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879

Enantioselective Henry Reaction Catalyzed By Supported Transition Metal Complexes Pavel Drabina, Lydie Harmand and Milo Sedlák* Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic Abstract: This present mini-review summarizes the recently published development and trends concerning catalysts based on immobilized complexes of transition metals which are designated for asymmetric Henry reaction. The paper presents examples of originally homogeneous catalysts which were anchored to organic carriers (such as dendrimers, polymers) or inorganic materials. The catalysts based on organic carriers are classified and discussed in accordance to their solubility in reaction medium. In the case of inorganic materials, the discussed catalysts were anchored to different types of surface of micro- and nano-particles. The discussion concerns the effect of immobilization of catalysts on the overall reaction yield, enantioselectivity, possible recycling, and the effect of recycling on the abovementioned parameters.

Keywords: Enantioselective catalysis, Henry reaction, nitroaldol reaction, recyclable catalysts, solid supported catalyst, transition metal complexes. 1. INTRODUCTION The Henry reaction catalyzed by chiral, optically pure complexes of transition metals represents one of the basic stereoselective reactions where a new carbon-carbon bond is created [1]. This reaction finds applications in the synthesis of optically pure functionalized 2-nitroalcohols, which are used, e.g., for preparation of biologically active compounds [2-6]. In the past decade, a number of highly efficient homogeneous catalysts have been developed on the basis of coordination compounds [1]. The Henry reaction catalyzed in this way is based on addition reaction of -carbon atom of nitroalkane on the electron-deficient carbonyl carbon atom of aldehyde. The key step of the catalysis with transition metal complexes is simultaneous coordination of oxygen atoms of nitroalkane and aldehyde to the transition metal ion (Lewis acid) [7]. The coordination results in approaching and activation of both reacting species. The present suitable chiral ligand enables such an orientation of both reactants that leads to preferred formation of one of the stereoisomers. The catalyst usually contains a suitable basic centre for trapping the proton from -carbon atom of nitroalkane [7] (Scheme 1). CH3N

O

L

O

L

L

OH R

X X

M

O N

H

L

X X O CH3 N

M

O R

HX O

HX

L

L

X

M

O N

H2C

O

O

H

L R

L O

M

X O N

H

O

Scheme 1.

*Address correspondence to this author at the Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic; Tel: +420 466 037 010; Fax: +420 466 037 068; E-mail: [email protected] 1875-6271/14 $58.00+.00

The successful applications of different highly efficient ligands can be documented by a number of papers [8-13]. The most efficient optically pure ligands include, e.g., binol [8], amino alcohols [9] Schiff bases [10], salens [11]. Out of the heterocyclic ligands, the most important are oxazolines [7, 12], thiazolines [12], imidazolidines [13, 14] and macro-heterocyclic compounds [15]. The most often used transition metal ions are La(III) [8], Cr(III) [11], Cu(I) [3], Cu(II) [4-7, 10, 12-14] and Zn(II) [9, 10, 15]. The applications of homogeneous catalysts are often restricted by practical impossibility of their reuse. This disadvantage is overcome by the application of recoverable and recyclable catalysts [16]. The term recyclable catalyst covers different types of catalysts, both solid supported catalysts and easily isolated catalysts [16-21]. For example, a catalyst can be composed of large molecules, such as macrocycles [5, 6, 15, 20, 22] or fragments of ionic liquids [23-24]. The molecules designed in this way often operate under homogeneous conditions. Then, the separation of such catalysts is based on their precipitation or extraction from reaction mixture by addition of suitable solvent [16-20]. For example, a unique system was described recently [25] for the ditopic chiral optically pure bisoxazoline Cu(II) complex, which is polymerized by addition of noncoordinating solvent and thus it can easily be isolated. Afterwards, the formed coordination polymer is depolymerized back to active complex in a coordinating solvent. This catalyst was successfully recycled for the Henry reaction in up to 11 catalytic cycles with maintaining high reaction yields (>90%) as well as enantioselectivity (>90% ee) [25]. Another variant consists in the process of heterogenization, i.e. support of a well-known homogeneous catalyst by connecting it to a suitable inert carrier [16-19]. A homogeneous catalyst designated for immobilization must meet a number of requirements, in particular it must provide excellent results already under the conditions of the homogeneous catalysis itself [17]. Suitable carriers include specific molecules, such as dendrimers [26], polymers [19] or various inorganic materials [20]. Usually, in the case of dendrimers and some polymers, these systems are soluble completely or partially in the reaction medium. However, there is a number of polymeric carriers that are insoluble in the reaction medium. The traditionally used insoluble polymeric carriers are spherical particles of styrenedivinylbenzene copolymer, or its modified forms [19]. These catalysts have the advantage of being easily isolated after a reaction in batch-wise arrangement or being usable in flow-type reactors (flow© 2014 Bentham Science Publishers

880 Current Organic Synthesis, 2014, Vol. 11, No. 6

Drabina et al.

OH

O H

1 mol% Cu(OAc)2 1.2 mol% L

+ CH3NO2

i-PrOH/trifluoroethanol 22 °C, 3d

NO2

74% ee

1

Si

Si

87% 87% ee

86% 84% ee

Cu

O

AcO

Cu

OAc

Si

Si

87% 83% ee

N

N

N

N AcO

Si

O

O

O

2

Cu

OAc

4

OAc

Si

3

Si

N

N AcO

Si

NO2

O

O

95%

NO2

Si

O

O 4

AcO 2

4

N

N Cu

OAc 2

2

4

Scheme 2.

technology) [16]. The solid supported of ligands on polymer can be performed either by the method of anchoring of the catalyst to a chemically suitably activated surface of polymeric carrier (PostModification Strategy) [19], or by copolymerization of a ligand containing a double bond with monomers that form the basic skeleton of polymeric carrier (Copolymerization Strategy) [19]. Generally, the preference of the strategy cannot be stated - it depends on a number of factors in particular cases. Moreover, a new alternative is the utilization of inorganic micro- and nano-particles as carriers [20]. Due to their dimensions, these colloid catalysts operate on the boundary between homogeneous and heterogeneous catalysis, and slowing down of the reaction by diffusion is negligible [20]. However, the colloid systems do not offer the possibility of their simple separation from the reaction products. This disadvantage can easily be overcome, e. g., by the application of functionalized magnetic nano-particles, which so far have been used predominantly in biology and medicine [20]. Such particles can be easily separated from the reaction medium by means of external magnet. The aim of this review article is to summarize the so far published results in the area of solid supported enantioselective complex catalysts designated for the Henry reaction and to evaluate their properties in comparison with analogous homogeneous catalysts. 2. CATALYSTS ANCHORED ON DENDRIMERS AND POLYMERS 2.1. Homogeneous and Semi-homogeneous Systems Functionalized bis- and tris(oxazolines) were attached by means of alkynyl linker to carbosilane dendrimers which were subsequently complexed with Cu(OAc)2 (Scheme 2) [26]. The prepared catalysts 1-4 were tested for the reaction of 2-nitrobenzaldehyde with nitromethane giving (S)-1-(2-nitrophenyl)-2-nitroethanol. The results obtained show that the catalysts derived from bis(oxazoline) 1-4 exhibited a higher enantioselectivity (74-87% ee) than the corresponding tris(oxazolines) (52-53% ee). The recycling of dissolved dendrimer catalysts was implemented by placing into the commercially available dialysis membrane bags (“tea bags”) (Scheme 2) [26]. The bis(oxazoline) catalyst 1 alone exhibited a lower enantioselectivity (ca  10% ee) than the dendrimer forms 2-4 [26]. However, the possibility of recycling was only for the asymmetric hydrazination of ethyl 2-methylacetoacetate. In this case with cata-

lyst 4, a mild increase in enantioselectivity was observed after the third and the fourth cycles, namely from 82% ee to 88% ee. After further cycles, there took place a monotonous decrease down to 77% ee after the seventh cycle [26]. Another example of highly efficient soluble polymeric catalyst of the Henry reaction is the recoverable catalyst 6 based on the Cu(II) complex of PEG-modified analogue of C2-symmetrical diaminobithiophene (Scheme 3) [27]. Catalyst 6 was tested for reactions of a series of functionalized aldehydes with excess nitromethane under homogeneous conditions in ethanol at the temperature of 0 °C. The reaction proceeded with high conversions 8589 % and with high enantioselectivity 83-93% ee. The solid supported catalyst 6 exhibited an only mild decrease in enantioselectivity (6% ee) as compared with the original nonimmobilized catalyst 5 [28] (Scheme 3). The principle of recycling of the catalyst is based on precipitation of the catalyst with diethyl ether. The catalyst was repeatedly used five times altogether in the reaction of benzaldehyde with nitromethane. In comparison with the fresh catalyst, there took place a decrease in conversion by ca 30 % after the fifth cycle, which was explained by mass loss of the catalyst (19 %) due to repeated isolation. The enantioselectivity of recycled catalyst was the same as that of the fresh catalyst: 88% ee after the fifth cycle. After removing Cu(II) ions, the five-times used catalyst was analyzed by of 1H NMR spectroscopy and obtained results were identical with those obtained with the fresh catalyst [27]. In another paper [29] we described preparation and characterization of the recyclable catalyst 8, in which the ligand, i.e. (2R,5S)5-isopropyl-5-methyl-2-(pyridine-2-yl)imidazolidine-4-one, was coordinatively attached to the Cu(II) salt of block copolymer methoxypoly(ethylene glycol)-b-poly(L-glutamic acid) (Scheme 4). The reaction of functionalized aldehydes with nitromethane catalyzed by 8 gave the corresponding functionalized (S)-2nitroethanols in high yields (70-98%) and with high enantioselectivity (61-92% ee), comparable with the original catalyst (90-96% ee) [13]. The reaction mixture represents a colloidal system formed by self-assembled aggregates of the catalyst with average hydrodynamic magnitude of particles 189 ± 3 nm (DLS). After partial removal of ethanol by distillation and after addition of diethyl ether, the catalyst was isolated from the reaction mixture by centrifuging. Only a minor decrease in the enantioselectivity ( 9% ee) and the

Current Organic Synthesis, 2014, Vol. 11, No. 6 881

Enantioselective Henry Reaction Catalyzed By Supported Transition Metal Complexes

O R

H

OH

cat. 5 mol %

+ CH3NO2

EtOH, 0 °C, 40h

NO2

R

R: Ph, 4-FPh, n-C7H15, 2-NO2Ph, 4-ClPh, C6F5, 3,5-(MeO)2Ph, c-C6H11, t-Bu S S NH

OAc

Cu

58–99% 81–99% ee

5

OAc

NH

S S S S

NH

85–89%

OAc

Cu

83–93% ee

6

OAc

NH

S

O

3

MeO-PEG5000

Scheme 3.

O R

H

OH

cat. 5 mol %

+ CH3NO2

EtOH, 10 °C, 72h

NO2

R

R: Ph, 2-MeOPh, 4-ClPh, 4-BrPh, 4-PhPh, t-Bu H N Cu AcO

H N N H OAc

O

H

CH3

CH3

N

CH3

Cu O

7

N H

O

N H

CH3 O

O

87–97% 90–96% ee 70–98% 61–92% ee

H N

CH3 CH3

O

8

H N O

NH O 8

MeO-PEG5000

Scheme 4.

yield ( 22%) was observed after ten cycles. The kinetic dependences show that the conversion decrease is caused by a lowered activity of the catalyst and also by the mass loss of the catalyst during recovery (Scheme 4) [29]. In the same research [29] we also tested the Cu(II) derivative of the used block copolymer (methoxypoly(ethylene glycol)-b-poly(L glutamic acid)). Although this polymeric complex successfully catalyzed the Henry reaction (92 %), the enantioselectivity was insignificant (2%), which was explained by the relatively large distance between chiral centre of poly(L -glutamic acid) and Cu(II) ion. 2.2. Heterogeneous Polymeric Systems A heterogeneous variant of the catalyst is represented by the complex of Cu(II) acetate with polystyrene-supported chiral 2,2methylenebis[(4S)-tert-butyl-2-oxazoline] [30] (Scheme 5). Cata-

lyst 10 was prepared by copolymerization of 4-vinylbenzyl derivative of 2,2-methylenebis[(4S)-tert-butyl-2-oxazoline] with styrene and divinylbenzene. The authors prepared the copolymer with relatively high content of DVB (15%). The complex of bisoxazoline 9 as such catalyzed the reaction of 4-chlorobenzaldehyde with nitromethane to (S)-1-(4-chlorophenyl)-2-nitroethanol with low yield (66 %) and only moderate enantioselectivity (49% ee) as compared with excellent results of the polymeric catalyst 10 (95 %, 86% ee). This remarkable difference was explained by the fact that in the case of catalyst 9, the little efficient dimer form be formed, which is not possible in the case of catalyst 10 [30]. The catalyst 10 was successfully recycled five times with minimum decrease in enantioselectivity (86% ee). In the case of non-porous pearl-like copolymers PS-DVB with high content of cross-linker, a marked decrease is usually observed in diffusion of reactants toward the catalytic centre and hence retardation of the reaction with concomitant lowering of conversion [19, 31]. Therefore, in this case [30], the high

882 Current Organic Synthesis, 2014, Vol. 11, No. 6

O R

14 [31] and 15 [33] was prepared. The effectiveness of these catalysts was compared under the same conditions for the reaction of 2nitrobenzaldehyde with nitromethane (Scheme 7).

OH

cat. 5 mol %

+ CH3NO2

H

Drabina et al.

EtOH, 25 °C, 24h

NO2

R

R: Ph, 4-MeOPh, 2-ClPh, 3-ClPh, 4-ClPh, 4-BrPh, 4-NO2Ph

N t-Bu

x

O

O

AcO

z

y

N Cu

OAc

9 R: 4-ClPh: 66%, 49% ee

t-Bu O

O 10

N

N

56–97% 49–88% ee

Cu

t-Bu

AcO

OAc

t-Bu

In the given series of catalysts 12-15, catalyst 12 was the most efficient (79% ee). The difference in enantioselectivity of catalyst 14 and the sterically more demanding catalyst 15 ( 21% ee) [33] can also be ascribed to the fact that the active complex of catalyst 15 was anchored on the commercially available pearl-like (4methoxy-phenyl)diisopropylsilylpropylpolystyrene (Novabiochem) [33]. The same methods as for catalysts 12-14 were used for preparation and characterization of analogous catalysts [31], but with the use of commercially available chloromethylated polymers MerrifieldTM and JandaJelTM [19]. However, in both these cases the reaction of 2-nitrobenzaldehyde with nitromethane proceeded significantly slowly: after one week (25 °C) an only ca 5% conversion was reached [31]. The given findings document that also the polymeric matrix itself can distinctly affect the overall yield and enantioselectivity of the reaction. 3. CATALYSTS ANCHORED ON INORGANIC MATERIALS

Scheme 5.

values of conversion (24 hours; 56-97%) can be interpreted by the fact that the authors used a powder form of the catalyst; however, they did not specify the porosity and specific surface of the catalyst. Another paper [31] describes the method in which a swelling pearl-like copolymer styrene - 4-vinylbenzyl chloride cross-linked by tetra(ethylene glycol)-bis(4-vinylbenzyl)ether (2%) (200-800 μm) was connected with covalently anchored (1R,2R)-2-(2,3dihydro-1H-isoindole-2-yl)-1,2-diphenylethane-1,2-diamine. Subsequent complex formation with Cu(II) acetate gave catalyst 12, which was tested for the reactions of functionalized aldehydes with nitromethane in ethanol (25 °C) (Scheme 6) [31]. The nitroaldolization reactions proceeded in the polymeric matrix of swelling catalyst at a rate comparable with that of the reaction using catalyst 11 in homogeneous medium [32]. The respective functionalized (S)-2-nitroethanols were formed quantitatively within 24 hours with the excesses as high as 96% ee. After fivefold recycling of the catalyst, no lowering of yields and enantioselectivity took place; however, a partial mechanical degradation of polymeric matrix was observed [31]. By anchoring complexes of different diamines on the above-mentioned polymer, a group of catalyst 12O R

Among the first successful catalysts of asymmetric Henry reaction of aliphatic aldehydes belongs the La(III) complex of (R)BINOL 16 (79-91%; 73-90% ee) (Scheme 8) [8]. Its immobilized variant 17 was prepared by covalent anchoring to mesoporous silica MCM-41 (6750 m2 g-1, pore size 3 nm) [34]. The solid supported complex 17 provides slightly lower yields and enantioselectivity of the respective functionalized (S)-2-nitroethanols (64-78 %, 62-84% ee). On the other hand, the catalyst can be separated very easily by simple filtration, and no significant loss of catalytic activity was observed after 3 recyclings (Scheme 8) [34]. Another successful variant is the application of catalyst 18 (Scheme 9) [35], which represents binol anchored on nanocrystalline MgO. Magnesium oxide fulfils both the necessary function of Lewis acid at the side of Mg2+ ion and the necessary activity of Lewis base at the side of O2- and O- [35]. The Henry reaction of nitromethane with functionalized benzaldehydes and some aliphatic aldehydes catalyzed by 18 provides the corresponding functionalized (S)-2-nitroethanols in excellent yields (70-95%, 60-98% ee) [35]. Catalyst 18 was also successfully adopted for asymmetric Henry reaction of -ketoesters with nitromethane (75-70 %, 98% ee). A certain disadvantage was the necessity to perform both reactions at the temperature of -78 °C [35]. OH

cat. 5 mol % H

+ CH3NO2

EtOH, 18 °C, 24h

NO2

R

R: Ph, 2-MeOPh, 4-CNPh, 4-ClPh, 4-PhPh, 2-NO2Ph, 4-NO2Ph, n-C4H9, t-C4H9, c-C6H11

N

N

N Cu

Cu AcO OAc

AcO

Scheme 6.

OAc

y

12

11 92–99% 91–97% ee

x

NH

97–99% 76–96% ee

z

O 4

Current Organic Synthesis, 2014, Vol. 11, No. 6 883

Enantioselective Henry Reaction Catalyzed By Supported Transition Metal Complexes

O

OH NO2

cat. 5 mol %

H + CH3NO2

EtOH, rt, 24h

NO2

NO2

PS

PS N

H2N

N

N Cu

Cu AcO

AcO

OAc

99% 79% ee

12

OAc

99% 76% ee

13

PS

PS N

N

H2N

Cu

Cu AcO

AcO

OAc 99% 53% ee

14

N

15

OAc

99% 32% ee

Scheme 7.

O R

H

OH

cat. 5 mol %

+ CH3NO2

THF, –42 °C, 18–34h

NO2

R

R: PhCH2CH2, i-Pr, c-C6H11

Li O O O

La Li O

O O

Li

16

79–91% 73–90% ee

Li O O

m2·g–1

MCM-4: 675 pore size: 3 nm 64–78% 62–84% ee

Scheme 8.

O

O O Si O

17

La Li O

O O

Li

884 Current Organic Synthesis, 2014, Vol. 11, No. 6

O R

H

Drabina et al.

means of hydrogen bonds (Scheme 11). In a series of reactions of different aldehydes with nitromethane catalyzed by the solid supported catalyst 22, the yields attained with individual aldehydes were lower in average by ca 10-20 % (down to 59%) as compared with those obtained with the homogeneous catalyst. However, the attained enantioselectivity was comparable with that obtained with the homogeneous catalyst 21 (up to 97% ee). Moreover, the catalyst was successfully recycled seven times without any loss of enantioselectivity (Scheme 11).

OH

10 mol %

+ CH3NO2

THF, –78 °C, 48-72 h

NO2

R

R: Ph, 2-NO2Ph, 4-NO2Ph, 2-ClPh, 4-ClPh, 4-MeOPh, 2-MeOPh, 4-MePh, 2-MePh, n-C5H11, t-Bu, n-Bu H O H

H

O

Another alternative of the usage of an analogous bis(oxazoline) Cu(II) complex [38] was its anchoring by means of charge-transfer interaction between the ligand containing an anthracene fragment and SiO2 carrying a trinitrofluorene fragment (Scheme 12). The bis(oxazoline) complex catalyst 23 alone was very efficient for the Henry reaction of a series of aldehydes with nitromethane and nitroethane (59-89%; 82-90% ee). After immobilization via CTcomplex, the reaction of benzaldehyde with nitromethane catalyzed by 24 attained comparable results (84%; 89% ee). On the other hand, recycling of catalyst 24 caused stepwise lowering of both yield and enantioselectivity: after the seventh cycle, the yield of (R)-2-nitro-1-phenylethanol was 51% with 82% ee [38].

70–95% 60–98% ee

O

O Nanocrystalline-MgO H

18

Scheme 9.

In another research work, the analogue of chiral optically pure bis(oxazoline) Cu(II) complex 19 was immobilized on magnetically separable hierarchically-ordered meso-cellular mesoporous silica (pore size: 7 nm; 142 m2 g-1) (Scheme 10) [36].

The same bis(oxazoline) Cu(II) complex containing anthracene fragment 23 (Scheme 12) with coordinated Cu(OAc)2 was anchored on surface of charcoal by means of - stacking [39]. The resulted catalyst 23-Cu(OAc)2@charcoal was tested for the reaction of benzaldehyde with nitromethane (20 °C, EtOH, 24-120 hours). (R)-2Nitro-1-phenylethanol was obtained in the yield of 68 % and 69% ee. After seven cycles, a decrease down to 56 % and 40% ee was observed. The catalyst was isolated by filtration. The application of “tea-bag” technique and prolongation of reaction time to 120 hours led to increase in yield to 76 %, however, with concomitant decrease of enantioselectivity to 62% ee. After seven cycles, the yield decreased to 60% and 46% ee [39]. Another tested variant of catalyst 23 was the complexes with Cu(OTf)2 anchored on charcoal,

The prepared catalyst 20 was tested for Henry reaction of a series of aldehydes with nitromethane in high yields comparable with those obtained with the homogeneous catalyst 19 (97 %) [7]. The attained enantioselectivity was lower (84% ee) than that obtained with the homogeneous catalyst (94% ee). The recyclability of catalyst 20 was verified for the reaction of 2-methoxybenzaldehyde with nitromethane: the enantioselectivity was not significantly lowered even after five cycles (84 82 % ee) (Scheme 10) [36]. The more recently published paper [37] describes preparation of similar bis(oxazoline) Cu(II) complex 21, whose molecule contains fragments of ionic liquid enabling anchoring of the complex on mesoporous silica SBA-15 (600-800 m2 g-1, pore size 6-11nm) by O R

H

OH

cat. 5 mol %

+ CH3NO2

EtOH, 0–25 °C, 24–60h

NO2

R

R: Ph, 4-NO2Ph, 2-NO2Ph, 4-FPh, 4-ClPh, 2-MeOPh, 1-Napht O

O N

N AcO

OAc

OTMS

O

O O Si O SiO2: 145 m2·g–1 pore size: 7 nm O O Si O OTMS Scheme 10.

70–98% 57–94% ee

19

Cu

O

N H

N N

O O

N 6 O

20

OAc Cu

O N H

N N

O O

N N 56–97% 49–84% ee

N

6 O

O

OAc

Current Organic Synthesis, 2014, Vol. 11, No. 6 885

Enantioselective Henry Reaction Catalyzed By Supported Transition Metal Complexes

O R

H

OH

cat. 5 mol %

+ CH3NO2

MeOH, 0 °C, 48h

NO2

R

R: Ph, 4-NO2Ph, 4-BrPh, 4-FPh, 4-ClPh, 2-MeOPh, 3,5-(MeO)2Ph, 3,4,5-(MeO)3Ph, 4-MePh N

N

N

40–70% 77–94% ee

21

O

O

N

S

O

H

O

H

N

N

O

SBA-15:

N

600–800 m2·g–1

O O

22

OAc

N

15–59% 41–94% ee

O

N

N H

OAc Cu

pore size: 6–11 nm

O

OAc

N

O

H

O

OAc Cu

2 OTs

N O

O

O S

O

H

O

H

Scheme 11.

O R1

NO2 H

+

OH

cat. 10 mol %

NO2

R1

EtOH, 20 °C, 24–120h

R2

R2 R1:

Ph, 4-NO2Ph, 4-CF3Ph, c-C6H11, n-C6H13

R2:

H, CH3

O O

N

OAc Cu

3

23

59–89% 82–90% ee

OAc

N O NO2

SiO2

O

O

O

O Si O 24 R1 = Ph 84% R2 = H 89% ee Scheme 12.

O

N H

O

N N

O2N

O NO2

OAc Cu

3

OAc

886 Current Organic Synthesis, 2014, Vol. 11, No. 6

O Ph

H

Drabina et al.

+ CH3NO2

R = OCH3 83%, 31% ee N

N

O

R

Cu

CH3

O Si

NO2

Ph

R=H 99%, 5% ee SiO2

catalysts of the Henry reaction are Cu(II) complexes of (1S,2S)N1,N2-bis(3-chlorobenzyl)cyclohexane-1,2-diamine 27 and (1S,2S)N1,N2-bis(4-chlorobenzyl)cyclohexane-1,2-diamine 28. Their application to reactions of nitromethane with a series of aldehydes gave the yields of the respective(R)-2-nitroethanols 78-90% and 81-94% ee (Scheme 15) [43]. The reaction was carried out in ionic liquid, which enabled subsequent separation of catalysts after extraction of the products. These catalysts were recycled up to five times without any loss of effectiveness [43]. Complexes 27 and 28 were also encapsulated in zeolite Y and in mesoporous zeolite ZSM-5 (Scheme 15) [44]. The yields and enantioselectivities attained with both zeolite carriers were comparable with both dichloro derivatives. In the reactions of functionalized benzaldehydes and cyclohexanecarbaldehyde with nitromethane, the obtained yields were as high as 99 % with ee as high as 94 % for the corresponding (R)-2-nitroalcohols. The catalysts were also recycled five times without observable lowering of yields and enantioselectivity (Scheme 15) [44].

OH

NEt3, rt, 24 h

OTf

TfO

R = CH3

25 R: H, OCH3, CH3

N

O

99%, 34% ee

Scheme 13.

fullerene and on single-walled carbon nano-tubes, which were tested for the reaction of ethyl glyoxylate with -methylstyrene (up to 89 %, up to 69% ee) [39].

O R

H

Salens and their complexes with transition metals represent a class of highly efficient catalysts that are used for a number of asymmetric reactions [40]. The prepared salen derivatives were immobilized by anchoring on SiO2 25 (Scheme 13) [41]. After complex formation with Cu(OTf)2, the catalyst was tested for the reaction of benzaldehyde with nitromethane giving (R)-2-nitro-1phenylethanol. The best results were obtained with methyl derivative 25. Although, after 24 hours, the attained conversion was very high (99%), the enantioselectivity of reaction was low (34% ee; Scheme 13) [41].

NH Cu

R

H

R2

R1

R1

Another efficient variant of solid supported catalyst is the Cu(II) complex derived from chiral amino alcohol anchored on mesoporous SiO2 [45]. Catalyst 29 represents a complex immobilized on mesoporous hexagonal silica SBA-15 (pore size 3-13 nm), OH

Morpholine EtOH, rt, 8–18h

MCM-41: up to 1500 m2·g–1 pore size: 2–10 nm

R

N

N Cu

O

O O

O Scheme 14.

Si

ZSM-5: 78–99 % 68–93 % ee

Scheme 15.

4-BrPh, 4-FPh, 4-NCPh, 4-HOPh, 1-Napht, 2-Napht, 2-Furyl, 2-Thioph, c-C6H11, n-C6H13, n-C4H9, PhCH=CH, t-Bu

O

R2

27: R1 = Cl, R2 = H 28: R1 = H, R2 = Cl

R: Ph, CH3Ph, 4-CH3OPh, 2-CH3OPh, 4-NO2Ph, 3-NO2Ph, 4-ClPh,

60–92% 60–90% ee

NO2

N H OAc

AcO

Complexes of N ,N -dibenzyl derivatives of (1S,2S)cyclohexane-1,2-diamine as reduced variants of previous Schiff bases represent comparable and in some parameters even better catalysts for the Henry reaction [43, 44]. For instance, very efficient + CH3NO2

R

56–99 % zeolite Y: 65–94 % ee

2

O

EtOH, 0–50 °C, 48–72h

R: Ph, 3-NO2Ph, 3-ClPh, 4-ClPh, 4-MeOPh, PhCH2CH2, n-C5H11

Much more efficient was catalyst 26, i.e. Cu(II) complex of tert-butyl derivative of salen anchored on mesoporous silica MCM41 (up to 1500 m2 g-1, pore size 2-10 nm) [42]. In a series of reactions of both aromatic and aliphatic aldehydes using catalyst 26, the attained yields of the corresponding (R)-2-nitroethanols were as high as 92 % with 90% ee. The recycling of catalyst 26 was not accompanied by any significant lowering of yield and enantioselectivity (Scheme 14) [42]. 1

OH

cat. 10 mol %

+ CH3 NO2

N

26

NO2

Current Organic Synthesis, 2014, Vol. 11, No. 6 887

Enantioselective Henry Reaction Catalyzed By Supported Transition Metal Complexes

and catalyst 30 represents a complex solid supported on mesostructured cellular foams (MCF) (pore size 7-35 nm) (Scheme 15). Both these catalysts were tested for the reaction of a series of aldehydes with nitromethane giving functionalized (S)-2-nitroethanols (29 up to 99% ee; 30 up to 96% ee). The better enantioselectivity of catalyst 29 was explained by the fact that the carrier MCF has greater pore sizes and a more robust construction (framework) than carrier SBA-15. Catalyst 29 was recycled four times for the reaction of benzaldehyde with nitromethane: the yield mildly decreased (97  95 %), and the decrease in enantioselectivity was also insignificant (97  96%) [45] (Scheme 16). O R

OH

cat. 10 mol % H

+ CH3NO2

EtOH, rt, 40h

R

NO2

R: Ph, 2-MeOPh, 3-MeOPh, 4-MeOPh, 4-ClPh, 4-FPh, 4-BrPh, 4-F3CPh, 4-NO2Ph, 4-MePh, c-C6H11, n-C5H11, PhCH=C(CH3)

29

O

61–97% 5–99% ee

Si O O

HN

30

75–94% 9–96% ee

NH

Cu O

29 SBA-15 30 MCF

of asymmetric Henry reaction is intensively developing at present, and in the future it can be expected that the number of papers dealing with immobilized catalysts will increase. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS The authors acknowledge financial support from The Czech Science Foundation (GAR) Project No 14-00925S. ABBREVIATIONS AcOH

=

Acetic acid

AIBN

=

Azobisisobutyronitrile

BET

=

BINOL = CT

=

Charge transfer

DVB

=

1,4-Divinylbenzene or a technical mixture of 1,3and 1,4-divinylbenzene

MCF

=

Meso-structured cellular foams (pore size 7-35 nm).

MCM-41 =

O O

R: Ph, 4-MeOPh, 4-ClPh, 4-F3CPh, 4-MePh, n-C5H11 Scheme 16.

CONCLUSION This mini-review was focused on the coordination catalytic systems originally homogeneous, designated for the Henry reaction, which were later supported by various types of carriers. The aim of their immobilization was the possibility of their reuse and easy separation from the reaction mixture. The summarization of results showed that the key properties of the chosen polymeric carriers are their solubility or swelling ability of polymeric matrix. A significant role plays the character of surfaces of highly cross-linked macroporous polymers orinorganic carriers, particularly specific surface, size and shape of the pores. The methodology of the way of anchoring of the complexes was not discussed in detail; however, it follows from the given chemical structures of the catalysts. The main criteria in evaluation of the effect of solid supporting upon the effectiveness of the catalyst were the influence on overall yield of reaction, enantioselectivity, possibility of recycling, and effect of the recycling on the above-mentioned parameters. Efforts were made to compare the original catalysts with their solid supported versions under comparable reaction conditions. Generally, it can be stated that in predominant number of examples, the solid supporting led to a certain lowering of chemical yields as well as to lowered enantioselectivity. In several cases, we described a distinct negative effect of carrier, especially upon the reaction rate. In fewer cases, we observed increases in both chemical yield and enantioselectivity upon solid supporting. However, interpretation of such slightly better results obtained with solid supported versions of catalysts is complicated also by the fact that the compared reactions were not performed by the same authors, which could lead to differences in reaction conditions. On the other hand, a certain example of reaction was described in which the monitored parameters were influenced positively to a very distinct extent, due to prevention of formation of little efficient dimeric complex [30]. The application area

Brunauer-Emmett-Teller adsorption isotherm 1,1-Bis(2-naphthol)

Mesoporous silica (up to 1500 m2 g-1, pore size 2-10 nm)

PEG

=

Poly(ethylene glycol)

PS

=

Poly(styrene)

PS-DVB =

Styrene-divinylbenzene copolymer

SBA-15 =

Mesoporous silica

ZSM-5

Mesoporous zeolite

=

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Accepted: July 25, 2014