (Scheme 48a) [96].example The reaction was found to have much broader scope by than a Pt(II)/Pt(IV) manifold (Scheme 48a) [96]. The reaction was found to have a much broader scope a Pt(II)/Pt(IV) manifold (Scheme 48a) [96]. The reaction was found to have a much broader scope than the analogous palladium-catalyzed transformations, being tolerant of a wide range of both electron than the analogous palladium-catalyzed transformations, being tolerant ofrange a wide range of both analogous transformations, being tolerant ofreversal a wide of both electron richtheand electronpalladium-catalyzed deficient arenes, and furthermore, a complete in site-selectivity was electron rich electron and electron deficient arenes, furthermore, a complete reversal site-selectivity was rich and and and furthermore, a complete reversal in in site-selectivity was observed when usingdeficient Na2PtClarenes, 4 versus Na2PdCl4 (Scheme 48b). The proposed mechanism proceeds observed when using NaNa Na2 2PdCl 48b). The Theproposed proposedmechanism mechanismproceeds proceeds 2 PtCl 4 versus 44 (Scheme when using 2PtCl 4 versus Na PdClC–C (Scheme viaobserved a Pt(II)/Pt(IV) redox cycle with two-electron bond 48b). forming reductive elimination, analogous via via a Pt(II)/Pt(IV) redox cycle with two-electron C–C formingreductive reductiveelimination, elimination,analogous analogous a Pt(II)/Pt(IV) cycle two-electron C–C bond bond forming to previous work onredox Pd(IV) (seewith Section 2.2.5) [72]. to previous work onon Pd(IV) (see Section to previous work Pd(IV) (see Section2.2.5) 2.2.5)[72]. [72]. a.
R
a.
R
Na 2PtCl 4 (2.5–5 mol %) Na 2PtCl (2.5–5 mol %) [Ar 24I]TFA [Ar 2I]TFA Ar R Ar R AcOH or TFA AcOH or TFA Bu 4NOTf 42-85% Bu 4NOTf 42-85% Tolerant of both electron-rich and Tolerant of both electron-rich and electron-deficient arenes electron-deficient arenes
b. b.
Ph
[M]IV
Ph
[M]IV [Ph 2I]TFA [Ph 2I]TFA
+
Bu 4NOTf
Bu 4NOTf
[M]IV
[M]IV
Na 2PdCl PdCl 4 Na 2 4 Na 2PtCl PtCl 4 Na 2 4
Ph
105
Ph
+ 106
105
106
25 25
1 1
11
1010
OptimizedNa Na2PtCl - 1:35with with 65% yield 2PtCl Optimized - 1:35 65% yield 44
Scheme 48. (a)(a) Intermolecular C–H arylation with Pt(II) and diaryliodonium diaryliodonium salts;(b) (b) Reversal of site Scheme IntermolecularC–H C–Harylation arylationwith withPt(II) Pt(II) and site Scheme 48.48. (a) Intermolecular diaryliodoniumsalts; salts; (b)Reversal Reversalofof site selectivity forfor Pt(II) vs.vs. Pd(II) selectivity Pt(II) Pd(II)ininarene areneC–H C–Hactivation. activation. selectivity for Pt(II) vs. Pd(II) in arene C–H activation.
Conclusion 3.3.3.3. Conclusion 3.3. Conclusions The true strength thePt(II)/Pt(IV) Pt(II)/Pt(IV)redox redoxcouple couple remains remains in forfor more The true strength ofof the inits itsuse useas asaamodel modelcomplex complex more The true strength of the Pt(II)/Pt(IV) redox couple remains in its use as a model complex for reactive Pd(II)/Pd(IV) speciesdue duetototheir theirincreased increased stability. stability. However, utility reactive Pd(II)/Pd(IV) species However,the thepotential potentialsynthetic synthetic utility more reactive Pd(II)/Pd(IV) species due to their increased stability. However, the potential synthetic of platinum catalyzed reactionsisisevident, evident,having havingdisplayed displayed enhanced selectivity of platinum catalyzed reactions enhancedreactivity, reactivity,and anddivergent divergent selectivity utility of platinum catalyzed reactions isthat evident, having displayed enhanced λ reactivity, and divergent 3-iodanes produce relative to palladium. Dutton’s findings oxidation employing poly(cationic) less 3 relative to palladium. Dutton’s findings that oxidation employing poly(cationic) λ -iodanes produce less selectivity relative to palladium. Dutton’s findings thatmay oxidation employing poly(cationic) λ3 -iodanes stable Pt(IV) centers relative to traditional oxidants lead to more reactive Pt(IV) intermediates stable Pt(IV) centers relative to traditional oxidants may lead to more reactive Pt(IV) intermediates produce less stable Pt(IV) centers relative to traditional oxidants may lead to more reactive Pt(IV) and expand their applications in oxidative couplings. and expand their applications in oxidative couplings. intermediates and expand their applications in oxidative couplings. 4. Gold 4. Gold Gold 4. 4.1. Introduction 4.1. Introduction Introduction 4.1. Gold catalysis has historically proceeded through redox neutral pathways relying on its high Gold catalysis catalysis has historically historically proceeded through redox neutral neutral pathways relying on its itsand high Gold has through redox pathways relying on high efficiency as a carbophilic pi acid. proceeded This has seen wide application in the activation of alkynes efficiency as carbophilicattack pi acid. acid. This has seen seen wide wide cascades, application insynthetic the activation activation of alkynes alkynes and efficiency carbophilic pi has application the of and alkenes as foraanucleophilic andThis cycloisomerization andin applications of these alkenes for nucleophilic attack and cycloisomerization cascades, and synthetic applications of these alkenes for nucleophilic attack and cycloisomerization cascades, and synthetic applications of these pathways have been recently reviewed [97–100]. Reactions containing Au(I)/Au(III) redox cycles are pathways have been recently recently reviewedof [97–100]. Reactions containing Au(I)/Au(III) redox cycles are rare byhave comparison, a consequence the high barrier for oxidation of Au(I) to redox Au(III)cycles (redox pathways been reviewed [97–100]. Reactions containing Au(I)/Au(III) are rare bycomparison, comparison, aTypical consequence of high the high barrier forelimination, oxidation oftoAu(I) Au(III) (redox potential +1.41 V). oxidative addition/reductive areto(redox ubiquitous in rare by a consequence of the barrier for oxidation of Au(I)which Au(III) potential potential +1.41chemistry, V). Typical oxidative addition/reductive elimination, which ubiquitous Pd(0)/Pd(II) areaddition/reductive thereby challenging with Au(I)/Au(III) (Scheme 49a, 107are toin 109) [101,102] in +1.41 V). Typical oxidative elimination, which are ubiquitous Pd(0)/Pd(II) and examples of suchare reactivity are scarce [103–105]. Instead, Au(III) complexes via Pd(0)/Pd(II) chemistry, thereby challenging with Au(I)/Au(III) (Scheme 49a, can 107 be to accessed 109) [101,102] Au(I) species,are Lnscarce –Au–X,[103–105]. to a powerful external and contextvia andexposure examplesofofan such reactivity Instead, Au(III) oxidant, complexes caninbethis accessed hypervalent iodine reagents haveLnfound widespread use, along with hydroperoxides, and exposure of an Au(I) species, –Au–X, to a powerful external oxidant, andSelectfluor, in this context
hypervalent iodine reagents have found widespread use, along with hydroperoxides, Selectfluor, and
Molecules 2017, 22, 780
31 of 54
chemistry, are thereby challenging with Au(I)/Au(III) (Scheme 49a, 107 to 109) [101,102] and examples of such reactivity are scarce [103–105]. Instead, Au(III) complexes can be accessed via exposure of an Au(I) species, Ln –Au–X, to a powerful external oxidant, and in this context hypervalent iodine Molecules 2017, 22, 780widespread use, along with hydroperoxides, Selectfluor, and others. A 31 of 54 reagents have found catalytic cycle based on this approach is shown in Scheme 49a; oxidation of Ln –Au–X gives Au(III) species 107, others. A catalytic cycle based on this approach is shown in Scheme 49a; oxidation of Ln–Au–X gives followed by two subsequent ligand exchanges to access 109, which would then undergo rapid reductive Au(III) species 107, followed by two subsequent ligand exchanges to access 109, which would then elimination strategy has led[102]. to developments in led alkynylation, olefininfunctionalization, undergo[102]. rapid This reductive elimination This strategy has to developments alkynylation, cross-couplings and dimerization/homo-coupling reactions using a wide reactions range of using oxidants beyond olefin functionalization, cross-couplings and dimerization/homo-coupling a wide hypervalent species these reports been recently reviewed Gouverneur [102]. range of iodine oxidants beyondand hypervalent iodinehave species and these reports have by been recently reviewed Unfortunately, advancements in reaction development centered on Au(I)/Au(III) have not by Gouverneur [102]. Unfortunately, advancements in reaction development centered on Au(I)/Au(III) have not or coincided with equivalent mechanistic understanding of the oxidation/reduction pathways coincided with equivalent mechanistic understanding of the oxidation/reduction pathways speciation of the organometallic gold complexes involved. Oxidation potentials of activeorAu(I) speciation of theand organometallic gold Oxidation potentials active Au(I) species species are varied, can depend oncomplexes both theinvolved. counterion and the ligandofsphere, making proper are varied, and can depend on both the counterion and the ligand sphere, making proper oxidant oxidant selection delicate and the generation of isolable Au(III) species challenging and unpredictable. selection delicate and the generation of isolable Au(III) species challenging and unpredictable. However, pioneering studies by Hashmi [106,107], along with the synthesis and characterization of However, pioneering studies by Hashmi [106,107], along with the synthesis and characterization of stoichiometric Au(III) complexes Lippert[109], [109],Fuchita Fuchita [110], Constable stoichiometric Au(III) complexesbybyBennett Bennett [108], [108], Lippert [110], andand Constable [111],[111], have have laid the foundations for a more in depth understanding of the chemistry of these highly reactive laid the foundations for a more in depth understanding of the chemistry of these highly reactive intermediates (Scheme 49b). intermediates (Scheme 49b). b. Stoichiometric Au(III) complexes
a. Au(I)/Au(III) catalytic cycle R
R
L
Reductive Elimination
AuI
Cl
X [O]
R
Cl
109
X
R
AuIII
R
Direct Oxidative Addition
X
AuIII 107
L R
X
AuIII 108
X R
X X
AuIII
N
N
S
O NC
Me O
Cl Constable (1992)
X
Ligand Exchange
AuIII
N
Cl CN
N Me Lippert (1999)
R
X Ligand Exchange
Cl
X L
X
S
N N
L
AuIII
Me Me PPh 2 AuI AuIII Ph 2P
Me
Cl
AuIII
N Cl Me
[O] = hypervalent iodine, Selectfluor, peroxide Bennett (2001)
Fuchita (2001)
Scheme 49. (a) Plausible Au(I)/Au(III) catalytic cycle based on use of an external oxidant; (b) Pioneering
Scheme 49. (a) Plausible Au(I)/Au(III) catalytic cycle based on use of an external oxidant; examples of stoichiometric Au(III) complexes. (b) Pioneering examples of stoichiometric Au(III) complexes.
4.2. Complex Synthesis and Characterization
4.2. Complex Synthesis and Characterization
Au(I) cationic salts, along with phosphine and N-heterocyclic carbene (NHC) supported Au(I)
complexes, are the most utilized Au(I)/Au(III) catalysis. NHC–Au(I) chemistry in particular hasAu(I) Au(I) cationic salts, along withinphosphine and N-heterocyclic carbene (NHC) supported gained immense popularity in the last decade, making them arguably the most well studied of Au(I) has complexes, are the most utilized in Au(I)/Au(III) catalysis. NHC–Au(I) chemistry in particular complexes. strong σ–donation the NHC ligandthem aides arguably in stabilization of both Au(I) and gained immenseThe popularity in the lastofdecade, making the most wellthe studied of Au(I) Au(III) species, making these complexes ideal candidates for the synthesis of isolable complexes. The strong σ–donation of the NHC ligand aides in stabilization of both the Au(I) Au(III) and Au(III) complexes [112]. species, making these complexes ideal candidates for the synthesis of isolable Au(III) complexes [112]. Limbach and Nolan reported the synthesis of a range of NHC-Au(III) complexes via oxidation Limbach and Nolan reported the synthesis of a range of NHC-Au(III) complexes via oxidation of of an NHC–Au(I)–Cl with PhICl2 (Scheme 50, 110 → 113) [113,114]. The oxidation could also be an NHC–Au(I)–Cl PhICl 50, 110 →however 113) [113,114]. The oxidation also be carried 2 (Scheme at cryogenic temperatures, the oxidation with PhIClcould 2 proceeded more carried out withwith Cl2(g) out with Cl2 (g) at cryogenic temperatures, howevermaking the oxidation with PhICl2 proceeded more cleanly, cleanly, in higher yield, and at room temperature, it far more advantageous. This advantage in higher yield, andtoatthe room temperature, making itconditions far more when advantageous. This advantage was attributed relatively milder oxidizing using PhICl 2 versus Cl2(g). was Interestingly, attemptedmilder oxidations with other λ3-iodanes suchusing as Ph2PhICl IBr, PhI(OAc) 2 , and PhI(OTFA) 2 attributed to the relatively oxidizing conditions when versus Cl (g). Interestingly, 2 2 3 either failed to oxidize the Au(I) complexes or gave complex product mixtures. The authors assert attempted oxidations with other λ -iodanes such as Ph2 IBr, PhI(OAc)2 , and PhI(OTFA)2 either failed to that chlorine ligands are or crucial stabilizeproduct the Au(III) center and PhIClassert 2 is optimal as it acts as oxidize the Au(I) complexes gavetocomplex mixtures. Thethus authors that chlorine ligands both an oxidant and chlorinating agent. NHC–Au(I)–Ph complex 112 was also readily oxidized in and are crucial to stabilize the Au(III) center and thus PhICl2 is optimal as it acts as both an oxidant high yield with PhICl2 to give 113, which the authors state is the first reported Au(III) complex of the chlorinating agent. NHC–Au(I)–Ph complex 112 was also readily oxidized in high yield with PhICl2 to type [AuArCl2L] [113]. It is worth noting that trichloride Au(III) complex 111 could not be converted
Molecules 2017, 22, 780
32 of 54
give 113, which the authors state is the first reported Au(III) complex of the type [AuArCl2 L] [113]. It is 2017, 22, 780 32 of 54 worthMolecules noting that trichloride Au(III) complex 111 could not be converted to 113 via transmetalation with Molecules p-methoxyphenylmagnesium bromide, instead giving rise to 4,40 -bismethoxybiphenyl32via either 2017,transmetalation 22, 780 of 54 to 113 via with p-methoxyphenylmagnesium bromide, instead giving rise to 4,4′a radical coupling or inner-sphere reductive elimination pathway. bismethoxybiphenyl via either a radical coupling or inner-sphere reductive elimination pathway. to 113 via transmetalation with p-methoxyphenylmagnesium bromide, instead giving rise to 4,4′Mesor inner-sphere reductive elimination OMe Mes bismethoxybiphenyl via either a radical coupling pathway. N
AuI Cl
NMes NR AuI Cl 110 PhMgBr N 96% R 110 PhMgBr Mes 96% N AuI Ph NMes NR AuI Ph 112 N R
PhICl 2 92-94% PhICl 2 92-94%
PhICl 2 93% PhICl 2 93%
N
Cl N AuIII Mes Cl R ClN N 111AuIII Cl Cl R Cl 111 N
(p-OMeC6H 4)MgBr 81% (p-OMeC6H 4)MgBr MeO 81%
OMe
MeO
R = Mes,
Mes
Cl N Mes AuIII Ph R ClN
R = Mes, N N
113 III Cl N Au Ph R Cl
Scheme 50. Clean oxidation of NHC Au(I) 113complexes with PhICl2 by Nolan and Limbach. Scheme 112 50. Clean oxidation of NHC Au(I) complexes with PhICl2 by Nolan and Limbach. 50.shown Clean oxidation of NHC Au(I) complexes with PhICl2 are by Nolan and Limbach. It hasScheme also been by Nevado that Au(III) diacetate complexes less stable than the analogous
It has also been complexes shown by(Scheme Nevado51). that Au(III) diacetate complexes are less stable than the Au(III) dichloride Upon oxidation with PhICl 2, Au(III) dichloride complex It has also been shown by Nevado that Au(III) diacetate complexes are less stable than the analogous analogous complexes 51). Upon dichloride 115 is Au(III) isolated dichloride in high yields (Scheme (Scheme 51a), however use of oxidation PhI(OAc)2 with leadsPhICl to isolation of Au(III) 2 , Au(III) Au(III) complexes 51). Upon oxidation withuse PhICl , (Scheme Au(III) dichloride complex bispentafluorophenyl 119, via Au(I) mediated transmetalation 51b) [115]. Ligand complex 115dichloride is isolated incomplex high (Scheme yields (Scheme 51a), however of 2PhI(OAc) leads to isolation of 2 115 is isolated in highto yields (Scheme 51a), however use of PhI(OAc) 2 leads to isolation of Au(III) exchange is proposed occur through119, twovia possible either via oxidation of Au(I) species Au(III) bispentafluorophenyl complex Au(I)pathways, mediated transmetalation (Scheme 51b) [115]. bispentafluorophenyl 119,[Au(PPh via Au(I) transmetalation 51b) [115]. Ligand 116exchange to give 117isor of acomplex dissociated 3)2mediated ]+two species to givepathways, 118. Both(Scheme of thesevia complexes would Ligand proposed to occur through possible either oxidation of Au(I) exchange is proposed occur through two possible pathways, oxidation of Au(I) species then converge to give to 119, where the chloride atom (observed ineither X-rayvia crystallography) is proposed + species 116 to give 117 or of a dissociated [Au(PPh ) ] species to give 118. Both of these complexes 3 2 to give 118. Both of these complexes would 116 to give 117solvent or of aactivation. dissociated [Au(PPh3)2]+ species to come from would then converge to give 119, where the chloride atom (observed in X-ray crystallography) is then converge to give 119, where the chloride atom (observed in X-ray crystallography) is proposed a. proposed to come from solvent activation. to come from solvent activation. a. C6F 5
AuI PPh 3
Ph 3P
93% PhICl 2
C6F 5 Ph 3P
93%
C6F 5
AuIII
Cl Cl
115 Cl AuIII Cl b. 114 115 Ph 3P PhI(OAc) 2 Cl AuIII C6F 5 AuI PPh 3 C6F 5 39% C6F 5 b. 119 116 Ph 3P PhI(OAc) 2 Cl AuIII C6F 5 AuI PPh 3 C F 39% C 6 5 6F 5 Ph 3P 119 116 AuIII OAc [AuIII(OAc) 4] or C6F 5 OAc 118 Ph 3P 117 OAc III after III(OAc) directAu oxidation or oxidation [Au 4] dissociation C6F 5 OAc 117 118 after Scheme 51. (a) Oxidation of [Au(I)(C6F5direct )PPhoxidation 3] with PhICloxidation 2; (b) Oxidation of [Au(I)(C6F5)PPh3] with PhI(OAc)2. dissociation C6F 5
114 AuI PPh 3
PhICl 2
Scheme 51. (a) Oxidation of [Au(I)(C 6F52)PPh 3]become with PhICl (b) Oxidation of [Au(I)(C 6F5generation )PPh3] with PhI(OAc) 2. Based these findings, hasF the2; reagent of choice for the of isolable Scheme 51.on(a) Oxidation of PhICl [Au(I)(C 6 5 )PPh3 ] with PhICl2 ; (b) Oxidation of [Au(I)(C6 F5 )PPh3 ] Au(III) complexes, particularly in the context of NHC complexes [112,116,117]. A noteworthy report with PhI(OAc)2 . on and theseco-workers findings, PhICl 2 has become for the investigation generation of into isolable fromBased Huynh utilized PhICl 2 as the thereagent oxidantofinchoice a thorough the Au(III) complexes, particularly inproperties the context A noteworthy report structural and electrochemical ofofa NHC rangecomplexes of mono-[112,116,117]. and bis-NHC Au(I) and Au(III) Based on these findings, become the reagent of achoice for the generation of the isolable from Huynh and co-workers utilized PhICl 2 as theonoxidant in thorough investigation 2 has complexes. This report is an PhICl excellent source for data how oxidation state, NHC, and halideinto ligands structural and particularly electrochemical properties a range of monoand Au(I) discussion and Au(III) Au(III) complexes, in the contextand ofofNHC complexes [112,116,117]. noteworthy report effect the properties of NHC-Au species, the reader is directed therebis-NHC for a A detailed of from complexes. This report is an excellent source for data on how oxidation state, NHC, and halide ligands their findings [112]. Huynh and co-workers utilized PhICl2 as the oxidant in a thorough investigation into the structural and effectDutton the properties of NHC-Au species, and the reader is directed there forasa detailed discussion and co-workers recently utilized (poly)cationic λ3-iodanes neutral ligand-donor electrochemical properties of a range of monoand bis-NHC Au(I) and Au(III) complexes. Thisofreport their findings [112]. oxidants to access tricationic Au(III) complexes [118]. The same group has used these oxidants for the is an excellent source for data on how oxidation state, NHC, and3halide ligands effect the properties of andand co-workers recently (see utilized (poly)cationic λ -iodanes as neutral ligand-donor studyDutton of Pd(IV) Pt(IV) complexes Section 3.1) and Ritter also employed these reagents as NHC-Au species, and the reader is directed there for a detailed discussion of their findings [112]. oxidants Au(III) complexes [118].C–H The same group has these oxidants for the oxidants to in access a high tricationic profile study on Pd(IV)-catalyzed fluorination (seeused Section 2.2.3.1). Oxidation Dutton and co-workers recently utilized (poly)cationic λ3also -iodanes as neutral ligand-donor 3 study of Pd(IV) and Pt(IV) complexes (see Section 3.1) and Ritter employed these reagents as of Au(I) complex 120 with three different (poly)cationic λ -iodanes possessing varied pyridine oxidants to access tricationic Au(III) complexes [118]. The same group has used these oxidants oxidants in a high profile study on Pd(IV)-catalyzed C–H fluorination (see Section 2.2.3.1). Oxidation ligands resulted in complexes 121–123 (Scheme 52). This discovery is significant as prior attempts to for the study of Pd(IV) complexes Section 3.1) and Ritter also of Au(I) complex 120 and with three different (poly)cationic λ3-iodanes possessing varied access similar complexes viaPt(IV) salt metathesis on (see halogenated Au(III) intermediates ledemployed to pyridine complexthese ligands resulted inincomplexes 121–123 (Scheme 52). This discovery is significant as prior to reagents as oxidants a high profile study on Pd(IV)-catalyzed C–H fluorination (seeattempts Section 2.2.3.1). access similar complexes via salt metathesis on halogenated Au(III) intermediates led to complex
Molecules 2017, 22, 780
33 of 54
Oxidation of Au(I) complex 120 with three different (poly)cationic λ3 -iodanes possessing varied pyridine ligands resulted in complexes 121–123 (Scheme 52). This discovery is significant as prior attempts to access similar complexes via salt metathesis on halogenated Au(III) intermediates led to Molecules 2017, 22, 780 33 of 54 complex decomposition, emphasizing the power of halide-free external oxidants in Au(I)/Au(III) redox chemistry [117]. Cyclic voltammetry reveals Au(III)/Au(I) reduction potentials ranging from − 0.41 decomposition, emphasizing the power of halide-free external oxidants in Au(I)/Au(III) redox to + for 121–123 (reference 0.069 V vs. [Fc\Fc+ ] for [Au(III)/Au(I)][(dppe) ]), showing −0.04chemistry V vs. [Fc\Fc [117].]Cyclic voltammetry reveals Au(III)/Au(I) reduction potentials ranging from 2−0.41 to + a trend that mirrors the electron donating ability ofVthe pyridine ligands ((121)NMe −0.04 V vs. [Fc\Fc ] for 121–123 (reference 0.069 vs.different [Fc\Fc+] for [Au(III)/Au(I)][(dppe) 2]), showing 2 > (122)H > a trend that mirrors the electron donating ability the different pyridine ligands ((121)NMe 2 > (122)H (123)CN) and highlighting the potential to tune theofreactivity of tricationic Au(III) complexes by varying > (123)CN) and highlighting potential to tune the reactivity of tricationic Au(III) by the heterocyclic ligands. Facile the ligand exchange from 123 was demonstrated withcomplexes 2,2,2-tripyridine varying the heterocyclic ligands.subsequent Facile ligand exchangewith fromH123 was demonstrated with 2,2,2- 125. to give 124, which could undergo exchange O to give Au(III)–OH complex 2 tripyridineoftoterminal give 124,Au(III)–OH which could complexes undergo subsequent exchange with H2O to give Au(III)–OH The synthesis is rare and previous complexes were accessed via complex 125. The synthesis of terminal Au(III)–OH complexes is rare and previous complexes were salt metathesis with AgClO4 [119]. Intriguingly, homoleptic Au(III) complex (121) is stable to aqueous accessed via salt metathesis with AgClO4 [119]. Intriguingly, homoleptic Au(III) complex (121) is conditions, unlike Au(III) complexes (122 and 123). This distinction in reactivity also indicates that stable to aqueous conditions, unlike Au(III) complexes (122 and 123). This distinction in reactivity chemoselective ligand exchange could be possible these also indicates that chemoselective ligand exchangein could beAu(III) possibletrications. in these Au(III) trications. NMe 2
R
OTf
2
2OTf 3 R
N AuI
N I N
+
N
N N
- PhI
N AuIII N
Me 2N
Potential vs Fc/Fc +
R
R
see table
R
NMe 2 120
R = CN
NMe 2 3
3
3OTf
OH N
AuIII
H 2O N
N
N AuIII
N
N
125
124
3OTf
NMe 2
3OTf
N
E p,red (AuIII/I )
(121) –NMe2 (122) –H (123) –CN
–0.41 V –0.22 V –0.04 V
125
0.21 V
N N
N
Scheme 52. Dutton’s synthesis of tricationic Au(III) complexes and evaluation of their electrochemical properties. Scheme 52. Dutton’s synthesis of tricationic Au(III) complexes and evaluation of their electrochemical properties. 4.3. Synthetic Applications
In 2008, Beller and Tse developed the first gold catalyzed homo-coupling of arenes using HAuCl4 4.3. Synthetic Applications with PhI(OAc)2 as the external oxidant (Scheme 53a) [120,121]. Mechanistic insights hint that Au(III) is
In Beller and Tse developed the that firsta free goldradical catalyzed of arenes using the2008, active C–H functionalization catalyst and cation homo-coupling is not likely. PhI(OAc) 2 showed the highest conversion compared to other oxidants including PhI(OTFA) 2 , IBX and Oxone and HAuCl4 with PhI(OAc)2 as the external oxidant (Scheme 53a) [120,121]. Mechanistic insights hint subsequent studies found it was essential that the external be hypervalent iodine derived [121].likely. that Au(III) is the active C–H functionalization catalystoxidant and that a free radical cation is not More recently, Larrosa and co-workers demonstrated that electron deficient arene-Au(I) species are, IBX PhI(OAc)2 showed the highest conversion compared to other oxidants including PhI(OTFA) 2 capable of mediating hetero-coupling reactions with unactivated, electron-rich arenes utilizing and Oxone and subsequent studies found it was essential that the external oxidant be hypervalent Koser’s reagent, PhI(OH)OTs, as the external oxidant (Scheme 53b) [122]. PhI(OPiv)2 also gave very iodine derived [121]. More recently, Larrosa and co-workers demonstrated that electron deficient high yields in this transformation, however “F+” based oxidants Selectfluor and XeF2, as well as other arene-Au(I) species are capable of mediating hetero-coupling reactions with unactivated, electron-rich acetate-ligated hypervalent iodine reagents (PhI(OAc)2, PhI(OTFA)2), were ineffective, again emphasizing arenes as the external oxidantIt(Scheme [122]. PhI(OPiv) 2 theutilizing delicate Koser’s nature ofreagent, oxidant PhI(OH)OTs, selection in high-valent metal catalysis. has also 53b) demonstrated that + ” based oxidants Selectfluor and also gave very high yields in this transformation, however “F silylated arenes are capable of undergoing arylation with a range of electron-deficient and electronXeF2 ,rich as well otherAu(I)/Au(III) acetate-ligated iodine (PhI(OAc) arenesasunder redoxhypervalent conditions, using anreagents in situ formed oxidant from PhI(OAc) 2 , PhI(OTFA) 2 ), 2were and camphor sulphonic acid the (CSA) (Scheme 53c) [123]. Other selection carboxylate of the PhI(O 2 ineffective, again emphasizing delicate nature of oxidant inligands high-valent metal2CR) catalysis. also effective,that however, λ3-iodane acidundergoing and the “F+”arylation oxidant Selectfluor It hastype alsowere demonstrated silylated arenesiodosylbenzoic are capable of with a range were completely ineffective, producingarenes none ofunder the desired product. redox conditions, using an in of electron-deficient and electron-rich Au(I)/Au(III) situ formed oxidant from PhI(OAc)2 and camphor sulphonic acid (CSA) (Scheme 53c) [123]. Other carboxylate ligands of the PhI(O2 CR)2 type were also effective, however, λ3 -iodane iodosylbenzoic acid and the “F+ ” oxidant Selectfluor were completely ineffective, producing none of the desired product.
Molecules 2017, 22, 780
34 of 54
Molecules 2017, 22, 780
34 of 54
a. Beller, Tse (2008) Me 2 mol% HAuCl 4 PhI(OAc) 2
Me
Me
81% Me
Me
[O]
Me
b. Larrosa (2012) F OMe
3
NO 2
I
69% 1% 0%
OMe
F AuIPHPh
Yield
PhI(OTFA) 2 IBX Oxone
I
PhI(OH)OTs 85%
NO 2
c. Russell (2012) TMS R
R
1 mol% PPh 3 AuOTs PhI(OAc) 2, CSA 29-97%
R R
Scheme 53. 53. (a) (a) First First example example of of aa gold gold catalyzed catalyzed homo-coupling homo-coupling by by Tse Tse and and Beller; Beller; (b) (b) Stoichiometric Stoichiometric Scheme Au(I)-arene hetero-coupling of unactivated arenes using Koser’s reagent; (c) Gold catalyzed arylation Au(I)-arene hetero-coupling of unactivated arenes using Koser’s reagent; (c) Gold catalyzed arylation of silylated arenes with electron-rich and electron-poor arenes by Lloyd-Jones and Russell. of silylated arenes with electron-rich and electron-poor arenes by Lloyd-Jones and Russell.
Nevado recently provided mechanistic insights into the role of various oxidants in Au(I)/Au(III) Nevado recently provided mechanistic insights into the role of various oxidants in Au(I)/Au(III) oxidative couplings (Scheme 54) [124]. Oxidation of Au(I) complex with PhI(OAc)2 in the presence of oxidative couplings (Scheme 54) [124]. Oxidation of Au(I) complex with PhI(OAc)2 in the presence of N-methylindole cleanly gave the hetero-coupling product (126) in 80% yield; this transformation was N-methylindole cleanly gave the hetero-coupling product (126) in 80% yield; this transformation was also successful using electron-rich arenes 1,3,5- and 1,2,5-trimethoxybenzene. Interestingly, the same also successful using electron-rich arenes 1,3,5- and 1,2,5-trimethoxybenzene. Interestingly, the same transformation using PhICl2 as the external oxidant was only applicable to N-methylindole as a transformation using PhICl2 as the external oxidant was only applicable to N-methylindole as a substrate. Upon oxidation to Au(III) 127, arene-auration can occur through two modes: (1) electrophilic substrate. Upon oxidation to Au(III) 127, arene-auration can occur through two modes: (1) electrophilic aromatic substitution to give 128 or (2) concerted C-H activation via 129, both of which converge to aromatic substitution to give 128 or (2) concerted C-H activation via 129, both of which converge give intermediate 130, which undergoes C–C bond-forming reductive elimination. The reactivity to give intermediate 130, which undergoes C–C bond-forming reductive elimination. The reactivity difference between PhI(OAc)2 and PhICl2 indicate that the basicity of the in-situ generated counterion difference between PhI(OAc)2 and PhICl2 indicate that the basicity of the in-situ generated counterion may play a key role and, analogous to Sanford’s work in Pd(II)/PhI(OAc)2 C–H activation [2], the may play a key role and, analogous to Sanford’s work in Pd(II)/PhI(OAc)2 C–H activation [2], acetate group may assist in the key activation step (129). This would account for the diminished the acetate group may assist in the key activation step (129). This would account for the diminished reactivity seen with PhICl2 in the case of substrates possessing less acidic C–H bonds such as 1,3,5reactivity seen with PhICl2 in the case of substrates possessing less acidic C–H bonds such as 1,3,5- and and 1,2,5-trimethoxybenzene, and suggest that PhI(OAc)2 may be superior to PhICl2 for Au(III)1,2,5-trimethoxybenzene, and suggest that PhI(OAc)2 may be superior to PhICl2 for Au(III)-mediated mediated C–H activation. C–H activation. Au(III)-catalyzed arene alkynylations have been reported by both Nevado [125] and Waser [126], Au(III)-catalyzed arene alkynylations have been reported by both Nevado [125] and Waser [126], employing PhI(OAc)2 and an alkynyl benziodoxolone respectively (Scheme 55). The development of employing PhI(OAc)2 and an alkynyl benziodoxolone respectively (Scheme 55). The development gold-mediated alkynylations of this type has been recently reviewed [127]. Nevado’s work used of gold-mediated alkynylations of this type has been recently reviewed [127]. Nevado’s work used PhI(OAc)2 as an oxidant to couple electron-withdrawn alkynes and unactivated, electron-rich arenes; PhI(OAc)2 as an oxidant to couple electron-withdrawn alkynes and unactivated, electron-rich arenes; oxidants including PhIO, Selectfluor, and TBHP gave significantly lower yields. Waser utilized a oxidants including PhIO, Selectfluor, and TBHP gave significantly lower yields. Waser utilized slightly different approach wherein 1-[(triisopropylsilyl)ethynyl]-1λ3,2-benziodoxol-3(1H)-one (TIPSa slightly different approach wherein 1-[(triisopropylsilyl)ethynyl]-1λ3 ,2-benziodoxol-3(1H)-one EBX) served as both the oxidant and alkyne source, in the direct C3-alkynylation of indoles. Waser later (TIPS-EBX) served as both the oxidant and alkyne source, in the direct C3-alkynylation of indoles. extended this method to the alkynylation of other electron-rich heterocycles including thiophenes, Waser later extended this method to the alkynylation of other electron-rich heterocycles including anilines, and furans [128–130]. Although Au(I)/Au(III) redox couples are proposed in these cases via thiophenes, anilines, and furans [128–130]. Although Au(I)/Au(III) redox couples are proposed in these intermediates such as 132, Au(I) carbophilic pi activation cannot dismissed as subsequent α- or cases via intermediates such as 132, Au(I) carbophilic pi activation cannot dismissed as subsequent αβ-elimination could provide the desired products via iodo-Au(I) intermediate 131. or β-elimination could provide the desired products via iodo-Au(I) intermediate 131.
Molecules 2017, 22, 780 Molecules 2017, 22, 780 Molecules 2017, 22, 780
35 of 54 35 of 54 35 of 54
N N Me Me
C6F 5 C6F 5 PhI(OAc) 2 PhI(OAc) 2 AuI AuI PPh 3 PPh 3
N NMe Me Ph 3P Ph 3P AuIII C6F 5 AuIII C6F 5 127 127
N 126 NMe 126 Me
Ph 3P OAc Ph 3P AuIII OAc C6F 5 AuIII N Me H C6F 5 N Me H OAc OAc 128 128
OAc OAc OAc OAc
N NMe Me
C6F 5 C6F 5
Ph 3P AuI C6F 5 Ph 3P AuI C6F 5 PhI(OAc) 2 PhI(OAc) 2 80% 80%
Me Me
Ph 3P Ph C63FP5 C6F 5
O O O H O H Au Au OAc N OAc N Me Me 129 129
HOAc HOAc Ph 3P OAc Ph 3P AuIII OAc III C6F 5 Au N Me C6F 5 N Me 130 130
126 126
HOAc HOAc
Scheme 54. 54. Proposed Proposed mechanism mechanism of of Au(I)/Au(III) Au(I)/Au(III) catalyzed Scheme catalyzed hetero-coupling hetero-coupling of of electron electron rich rich arenes. arenes. Scheme 54. Proposed mechanism of Au(I)/Au(III) catalyzed hetero-coupling of electron rich arenes. Nevado (2010) Nevado (2010) OMe OMe MeO MeO
OMe OMe
5.0 mol% [Au(PPh 5.0 mol% 3)Cl] [Au(PPh PhI(OAc) 3)Cl] 2 PhI(OAc) 2 R R
OMe OMe MeO MeO
R R OMe OMe Si(iPr) 3 Si(iPr) 3
Waser (2009) Waser (2009)
AuIII AuIII
5.0 mol% AuCl 5.0 mol% AuCl N H N H
(iPr) 3Si (iPr) 3Si
I O I O
Possible Intermediates Possible Intermediates AuI [I] AuI [I] R R 131 131 via pi activation/elimination via pi activation/elimination
O O
N H N H
Si(iPr) 3 Si(iPr) 3
132 132 via Au(I)/Au(III) redox cycle via Au(I)/Au(III) redox cycle
Scheme 55. Oxidative alkynylation reactions by Nevado and Waser. Two potential pathways for Scheme 55. 55. Oxidative Oxidative alkynylation alkynylation reactions reactions by by Nevado Nevado and and Waser. Two potential pathways for Scheme alkynylation based upon Au(I) oxidation or Au(I) pi activation. Waser. Two potential pathways for alkynylation based based upon upon Au(I) Au(I) oxidation oxidationor orAu(I) Au(I)pi piactivation. activation. alkynylation
In 2009, Muñiz and Iglesias took advantage of highly reactive Au(III) complexes to develop a In 2009, Muñiz and Iglesias took advantage of highly reactive Au(III) complexes to develop a gold-catalyzed alkeneand diamination reaction (Schemeof 56), analogous to their previous report to employing In 2009, Muñiz Iglesias took advantage highly reactive Au(III) complexes develop gold-catalyzed alkene diamination reaction (Scheme 56), analogous to their previous report employing (see alkene Section diamination 2.2.4.1) [131].reaction Alkenes (Scheme underwent intramolecular withreport tosylaPd(II)/Pd(IV) gold-catalyzed 56), analogous todiamination their previous Pd(II)/Pd(IV) (see Section 2.2.4.1) [131]. Alkenes underwent intramolecular diamination with tosylprotected ureas 133 under basic conditions using [Au(OAc)PPh 3 ] to give bicyclic ureas (136) in high employing Pd(II)/Pd(IV) (see Section 2.2.4.1) [131]. Alkenes underwent intramolecular diamination protected ureas 133 under basic conditions using [Au(OAc)PPh3] to give bicyclic ureas (136) in high yield.tosyl-protected Redox neutralureas anti-aminoauration Au(I) using intermediate 134, followed irreversible with 133 under basicgives conditions [Au(OAc)PPh bicyclic ureas 3 ] to giveby yield. Redox neutral anti-aminoauration gives Au(I) intermediate 134, followed by irreversible oxidation by PhI(OAc) 2 to Au(III) 135. Following deprotonation, SN2-type intramolecular (136) in high yield. Redox neutraldiacetate anti-aminoauration gives Au(I) intermediate 134, followed by oxidation by PhI(OAc)2 to Au(III) diacetate 135. Following deprotonation, SN2-type intramolecular cyclization provides cyclic 136diacetate and regenerates the Au(I) catalyst. Although the irreversible oxidationthe by desired PhI(OAc) Au(III) 135. Following deprotonation, SN 2-type 2 to urea cyclization provides the desired cyclic urea 136 and regenerates the Au(I) catalyst. Although the proposed Au(III) intermediates werethe toodesired reactivecyclic to beurea isolated, mechanistic studies using catalyst. a PPh3– intramolecular cyclization provides 136 and regenerates the Au(I) proposed Au(III) intermediates were too reactive to be isolated, mechanistic studies using a PPh3– Au(I)–Me the complex 137 Au(III) gave an isomeric mixtures ofreactive Au(III) to intermediates 138 and 139studies upon Although proposed intermediates were too be isolated, mechanistic Au(I)–Me complex 137 gave an isomeric mixtures31 of Au(III) intermediates 138 and 139 upon oxidation with PhI(OAc)complex 2, which 137 weregave detectable by P-NMR. using a PPh an isomeric mixtures of Au(III) intermediates 138 and 139 3 –Au(I)–Me oxidation with PhI(OAc)2, which were detectable by 31P-NMR. upon oxidation with PhI(OAc)2 , which were detectable by 31 P-NMR.
Molecules 2017, 22, 780 Molecules 2017, 22, 780 Molecules 2017, 22, 780
36 of 54 36 of 54 36 of 54 O O R RR R
H N H Ts N NH Ts NH
136 136
Ts Ts N O N O N N
7.5 mol% [Au(OAc)PPh 7.5 mol% 3] PhI(OAc) [Au(OAc)PPh 2, NaOAc 3] PhI(OAc) 2, NaOAc 62-93% 133 62-93% 133
L AuIOAc L AuIOAc
AcOH AcOH
O O N N
AuI L AuI L 134 134
Ts Ts NH O NH O N N
AcOH AcOH OAc OAc
136 136
133 133
Ts HN Ts HN
L OAc LAuIII OAc AuIIIOAc OAc
R RR R
O O Ts N N Ts N N
L OAc L AuIII OAc AuIII OAc OAc
PhI(OAc) 2 PhI(OAc) 2
Ph 3P Ph 3P
Au(I) Me Au(I) Me 137 137 PhI(OAc) 2 PhI(OAc) 2
Ph 3P Me III Ph 3P Au Me 138 AcO AuIII OAc 138 AcO OAc Ph 3P OAc III Ph 3P Au OAc 139 AcO AuIII Me 139 AcO Me Detected by 31P NMR Detected by 31P NMR
135 135
Scheme 56. 56. Au(I)/Au(III) Au(I)/Au(III) catalytic Scheme catalyticcycle cycle for for intramolecular intramolecular diamination diamination of of olefins. olefins. Scheme 56. Au(I)/Au(III) catalytic cycle for intramolecular diamination of olefins.
An interesting example of C–C bond cleavage was demonstrated by Shi and co-workers with An example of by with An interesting interesting example of C–C C–C bond bond cleavage cleavage was was demonstrated demonstrated by Shi Shi and and co-workers co-workers with Au(I)/Au(III) catalysis and methylenecyclopropanes (Scheme 57) [132]. Precomplexation of alkene to Au(I)/Au(III) (Scheme 57) [132]. Precomplexation of alkene to Au(I)/Au(III) catalysis and methylenecyclopropanes Precomplexation of alkene to Au(I) leads to a redox neutral allylic rearrangement to give 140, which is oxidized with PhI(OAc)2 to Au(I) leads to a redox neutral allylic rearrangement to give 140, which is oxidized with PhI(OAc) to Au(I)Au(III) leads to a redox141. neutral allylic elimination rearrangement to141 givewould 140, which is oxidized with PhI(OAc) 22 to give diacetate Reductive from give the desired diacetate 142 with give Au(III) diacetate 141. Reductive elimination from 141 would give the desired diacetate 142 with give Au(III) diacetate 141. desired diacetate 142 with regeneration of a [Au(I)(PMe3)OAc] catalyst. regeneration regeneration of of aa [Au(I)(PMe [Au(I)(PMe33)OAc] catalyst. 5.0 mol% [Au(PMe 3)Cl] 5.0 mol% PhI(OAc) [Au(PMe 2 3)Cl] PhI(OAc) 40-99% 2
Ar Ar Ar Ar Ar Ar Ar Ar 142 142
Ph Ph Ph Ph
Ar Ar Ar Ar 142 142
40-99% OAc OAc OAc OAc L AuIOAc L AuIOAc
AuClL Ar AuClL Ar Ar Ar HCl HCl Ph Ph Ph I PhAu I Au
OAc L OAc III L Au OAc AuIIIOAc OAc OAc 141 141
PhI(OAc) 2 PhI(OAc) 2
Ph Ph Ph Ph 140 140
OAc OAc OAc OAc
OAc OAc
OAc OAc AuI L AuI L
Scheme 57. Au(I)/Au(III) catalyzed diacetoxylation of methylenecyclopropanes via C–C bond cleavage. Scheme Au(I)/Au(III) catalyzed diacetoxylation of methylenecyclopropanes via C–C bondvia cleavage. Scheme57.57. Au(I)/Au(III) catalyzed diacetoxylation of methylenecyclopropanes C–C bond cleavage.
Molecules 2017, 22, 780 Molecules 2017, 22, 780
37 of 54 37 of 54
4.4. Conclusions 4.4. Conclusions Although advancements have been made toward mechanistic understanding of Au(I)/Au(III)mediated oxidative couplings, it ishave clear that of gold redoxunderstanding chemistry are still Although advancements beensynthetic made applications toward mechanistic of in their infancy. As methods development and mechanistic elucidation in this field the Au(I)/Au(III)-mediated oxidative couplings, it is clear that synthetic applications continues, of gold redox choice of external oxidant play a As crucial role as it affects both stability elucidation of the reactive Au(III) chemistry are still in their infancy. methods development and the mechanistic in this field intermediates and their subsequent reactivity in organic transformations. Thus far, PhICl 2 has emerged as continues, the choice of external oxidant play a crucial role as it affects both the stability of the reactive aAu(III) leaderintermediates for the isolation Au(III) complexes, due the reaction conditions and stabilization andoftheir subsequent reactivity in mild organic transformations. Thus far, PhICl2 imparted by the transfer of chloride ligands. Conversely, PhI(OAc) 2 is more efficient in catalytic has emerged as a leader for the isolation of Au(III) complexes, due the mild reaction conditions and manifolds as imparted the resultant complexes more reactive andConversely, acetate ligands can assist in key C–H stabilization by the transfer are of chloride ligands. PhI(OAc) 2 is more efficient 3 activation The work of resultant Dutton incomplexes the use of (poly)cationic λ -iodanes has laid the groundwork in catalyticsteps. manifolds as the are more reactive and acetate ligands can assist in for the exploration of highly tunable Au(III) complexes and this discovery could lead to new 3 key C–H activation steps. The work of Dutton in the use of (poly)cationic λ -iodanes has laid the developments in the chemistry of cationic Au(III) intermediates. groundwork for the exploration of highly tunable Au(III) complexes and this discovery could lead to new developments in the chemistry of cationic Au(III) intermediates. 5. Nickel 5. Nickel 5.1. Introduction 5.1. Introduction Nickel is a group 10 metal, the first-row counterpart of palladium. Low oxidation state nickel is a group 10Ni(0)//Ni(I)/Ni(II), metal, the first-rowhave counterpart of palladium. Low oxidation state nickel redox redoxNickel couples, such as seen significant applications in catalysis, including couples, such as Ni(0)//Ni(I)/Ni(II), have seen significant applications in catalysis, including Suzuki, Suzuki, Negishi, and Kumada couplings, as well as recent advancements in radical mediated crossNegishi, and Kumada couplings, well as recent advancements processes in radical has mediated cross-coupling coupling reactions [133]. A recent as resurgence in nickel-catalyzed been fueled not only reactions [133]. A recent resurgence in nickel-catalyzed processes hasunique been fueled not only by its by its greater sustainability and economic advantages, but also by its electronic properties greater sustainability economic to advantages, but counterpart. also by its unique properties that that facilitate reactivityand inaccessible its palladium Unlikeelectronic palladium, which relies facilitate reactivity inaccessible to itsredox palladium Unlike palladium, relies almost almost exclusively on two-electron cycles,counterpart. nickel can undergo facile one-which and two-electron exclusively onproviding two-electron redox nickel Ni(II), can undergo oneand two-electron redox events, redox events, access to cycles, Ni(0), Ni(I), Ni(III),facile and in rare examples, Ni(IV) oxidation providing access58). to Ni(0), Ni(III), and in rare examples, Ni(IV)ofoxidation states 58). states (Scheme WhileNi(I), theseNi(II), numerous pathways expand the scope reactivity, they(Scheme also make While these numerous pathways investigations expand the scope of reactivity, they alsorange makeof characterization and characterization and mechanistic difficult due to the wide redox couples that mechanistic investigations difficult due to rangestate of redox couples that could invoked. could be invoked. Recent advancements in the highwide oxidation palladium chemistry hasbesparked a Recent advancements high chemistry has and sparked a renewed renewed investigationininto theoxidation synthesisstate andpalladium characterization Ni(III) Ni(IV) species, investigation which could into theexpand synthesis characterization Ni(III) and Ni(IV) species, which could expand the the greatly theand current scope of nickel-catalyzed reactions. In this section, wegreatly will highlight current of nickel-catalyzed reactions. thisserve section, weselection will highlight theeither key role both key role scope that both ligand scaffold and oxidantIncan in the between one-that or twoligand scaffold andas oxidant in the between or two-electron pathways as electron pathways well ascan theserve stability of selection the resultant higheither valentonecomplexes. Hypervalent iodine well as the stability of athe resultant highadvancement valent complexes. iodine have played oxidants have played key role in the of thisHypervalent field, analogous to oxidants their prominent role a key in the advancement of thiscatalysis field, analogous to their role the development of in the role development of Pd(II)/Pd(IV) (see Section 2.1).prominent In order to putinmodern approaches Pd(II)/Pd(IV) catalysis (see with Section 2.1). history In orderoftoisolated put modern approaches context, will begin into context, we will begin a brief Ni(IV) complexesinto accessed viawe a variety of with a brief history of isolated Ni(IV) complexes accessed via a variety of methods. methods. Traditional Modes of Ni catalysis: Ni(0)/Ni(I)/Ni(II) Ni
0
1 e–
Ni
I
1 e–
Ni
direct 2 e – Ni
I
2
e–
II
Emerging area of research: Ni(II)/Ni(III)/Ni(IV) Ni III
1 e–
Ni
II
2 e–
Ni IV
Common Oxidants:
HVI: PIDA, PhICl2 Non-HVI: TDTT, NFTPT, Br2
Ni
III
Scheme Scheme 58. 58. Traditional Traditional and and emerging emerging nickel nickel oxidation oxidation states states invoked invoked in in catalysis, catalysis, and and common common oxidants reported in the literature. oxidants reported in the literature.
Reports on the isolation of Ni(IV) complexes date back to the mid 1970’s, and these species were Reports on the isolation of Ni(IV) complexes date back to the mid 1970’s, and these species even proposed as intermediates in some of the first Ni(0) catalyzed cross coupling reactions. The first were even proposed as intermediates in some of the first Ni(0) catalyzed cross coupling reactions. diorganonickel (IV) complex was synthesized and isolated by Cordier in 1994 through oxidative The first diorganonickel (IV) complex was synthesized and isolated by Cordier in 1994 through addition of methyl iodide to a Ni(II) precursor with acylphenolato and trimethylphosphine ligands oxidative addition of methyl iodide to a Ni(II) precursor with acylphenolato and trimethylphosphine (Scheme 59) [134]. In this case, the rigid chelate ring contains a hard base and powerful σ-donating ligands (Scheme 59) [134]. In this case, the rigid chelate ring contains a hard base and powerful phosphine ligands that provide substantial stability to the high-oxidation nickel complex. In 1999,
Molecules 2017, 22, 780
38 of 54
σ-donating phosphine ligands that provide substantial stability to the high-oxidation nickel complex. In 1999, Tanaka et al reported the first silylnickel(IV) complex (Scheme 59) through a proposed Molecules 2017, 22, 780 of 54[135]. oxidative addition of 1,2-disilylbenzene to a Ni(dmpe)2 and subsequent elimination of38H 2 Again, this complex was stabilized by strong σ-donor ligands and a rigid chelate similar to Tanaka et al reported the first silylnickel(IV) complex (Scheme 59) through a proposed oxidative that of Cordier, and in both reports, X-ray2 crystallographic analysis of confirmed an octahedral addition of 1,2-disilylbenzene to a Ni(dmpe) and subsequent elimination H2 [135]. Again, this geometry. Inwas 2003, Linden an isolable complex three sigma and bonded complex stabilized byreported strong σ-donor ligands Ni(IV) and a rigid chelatecontaining similar to that of Cordier, norbonyl ligands in a pseudo-tetrahedral geometry (Scheme 59) [136]. This species was accessed in both reports, X-ray crystallographic analysis confirmed an octahedral geometry. In 2003, Linden ◦ C. Along via oxidation of isolable a tris(1-norbornyl)nickel(II) complex anionbonded with Onorbonyl with being reported an Ni(IV) complex containing three sigma in a pseudo2 at −60ligands tetrahedral 59) [136]. species was accessed via oxidation of ashielding tris(1-norbornyl) strong σ-donorgeometry ligands,(Scheme the steric bulkThis of the 1-norbonyl ligands provides necessary nickel(II) complex anion with O2 at −60 °C. Along withMost being recently, strong σ-donor ligands, the steric bulk isolated of the for stabilization of the trialkylnickel(IV) species. the Steigerwald group a 1-norbonyl ligands provides shielding necessary for stabilization of the trialkylnickel(IV) species. Most tetraalkyl aspirocyclononane Ni(IV) complex that is remarkably stable to oxygen and high temperatures recently, the Steigerwald group isolated a tetraalkyl aspirocyclononane Ni(IV) complex that is (Scheme 59) [137]. The complex was isolated as an intermediate in a Ni(0) catalyzed strain release remarkably stable to oxygen and high temperatures (Scheme 59) [137]. The complex was isolated as an ring-opening polymerization, formed via the combination of two molecules of substrate into a dimeric intermediate in a Ni(0) catalyzed strain release ring-opening polymerization, formed via the bismetallocyclopentane nickel species. The remarkable stability of this complex isnickel attributed to The the high combination of two molecules of substrate into a dimeric bismetallocyclopentane species. degree of steric shielding afforded by the large alkyl ligands. From these reports, common features remarkable stability of this complex is attributed to the high degree of steric shielding afforded by emerge stabilize high oxidation state nickel complexes: strong which σ-donor ligands, chelation, thewhich large alkyl ligands. From these reports, common features emerge stabilize highrigid oxidation and steric of the metal Additionally, these examples the unusual state shielding nickel complexes: strongcenter. σ-donor ligands, rigid chelation, andhighlight steric shielding of the stability metal of center. Additionally, these examples highlight the unusual stability the nickel–alkyl complexes. bond, a feature the nickel–alkyl bond, a feature not common in late transition metaloforganometallic Finally, not common in late transition metal organometallic complexes. Finally, a wide range of oxidants, a wide range of oxidants, varied in mechanism and strength, were utilized to access these systems, varied in mechanism and addition strength, were to access these systems, ranging C–X oxidative ranging from C–X oxidative to O2utilized oxidation at low temperature. Whilefrom these studies provide addition to O2 oxidation at low temperature. While these studies provide valuable evidence for the valuable evidence for the viability of Ni(IV) species and the oxidants capable of accessing them, they viability of Ni(IV) species and the oxidants capable of accessing them, they are not readily translatable are not readily translatable to catalytic manifolds. to catalytic manifolds.
O Ni II
[O]
NiIV
[O] = MeI, RSi–H, O 2
PMe 3 Me Ni IV I PMe 3
H 2Si
O XX Cordier (1994)
H 2Si
SiH2 Me 2 P Ni IV P SiH Me 2 2
Tanaka (1999)
Ni IV
Br
R
Ni IV
R
Steingerwald (2009)
R=
Linden (2003)
Scheme 59. Examples of isolated organonickel complexes. Highly rigid ligand scaffolds and Scheme 59. Examples of isolated organonickel (IV)(IV) complexes. Highly rigid ligand scaffolds and strong strong σ-donor ligands are common features that aid in stabilization of these species. σ-donor ligands are common features that aid in stabilization of these species.
Over the last 10 years, a renewed interest in this has focused on the subsequent reactivity and
Over thesynthetic last 10 years, interest in this focused on the possible utility aofrenewed Ni(IV) species, along withhas ligand scaffolds andsubsequent oxidants thatreactivity could be and possible synthetic of Ni(IV) species, with ligand scaffolds and oxidants that could developed into utility viable catalytic systems. In along this effort, hypervalent iodine reagents have emerged as be developed intocandidates viable catalytic In this effort, hypervalent iodine reagents have emerged as promising for bothsystems. species isolation and catalysis. promising candidates for both species isolation and catalysis. 5.2. Stoichiometric Studies
5.2. Stoichiometric Studies Continued efforts in the stoichiometric synthesis and characterization of Ni(IV) complexes have expanded toefforts includein their reactivity, particularly in C–C and C–X bond forming reductive have Continued thesubsequent stoichiometric synthesis and characterization of Ni(IV) complexes eliminations. The most challenging element of this work has been species isolation and proof of either expanded to include their subsequent reactivity, particularly in C–C and C–X bond forming reductive Ni(II)/Ni(III) or Ni(II)/Ni(IV) redox couples, as these intermediates are highly reactive and short lived. eliminations. The most challenging element of this work has been species isolation and proof of either The Sanford group has spearheaded these efforts, building on their extensive work in high Ni(II)/Ni(III) Ni(II)/Ni(IV) redox couples, these intermediates are highly short lived. oxidationor state palladium chemistry. Theyas have explored the viability of bothreactive Ni(III) and and Ni(IV)
Molecules 2017, 22, 780
39 of 54
The Sanford group has spearheaded these efforts, building on their extensive work in high oxidation state palladium chemistry. They have explored the viability of both Ni(III) and Ni(IV) manifolds as inexpensive analogues to Pd(IV) in C–X reductive eliminations. A 2010 report studied Molecules 2017, 22, 780 39 of 54 the oxidation of complex 143 with an excess of PhICl2 , which yielded an approximately 2:1 mixture of C–Clmanifolds and C-Bras reductive elimination 60a) [138]. This represented the first example inexpensive analoguesproducts to Pd(IV) (Scheme in C–X reductive eliminations. A 2010 report studied of C–X forming reductive elimination nickel. They propose the likely intermediacy thebond oxidation of complex 143 with an excessfrom of PhICl 2, which yielded an approximately 2:1 mixture of a of species C–Cl and C-Br reductive elimination products oxidation (Scheme 60a) [138]. This the first Ni(III) (144), however note that two-electron to Ni(IV) (145)represented cannot be ruled out due example of C–X bond forming reductive elimination from nickel. They propose the likely to an inability to isolate the reactive intermediates. It is also noteworthy that both the chlorine ligands intermediacy of a Ni(III)and species (144), however that in two-electron Ni(IV) introduced upon oxidation, the bromine ligandnote present complexesoxidation 144 and to 145, were(145) available cannot be ruled out due to an inability to isolate the reactive intermediates. It is also noteworthy that for C–X reductive elimination. both the chlorine ligands introduced upon oxidation, and the bromine ligand present in complexes A subsequent study by Sanford examined the oxidation of a Ni(PhPy)2 complex (146), which 144 and 145, were available for C–X reductive elimination. can then undergo competitive C–X orexamined C–C reductive elimination (Scheme 60b) [139]. The analogous A subsequent study by Sanford the oxidation of a Ni(PhPy) 2 complex (146), which can palladium complex had previously been shown to undergo preferential C–X reductive elimination then undergo competitive C–X or C–C reductive elimination (Scheme 60b) [139]. The analogous whenpalladium exposed complex to a variety of oxidants, including iodine reagents (see elimination Section 2.2.3.1), had previously been shown tohypervalent undergo preferential C–X reductive and thus studytoprovided anoxidants, excellent comparison of the iodine reactivity of these two metals. It was whenthis exposed a variety of including hypervalent reagents (see Section 2.2.3.1), and thus this study provided an excellent comparison of the reactivity of these two metals. It was shown that upon oxidation with PhICl2 , complex 146 gave only trace C–Cl bond formation (3%, 147) shown that product upon oxidation withthat PhICl complex 146 gave only trace C–Cl bond formation (3%, 147)of 146 and the major 148 was of2,C–C reductive elimination. Interestingly, oxidation and the major product 148 was that of C–C reductive elimination. Interestingly, oxidation of with but + with other Cl sources, NCS or CuCl2 , provided no observable C–Cl products, showing146 a slight other Cl+ sources, NCS or CuCl2, provided no observable C–Cl products, showing a slight but significant divergence in reactivity of the hypervalent iodine oxidant. Again, in this study, no high significant divergence in reactivity of the hypervalent iodine oxidant. Again, in this study, no high oxidation state intermediates could be isolated or observed in situ, although the divergent reactivity oxidation state intermediates could be isolated or observed in situ, although the divergent reactivity fromfrom that that of palladium may suggest of palladium may suggesta adifference difference in in mechanism. mechanism. a. Sanford (2009) Cl
Path A: 1 e – Ni (III) N
4 equiv. PhICl 2
Br
12 hr, 25 °C
Me Ni II N
143
Me
NiIII N
144
N 53%
Br N Cl
Path B: 2 e – Ni (IV)
N
+
Me Cl N NiIV Cl Br
25%
N Br
145
b. Sanford (2010) via Ni II N 146
[O] N
C 6H 6, rt
Ni III
or Ni IV
+
N N
N No isolable intermediates [O]
147
Cl
yield C–X (147) (%)
148 yield C–C (148) (%)
PhICl 2
3
69
NCS
nda
68
nda
79
CuCl 2 and =
not detected
Scheme 60. C–X versus C–C bond forming reductive elimination from high oxidation state nickel.
Scheme 60. C–X versus C–C bond forming reductive elimination from high oxidation state nickel. (a) Competitive C–Br vs. C–Cl bond formation; (b) Competitive C–Cl vs. C–C bond formation. (a) Competitive C–Br vs. C–Cl bond formation; (b) Competitive C–Cl vs. C–C bond formation.
Recently, in a high-profile report, the same group succeeded in the isolation and characterization of a Ni(IV)in intermediate uponreport, oxidation a diorganonickel(II) complex (Schemeand 61) [140]. At the Recently, a high-profile theofsame group succeeded in the149 isolation characterization outset intermediate of their study,upon key insights intoofthe stability of 149 to complex oxidation 149 were obtained cyclicAt the of a Ni(IV) oxidation a diorganonickel(II) (Scheme 61)via [140]. + voltammetry. Observation of two quasi-reversible oxidation peaks at −0.61 V and +0.27 vs. Fc/Fc outset of their study, key insights into the stability of 149 to oxidation were obtained via ,cyclic representative of Ni(II)/Ni(III) and Ni(III)/Ni(IV) redox couples, is noteworthy as these potentials are voltammetry. Observation of two quasi-reversible oxidation peaks at −0.61 V and +0.27 vs. Fc/Fc+ , relatively low and hints that catalytic cycles with interchange between these redox states could be representative of Ni(II)/Ni(III) and Ni(III)/Ni(IV) redox couples, is noteworthy as these potentials + plausible. Initially, oxidation of 149 with hypervalent iodine reagents PhI(OAc)2, PhICl2, or F source
Molecules 2017, 22, 780
40 of 54
are relatively low Molecules 2017, 22, 780and hints that catalytic cycles with interchange between these redox states could 40 ofbe 54 plausible. Initially, oxidation of 149 with hypervalent iodine reagents PhI(OAc)2 , PhICl2 , or F+ source 3 2 reductive elimination NFTPT resulted in rapid rapid C(sp C(sp3)–C(sp2)) reductive elimination to give cyclobutane 151, not allowing allowing for study of any any intermediates. intermediates. However, use of of TDTT TDTT resulted resulted in in formation formation of of aa stable stable Ni(IV)–CF Ni(IV)–CF33 1 19 F-NMR, before giving rise to the same cyclobutane intermediate intermediate 152 152 that that was was detectable detectable by by 1H- and 19F-NMR, before product 151. Further evidence for the intermediacy of a Ni(IV) species was provided through X-ray crystallographic characterization of a complex possessing a tridentate tris(2-pyridyl) methane ligand, ligand, again upon oxidation oxidation with TDTT TDTT (not (not shown). shown). Although products of oxidation oxidation with hypervalent hypervalent iodine reagents were not directly observed in this case, the analogous reactivity of complex 150 to that of hypervalent iodine-mediated oxidation provides strong evidence that those reactions are also proceeding evidence forfor thethe accessibility of proceedingvia viaaaNi(II)/Ni(IV) Ni(II)/Ni(IV) redox redoxcouple. couple.This Thisstudy studyprovides providesclear clear evidence accessibility Ni(II)/Ni(IV) catalysis manifolds andand supports further efforts for developing reactions that that invoke this of Ni(II)/Ni(IV) catalysis manifolds supports further efforts for developing reactions invoke pathway. It also that the of novel C–X bond-forming reactions, analogous to those this pathway. It shows also shows thatdevelopment the development of novel C–X bond-forming reactions, analogous to with will likely that are not capable of undergoing competitive thosePd(II)/Pd(IV), with Pd(II)/Pd(IV), willrequire likely ligand requirescaffolds ligand scaffolds that are not capable of undergoing C–C reductive elimination. competitive C–C reductive elimination. Me N N
Ni II
Me Me
[O]
N N
149 E1 /2 = –0.61V, +0.27V vs Fc/Fc+
S OTf – TDTT CF3
Me
Ni IV X1
C–C reductive elimination
X2
-[NiII ]
150 X = F/OTf, OAc, Cl
[O]
X1, X2
PhI(OAc) 2
OAc, OAc
PhICl 2
Cl, Cl
NFTPT
F, OTf
TDTT
OTf, CF 3
Me Me 151 C(sp 3 ) – C(sp 2 ) bond formation Me
rapid RE no isolable intermediates
N N
Me
NiIV OTf
CF 3
152 characterized in situ
Scheme 61. 61.Oxidation Oxidationand and reductive elimination a Ni(IV) intermediate using hypervalent Scheme reductive elimination fromfrom a Ni(IV) intermediate using hypervalent iodine, + + + + , and CF 3 sources. Rapid and selective C–C bond formation was observed in the case of all all iodine, F , andFCF sources. Rapid and selective C–C bond formation was observed in the case of 3 hypervalent iodine oxidants.
Sanford’s group has recently utilized tripyrazoylborate (Tp) as a supporting ligand to isolate Ni(IV) Sanford’s group has recently utilized tripyrazoylborate (Tp) as a supporting ligand to isolate Ni(IV) complex 153 and examine its competency in C(sp2)–CF3 reductive eliminations (Scheme 62a) [141]. complex 153 and examine its competency in C(sp2 )–CF3 reductive eliminations (Scheme 62a) [141]. Oxidation was examined with a variety of Ar–X species and it was found that only aryldiazonium Oxidation was examined with a variety of Ar–X species and it was found that only aryldiazonium salts salts and diaryliodonium salts were competent oxidants, with diaryliodonium salts giving superior and diaryliodonium salts were competent oxidants, with diaryliodonium salts giving superior yields yields of complex 154. This represents the first evidence of the accessibility of the Ni(II)/Ni(IV) of complex 154. This represents the first evidence of the accessibility of the Ni(II)/Ni(IV) manifold manifold under mild conditions with diaryliodonium salts. 154 was further shown to undergo clean under mild conditions with diaryliodonium salts. 154 was further shown to undergo clean C–CF3 bond C–CF3 bond formation upon heating and results suggest a two-electron reductive elimination formation upon heating and results suggest a two-electron reductive elimination pathway. Sanford pathway. Sanford also recently reported the oxidation of a TpNi(II)biphenyl complex 155 and its also recently reported the oxidation of a TpNi(II)biphenyl complex 155 and its subsequent reactivity in subsequent reactivity in C–O bond reductive elimination (Scheme 62b) [142]. Oxidation with C–O bond reductive elimination (Scheme 62b) [142]. Oxidation with PhI(OTFA)2 for just 10 min at PhI(OTFA)2 for just 10 min at 25 °C cleanly produced 156 in 50% isolated yield. In line with their 25 ◦ C cleanly produced 156 in 50% isolated yield. In line with their previous studies, intramolecular previous studies, intramolecular C(sp2)–O coupling from 156 was slow, however 157 could be C(sp2 )–O coupling from 156 was slow, however 157 could be detected upon heating, the result of C–O detected upon heating, the result of C–O bond-forming reductive elimination followed by cyclization bond-forming reductive elimination followed by cyclization and hemiketalization. It was also found and hemiketalization. It was also found that the –OTFA ligand could under rapid displacement with that the –OTFA ligand could under rapid displacement with a nucleophilic source of –CF3 , which a nucleophilic source of –CF3, which would then undergo slow C–CF3 reductive elimination (not would then undergo slow C–CF3 reductive elimination (not shown). Taken together, these two reports shown). Taken together, these two reports show the relatively facile access of the Ni(II)/Ni(IV) show the relatively facile access of the Ni(II)/Ni(IV) manifold with hypervalent iodine reagents and manifold with hypervalent iodine reagents and lend strong support for the development of diverse lend strong support for the development of diverse catalytic reactions via this pathway. Furthermore, catalytic reactions via this pathway. Furthermore, the tripyrazoylborate ligand is emerging as an the tripyrazoylborate ligand is emerging as an excellent choice for stabilization and isolation of high excellent choice for stabilization and isolation of high oxidation state nickel complexes. oxidation state nickel complexes.
Molecules 2017, 22, 780
41 of 54
Molecules 2017, 22, 780 Molecules 2017, 22, 780
41 of 54 41 of 54
a. Sanford (2015) a. Sanford (2015) N H N H B B N NN N N N N N
I I
NBu 4 NBu 4
N N
BF 4 BF 4
N H NN H B N B N CF3 N N N NiIV CF3 N N NiIV CF3 N N CF3
CF3 55 °C Ni II CF3 55 °C –35 °C, 10 min Ni II CF3 –Ni(II) –35 °C, 10 min –Ni(II) CF3 77% 153 77% 154 153 154 1 13 Use of other Ar–X oxidants characterized by H, C, 11 B, 19 F NMR 1 H, 13C, 11 B, 19 F NMR Use of other Ar–X oxidants characterized by and x-ray crystallography X N 2 BF 4 and x-ray crystallography X N 2 BF 4 X= Br, I, OTf X= Br, E1 /2 = +0.57 V vs Fc/Fc +
iPr iPr
N N NCl N IV N N Cl Ni IV Cl Ni N Cl Cl N iPr Cl iPr iPr iPr
iPr iPr
159 159
Scheme 63. Two electron oxidation of Ni(II) monoanionic bis(carbene) pincer complex with PhICl2 Scheme 63. Two electron oxidation of Ni(II) monoanionic bis(carbene) pincer complex with PhICl2 Scheme 63. Two electron oxidation Ni(II) monoanionic bis(carbene) pincer complex with PhICl2 and characterization of stable Ni(IV)ofintermediate 159. and characterization of stable Ni(IV) intermediate 159. and characterization of stable Ni(IV) intermediate 159.
5.3. Synthetic Applications 5.3. Synthetic Applications 5.3. Synthetic Applications Catalytic cycles, and thus also synthetic applications, invoking high oxidation state nickel Catalytic cycles, and thus also synthetic applications, invoking high oxidation state nickel intermediates remain quitethus rare. However, building on the body of work high in stoichiometric complex Catalytic cycles, synthetic applications, invoking oxidation state nickel intermediates remainand quite rare.also However, building on the body of work in stoichiometric complex synthesis, such reports are beginning to emerge, but the oxidation states in play are difficult to intermediates remain quite rare. However, building on the body of work in stoichiometric complex synthesis, such reports are beginning to emerge, but the oxidation states in play are difficult to
Molecules 2017, 22, 780
42 of 54
Molecules 2017, 22, 780 42 of 54 to synthesis, such reports are beginning to emerge, but the oxidation states in play are difficult Molecules 2017, 22, 780outline recent catalytic reports invoking both Ni(III) and Ni(IV) intermediates 42 of 54 discern. Below we via discern. Below we outline recent catalytic reports invoking both Ni(III) and Ni(IV) intermediates via oxidation with hypervalent iodine reagents as well as synthetically relevant applications involving oxidation with hypervalent iodinecatalytic reagentsreports as well as synthetically relevant applications involving discern. Below we outline recent invoking both Ni(III) and Ni(IV) intermediates via stoichiometric complexes. stoichiometric oxidation withcomplexes. hypervalent iodine reagents as well as synthetically relevant applications involving The Nocera group reported the catalytic photoelimination of Cl2 from a Ni(III) intermediate, The Nocera group reported the catalytic photoelimination of Cl2 from a Ni(III) intermediate, stoichiometric complexes. accessed via one-electron oxidation of a Ni(II) species with PhICl2 (Scheme 64) [144]. In this case, accessed via one-electron oxidation a Ni(II) photoelimination species with PhIClof2 (Scheme [144]. intermediate, In this case, The Nocera group reported theofcatalytic Cl2 from 64) a Ni(III) careful control of the equivalents ofPhICl PhICl for selective transfer of a single chlorineItatom. 2 allows careful control of the equivalents of 2 allows for selective transfer of a single chlorine is accessed via one-electron oxidation of a Ni(II) species with PhICl2 (Scheme 64) [144]. Inatom. this case, It is also possible that the ligand scaffold does not lower the oxidation potential far enough to make also possible that ligand scaffold does2 not lower oxidation potential far enough makeIt ais a careful control of the equivalents of PhICl allows for the selective transfer of a single chlorinetoatom. subsequent Ni(III)/Ni(IV) oxidation accessible in this case, however electrochemical measurements subsequent Ni(III)/Ni(IV) oxidation accessible this case, however potential electrochemical measurements also possible that the ligand scaffold does notinlower the oxidation far enough to make aon this complex were not provided. on this complex were not provided. subsequent Ni(III)/Ni(IV) oxidation accessible in this case, however electrochemical measurements
on this complex were not provided.
0.5 PhICl 2
0.5 PhI 0.5 PhI
0.5 PhICl 2
Cl P Cl P Cl
P Ni II
P Cl
Ni II
ΔH = + 23.7kcal.mol-1 ΔH = + 23.7kcal.mol-1
Cl
P
ClP NiIII P Cl P Cl NiIII Cl Cl hν
0.5 Cl 2
hν
0.5 Cl 2
Cl Cl P
Cl
P
P
ClP NiIII P Cl NiIII Cl Cl
Ni III P Cl Ni III Cl Cl
Cl P
Cl P
Scheme Photoeliminationof ofCl Cl2 via via single single electron cycle mediated by by PhICl 2. Scheme 64.64. Photoelimination electronNi(II)/Ni(III) Ni(II)/Ni(III) cycle mediated PhICl 2 2. Scheme 64. Photoelimination of Cl2 via single electron Ni(II)/Ni(III) cycle mediated by PhICl2.
Chatani’s group invoked a Ni(II)/Ni(IV) catalytic cycle in their development of a directed C(sp3)– Chatani’s group invoked a Ni(II)/Ni(IV) catalytic cycle in their developmentsalts of a directed H arylation with diaryliodonium salts (Scheme 65) [145]. of diaryliodonium Chatani’s group invoked a Ni(II)/Ni(IV) catalytic cycleAinvariety their development of a directedbearing C(sp3)– 3 )–H C(sp arylation with diaryliodonium salts (Scheme 65) [145]. A variety of diaryliodonium salts different counter ions such were examined, however high yields only in the case of triflate, H arylation with diaryliodonium salts (Scheme 65) [145]. A were variety of obtained diaryliodonium salts bearing bearing different counter ions such were examined, however high yields were only obtained in presumably due toions thesuch necessary regeneration of the Ni(OTf) 2 catalyst. Although no in intermediates werethe different counter were examined, however high yields were only obtained the case of triflate, case of triflate, presumably due with to the necessary regeneration of theproviding Ni(OTf) catalyst. Although isolated, radical trap experiments TEMPO gave TEMPO-adducts, least preliminary presumably due to the necessary regeneration of theno Ni(OTf) 2 catalyst. Although no2at intermediates were noevidence intermediates were isolated, radical trap experiments with TEMPO gave no TEMPO-adducts, againsttrap one-electron pathways. isolated, radical experiments with TEMPO gave no TEMPO-adducts, providing at least preliminary providing least preliminary against one-electron pathways. evidenceatagainst one-electronevidence pathways. R
Ni(OTf) 2, Na 2CO3
O
ON R H R H N H H
MesCONa 2H CO Ni(OTf) 2, 2 3
R
N N
2H ArMesCO I OTf Ar Ph I OTf 21-72% yield Ph 21-72% yield
R
O
ON R H R Ar N H Ar
R
N N
Ph Ph Ph Ph
O ON
N Ni N Ar Ni OTfN Possible Ni(IV) Arintermediate OTf Possible Ni(IV) intermediate
Scheme 65. C(sp3)–H arylation with diaryliodonium salts through a proposed Ni(II)/Ni(IV) catalytic cycle. 3)–H arylation with diaryliodonium salts through a proposed Ni(II)/Ni(IV) catalytic cycle. Scheme 65. 3 )–H arylation with diaryliodonium salts through a proposed Ni(II)/Ni(IV) Scheme 65.C(sp C(sp Continuing their work in oxidative fluorination chemistry (for palladium analogue see Section catalytic cycle.
2.2.3.1), the Ritter group reported the development of achemistry one-step oxidative radiofluorination of arenes Continuing their work in oxidative fluorination (for palladium analogue see Section 18F, and poly(cationic) N-ligated λ3-iodane 161 as the employing 160 with 2.2.3.1), theNi(II) Rittercomplex group reported theaqueous development of a one-step oxidative radiofluorination of arenes Continuing their work inone-step oxidative fluorination chemistry (for palladium analogue 18F, and bond 3-iodane oxidant (Scheme 66) [146]. This oxidation/C–F formationN-ligated proceeds via the two-electron employing Ni(II) complex 160 with aqueous poly(cationic) λ 161 as thesee Section 2.2.3.1), the66) Ritter group reportedoxidation/C–F theiodine development of a one-step oxidative radiofluorination oxidation of 160 by[146]. cationic 161,bond which undergoes ligand exchange with oxidant (Scheme This hypervalent one-step formation proceeds via the two-electron 18 F, and poly(cationic) N-ligated 3 -iodane 3-iodane ofnucleophilic arenes employing Ni(II) complex 160 with aqueous λ fluoride and subsequent reductive elimination. The use of (poly)cationic λ 161 oxidation of 160 by cationic hypervalent iodine 161, which undergoes ligand exchange with 18 3 161 as the oxidant (Scheme 66) [146]. This one-step oxidation/C–F bond formation proceeds via elimination could easily be outcompeted by the –X isnucleophilic critical in this case asand thesubsequent desired F reductive elimination. fluoride The use of (poly)cationic λ -iodane 161the 18F two-electron of the 160 bycenter cationic hypervalent iodine could 161, which undergoes ligand ligands transferred to the metal with traditional hypervalent iodine (i.e., –Cl, –OAc, reductive elimination easilycomplexes be outcompeted byexchange the –X is critical inoxidation this case as desired –OTFA) [40]. Instead transfers two “innocent” heterocyclic ligands, allowing the ligands transferred to complex the metal161 center with traditional hypervalent iodine complexes (i.e., –Cl,for –OAc, –OTFA) [40]. Instead complex 161 transfers two “innocent” heterocyclic ligands, allowing for the
Molecules 2017, 22, 780
43 of 54
with nucleophilic fluoride and subsequent reductive elimination. The use of (poly)cationic λ3 -iodane 161 is critical in this case as the desired 18 F reductive elimination could easily be outcompeted by the –X ligands transferred to the metal center with traditional hypervalent iodine complexes (i.e., –Cl, Molecules 2017, 22, 780 43 of 54 –OAc, –OTFA) [40]. Instead complex 161 transfers two “innocent” heterocyclic ligands, allowing for the challenging challengingC–F C–F reductive reductiveelimination eliminationto toproceed proceedin inhigh high yield. yield. Even Even though though the the detailed detailed mechanistic mechanistic studies studies were were not not reported, reported, the the formation formation of of Ni(IV) Ni(IV) intermediate intermediate 162 162 is is reasonable reasonable by by analogy analogy to to their their 3 -iodanes as two-electron oxidants prior reports, as well as work by Dutton employing (poly)cationic λ prior reports, as well as work by Dutton employing (poly)cationic λ3-iodanes as two-electron in the generation of Pd(IV)ofand Pt(IV) species Section 3.1). This work promise of the oxidants in the generation Pd(IV) and Pt(IV)(see species (see Section 3.1). Thisshows work the shows the promise 3 -iodanes as powerful oxidants that allow for challenging relatively unexplored class of (poly)cationic λ 3 of the relatively unexplored class of (poly)cationic λ -iodanes as powerful oxidants that allow for reductive eliminations that would be precluded with the use with of more oxidants. challenging reductive eliminations that would be precluded thecommon use of more common oxidants. O N
S O
S NO 2
N Ni II N
1.5 equiv N-HVI (161) aqueous 18F –, 18-cr-6 MeCN 23 oC,
