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Recent Advances in Organocatalytic Asymmetric Morita−Baylis− Hillman/aza-Morita−Baylis−Hillman Reactions Yin Wei and Min Shi* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China 10. Latest Developments Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

1. INTRODUCTION The carbon−carbon bond forming reactions is one of the most important reactions in organic chemistry, and therefore has been and remains an important and a fascinating area in organic synthesis. Among these carbon−carbon bond forming reactions, the Morita−Baylis−Hillman (MBH) reaction has become one of the most useful and popular carbon−carbon bond forming reactions with enormous synthetic utility, promise, and potential. The classical MBH reaction can be broadly defined as the formation of α-methylene-β-hydroxycarbonyl compounds by addition of α,β-unsaturated carbonyl compounds to aldehydes catalyzed by tertiary amine or phosphine (Scheme 1). Instead of aldehydes, imines can also participate in the reaction if they are appropriately activated, and in this case the process is commonly referred to

CONTENTS 1. Introduction 2. Recent Mechanistic Insights into the MBH/azaMBH Reaction and Its Asymmetric Version 2.1. Amine-Catalyzed Mechanism 2.2. Phosphine-Catalyzed Mechanism 2.3. Mechanistic Insights into the MBH/aza-MBH Reaction Using Cocatalytic Systems or Multi-/ Bifunctional Catalysts 2.4. Stereoselectivity of MBH/aza-MBH Reaction 3. Asymmetric Induction with Substrates 4. Catalytic Asymmetric Induction with Chiral Lewis Bases 4.1. Catalytic Asymmetric Induction with Chiral Amine Catalysts 4.2. Catalytic Asymmetric Induction with Chiral Phosphine Catalysts 5. Catalytic Asymmetric Induction with Chiral Lewis Acids 6. Catalytic Asymmetric Induction with Chiral Brønsted Acids 6.1. Catalytic Asymmetric Induction with Chiral Thioureas 6.2. Catalytic Asymmetric Induction with Proline Derivatives 6.3. Catalytic Asymmetric Induction with Chiral Thiols 7. Recent Transformation of MBH Adducts Catalyzed by Organocatalysts 7.1. Allylic Substitution Reactions of MBH Acetates and Carbonates 7.2. Annulation of MBH Acetates and Carbonates with Electron-Deficient Olefins 8. Recent Developments in Asymmetric Rauhut− Currier Reaction 9. Conclusions © XXXX American Chemical Society

AA AD AD AD AD AD AD AD

A C C F

F G H I

Scheme 1

I M O P

Scheme 2

P R T T T X Y AA

Received: May 13, 2012

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dx.doi.org/10.1021/cr300192h | Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 3

Scheme 4

MBH reaction can be attributed to its several advantages as follows: (i) the starting materials are commercially available and the reaction is suitable for large-scale production; (ii) the atomeconomic nature; (iii) the MBH adducts are flexible and multifunctionalities which could be easily transformed to other synthetically interesting products; (iv) it usually involves a nucleophilic organocatalytic system without the heavy-metal pollution; (v) it can occur under mild reaction conditions. In recent years, several major reviews4 and mini reviews5 on this fascinating reaction regarding the development of this reaction and its applications have been published. Since Basavaiah4e and Lamaty’s major review4d on MBH/aza-MBH reactions were published, which covers the advances of MBH/ aza-MBH reactions before the end of 2008, there still has been a boom of research results on MBH/aza-MBH reactions in recent

as the aza-Morita−Baylis−Hillman (aza-MBH) reaction. The origin of the MBH reaction can date back to 1968 to a pioneering report presented by Morita (phosphine-catalyzed reaction),1 and then Baylis and Hillman described a similar amine-catalyzed reaction in 1972.2 Though this reaction is promising and fascinating, unfortunately, it has been ignored by organic chemists for almost a decade after its discovery. At the beginning of the 1980s, organic chemists such as Drewes, Hoffmann, Perlmutter, and Basavaiah started looking at this reaction and exploring various aspects of this important reaction.3 In particular, since the mid-1990s, particularly in recent decades this reaction and its applications have received remarkable growing interest, and the exponential growth of this reaction and the importance of this reaction are evidenced by numerous research papers. The reasons for the fast growth of MBH/azaB


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years. This review mainly focuses on the advances of asymmetric MBH/aza-MBH reactions from 2009 to 2011. We hope that this review will satisfy the expectations of readers who are interested in the development of the field and looking for up-to-date information on the chemistry of MBH/aza-MBH reactions.

Scheme 8

2. RECENT MECHANISTIC INSIGHTS INTO THE MBH/AZA-MBH REACTION AND ITS ASYMMETRIC VERSION

Scheme 9

2.1. Amine-Catalyzed Mechanism

Although the elementary steps of the MBH reaction have been described in the earliest publications,1a the exact reaction Scheme 5

mechanism, in particular those controlling the asymmetric induction, has been debated frequently and remains as the core of the mechanistic discussion. The commonly accepted mechanism for the MBH reaction was first proposed by Hoffmann3b and supported by kinetic data studied by Hill and Isaacs6 in the late 1980s and others.7 Their proposed mechanism is described in Scheme 2. The catalytic cycle is initiated by the conjugate addition of a tertiary amine 1 to an electron-deficient alkene 2, such as acrylonitrile, to generate the zwitterionic amine-acrylate 3. In step II, the species 3 then attacks the aldehyde 4, leading to formation of the intermediate 5 via an aldolic addition reaction. The following intramolecular proton shifts within 5 to form 6 in step III, which subsequently generates the final MBH adduct and releases the catalyst 1 via E2 or E1cb elimination in step IV. Through the kinetic studies by Hill and Isaacs, using acrylonitrile as an electron-deficient alkene and acetaldehyde as a carbon nucleophile for the MBH reaction, step II was initially suggested as the MBH rate-determining step (RDS, Scheme 2), due to the low kinetic isotopic effect (KIE = 1.03 ± 0.1). This suggested mechanism was also supported by subsequent independent investigations including isolation of one intermediate in the

Scheme 6

Scheme 7

C


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Scheme 10

both mechanisms proposed by McQuade et al. and Aggarwal et al. are possible. Besides kinetic studies, theoretical studies on the MBH mechanism were conducted initially by Xu13 and Sunoj.14 Subsequently, Aggarwal and co-workers performed an extensive theoretical study, which supported their own kinetic observations and those of McQuade about the proton transfer step.15 Two distinct pathways leading to the products were proposed: (i) a second molecule of aldehyde participates the reaction to form a hemiacetal alkoxide hemi1 followed by rate-limiting proton transfer as proposed by McQuade (non-alcohol-catalyzed pathway) and (ii) an alcohol acts as a shuttle to transfer a proton from the α-position to the alkoxide of int2 (Scheme 4). In addition, a few computational studies recently appeared in the literature attempting to address mechanistic questions in the MBH reaction, including the addition of explicit water or methanol molecules.16 However, some limitations exist in all these studies. The popular B3LYP functional which may not be appropriate for this system was always used, and only potential electronic energies were employed to describe the energetics. It should be mentioned that Sunoj et al.14,17 have employed interesting CBS-4 M and mPW1K methods to compute the free energies for the reaction pathways. However, they just compared the energetics of the direct proton transfer pathways (via a fourmembered transition structure) and the 1,3-proton transfer assisted by water and did not consider the possibility of the influences by a second molecule of aldehyde or other protic species. More recently, Cantillo and Kappe presented a detailed computational and experimental reinvestigation on the aminecatalyzed MBH reaction of benzaldehyde with methyl acrylate.18 They have proven that it was impossible to accelerate the reactions through variable-temperature experiments and MP2 theoretical calculations of the reaction thermodynamics. The complex reaction mechanism for the MBH reaction has been revisited using the M06-2X computational method. The results provided by this theoretical approach are in agreement with all the experimental/kinetic evidence such as reaction order, acceleration by protic species (methanol, phenol), and autocatalysis. They also pointed out that the suggested pathways (Aggarwal and McQuade pathways) are competing mechanisms, and either of two mechanisms is more favored depending on the specific reaction conditions. Very recently, Eberlin and Coelho have investigated the mechanism of aza-MBH via the ESI-MS(/MS) technique and proposed a rational mechanism for the aza-MBH reaction.19 They monitored the DABCO-catalyzed aza-MBH reaction of

Figure 1.

catalytic cycle which was confirmed by X-ray analysis8 and the interception of all key intermediates using electrospray ionization with mass and tandem mass spectrometry.9 However, McQuade et al.10 and Aggarwal et al.11 re-evaluated the MBH mechanism through kinetic and theoretical studies, focusing on the proton-transfer step and proposed the protontransfer step as RDS. McQuade observed that the MBH reaction was second order relative to the aldehyde and showed a significant kinetic isotopic effect (KIE: kH/kD = 5.2 ± 0.6) in DMSO, and primary KIE (>2) was found in other tested solvents (DMF, MeCN, THF, CHCl3), indicating the relevance of proton abstraction on the RDS. On the basis of these kinetic data, McQuade proposed a new mechanism view for the protontransfer step (Scheme 3), suggesting the proton transfer step as the RDS. Aggarwal also proposed that the proton transfer step was the rate-determining step based on their kinetic studies, but only at its beginning (≤20% of conversion), and then step II was the RDS when the product concentration built and proton transfer became increasingly efficient. They suggested that the MBH adducts 10 may act as a proton donor and therefore can assist the proton-transfer step via a six-membered intermediate (Scheme 3). This model also explained the autocatalytic effect of the product. More recently, Elberlin and Coelho performed complementary investigations on the MBH reaction mechanism via electrospray ionization mass spectrometry (ESI-MS) (/MS).12 New key intermediates for the RDS of the MBH reaction have been successfully intercepted and structurally characterized, which provide strong experimental evidence that D


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Scheme 11. Proposed Catalytic Paths with Reaction Intermediates and Transition States Computationally for the L-Proline/ Imidazole-Catalyzed Formation of (R)-32a

Free energy (kJ mol−1) calculated at 0 °C at the B3LYP/6-31G(d,p) (plain text) and PCM B3LYP/6-31++G(d,p)B3LYP/6-31G(d,p) (italic) levels of theory. The lowest-energy reaction path is indicated by bold arrows.

a

Miller and co-workers have performed kinetic studies on a pyridylalanine-peptide catalyzed enantioselective coupling of allenoates 16 and N-acyl imines 17 to investigate the mechanism of the aza-MBH reaction.20 In the catalytic cycle of a typical MBH/aza-MBH reaction, the proton transfer step is often

methyl acrylate 2 with imine 12 by ESI-MS(/MS) spectrometry and intercepted the key intermediates 13, 15 and a unique bissulfonamide intermediate 14. On the basis of their results, they proposed the mechanistic cycle for the aza-MBH reaction as depicted in Scheme 5. E


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Scheme 12

temperatures, since it is not observed in reactions such as the MBH reaction involving the more reactive α,β-unsaturated ketones under mild conditions. Sunoj and co-workers have also done theoretical studies on the mechanism of the trimethylphosphine catalyzed aza-MBH reaction between acrolein and mesyl imine.17b They found that the relative energies of the crucial transition states for the PMe3catalyzed reaction are lower than those of the corresponding NMe3-catalyzed reaction. The kinetic advantage of the PMe3catalyzed reaction is also evident in the proton transfer step, where the energies of the transition states are much lower than those of the corresponding NMe3-catalyzed reaction. These predictions are consistent with the available experimental reports where faster reaction rates are in general noticed for the phosphine-catalyzed aza-MBH reaction.21 Most recently, Tong et al. isolated a stable phosphonium− enamine zwitterion 22, which has long been postulated as one of the key intermediates in the aza-MBH reaction, from the PPh3catalyzed reaction between propiolate and N-tosylimine (Scheme 8), which provides some experimental evidence to support the postulated reaction mechanism of the phosphinecatalyzed MBH reaction.22

Scheme 13

considered as the RDS. In comparison to typical MBH/azaMBH reactions through mechanistic experiments, including kinetics and hydrogen/deuterium kinetic isotope effects, they have found that the catalyst addition to allenoate becomes the RDS in this pyridylalanine-peptide catalyzed aza-MBH reaction (Scheme 6). 2.2. Phosphine-Catalyzed Mechanism

The most likely mechanism of the MBH/aza-MBH reaction catalyzed by tertiary phosphines is identical to that of the aminecatalyzed reaction via path a shown in Scheme 7, giving the normal MBH/aza-MBH adducts 19. In principle, the initially formed zwitterionic intermediate 18 during the phosphinecatalyzed MBH/aza-MBH reaction can isomerize to phosphorus ylide 20, which can then undergo a Wittig reaction to give olefins 21 (Scheme 7, path b). The latter process may require elevated

2.3. Mechanistic Insights into the MBH/aza-MBH Reaction Using Cocatalytic Systems or Multi-/Bifunctional Catalysts

Besides using traditional Lewis base catalysts in the MBH/azaMBH reaction, the cocatalysts have been often used in the MBH/aza-MBH reaction to accelerate the reaction. Recently, F


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phenolic Brønsted acid, in an asymmetric aza-MBH reaction.28 They have performed the kinetic experiments to investigate the mechanism of asymmetric aza-MBH reaction catalyzed by trifunctional catalyst. The catalysis was found to be first order in the trifunctional catalyst with the Michael addition as the ratelimiting step.

several mechanistic studies have been carried out to investigate the mechanism of the MBH/aza-MBH reaction using cocatalytic systems. Eberlin and Coelho studied the MBH reaction between 2-thiazolecarboxaldehyde and methyl acrylate in the presence of DABCO and thiourea by ESI-MS(/MS) and density functional theory (DFT) techniques.23 The key intermediates were intercepted and characterized by ESI-MS(/MS) and suggested that thiourea 23 acted as an organocatalyst in all steps of the MBH reaction cycle shown in Scheme 9, including the ratelimiting proton-transfer step. The DFT calculations confirmed this suggested a catalytic cycle and also revealed that the thiourea did not act as a proton shuttle in the rate-limiting proton-transfer step; instead, it acted as a Brønsted acid stabilizing the basic oxygen center being formed in the transition state and decreased the barrier of the RDS, thus accelerating the reaction. Sunoj and co-workers have identified the role of protic cocatalysts such as water, methanol, and formic acid in the MBH/aza-MBH reaction by theoretical studies.17b They found that the protic cocatalysts had a profound influence in decreasing the activation barriers associated with the key elementary steps due to the improved stabilization of the proton transfer transition state through a relay mechanism. The MBH/aza-MBH reaction involving proline as a catalyst with imidazole as a cocatalyst was proposed to proceed through an iminium ion intermediate. Recently, Santos and co-workers have studied the mechanism of proline-catalyzed and imidazoleco-catalyzed intramolecular MBH reaction by DFT calculations.24 They first investigated the catalytic path for the MBH reaction of the α,β-unsaturated dialdehyde catalyzed by L-proline in the absence of imidazole and found that water acted as an important catalyst when imidazole was not present. When imidazole was used as a cocatalyst, water was still important in the imidazole addition step. Their results rationalized the experimental outcome of the intramolecular MBH reaction and provided theoretical evidence to some mechanistic proposals. Oh and co-workers have performed the mechanistic investigation on the proline-catalyzed asymmetric MBH reactions of vinyl ketones in the presence of brucine N-oxide 24 as a cocatalyst.25 In this dual catalytic system, proline is believed to form iminium intermediates A with electron-deficient aryl aldehydes, while the N-oxide activates vinyl ketones to provide enolates B through conjugate addition (Scheme 10). Upon the combination of these two intermediates, the MBH products with high enantioselectivities are obtained by controlling the RDS through the H-bridged chairlike transition state C. The bifunctional strategy has been successfully used to design new organocatalysts for the MBH/aza-MBH reaction in recent years. In the bifunctional strategy, a Lewis base and a Brønsted acid can be crafted onto one chiral backbone to act cooperatively in the MBH reaction cycle. The Lewis base functionality serves to initiate the Michael addition step of the reaction, and the Brønsted acidity is thought to stabilize the zwitterionic intermediates and promote the subsequent aldol and protontransfer-elimination step. The first bifunctional catalyst, which is a hydroxylated chiral amine derived from cinchona alkaloids, for high enantioselective MBH reaction was reported by Hatakeyama in 1999.26 Subsequently, Shi and co-workers first reported a bifunctional phosphorus catalyst bearing an artificial chiral backbone for the high enantioselective aza-MBH reaction.27 Recently, Liu and co-workers developed this strategy and employed a trifunctional catalyst 25 (Figure 1), which involves the phosphine Lewis base, the nitrogen Brønsted base, and the

2.4. Stereoselectivity of MBH/aza-MBH Reaction

The MBH/aza-MBH adduct has only one stereogenic center; however, several intermediates and transition states during the reaction process have more than one chiral center, which makes the studies on the stereoselectivity of MBH/aza-MBH reaction more complicated. Aggarwal first proposed models to account for the stereoselectivity of the MBH/aza-MBH reaction.15 They suggested that all four diastereomers of the intermediate alkoxide are formed in the reaction, but only one has the hydrogen-bond donor suitably positioned to allow fast proton transfer, while the other diastereomers revert back to starting materials. Although several mechanistic studies also proposed similar transition states to account for the stereoselectivity of the MBH/aza-MBH reaction, there are no studies to investigate the full reaction pathway of the enantioselective MBH/aza-MBH reaction until recently. Santos and co-workers have first investigated the proline-catalyzed and imidazole-co-catalyzed enantioselective intramolecular MBH reaction by DFT calculations.24 Their results indicated that proline played important role for selectivity in two different reaction steps, the cyclization and the addition of imidazole (Scheme 11). They also demonstrated that the imidazole addition step was the rate-limiting step, and the calculated results indicated that both imidazole addition and cyclization steps influenced overall selectivity of the reaction. Hu and co-workers investigated the mechanism of the MBH reaction between formaldehyde and methyl vinyl ketone (MVK) catalyzed by N-methylprolinol using the DFT method.16c They focused on the two reaction steps: C−C bond formation and hydrogen migration, which were commonly considered as the RDS under different reaction conditions, to investigate the stereoselectivity. In the presence of water, the hydrogen migration occurs via a six-membered ring transition state, and the corresponding energy barrier decreases dramatically, and therefore the RDS is the C−C bond formation step. The calculations indicate that the C−C bond formation step controls the stereochemistry of the reaction. In this step, the hydrogen bonding induces the direction of the attack of enamine to aldehyde from the -OH group side of N-methylprolinol. The energy-favored transition states are mainly stabilized by hydrogen bonding, while the chirality of the products is affected by the hydrogen bonding and the steric hindrance. The calculations correctly reproduce the major product in (R)-configuration, which is consistent with the experimental observation. In 2011, Chen and co-workers investigated the detailed mechanism for the thiourea-tertiary amine-catalyzed enantioselective aza-MBH reaction of nitroalkene and N-tosylimine by DFT calculations.29 They proposed the mechanism as shown in Scheme 12. The Lewis base catalyst 33 activates the nitroalkene 34 by the tertiary amine moiety via a weak covalent bond, generating the zwitterionic intermediate Int0. The nucleophilic addition of Int0 to N-tosylimine 35 affords Int2. Then the proton is transferred from the methyl group of 34 to the electronegative nitrogen of 35, which is followed by the generation of β-nitro-γ-enamine 36 and the recovery of catalyst. The proton transfer from methyl group of nitroalkene to the electronegative nitrogen of imine was identified as the rateG


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Scheme 14

Scheme 15

Scheme 16

limiting step via DFT calculations. The more favored TSs were stabilized by the noncovalent interactions (H-bonds, π−π stacking), leading to (3S,4R)-36 as the major product.

regarding organocatalytic asymmetric MBH/aza-MBH reactions will be reviewed in the following sections. Krishna and co-workers first reported that chiral aldehydes trans-(2R,3R) cyclopropanecarbaldehydes 37 underwent a facile MBH reaction with a variety of activated olefins in the presence of a catalytic amount of DABCO to furnish the corresponding adducts 38 in good yields and selectivities (Scheme 13).30 It was found that the ring conformation and substituents played a decisive role in the stereoselection of the product. Subsequently, they reported an innovative synthesis of 3-deoxy sugars in both D and L forms as exclusive products in high yield through a sequential MBH reaction of sugar-derived aldehyde 39 or 42 with ethyl acrylate and Lewis acid catalyzed reaction.31 It was

3. ASYMMETRIC INDUCTION WITH SUBSTRATES Since 2009, considerable efforts have been devoted to development of asymmetric MBH/aza-MBH reactions. Either employing chiral substrates or using chiral catalysts are straightforward strategies to achieve asymmetric MBH/aza-MBH reactions. Actually, only a few examples using chiral substrates to obtain asymmetric MBH/aza-MBH adducts were reported in recent years, which are summarized in this section. Numerous reports H


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demonstrated that sugar-derived aldehyde 39 or 42 with ethyl acrylate underwent the DABCO-catalyzed MBH reaction, producing divinyl carbinols 40 or 43, which were further transformed to the corresponding 3-deoxy sugars 41 or 44 as exclusive isomers in high yields in the presence of Lewis acid (Scheme 14). These products could also be employed as valuable components in the synthesis of sugar-modified nucleosides. Pinho e Melo et al. has demonstrated a DABCO-catalyzed azaMBH reaction of chiral allenes with imines to synthesize optically active α-allenylamines and 2-azetine derivatives.32 The use of (1R)-(−)-10-phenylsulfonylisobornyl buta-2,3-dienoate 45 with imines 46 as starting materials underwent the DABCO-catalyzed aza-MBH reaction smoothly, affording aza-MBH adducts 47 with S configuration and 2-azetine 48, whereas (1S)-(+)-10phenylsulfonylisobornyl buta-2,3-dienoate 49 leads to aza-MBH adducts 50 with R configuration and 2-azetine 51 (Scheme 15). The yields of products and the ratios of products 47:48 and 50:51 can be controlled by carefully selecting the reaction conditions or by tuning the electronic properties of the imines.

Scheme 19

Scheme 20

Scheme 21

4. CATALYTIC ASYMMETRIC INDUCTION WITH CHIRAL LEWIS BASES 4.1. Catalytic Asymmetric Induction with Chiral Amine Catalysts

The chiral tertiary amine catalysts based on the quinidine framework such as β-ICD for asymmetric MBH/aza-MBH Scheme 17 Scheme 22

Scheme 23

Scheme 18

reaction have been intensively investigated.5b Recent reports demonstrate that β-ICD is still an efficient chiral amine catalyst with good selectivity for asymmetric MBH/aza-MBH reaction

with respect to various substrates. Zhu’s group reported a β-ICD 52a or β-ICD-amide 52b catalyzed aza-MBH reaction between I


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Scheme 24

Scheme 25

Scheme 27

Scheme 26

Scheme 28

N-sulfonylimines and alkyl vinyl ketones, affording (R)-enriched product 53.33 This reaction was suitable for aromatic imines and aliphatic imines, affording the corresponding products in moderate to good yields with excellent enatioselectivities.

Interestingly, they found that adding a catalytic amount of βnaphthol 54 led to the same reaction with reversed J


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Scheme 29

Figure 2. Proposed transition structures.

Scheme 30

enantioselectivies, affording the product (S)-53 in excellent yields and enantioselectivities (Scheme 16). They hypothesized that the Mannich-type coupling step may become the RDS in the presence of achiral protic additive β-naphthol in this reaction, affording the (2S,3S) Mannich adduct 56 via a ternary Z-enolate complex 55. Subsequent β-naphthol-assisted β-elimination of (2S,3S) Mannich adduct 56 via a plausible six-membered cyclic transition state would then provide the observed (S)-aza-MBH adduct 53 (Scheme 17). They also performed a control experiment which indicated that both the amide-NH in 52b and phenol-OH in 54 were important for the high enantioselectivity observed in this catalytic system. Subsequently, Zhu’s group reported another β-ICD-amide catalyzed and β-naphthol cocatalyzed aza-MBH reaction using readily available α-amidosulfones 57 as substrates to afford uniformly the (S)-adducts 53 in high yields and excellent enantioselectivities (Scheme 18).34 This is a domino process in which the catalyst 52 served both as a base to trigger the in situ generation of N-sulfonylimine and then as a nucleophile to initiate the azaMBH reaction. This reaction underwent smoothly with respect to various substrates under mild reaction conditions, which provided an easy access to α-methylene-β-amino-β-alkyl carbonyl compounds with simple operations. Shi and co-workers almost simultaneously demonstrated the similar asymmetric aza-MBH reaction of N-protected imines 58 or N-protected α-amidoalkyl phenyl sulfones 57 with MVK or EVK catalyzed by β-ICD, affording highly enantioselective azaMBH products 59 in good yields with high enantioselectivities (Scheme 19).35 Besides mild reaction conditions and operational simplicity since it avoided handing of unstable preformed imines, the reaction was found to be general with respect to various Nprotected imines. Subsequently, Shi’s group reported a β-ICDcatalyzed asymmetric MBH reaction of isatin derivatives 60 with acrylates to afford 3-substituted 3-hydroxy-2-oxindoles 61 in good yields with high enantioselectivities (Scheme 20).36 This is a first example to employ isatin derivatives as activated ketones in MBH reaction, which demonstrates an efficient synthetic method for the catalytic asymmetric construction of quaternary stereocenter. The obtained MBH adducts 3-substituted 3hydroxy-2-oxindoles could be facilely transformed to 3-aryl-3hydroxypyrrolidin-2-ones 62 with chirality remaining, which were precursors of promising drug candidates for treatment of HIV-1 infection. Soon after Shi’s report, Lu’s group demonstrated the almost same β-ICD-catalyzed asymmetric MBH reaction of isatin derivatives 63 with acrylates. They also showed that β-ICD was an efficient catalyst for this reaction, affording 3-substituted 3-hydroxy-2-oxindoles 64 in good yields with high enantioselectivities (Scheme 21).37 They pointed out that the C6′-OH group of β-ICD is probably to facilitate the key proton transfer step in the MBH reaction, via an intramolecular proton relay process. Meanwhile, Zhou and co-workers presented a β-ICD catalyzed MBH reaction of isatin derivatives 65 and acrolein to provide enantiomerically enriched 3-substituted 3-hydroxyox-

Scheme 31

Scheme 32

K


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Scheme 33

affording the MBH products in good yields with enantiomeric excesses up to 73%. Liebscher and co-workers have demonstrated a novel dual catalytic system composed of chiral α-guanidininoester 98 and triphenylphosphine as an efficient catalytic system for the asymmetric MBH reaction (Scheme 25).41 This catalytic system provided the MBH products 99 in high yields with good enantionselectivities up to 88% in the asymmetric MBH reactions of aromatic aldehydes with methyl acrylate. However, other Michael acceptors such as acrylonitrile or MVK were not suitable for these asymmetric MBH reactions. Very recently, Terada reported an efficient guanidine/azole cocatalytic system for the MBH reaction and achieved fairly good enantioselectivies by using an axially chiral guanidine/azole binary catalytic system.42 The MBH reaction was investigated with a series of aldehydes 100 and cyclic enones 101 using tetramethylguanidine 102 and an azole 103 as the cocatalytic system in THF at room temperature (Scheme 26). A range of aromatic aldehydes having different substituents underwent the reaction smoothly, giving the products 104 in moderate to high yields. The related enantioselective MBH reaction of 4chlorobenzaldehyde and cyclpentenone was also investigated using chiral guanidine 105/azole 106 cocatalytic system, affording the product in high yield with moderate enantioselectivity (Scheme 26). The proposed mechanism is shown in Scheme 27. In contrast to the generally accepted mechanism in which stable zwitterionic intermediates should be formed, the guanidine/azole binary system can be embedded into the catalytic cycles of the MBH reaction without the formation of zwitterionic intermediates, just through electrostatic interaction and hydrogen bonding. Takizawa and Sasai developed a new class of acid−base bearing an imidazole unit chiral organocatalysts 107 and 108 for aza-MBH reaction of conjugated nitroalkenes 109 with imines 110 (Scheme 28).43 They investigated the substrate scope under

Scheme 34

indoles 66, which could serve as valuable synthetic building blocks.38 They have shown that a variety of isatins worked well with acrolein to give the MBH adducts in good yields with excellent ee’s (Scheme 22). The obtained MBH adducts could be easily transformed into other compounds which can be potentially used for the synthesis of the analogues of natural products. In 2010, Connell and co-workers screened a series of chiral amine nucleophiles 67−72 for the asymmetric MBH reaction of aromatic and aliphatic aldehydes with cyclopentenone in the presence of MgI2 as a cocatalyst (Scheme 23).39 They identified that the Fu’s planar chiral DMAP catalyst 67 was the most efficient catalyst for this asymmetric MBH reaction of a variety of aromatic aldehydes or aliphatic aldehydes with cyclopentenone, affording the MBH products 74 in good to excellent yields and moderate to excellent enantioslectivities. They have mentioned that Lewis acid MgI2 as a cocatalyst could accelerate the reaction rate. Rouden and Maddaluno screened 20 new and easily prepared diamines 75−94 for the asymmetric MBH reaction of MVK and substituted benzaldehydes (Scheme 24).40 Chiral nonracemic 3(N,N-dimethylamino)-1-methylpyrrolidine 80 was found to promote efficiently the reaction of MVK with a variety of ortho- and para-substituted electron-deficient benzaldehydes, L


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Scheme 35

catalysis, namely, the combination of Lewis basic and Brønsted acidic sites within one chiral backbone, has proved a powerful strategy for design of novel efficient catalysts in MBH/aza-MBH reactions and its related reactions.5c,44 In 2009, Liu’s group further developed and applied this multifunctional strategy to design new chiral phosphines 25 containing a Lewis base, a Brønsted base, and a Brønsted acid moiety as shown in Figure 1. They first used these trifunctional chiral phosphine catalysts 25 to catalyze an asymmetric aza-MBH reaction between N-tosylimines and MVK. The reactions underwent smoothly with fast reaction rates at room temperature, affording the corresponding products in good yields and enantioselectivities (Scheme 29).45 This catalytic system required an acidic additive such as benzoic acid to confer its enantioselectivity and rate improvement for both electron-rich and electron-deficient imine substrates. They further rationalized the role of benzoic acid based on the hypothesis in which they proposed the favored transition state involving formation of hydrogen bonding and chiral ion pair between the catalyst and the benzoic acid after protonation (Figure 2, proposed transition structures). In the disfavored transition structure, the desired hydrogen bonding was not formed, and the rate of this pathway did not depend on the presence or absence of an ion pair. A counterion-facilitated proton transfer process required the lower energy barrier, which allowed the substrates to pass through this pathway faster than other competing pathways and accounted for

Scheme 36

the optimized reaction conditions. Regardless of whether the aromatic substituent R2 of imine 110 is electron-withdrawing or electron-donating, organocatalysts 107 and 108 which include the phenolic hydroxy groups as acidic functionalities and basic imidazole unit cooperatively activate nitroalkenes, promote this aza-MBH reaction, affording the products 111 in good yields with moderate enantioselectivities. This reaction has limitations for some substrates, such as 4-methoxy-1-(2-nitrovinyl)benzene, 2,4-dimethoxy-1-(2-nitrovinyl)benzene, (2-nitrovinyl)benzene, either leading to the desired products in low yields or without formation of the desired products. 4.2. Catalytic Asymmetric Induction with Chiral Phosphine Catalysts

Chiral phosphines have been intensively used as efficient catalysts in MBH/aza-MBH reactions.40 In a recent development of new chiral phosphines, the concept of multifunctional M


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Scheme 37

Figure 3.

Scheme 38

they developed a series of trifunctional chiral phosphine catalysts by tuning the acidity of the phenolic Brønsted acid group to improve catalytic efficiency.46 They found that compared to the catalyst 25a, the catalyst 25b with more acidic phenolic Brønsted acid group was more efficient for the aza-MBH reaction between N-tosylimines and MVK (Scheme 30). Furthermore, they designed and synthesized a series of new trifunctional catalysts 112b−112e with a NH2 or NHTs Brønsted acid moiety and investigated their performance in generic aza-MBH reactions.47 Using catalyst 112d, better enantioselectivity was observed for aza-MBH reactions at relatively low catalyst loading (2.5 mol %) under facile conditions, and the substrate scope of catalyst 112d was also investigated using a representative set of aryl imines and aryl aldehydes (Scheme 31). The para-substituted aryl imines could undergo this reaction smoothly in the presence of catalyst

Scheme 39

the observation that the rate enhancement and enantioselectivity improvement by using benzoic acid as an additive. Subsequently, N


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aromatic substituent of 123 is electron-withdrawing or electrondonating, acid−base organocatalyst (S)-121 promotes the reaction, affording products 125 in moderate yields with good to high enantioselectivities (Scheme 36). α,β-Unsaturated Ntosylimine was able to be used as a substrate for this reaction.? Wu and co-workers recently developed a new class of chiral phosphine-squaramide catalysts to promote the asymmetric intramolecular MBH reactions of ω-formyleneones.50 They first tested these new developed chiral phosphine-squaramide catalysts 126 and several other related phosphine catalysts 127−129 (Scheme 37) in this intramolecular MBH reaction. The chiral phosphine-squaramides 126a−c, containing different alkoxyl scaffolds, all exhibited high catalytic activities, and the MBH adducts were obtained in very good yields and enantioselectivities. Subsequently, the substrate scope was explored in the presence of phosphine-squaramide 126a. As shown in Scheme 37, the reactions worked well with acyclic substrates, bearing hydrogen, electron-withdrawing or electrondonating substituents on the phenyl group, to give the desired adducts in good to high yield (64−98%) and excellent enantioselectivity (88−93% ee). More recently, Lu’s group designed and prepared a series of novel bifunctional phosphine-sulfonamide organic catalysts 130−135 from natural amino acids as shown in Figure 3, which were utilized to promote enantioselective aza-MBH reactions.51 L-Threonine-derived phosphine-sulfonamide 135b was found to be the most efficient catalyst, and the reaction is applicable to a wide range of aromatic imines, affording the desired aza-MBH adducts in high yields and with excellent enantioselectivities (Scheme 38). Notably, the ortho-substituted aromatic imines, which are well-known to be difficult substrates for the aza-MBH reaction, were found to be suitable, and the products were obtained in nearly quantitative yields and with up to 97% ee. These results represent by far the best enantioselectivities attainable for the ortho-substituted substrates in the aza-MBH reaction. In addition, imines with heterocyclic rings were also applicable. A less satisfactory result was obtained for a cyclohexyl imine. Sasai and co-workers have developed a new spiro-type organocatalyst 136 having the Brønsted acid and Lewis base moieties for the enantioselective aza-MBH reaction.52 This bifunctional spiro-phosphine catalyst was found to show high asymmetric induction to yield aza-MBH products (Scheme 39).

112d, giving the desired products in higher ee values. However, for the ortho-substituted ones, the corresponding products were obtained in lower ee values, and for the aryl aldehydes, no reaction occurred under the standard conditions. They also demonstrated that the cooperativity between the counterion and the NHTs Brønsted acid of the trifunctional catalyst was required for good enantioselectivity and rate enhancement. Since Shi’s group first reported that 1,1′-bi-2,2′-naphthol (BINOL)-derived chiral bifunctional phosphine 113 could be used as an effective catalyst in asymmetric aza-MBH reaction of N-tosyl imines with MVK and phenyl acrylate,27 recently chiral bifunctional phosphine 113 has also been applied in aza-MBH with respect to other substrates and its related reactions. Shi and co-workers demonstrated the asymmetric aza-MBH reaction of N-protected α-amidoalkyl phenyl sulfones 114 with MVK catalyzed by catalyst 113, affording highly enantioselective azaMBH products 115 in good yields with high enantioselectivities (Scheme 32). Besides mild reaction conditions and operational simplicity since it avoided handling of unstable preformed imines, the reaction was found to be general with respect to various α-amidoalkyl phenyl sulfones.35 Sasai reported the first domino process based on the aza-MBH reaction catalyzed by bifunctional chiral phosphine (S)-113.48 On the basis of a bifunctional strategy, they envisioned that chiral 1,3-disubstituted isoindoline 118 could be acquired from enone 116 and Ntosylimine 117 with a Michael acceptor moiety at the ortho position in the presence of a bifunctional chiral organocatalyst bearing both Brønsted acid (BA) and Lewis base (LB) moieties. They proposed a catalytic cycle for the aza-MBH domino reaction as shown in Scheme 33. Initially, the Michael addition of the bifunctional chiral phosphine to enone 116 generates chiral enolate I which is stabilized by the BA moiety of catalyst. Subsequently, chiral enolate I reacts with N-tosylimine 117 to form intermediate II. At this stage, the reaction may undergo two divergent pathways. The first aza-MBH reaction pathway involves proton-transfer from the α position of the carbonyl group to the amine group and subsequent retro-Michael reaction of the organocatalyst, leading to the normal aza-MBH adduct 119. In the second pathway, the nitrogen anion of intermediate II could further react with the attached Michael acceptor intramolecularly to afford intermediate III, which undergoes proton-transfer and subsequent retro-Michael reaction to furnish the chiral isoindoline 118 along with regeneration of the organocatalyst. They screened a series of commonly used organocatalysts for this enantioselective aza-MBH domino reaction of enone with N-tosylimine, and they found that an acid−base organocatalyst (S)-113 was the most efficient catalyst to mediate this reaction. After optimization of the reaction conditions, they investigated the scope of aza-MBH domino reaction catalyzed by (S)-113, and the results are shown in Scheme 34. The synthetic utility of the highly functionalized azaMBH domino product was demonstrated through a variety of transformations. Subsequently, Sasai and co-workers developed another enantioselective aza-MBH domino process of α,β-unsaturated carbonyl compounds 122 and N-tosylimines 123 to afford tetrahydropyridine derivatives.49 They examined the catalyst (S)113 and several other known chiral organocatalysts for this azaMBH domino process (Scheme 35), and they found that the acid−base organocatalyst (S)-121 was the most efficient catalyst for this reaction, giving the product 124 or 125 in high enantioselectivity. The substrate scope under the optimized reaction conditions was investigated. Regardless of whether the

5. CATALYTIC ASYMMETRIC INDUCTION WITH CHIRAL LEWIS ACIDS Ryu and co-workers reported a highly enantioselective and Zstereocontrolled three-component coupling reaction of α,βacetylenic esters, aldehydes, and trimethylsilyl iodide (TMSI) using chiral cationic oxazaborolidinium catalysts (Scheme 40).53 Both the enantiomers of (Z)-β-iodo MBH esters (R/S) could be obtained enantioselectively by using an S- or R-oxazaborolidinium salt (137 or 137′) which behaves as chiral Lewis acids and has been proven to be an effective catalyst for Diels−Alder reactions, cyanosilylations, and Michael reactions. These esters can be directly converted into the optically active (Z)-βbranched derivatives with retention of configuration. These results are very useful in the synthesis of various optically active (Z)-β-branched MBH esters. Matsunaga, Berkessel, and Shibasaki found that La(O-iPr)3/ (S,S)-TMS-linked-BINOL 138 complex combined with a catalytic amount of DABCO could efficiently catalyze the azaMBH reaction of a broad range of N-diphenylphosphinoyl O


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Scheme 40

Scheme 42

Recently, Wu and co-workers developed a series of chiral bifunctional phosphinethioureas which were used as effective organocatalysts in the enantioselective MBH/aza-MBH reactions. They first synthesized a series of chiral bifunctional phosphinethioureas 142 (Scheme 43) and applied them to the enantioselective MBH reaction of aromatic aldehydes with acrylates.58 It is particularly noteworthy that this new catalytic system is suitable for various commercially available acrylates. With 8 mol % of phosphinothiourea 142e, the MBH reaction could proceed in 5−24 h under mild conditions and afford the desired products in moderate-to-excellent yields (up to 96%) with up to 77% ee (Scheme 43). Subsequently, they synthesized a new type of chiral bifunctional phosphinethioureas derived from L-valine which were also efficient for the asymmetric MBH reaction of acrylates with aldehydes.59 They evaluated these catalysts 142b−c, 143 in the asymmetric MBH reaction of 4nitrobenzaldehyde with methyl acrylate (Scheme 43) and found that catalyst 143a was the most efficient catalyst. Then, they found that the MBH reactions of various aldehydes with acrylates occurred smoothly in the presence of 10 mol % 143a, affording the desired product in moderate-to-excellent yields with good enantioselectivities up to 83% ee (Scheme 43). Later, they reported that these chiral cyclohexane-based phosphineureas and chiral bifunctional phosphinothioureas derived from L-amino

imines 139 with methyl acrylate.54 Aryl and heteroaryl imines were all suitable for this reaction, affording the desired products 140 in 77−99% yield and 81−95% ee. Alkenyl imines, the isomerizable alkyl imines, could be employed as well, giving products in 67−89% yield and 89−98% ee (Scheme 41). Kinetic studies pointed out the importance of both the nucleophilicity of La-enolate and the Brønsted basicity of a La-catalyst for promoting the reaction.

6. CATALYTIC ASYMMETRIC INDUCTION WITH CHIRAL BRøNSTED ACIDS 6.1. Catalytic Asymmetric Induction with Chiral Thioureas

Nagasawa first reported a highly efficient chiral thiourea catalyst for enantioselective MBH reaction in 2004.55 Subsequently, Jacobsen reported a chiral thiourea catalyst for the highly enantioselective aza-MBH reaction.56 In 2011, Ito found an efficient chiral biaryl-based bis(thiourea) organocatalyst 141 for asymmetric MBH reactions of 2cyclohexen-1-one with aldehydes.57 Good yields and high enantioselectivities were achieved in the reaction of 2-cyclohexen-1-one with both aromatic aldehydes and aliphatic aldehydes (up to 86% yield, 96% ee) (Scheme 42). Scheme 41

P


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Scheme 43

Scheme 44

Scheme 45

phosphinothiourea 142c, various ω-formyl-enone substrates underwent the enantioselective intramolecular MBH reaction smoothly, giving the desired products in good-to-excellent yields with up to 98% ee under mild reaction conditions (Scheme 44). These bifunctional phosphinothioureas 143 synthesized starting from different amino acids, including L-valine, L-alanine, and Lphenylalanine could promote the enantioselective intramolecular MBH reactions of ω-formyl-α,β-unsaturated carbonyl compounds, and the cyclic MBH products were obtained in good to-excellent yields with up to 84% ee in dichloromethane at room temperature (Scheme 44). More recently, they demonstrated that the chiral cyclohexane-based phosphineureas 142b were also efficient to catalyze the enantioselective MBH reaction of acrylates with isatins.62 In the presence of 10 mol % of phosphinothiourea 142b, the MBH reaction of acrylates with isatins could proceed smoothly to afford 3-substituted-3-

Scheme 46

acids were also efficient organocatalysts for the enantioselective intramolecular MBH reactions of ω-formyl-α,β-unsaturated carbonyl compounds.60,61 In the presence of 3 mol % of Q


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Scheme 47

in good yields (up to 95%) and high enantioselectivities (up to 95% ee) and diastereoselectivities (up to 1:99 dr) (Scheme 47). 6.2. Catalytic Asymmetric Induction with Proline Derivatives

Veselý have reported an organocatalytic highly enantioselective aza-MBH reaction of α,β-unsaturated aldehydes with in situ generated N-Boc and N-Cbz imines from the corresponding sulfones under mild and easy conditions.65 They first screened different catalysts 147a−e shown in Figure 4, solvents and base systems to achieve high enantioselectivities, diastereoselectivities, and yields. The (S)-proline 147a with DABCO was identified as the best catalytic system for this reaction, and CHCl3 was the best solvent choice. An excess amount of KF was also added, which was crucial for enhancement of the diastereoselectivity of the reaction. After the optimal reaction conditions were obtained, they investigated the reaction scope by using various α-amido sulfones and α,β-unsaturated aldehydes. Subsequently, the direct highly enantioselective addition of α,β-unsaturated aldehydes 148 to bench-stable N-carbamateprotected α-amido sulfones 149 was investigated again, giving the corresponding products 150 in good yields up to 87% with high enantioselectivities up to 99% ee (Scheme 48).66 They also reported a highly enantioselective organo-co-catalytic aza-MBH type reaction between N-carbamate-protected imines and α,βunsaturated aldehydes.66 Initially, they screened the cocatalytic systems, and the combination of (S)-proline 147a and DABCO was identified as the best cocatalytic system for this reaction. After optimization of reaction conditions, they investigated the scope of the catalytic entioselective addition of enals to N-Bocprotected imines. The corresponding β-amino aldehydes were obtained in good yields with high ee’s (86−99%) (Scheme 49). In 2010, Alemán and co-workers reported the first highly enantioselective oxa-Michael/aza-MBH tandem reaction between 2-alkynals and tosylimines leading to optically active 4amino-4H-chromenes using proline derivatives as organo-

Figure 4.

hydroxyl-oxindole derivatives in excellent yields (82−99%) and moderate enatioselectivities (up to 69% ee) (Scheme 45). Lu and co-workers also developed a series of chiral bifunctional phosphine-thiourea organocatalysts based on natural amino acid scaffolds.63 They first synthesized Lthreonine-derived bifunctional phosphine-thiourea catalyst 145 and found that it was an effective catalyst for asymmetric MBH reactions of acrylates with aromatic aldehydes. A range of aromatic aldehydes with different substituents were suitable for this asymmetric MBH reaction, affording the desired MBH adducts in good yields with up to 90% ee. They also investigated the influences of various additives such as MeOH, PhOH, 2naphthol, PhCOOH, and revealed that the hydrogen bonding interactions play a key role in the enantioselectivity. In 2009, Xu and co-workers reported the first example of a diastereo- and enantioselective aza-MBH-type reaction which was accomplished by the asymmetric synthesis of β-nitro-γenamines via a (1R,2R)-diaminocyclohexane thiourea derivative mediated tandem Michael addition and aza-Henry reaction.64 In the presence of catalyst 146, a variety of N-tosyl imines underwent this reaction smoothly, affording the desired products Scheme 48

R


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Scheme 49

Scheme 52

Figure 5.

Scheme 50

Scheme 51

Scheme 53

aromatic ring having electron-donating substituents (p-Me, oMeO, and p-MeO) did not affect the stereoselectivity, giving the corresponding products with ee’s in the range of 94−98%; electron-withdrawing group at the alkynal’s aromatic ring also gave the corresponding product with good enantioselectivity, however in low yield (55%). Decreasing the catalytic loading to 5 mol % did not diminish the yield and stereoselectivity. Other alkyl groups at para-position of alkynal’s aromatic ring, such as nPent and tBu also produced excellent ee’s with both 20 mol % and 5 mol % of catalyst. The reactions of alkynals bearing alkyl or alkenyl chains, instead of aryl ones, produced good stereoselectivity and isolated yields. Ramachary and co-workers have demonstrated the proline 147h/thiourea 154 cocatalyzed asymmetric MBH-type reactions from Hagemann’s esters 155 with nitroolefins 156 under

catalysts.67 A series of proline and its derivatives as shown in Figure 5 were screened for this reaction, and the results revealed that 147f was the best catalyst. In order to check the scope of the reaction, they explored reactions of different aryl, alkyl, and alkenyl alkynals 151 with 152 under the optimized conditions which were to carry out with the reaction catalyzed by 20 mol % or 5 mol % 147f in the presence of toluene at room temperature, affording the desired products 153 (Scheme 50). The alkynal’s S


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Scheme 54

ambient conditions.68 This novel asymmetric MBH-type reaction was suitable for a range of nitroolefins, affording the corresponding products 157 in fairly good yields with high enantioselectivities (Scheme 51). They also demonstrated the application of chiral MBH-type products in the synthesis of highly functionalized cyclohexenones.

Scheme 55

6.3. Catalytic Asymmetric Induction with Chiral Thiols

More recently, Miller and co-workers reported that orthomercaptobenzoic acid and ortho-mercaptophenols 158 could be used as efficient thiol catalysts in both the intramolecular MBH and Rauhut−Currier (RC) reaction, and they also demonstrated that chiral mercaptophenol afforded the reaction with low to moderate enatioselectivities (Scheme 52).69 Under established optimal reaction conditions, chiral catalysts (S)- and (R)-159 afforded high yields and moderate asymmetric inductions in the MBH reactions. The obtained enantioselectivities was not affected significantly by the amount of water and base added, catalyst loading, and substrate concentration but were markedly influenced by the reaction temperature. Interestingly, both increasing and decreasing the temperature from the established value of 70 °C resulted in lower ee values. The catalyst (R)-160 completely lost catalytic activity, indicating that a protic substituent in the ortho-position to the nucleophilic thiol plays a crucial role for catalytic activity.

7.1. Allylic Substitution Reactions of MBH Acetates and Carbonates

In 2001, Basavaiah first reported a SN2’ reaction on quinidinium salt of the Baylis−Hillman bromides to produce chiral Morita− Baylis−Hillman propargylic ethers.71 Recently, asymmetric transformations of MBH adducts via substitution of MBH adducts by nucleophiles as described in Scheme 53 have been commonly used and reported intensively. In 2004, Krische and co-workers reported the phosphinecatalyzed intermolecular allylic substitution reactions of MBH acetates, wherein N- and C-nucleophiles such as 4,5-dichlorophthalimide and 2-trimethylsilyloxyfuran (TMSOF) were utilized to generate allylic amines and γ-butenolides in high regioselectivities and in good yields, respectively.72 In 2008, Shi’s group developed catalyst 161, which was originally designed for aza-MBH reaction, achieved high yield and excellent ee for the allylic substitution of 2-trimethylsilyloxy furan, which is an effective approach for the asymmetric synthesis of γ-butenolides (Scheme 54).73 The experimental observations revealed that the active amide proton in 161 was critical to the catalytic reactivity and enantioselectivity. Subsequently, they designed and synthesized a series of novel multifunctional, chiral amide−

7. RECENT TRANSFORMATION OF MBH ADDUCTS CATALYZED BY ORGANOCATALYSTS The transformation of MBH adducts have attracted a lot of attention from organic chemists since they are synthetically important synthons.70 Organic chemists have continued to make their efforts on transformation of MBH adducts based on different strategies. Herein, we focus on recent reports for transformation of MBH adducts catalyzed by organocatalysts. T


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Scheme 56

(Scheme 54).74 NMR tracing experiments were conducted to

phosphane organocatalysts 162 for the allylic substitution of MBH acetate with 2-trimethylsilyloxyfuran for butenolide synthesis, which were suitable for a wide range of substrates in absolute MeOH or CH3CN, affording the desired products in good to excellent yield (42−98%) and high ee (85−99%)

identify the critical phosphonium intermediates in the catalytic cycles. Computational studies were also carried out to account for the origins of diastereo- and enantioselectivity, which U


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Scheme 57

suggesting that the active amino proton of the proline moiety might be dispensable in this reaction. More recently, Shi’s group further developed a series of chiral phosphine-thioureas 164−166, chiral amine 169 or used (DHQD)2PHAL 167, (DHQD)2PYR 168, as efficient organocatalysts for asymmetric substitutions of MBH adducts with various prenucleophiles 170−176 (Scheme 56).76 High yields and good enatioselectivities were achieved under mild reaction conditions with respect to a wide range of MBH adducts and various prenucleophiles such as phthalimide, oxazolones, diphenyl phosphite, pyrrole derivatives, carbamates, tosyl carbamates, and isatins.

revealed that the active proton of the amide moiety was the critical factor for the catalyst to have high enantiofacial control. Shi’s group also developed a series of L-proline derived chiral phosphine-amide catalysts and examined their performance. Using catalyst 163, they obtained the products in good yields with moderate ee’s, which could not be achieved successfully in previous studies (Scheme 55).75 Their results demonstrated that the chirality of proline moiety had some influences on the reaction outcomes but did not show any significant match/ mismatch between the chirality of binaphthol and proline. Replacing the active amino proton of proline by N-Boc group did not decrease the yield and enantioselectivity significantly, V


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Scheme 58

Scheme 59

Wang’s group developed an asymmetric organocatalytic allylic substitution reaction of MBH carbonates with phosphine oxides 185 or 186 using quinidine as a base catalyst.78 This organocatalytic approach provides an easy and efficient method for the direct preparation of optically allylic phosphine oxides with satisfactory yields and enantioselectivities (Scheme 58). More recently, they developed an asymmetric allylic substitution reaction of MBH carbonates with allylamines 187, affording Nallyl-β-amino-α-methylene esters in high yields and enantioselectivities (Scheme 58).79

Chen’s group has also developed a series of asymmetric substitutions of MBH adducts with various prenucleophiles 177−183 catalyzed by modified cinchona alkaloids β-ICD, (DHQD)2PHAL 167, (DHQD)2PYR 168, and (DHQD)2AQN 184 (Scheme 57).77 The corresponding products have been obtained in high yields (up to 98%) with good-to-excellent enantioselectivities (up to 99% ee). The allylic substituted products could be smoothly transformed into more complex compounds in good yields without any racemization. W


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Scheme 60

Scheme 64

Scheme 65

Scheme 61

Scheme 66

Scheme 62

Scheme 67

Scheme 63

because the in situ generated phosphorus ylides from MBH acetates and carbonates in the presence of tertiary phosphines are very reactive 1,3-dipoles in a variety of annulations. More recently, Zhang, Huang, and He and co-workers have also developed several MBH acetates and carbonates involving [4 + 1] annulations to give the corresponding annulation products in high yields, respectively (Scheme 59).80 In 2010, Tang and Zhou first reported the asymmetric version of intramolecular [3 + 2] annulation using the derivative of MBH adducts. They utilized spirobiindane-based chiral phosphines as catalysts to provide the corresponding products in good yields along with high ee values (Scheme 60).81

7.2. Annulation of MBH Acetates and Carbonates with Electron-Deficient Olefins

Among these transformations, annulation of MBH acetates and carbonates with electron-deficient olefins is an extremely useful synthetic method to construct multifunctional cyclic compounds X


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In 2011, Shi’s group reported a phosphine-catalyzed highly regio- and diastereoselective [3 +2] annulation of MBH carbonates with isatylidene malononitriles to produce spirocyclopenteneoxindoles in good yields under mild conditions (Scheme 61).82 They also demonstrated one example for an asymmetric version of the [3 + 2] annulation of MBH carbonate with isatylidene malononitriles catalyzed by the chiral bifunctional thiourea phosphine catalyst 191, affording the desired product in high yield with good enantioselectivity and diastereoselectivity (Scheme 62). Barbas and co-workers reported a similar enantioselective reaction of [3 + 2] annulation of MBH carbonates with methyleneindolines.83 The best catalyst for this reaction was chiral diphosphine 192. This reaction afforded the final spirocyclic products 193 in good yields with excellent stereoselectivities (Scheme 63). The clear limitation of this reaction is the low enantioselectivity obtained when alkyl MBH carbonates were used. Barbas and co-workers believe that the second diphosphine has a major impact on the stereoselectivity. In 2011, Lu’s group developed a threonine-derived phosphine thiourea catalyst 194 for the promotion of the stereoselective [3 + 2] cycloaddition process between the MBH carbonates and isatin-derived tetrasubstituted alkenes, giving the products in high yields with excellent enantioselectivities (Scheme 64).84 This strategy allows facile enantioselective preparation of biologically important 3-spirocyclopentene-2-oxindoles with two contiguous quaternary centers. Very recently, Shi and co-workers further developed a series of multifunctional thiourea-phosphines derived from natural amino acid and first applied them in asymmetric [3 + 2] annulations of MBH carbonates with trifluoroethylidenemalonates.85 The multifunctional thiourea-phosphine 195 was the best catalyst for this reaction, affording the highly functionalized trifluoromethyl- or pentafluoroethyl-bearing cyclopentenes in excellent yield (up to >99%) and enantioselectivity (up to 96%) (Scheme 65). Liu and co-workers reported a Me-DuPhos (196) catalyzed efficient asymmetric [3 + 2] cycloaddition reaction between MBH carbonates of isatins and N-phenylmaleimide.86 They first screened a series of chiral diphosphine reagents and then identified that the Me-DuPhos (196), which was commonly used as a ligand, was an efficient organocatalyst for [3 + 2] cycloaddition reaction between MBH carbonates of isatins and N-phenylmaleimide. A wide range of MBH carbonates derived from substituted isatins were suitable for this asymmetric [3 + 2] cycloaddition reaction, giving the corresponding spirooxindoles in good yields (up to 89%) with excellent diastereoselectivities and high enantioselectivities (up to 99% ee) (Scheme 66).

Scheme 68

Scheme 69

Scheme 70

Scheme 71

8. RECENT DEVELOPMENTS IN ASYMMETRIC RAUHUT−CURRIER REACTION The Rauhut−Currier (RC) reaction, also known as vinylogous MBH reaction, involves the coupling of one active alkene/latent enolate to a second Michael acceptor, creating a new C−C bond between the α-position of one activated alkene and the βposition of a second alkene under the influence of a nucleophilic catalyst.87 As a variant of MBH reaction, RC reaction, especially enatioselective RC reaction, has not been investigated intensively until now. Herein, we would like to overview the recent developments in asymmetric RC reaction since it may become another hot topic in the near future. The first enantioselective version of the intramolecular RC reaction was presented by Miller’s group using cysteine-based

catalyst.88 They examined a variety of traditional MBH catalysts and cysteine-based catalysts for the intramolecular RC reaction. They found that cysteine-based catalysts were effective catalysts and identified the catalyst 197 exhibits extraordinary reactivity and enantiotopic control ability. After extensive screening of reaction conditions, they demonstrated that electron-deficient Y


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Scheme 72

however, with low enantioselectivity (Scheme 69).69 Subsequently, they further developed a cysteine-based catalyst 197 catalyzed RC reaction, which was the key step in the quasibiomimetic synthesis of natural product Sch-642305 and related analogues (Scheme 70).89 In 2009, Christmann and co-workers developed an enantioselective organocatalytic RC-type cyclization of α,βunsaturated aldehydes catalyzed by the commercially available Jørgensen−Hayashi catalyst 147d with AcOH.90 The iridoid products, cyclopentene derivatives bearing a tetra-substituted olefin, were obtained in moderate to good yields (up to 73% yield) with good enantioselectivity (up to 96% ee) (Scheme 71). In 2011, Wu’s group also demonstrated an enantioselective RC reaction catalyzed by chiral phosphinothiourea derived from 91 L-valine. A wide substrate scope with respect to bis(enones) with symmetrical substituents 198 underwent this RC reaction smoothly, affording the desired products 199 in excellent yields (up to 99%) and enantioselectivities (90−99.4% ee) (Scheme 72, eq 1). In addition, the substrate 200 with unsymmetrical substituents, bearing both a strong electron-withdrawing group and a strong electrondonating group, was also investigated (Scheme 72, eq 2). A regioisomeric mixture in favor of 201a was obtained in total yield of 74%. Compared to the corresponding symmetrical substrates, the enantioselectivity was remarkably decreased. The results indicated that both the alkene activation by a nucleophilic catalyst and the coupling of an activated alkene to a second Michael acceptor had influence on the RC reaction. Meanwhile, Gu, Xiao, and co-workers extended this intramolecular RC reaction to nitroolefin enoates.92 The thiourea derivative 202 combined with achiral nucleophilic promoter 203 was identified as an efficient hydrogen-bonding catalyst for this intramolecular RC reaction with respect to a wide range of substrates, affording the corresponding RC product in high yields with good enantioselectivities (Scheme 73). Soon after, Shi’s group reported the first example of chiral amine catalyzed highly enantioselective intermolecular RC reaction of maleimides with allenoates and penta-3,4-dien-2one.93 The traditional catalyst β-ICD for MBH reaction could also catalyze this intermolecular RC reaction with respect to a wide range of substrates, affording the desired functionalized allene derivatives in good to high yields with good to excellent enantioselectivities (Scheme 74).

Scheme 73

Scheme 74

Scheme 75

and electron-rich aryl symmetrical bis(enones) as well as aliphatic and heteroaromatic bis(enones) were viable substrates in the presence of catalyst 197 under optimal reaction conditions (Scheme 67). Mechanistic studies also provided insights on the potential mechanism of the reaction and the suggested possible transition states shown in Scheme 68 that provide an explanation for the absolute stereochemistry formed in the observed products. More recently, Miller and co-workers showed that the aforementioned chiral thiol catalyst (R)-159 was also an efficient catalyst for intramolecular enantioselective RC reaction, Z


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Scheme 76

Very recently, Sasai developed a highly atom-economical, chemo-, diastereo-, and enantioselective Rauhut−Currier reaction catalyzed by amino acid based chiral phosphine 133 (Scheme 75).94a Aliphatic- and aromatic-substituted starting materials 204 were successfully cyclized to give the medicinally important product α-alkylidene-γ-butyrolactones 205 in good yields (up to 99%) with high enantioselectivities (up to 98% ee). Subsequently, they presented a report with more details on this highly chemo-, diastereo-, and enantioselective RC reaction and gave a few examples of RC product transformations.94b

Scheme 78

9. CONCLUSIONS During the past few years, different aspects of MBH/aza-MBH reactions, especially asymmetric MBH/aza-MBH reactions, have been studied intensively. In fact, significant developments have been made in the design of new chiral catalysts such as chiral amines, phosphines, and thioureas based on the concept of bi/ multifunctionality for the asymmetric version of the MBH/azaMBH reaction, and high enantioselectivities have been achieved. Although many important factors governing the reactions were identified, the present understanding of the basic factors, and the control of reactivity and selectivity, remains incomplete. There is no one catalyst which is suitable for all substrates so far, and thus the development of effective catalysts and catalyst diversity for asymmetric MBH/aza-MBH reactions that are applicable to most of the common activated alkenes and electrophiles still continues to be a challenging endeavor.

MBH reactions and the applications of MBH/aza-MBH adducts have appeared in the literature. Hatakeyama reviewed the applications of organocatalysts for enantioselective MBH/azaMBH reactions,95 and Takemoto also briefly overviewed the applications of bifunctional (thio)urea catalysts in aza-MBH reactions.96 Vasconcellos first highlighted the potentialities of MBH adducts as a new class of bioactive compounds to the discovery of new cheaper and efficient drugs.97 Wu’s group continually developed new chiral phosphinesquaramide catalysts 206 and employed them to catalyze the enantioselective MBH reaction of acrylates with isatins to construct 3-hydroxy-2-oxindoles with quaternary stereocenters.98 A variety of isatins and acrylates except for phenyl acrylate underwent this reaction smoothly, affording chiral 3-hydroxy-2oxindoles in good-to-excellent yields (up to 99%) with high enantioselectivities (up to 99% ee after a simple recrystallization) (Scheme 75). Very recently, they applied newly developed bifunctional phosphinothioureas 207 derived from saccharide to promote the enantioselective MBH reaction between acrylates and aldehydes.99 The desired MBH adducts were obtained in up to 96% yield and 83% ee under mild reaction conditions in the presence of glucose-based phosphineothiourea (Scheme 76). Zhou and co-workers first reported the asymmetric MBH reaction of aromatic aldehydes with acrolein catalyzed by β-ICD with 2,6-dimethoxybenzoic acid. The aromatic aldehydes with

10. LATEST DEVELOPMENTS Since this manuscript was submitted for publication, several mini reviews and new interesting reports on asymmetric MBH/azaScheme 77

AA


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Scheme 79

sequently, they developed AAA reaction of isatin-derived MBH carbonates with nitroalkanes catalyzed by β-ICD, affording the desired products in up to 92% yield with up to 92% ee (Scheme 79).105 Rios also reported the AAA reaction of MBH carbonates with 2-fluoromalonates catalyzed by β-ICD, affording the final fluorinated products in good yields and enantioselectivities (Scheme 79).106 Lu’s group107 and Shi’s group108 almost simultaneously reported the catalytic asymmetric [3 + 2] annulation of MBH carbonates with malaimides by using different chiral phosphine catalysts. Lu’s group used L-Thr-L-Val-derived phosphine 210 as catalyst to synthesize the functionalized bicyclic imides which were obtained in excellent yields, and with high diastereoselectivities and nearly perfect enantioselectivities (Scheme 80). Shi’s group developed a multifunctional thiourea-phosphine 211 which also efficiently catalyzed asymmetric [3 + 2] annulation of MBH carbonates with malaimides, affording the desired products in moderate to excellent yields and excellent diastereo- and enantioselectivities (Scheme 80). Subsequently, they applied this multifunctional thiourea-phosphine 211 in the asymmetric [3 + 2] annulation reactions of 2-arylideneindane-1,3-diones with MBH carbonates, producing the corresponding quaternary carbon centered spirocyclic cyclopentenes in moderate yields,

electron-withdrawing substituents would facilitate this reaction, giving the desired products in up to 87% yield with up to 81% ee (Scheme 77).100 Rinaldi also demonstrated to synthesize the azaMBH adducts mediated by cinchona alkaloids as promotors;101a they recently reported that β-ICD and its derivatives were efficient catalysts for MBH reactions of acrylates with aldehydes, and they also exploited the reaction mechanism through experimental and computational techniques.101b Shi’s group developed a new multifunctional chiral phosphine-amide type catalyst 208 which combined with the BINOL derivative could efficiently catalyze the asymmetric aza-MBH reaction of 5,5disubstituted cyclopent-2-enones with N-sulfonated imines, affording the corresponding products in good to outstanding yields with moderate to good ee’s under mild conditions (Scheme 78).102 They also demonstrated a highly enantioselective azaMBH reaction of isatin-derived ketimines with MVK catalyzed by chiral amine β-ICD or chiral phosphine 113.103 Lu’s group first applied phthalides in the asymmetric allylic alkylation (AAA) reaction with MBH carbonates to access optically enriched 3,3-disubstituted phthalides.104 By employing bifunctional chiral phosphines 194 or multifunctional tertiary amine-thioureas 209 as the catalyst, γ-selective or β-selective allylic alkylation products were obtained, respectively, in high yields with excellent enantioselectivities (Scheme 79). SubAB


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Scheme 80

Scheme 81

Scheme 82

with high diastereoselectivities and enantioselectivities under mild conditions (Scheme 80).109 Inspired by Lu’s work on chiral phosphine catalysts based on the skeletons of natural amino acids,84 Zhong and Loh synthesized the chiral phosphine catalysts derived from L-leucine and applied them on a catalytic asymmetric [4 + 2] annulation reaction initiated by an aza-RC reaction.110 A wide range of aryl 1-aza-1,3-dienes underwent the [4 + 2] annulation process with MVK or EVK smoothly in the presence of 10 mol % chiral phosphine 212, generating tetrahydropyridine adducts with exclusively trans diastereoselectivity and excellent enantioselectivity in high to excellent chemical yields (Scheme 81). Chi and co-workers also employed an amino acid derived chiral phosphine 133 to achieve an intramolecular aza formal [2 + 4] reaction between α,β-unsaturated imines and electron-deficient alkenes through a tandem RC/SN2-substitution sequence.111 More recently, Shi’s group reported a catalytic asymmetric intramolecular RC reaction catalyzed a highly nucleophilic

multifunctional chiral phosphine.112 They demonstrated that a highly nucleophilic multifunctional chiral phosphine 213 was an efficient catalyst for the asymmetric intramolecualr RC reaction of bis(enone)s, affording the corresponding cyclohexene 214 and cyclopentene 215 products in moderate to good yields and with good to high ee values under mild conditions (Scheme 82). AC


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AUTHOR INFORMATION

(21072206, 20472096, 20872162, 20672127, 21121062, 20732008, and 21102166).

Corresponding Author

*E-mail: [email protected]. Fax: 86-21-64166128.

ABBREVIATIONS Ac acetyl Ar aryl Bn benzyl Boc tert-butoxycarbonyl Bu butyl B3LYP Becke-3-Lee−Yang−Parr CBS-4M Complete Basis Set-4M Cbz carboxybenzyl Cy cyclohexyl DABCO 1,4-diazabicyclo[2.2.2]octane DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCE 1,2-dichloroethane DMF dimethyl formamide DMSO dimethyl sulfoxide Dppb 1,4-bis(diphenylphosphino)butane E2 bimolecular elimination E1cb 2-step, base-induced β-elimination Et ethyl EVK ethyl vinyl ketone EWG electron withdrawing group Hex hexyl β-ICD β-isocupreidine LG leaving group Me methyl MP2 Møller−Plesset perturbation theory for second order Ms mesyl MS Molecular sieve MVK methyl vinyl ketone Ph phenyl PMB para-methoxybenzyl Pr propyl TBDPS tert-butyldiphenylsilyl TBS tert-butyldimethylsilyl TDS thexyldimethylsilyl tert tertiary THF tetrahydrofuran TIPS triisopropysilyl TMS trimethylsilyl Tr trityl Ts tosyl PMP para-methoxyphenyl

Notes

The authors declare no competing financial interest. Biographies

Dr. Prof. Min Shi was born in Shanghai, China. He received his BS in 1984 (Institute of Chemical Engineering of East China, now named as East China University of Science and Technology) and PhD in 1991 (Osaka University, Japan). He had his postdoctoral research experience with Prof. Kenneth M. Nicholas at University of Oklahoma (1995-6) and worked as an ERATO Researcher in Japan Science and Technology Corporation (JST) (1996-8). He is currently a group leader of the State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (SIOC, CAS). His research interest is in photochemistry, total synthesis of natural products, asymmetric synthesis, Morita-Baylis-Hillman reaction, fixation of CO2 using transition metal catalyst.

REFERENCES (1) (a) Morita, K.; Suzuki, Z.; Hirose, H. Bull. Chem. Soc. Jpn. 1968, 41, 2815. (b) Morita, K. Japan Patent, 6803364, 1968. (2) Baylis, A. B.; Hillman, M. E. D. German Patent 2155113, 1972. (3) (a) Drewes, E.; Emslie, N. D. J. Chem. Soc., Perkin Trans. 1 1982, 2079. (b) Hoffmann, H. M. R.; Rabe, J. Angew. Chem., Int. Ed. Engl. 1983, 22, 795. (c) Basavaiah, D.; Gowriswari, V. V. L Tetrahedron Lett. 1986, 27, 2031. (d) Hoffmann, H. M. R.; Rabe, J. J. Org. Chem. 1985, 50, 3849. (e) Hoffmann, H. M. R.; Rabe, J. Helv. Chim. Acta 1984, 67, 413. (f) Perlmutter, P.; Teo, C. C. Tetrahedron Lett. 1984, 25, 5951. (4) (a) Drewes, S. E.; Roos, G. H. P. Tetrahedron 1988, 44, 4653. (b) Basavaiah, D.; Rao, P. D.; Hyma, R. S. Tetrahedron 1996, 52, 8001. (c) Basavaiah, D.; Satyanarayana, T.; Rao, A. J. Chem. Rev. 2003, 103, 811. (d) Declerck, V.; Martinez, J.; Lamaty, F. Chem. Rev. 2009, 109, 1. (e) Basavaiah, D.; Reddy, B. S.; Badsara, S. S. Chem. Rev. 2010, 110, 5447. (f) Singh, V.; Batra, S. Tetrahedron 2008, 64, 4511. (g) Ciganek, E. In Organic Reactions; Paquette, L. A., Ed.; John Wiley & Sons, Inc.: New York, 1997; Vol. 51, p 201.

Dr. Yin Wei was born in 1977 in Henan (P. R. China). She received her PhD from Ludwig-Maximilians-Universität in München (Germany) in 2009 under the direction of Professor Hendrik Zipse. Subsequently she joined in Professor Min Shi’s group at Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences (SIOC, CAS) as an assistant professor. She is currently working on the theoretical studies of organocatalysis.

ACKNOWLEDGMENTS We thank the Shanghai Municipal Committee of Science and Technology (11JC1402600), the National Basic Research Program of China (973)-2010CB833302, and the National Natural Science Foundation of China for financial support AD


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