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Recent progress in copper catalyzed asymmetric Henry reaction
Accepted Manuscript Title: Recent progress in copper catalyzed asymmetric Henry reaction Authors: Sheng Zhang, Yanan Li, Youguo Xu, Zhiyong Wang PII: DOI: Reference:
S1001-8417(17)30418-7 https://doi.org/10.1016/j.cclet.2017.10.001 CCLET 4274
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Chinese Chemical Letters
Received date: Revised date: Accepted date:
8-6-2017 26-8-2017 10-10-2017
Please cite this article as: Sheng Zhang, Yanan Li, Youguo Xu, Zhiyong Wang, Recent progress in copper catalyzed asymmetric Henry reaction, Chinese Chemical Letters https://doi.org/10.1016/j.cclet.2017.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Review
Recent progress in copper catalyzed asymmetric Henry reaction Sheng Zhang a,b, Yanan Li a, Youguo Xu a, Zhiyong Wanga,* a
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China b College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
Corresponding author.
E-mail address: [email protected] (Z. Wang)
Graphical Abstract Recent progress in copper catalyzed asymmetric Henry reaction Sheng Zhang a,b, Yanan Li a, Youguo Xu a, Zhiyong Wanga a
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China b
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
Henry reaction is one of the most classical reactions to construct synthetically useful product nitro alcohol, which as a privileged skeleton is widely distributed in various pharmaceuticals. This review summarizes the recent progress of copper-catalyzed asymmetric Henry reaction from 2011 to 2016. The significant progress that has been made in this area will be highlighted and some of challenges that the author believes may be hindering further progress will be revealed.
ARTICLE INFO: Article history:Received 8 June 2017 Received in revised form 26 August 2017 Accepted 21 September 2017 Available online ABSTRACT
Henry reaction is one of the most classical reactions to construct synthetically useful product nitro alcohol, which as a privileged skeleton is widely distributed in various pharmaceuticals. This review summarizes the recent progress of copper-catalyzed asymmetric Henry reaction from 2011 to 2016. The significant progress that has been made in this area will be highlighted and some of challenges that the author believes may be hindering further progress will be revealed. Keywords: Henry reaction Asymmetric catalysis Copper catalysis Nitro compounds C-C bond formation
1.
Introduction
The Henry or nitroaldol reaction represents a highly attractive synthetic method, which provides a facile and direct access to versatile building block β-nitroalcohol [1]. The difunctional group of β-nitroalcohol can be readily converted into other functional groups via a variety of transformations (Scheme 1) [2]. For the nitro group, it can be easily reduced to amino group under mild conditions (Path a). Additionally, the Nef reaction of nitroalkanes provides an alternative route to carbonyl compounds (Path b). With a radical condition, the nitro group can also be replaced with a hydrogen atom (Path c). With respect to the hydroxyl group, the dehydration process can be employed to prepare nitroolefin (Path d), which is one of the most popular Michael acceptors.
Due to the versatility of nitroalcohol, the Henry reaction has been widely applied in numerous synthetic ventures with the tremendous development of asymmetric Henry reaction (Fig. 1) [3]. For instance, the Henry reaction has been employed as a key step in the total synthesis of nucleoside antibiotics tunicamycin developed by Suami and coworkers [3a]. The synthesis of natural product tetrodotoxin starting from D-glucose was also achieved by using two-step Henry reactions in Sato group [3b]. Recently, an efficient method to construct marine alkaloid manzamine A, involving Henry and aza-Henry reaction, was reported by Dixon et al. [3c, 3d]. The above facts serve to emphasize that the great utility of Henry reaction in the practical synthesis. Consequently, considerable efforts have been devoted to the asymmetric Henry reaction since the pioneering work of Shibasaki in the first enantioselective Henry reaction [4]. Among the enormous reports, the copper catalyzed Henry reaction has occupied an important place for the advantages of flexible coordination mode, economical accessibility and the good tolerance for air and moisture [5]. Accordingly, in this respect, numerous chiral ligands were developed and tremendous progress was achieved. However, to the best of our knowledge, there was no specific review focused on the copper-catalyzed Henry reaction. Although Velmathi et al. summarized the transition metal catalyzed Henry reaction, covering some relative works on copper catalysis prior to 2011, a number of meaningful works dealing with new catalysts and concepts have recently emerged [6,7]. In this context, the recent advances in the field will be discussed in this digest and some previous papers will be introduced as background when necessary. To better illustrate the progress in the area, this review will be introduced according to the category of chiral ligands. 2.
Chiral diamine ligands
Chiral 1,2-diaminocyclohexane as a privileged skeleton has been widely employed in the synthesis of chiral diamine ligands, which turned out to be effective ligands for many metal-catalyzed asymmetric transformations [8]. For the Henry reaction, this catalytic system was first introduced by Arai in 2006 [9]. Recently, a chiral ligand bis(sulfonamide)-diamine L1 with multiple coordination sites was prepared by Wan and coworkers, which could readily coordinate with Cu(I) to form an active chiral catalyst (Scheme 2) [10]. This chiral copper catalyst was effective not only in the Henry reaction involving nitromethane but also for the reaction of substituted nitroalkanes. In particular, with the assistance of pyridine, the corresponding syn-selective adducts could be obtained with moderate to good diastereoselectivities (Scheme 2). In 2011, a range of C1-symmetrical diamine ligands were reported by Woggon to address the challenging diastereo-selectivity issue in the nitroaldol reaction of aliphatic aldehydes (Scheme 3) [11]. In order to find the optimal conditions for the reaction, a screening was performed using more than 60 chiral diamines-copper complexes. With the optimal ligand L2, various prochiral nitro compounds could react smoothly with 3-phenylpropionaldehyde giving the syn-selective product with 1.9/1-5.7/1 dr values and 90-99% ee values (Scheme 3). However, the shortcoming of this method was that a lower stereoselectivity was observed in the cases of aromatic aldehydes. Further investigation on the chiral diamine ligand structure showed that the alkyl substituent significantly affected the stereoselectivity in the Henry reaction [12]. The study of Gou et al. showed that branched diamine L3 was the optimal ligand in terms of the yield and enantioselectivity (Scheme 4). With this optimal ligand in hand, the generality of the substrate scope was examined in the presence of the corresponding copper complexes. Specifically, better enantioselectivities with 91-94% ee were observed for the aliphatic aldehydes compared with the aromatic counterparts (Scheme 4). Despite the admirable results in the Henry reaction of nitromethane, other nitroalkanes had not been tried in the report with the optimal catalyst. Very recently, the chirality induction mechanism in the diamine-copper catalyzed Henry reaction was investigated by Tanaka (Scheme 5) [13]. Interestingly, a significant positive nonlinear effect was observed when the reaction was performed in EtOAc, while a similar effect could not be detected in MeOH. This solvent-dependent nonlinear effect might originate from the difference solubility between the optically pure catalyst (R,R)-L4-Cu and the racemic one, especially in the aprotic organic solvents (Scheme 5). To the best of our knowledge, the strong solvent-dependent asymmetric amplification in Henry reaction was unprecedented in the literature. Because of the wide application of the vicinal diamine ligand in Henry reaction, the chiral 1,3-diamine L5 derived from cis-2benzamidocyclohexanecarboxylic acid was also explored by Kodama and Hirose in the asymmetric Henry reaction (Scheme 6) [14]. To achieve a satisfactory enantioselectivity, the racemization of the product induced by retro-Henry process was suppressed by using a lower reaction temperature (0 oC). With the optimized conditions, various substituted aromatic aldehydes were well tolerated to provide the Henry products with 83-91% ee, while the aliphatic aldehydes resulted in a dramatic decrease in the yield and ee value.
In continuation of their efforts to develop novel efficient catalysts, Arai and coworkers recently synthesized a series of binaphthylcontaining diamine ligand L6 starting from (R,R)-diphenylethylenediamine [15]. These chiral ligands combined with copper salts could be readily applied in the Henry reactions, regardless of the substituents of nitroalkanes. Remarkably, high levels of asymmetric
induction continued to be observed when chiral branched aldehydes were utilized as electrophile (Scheme 7). This method provided a direct access to a variety of highly functionalized nitroalcohols with multi-stereogenic centers (Scheme 7). To realize the recycling of catalyst, a polystyrene copolymer supported chiral copper catalyst (L7-Cu) was reported by Drabina based on the works of Arai (Scheme 8) [16]. With the high diffusion rate of the swelling pearl-like copolymer styrene, a comparable reaction rate and enantiomeric excess were achieved. Notably, the catalyst could be recycled for five times without losing its effectiveness with regard to the yield and enantioselectivity (Scheme 8).
In 2015, a simple chiral diamine ligand L8 was prepared from commercially available (1R,2R)-1,2-diphenyl-1,2-diaminoethane and tert-butylbromoacetate in Kureshy and Ganguly’s group (Scheme 9) [17]. The in-situ generated copper catalyst, obtained from the ligand L8 and copper triflate, was found to be an effective catalyst for the Henry reaction of trifluoromethyl ketones as well as aldehydes (Scheme 9). To get insight into the reaction mechanism, the DFT calculations were performed with B3LYP and M06-2X functional, which showed the crucial role of steric factors and noncovalent interactions in the stereoselectivity enhancement. The natural product camphor as one of accessible chiral sources has been attempted in chiral ligand design for numerous asymmetric transformations including the Henry reaction [18]. Particularly, an impressive progress in the camphor derived pyridinecontaining diamine ligands has been made by Pedro and Blay in the past decade [19]. Later, Gong et al. also achieved remarkable breakthrough in the Henry reaction with a C1-symmetric chiral secondary diamine (L9), which was derived from L-proline and camphor (Scheme 10) [20]. Taking advantage of the broad substrate generality exhibited by the corresponding catalyst, numerous of synthetically useful skeletons (6-9) were constructed by some cascade reactions. As part of Gong’s ongoing work on the synthesis and application of C1-symmetric ligands, a simplified chiral ligand L10 proved to be an alternative excellent catalyst in combination with CuCl2·2H2O for the nitroaldol reaction. Very recently, based on these reports, a 3D-printed flow reactor equipped with catalyst L11-Cu was developed by Benaglia and Puglisi (Scheme 10) [21]. To highlight potential industrial applications of this method, a multistep continuous synthesis of norephedrine has been readily achieved.
Amino acid derived diamine ligand recently received increasing attention. In 2011, 2-(pyridin-2-yl)imidazolidin-4-one L12 derived from amino amide was utilized as ligand by Drabina (Scheme 11) [22]. It was found that the relative configuration of the chiral ligand substantially impacted on the enantioselectivity. For example, the anti configuration resulted in >90% ee, while the syn configuration could only afford the product with 25% ee. Meanwhile, the geometry of the corresponding copper complex was unambiguously determined by single crystal X-ray diffraction (Scheme 11). The ongoing research in the same group revealed a heterogeneous catalyst, which could be recovered more than ten times without affecting the performance of catalysts [23]. In 2014, a cis-2-aminomethyl-5-phenylpyrrolidine ligand (L14), which could lead to a rigid bicyclic system complex, was found to be a highly efficient chiral ligand for Cu(II)-catalyzed Henry reaction (Scheme 12) [24]. An unprecedented enantiocontrol was observed with 98.5%-99.6% ee values for 36 substrates, including aromatic, heteroaromatic, vinylic, and aliphatic aldehydes (Scheme 12). To illustrate the superb performance of the copper catalyst, a plausible transition state model was proposed. As shown in the Scheme 12, the steric and electronic properties of the diamine ligand were believed to be a decisive factor for the perfect stereoselectivity.
The exploitation of chiral ligands with spiro-backbone has always been a hot topic due to the conformational rigidity and stable environment [25]. In this context, tunable spiro diamine ligands (L15-L16) were recently presented by Kałuża and coworkers (Scheme 13) [26]. Interestingly, both of the Henry product enantiomers could be selectively obtained by varying the substituents in the spiro diamine ligands. In addition, two reasonable transition state models accounted for the reverse enantioinduction were proposed based on the experimental findings (Scheme 13). To test the rigidity of chiral ligand on the catalytic efficiency, several non-rigid analogues were also attempted in the Henry reaction as a continued research of Kałuża (Scheme 14) [27]. The non-rigid pyrrolidine phenylmethanamine (L17) could also promote the Henry reaction when using in combination with copper acetate, albeit with a slightly lower stereocontol. Unexpectedly, an enhancement of enantioselectivity driven by the formation of a novel complex with a dicopper tetraacetate core was observed in the case of ligand L18. 3.
Chiral oxazoline based ligands
The effective asymmetric environments created by bis(oxazoline) ligand make it compatible with a wide range of mechanistically unrelated reactions [28]. These properties perfectly fulfill the requirements for “privileged ligands”, a term coined by Jacobsen [29]. Unexceptionally, bis(oxazoline)-copper catalyst has also been extensively explored in the Henry reaction. Since the pioneering work of Jørgensen and Evans, various structurally modified oxazoline-based ligands have been reported in Henry variants [30].
In 2011, a strategy to in-situ generate chiral copper catalyst was reported by Du et al. [31]. As an extention of previous work, a BINOL dicarboxaldehyde was used to replace achiral salicylaldehyde (in L19) to form a C2-symmetric modular BINOL-oxazoline Schiff-base ligand (L20), which could in-situ coordinate with two equivalents of copper acetate giving an active copper catalyst (Scheme 15). This modification obviously improved the catalytic performance, especially for aliphatic aldehydes (96%-98% ee).
To further expand the types of oxazoline-based catalysts, various chiral building blocks and synthetic methods have been utilized in the newly chiral ligand synthesis [32] (Fig. 1). For instance, the glucoBOX (L21), cyclopropane-based bisoxazoline (L22), and phenoloxazoline ligands (L23) were developed by Reddy, Zhong and Aydin, respectively, based on the structure of cyclopropane-1,2dicarbonyl dichloride, glucosamine and salicylic acid. As for the ligand 1,2,4-triazine-oxazoline (L24, L25), it was prepared through the cross-coupling reactions of oxazolylanilines with 3-halo-1,2,4-triazines. These structurally diverse oxazoline-based catalysts were generally suitable for the Henry reaction of aldehydes with nitromethane affording the product with moderate to excellent ee values, while only a limited substrate scope was observed for the Henry reaction of nitroethane. In 2014, the direct Henry reaction of nitromethane with 2-acylpyridine N-oxides was first disclosed by Pedro and Blay (Scheme 16) [33]. With the previously developed catalyst L26-Cu(OTf)2, a broad spectrum of tertiary nitroaldols bearing a quaternary stereocenter bonded to a pyridine ring were obtained with good to excellent stereoselectivity (Scheme 16). However, the catalytic system does have some limitations on the substrate scope: (1) the aryl substituted 2-acylpyridine N-oxides could not react smoothly to afford the products with satisfactory chemical or optical yields; (2) other nitroalkanes had never been employed in the reaction.
The recovery of chiral catalysts, as an enormous challenge and practical issue, has always been pursued by chemists. To address this pressing issue, the bis(oxazoline)-based catalyst was chosen as an ideal precursor of recyclable catalyst due to its versatile skeleton. In this respect, a great contribution has been made by Zhou and coworkers [34]. A library of chiral ionic liquid catalysts L27-L29-Cu was established by varying the counter anion, the aliphatic chain and the chiral scaffold, as shown in the Scheme 17. All of these catalysts could be recovered for more than six times without any notable loss in enantioselectivity. On the other hand, charge-transfer interaction between the anthracenyl group and trinitrofluorenone (TNF) has emerged as an efficient tool for recycling bis(oxazoline)-based catalysts (L30-Cu-TNF) since the first report of Schulz [35]. The asymmetric Henry reaction has also been attempted to demonstrate the general applicability of this novel concept (Scheme 18) [36]. Following the same recovery concept, a new heterogeneous catalyst (L31-Cu-SiO2TNF) was developed by immobilizing the trinitrofluorenone to a silica support. As a result, the immobilized catalyst could be recycled for 7 times with a maintained enantioselectivity through the reversible, non-covalent interactions. .
Almost at the same time, a polystyrene (PS) supported bis(oxazoline)-based catalyst was synthesized by Maggi via a facile procedure of copolymerizing monofunctionalized ligand (L32) with styrene and divinylbenzene (Scheme 19) [37]. With an appropriate catalyst loading value, the polystyrene supported catalyst (L32-Cu-Ps) could effectively promote the Henry reaction affording the product with high enantiopurity over at least five cycles. To probe the substrate generality, the heterogeneous catalysis was performed for several aldehydes substrates. In 2012, García has revealed an efficient release-capture strategy for the recovery of ditopic bisoxazoline catalysts in the Henry reaction (Scheme 20) [38]. In the non-coordinating solvent, the self-assembly effect of the ditopic bisoxazoline-copper catalysts L33Cu and L34-Cu resulted in an insoluble coordination polymer, whereas the coordination polymer could also release monomeric catalytic complexes through a reversible disassembly process in the coordinating solvent (Scheme 20). Taking advantage of the reversible equilibrium, the rationally designed ditopic catalyst could be recovered and reused for up to 11 reaction cycles keeping good yields and enantioselectivities. As an alternative method for the catalyst recovery, fluorous technology has drawn considerable attention since the first exploration of Curran [39]. Accordingly, the fluorous bis(oxazoline)-based (L35) catalyst recently has been designed for Henry reaction by Cai et al. (Scheme 21) [40]. The good solubility of this fluorous chiral catalyst made it totally different with the common heterogeneous catalyst, which could be easily recovered by fluorous solid-phase extraction (F-SPE). As shown in the report, 4 catalytic cycles had no significant effect on the product yields and ee values. Meanwhile, some aldehydes could be employed in the reaction as well, giving 72-99% ee (Scheme 21). .
4. Chiral Schiff-base ligands Chiral Schiff-base ligand was first developed by Aratani for the practical synthesis of chrysanthemic acid via an asymmetric cyclopropanation [41]. Since then, the applicability of the Schiff-base in various transformations has been widely explored [42].
Among these transformations, the Schiff-base catalyzed Henry reaction has caught growing attention since the groundbreaking research was realized in Wang’s group [43]. Encouraged by the promising result, the Schiff-base catalysts L36/L37-Cu derived from cinchona alkaloids were reported as efficient catalysts in Henry reaction by He and Zhang (Scheme 22) [44]. In the presence of copper-Schiff-base catalysts, a large number of aldehydes could smoothly reacted with nitromethane as well as nitroethane, although only a moderate dr value were detected. Moreover, the transition-state model accounting for the high stereocontrol was proposed in the report.
As shown in the aforementioned examples, the development of a robust Schiff-base catalyst, with lower catalyst loading and good substrate tolerance, is highly desirable. To meet this demand, Chen et al. reported a sterically bulky Schiff-base catalyst [L38-Cu]2 derived from aminoisoborneol (Scheme 23) [45]. As expected, the reaction was conducted with as little as 1 mol% catalyst loading and afforded 39 β-nitroalcohol products with good to excellent enantioselectivity (76%-98% ee). When this protocol extended to other nitroalkanes, the enantioselectivity was maintained while a lower dr value was observed. In 2012, a new class of chiral sulfoxide-Schiff base ligands (L39-L40) was developed by introducing an additional binding site chiral sulfoxide to the Schiff base scaffold (Scheme 24) [46]. The benzyl substituted sulfoxide-Schiff base (L39) was evaluated as the optimal ligand with respect to the yield and enantioselectivity. To demonstrate the generality of the rationally designed catalyst, numerous aldehydes, including aromatic, heteroaromatic, aliphatic and alkenyl aldehydes, were employed in the reaction giving the nitroalcohol products with uniformly high stereoselectivity. Unfortunately, expanding the substrate scope to nitroethane led to an unsatisfactory stereoinduction. To further explore the potential of the ligands in chiral heterocycle synthesis, a sequential one-pot Henry reaction and iodocyclization was discovered for tetrahydrofuran synthesis [47]. This novel methodology enabled a direct route to enantioenriched 2,5-polysubstituted tetrahydrofuran. Despite unappealing results in the diastereoslectivity, the following reduction dramatically improved the diastereopurity of the final amine product. Inspired by the great success in the homogeneous catalysis, the recyclable Schiff-base catalyst caught more and more attention. An array of ditopic Schiff-base ligands (L41) and polymer-supported ligands (L42) were reported as recyclable catalysts by Kureshy and Shen, repectively (Fig. 2) [48]. As previously mentioned, the ditopic Schiff-base ligands (L41) could be recovered via the equilibrium of assembly and disassembly. After 5 catalytic runs, a complete retention of activity and enantioselectivity was detected. While in the Shen’s work, the recovery of polymer-supported Schiff base catalyst (L42-Cu) was realized by a simple filtration and 8 catalytic recycles maintained the catalytic efficiency with 90% yield and 86% ee.
To further improve the catalyst performance as high as possible, the structurally modifying on the previously reported ligands was explored extensively (Fig. 3) [49]. Among these exploitations, the halogen atom substituted chiral ligands were prepared and screened. It was found that the iodine substituted Schiff-base ligands (L43, L44) were the better chiral ligands compared with previous results. While in the Xu’s work, a camphor-derived Schiff base ligand L45 was developed and applied in the Henry reaction. 5.
Chiral salen based ligands
The chiral salen-cobalt complex as an effective catalyst in Henry reaction was first revealed by Yamada [50]. In this context, it was anticipated that the modified chiral salen ligands would coordinate with copper to promote the asymmetric Henry reaction. Recently, Kureshy et al. reported a simply modified macrocylic salen ligand as a recyclable catalyst in copper catalyzed Henry reaction (Fig. 4) [51]. Interestingly, two types of macrocyclic ligands (L46, L47) could be selectively obtained by varying the reaction medium from the same reactants. With different chiral scaffolds and linked salicylaldehydes (L48, L49), a library of macrocylic salen ligands was achieved [52]. To evaluate the generality of these chiral ligands, a wide range of aldehydes were employed in the reaction. As expected, more than 90% ee values were observed for the most examples with an excellent recyclability (8 catalytic runs). In 2012, a chiral copper-salen catalyst with a rotatable aromatic π-wall was disclosed by Xu, which could selectively recognize simple aromatic aldehydes in the Henry reaction (Scheme 25) [53]. The further UV/vis spectroscopic study clearly illustrates the molecular interaction between chiral catalyst (L50-Cu) and benzaldehyde. This unique molecular recognition effect caused an excellent enantioselectivity for benzaldehyde and halide-containing aromatic aldehydes, and a diminished yield and selectivity for aliphatic and other aromatic aldehydes. In 2012, the chiral rigid scaffold cis-2,5-diaminobicyclo-[2.2.2]octane was employed by the group of White in a novel chiral salen ligand synthesis [54]. A high catalytic performance of the chiral ligand-copper complex in Henry reaction was detected (Scheme 26). The evaluation of reaction parameters showed that 10 mol% tetrahydrosalen ligand (L51) and 1 mol% copper (I) triflate was the optimal catalyst ratio, delivering the nitroalcohol products with 91%-98% ee for most examples. Notably, this efficient catalytic system could be directly used in the construction of the vicinal amino alcohol structure. Additionally, the syn-selective products could be
obtained with >20/1 dr value for the reaction of nitropropane with specific aldehydes. To rationalize the sense of the stereoinduction, the authors proposed a plausible transition-state model (Scheme 26).
To better understand the effect of N,N-substituents on the salen-copper catalyzed Henry reaction, a series of N-substituted tetrahydrosalen and dihydrosalen ligands was prepared via a concise approach [55]. As shown in the Scheme 27, the N,N-methylated tetrahydrosalen-copper (L52-Cu) catalyst proved to be the most efficient catalyst in terms of reaction yield and stereoselectivity. The further investigation on the substrate scope illustrated that this catalytic protocol was well compatible with aryl, heteroaryl, and alkyl aldehydes giving the corresponding products with moderate to good ee values.
To expand the category of the recyclable catalysts, some novel strategies were successfully applied in the salen-based catalysts, such as, MCM-41 supported salen catalyst (cat.1), salen oligomer catalyst (cat.2), zeolite-Y encapsulated salen catalyst (cat.3) (Fig. 5) [56]. Among these catalysts, cat.1 and cat.2 exhibited an excellent recyclability in Henry reaction with 5 and 10 catalytic runs, respectively. While for the encapsulated catalyst cat.3, the recovery of catalyst had not been studied. It is noteworthy that the encapsulated copper complex produced an obvious enhancement in enantioselectivity compared with the homogeneous counterpart. 6. Chiral amino alcohol ligands Chiral amino alcohol ligands were generally used in combination with ZnEt 2 to promote a broad range of asymmetric transformations. For the nitroaldol reaction, a dinuclear zinc-amino alcohol catalyst was first introduced by Trost as a powerful catalyst [57]. To avoid using the high reactive zinc reagent, copper salt was considered as an alternative Lewis acid to combine with amino alcohol. In 2011, a chiral amino acohol ligand (L53) containing an additional pyridine coordination site was prepared by Reddy and coworkers [58]. The activity of the catalyst L53-Cu in Henry reaction was next investigated with various substituted aldehydes. As shown in the Scheme 28, both aromatic and aliphatic aldehydes could react smoothly with nitromethane affording the corresponding adducts with 50%-86% ee. With respect to the mechanism, a reasonable transition-state model was proposed by the authors. To further improve the catalytic efficiency, an elegant amino alcohol-derived ligand (L54) was disclosed by Wang et al. and an ultimate success in Henry reaction was achieved with broad substrate scope and excellent stereoselectivity (Scheme 29) [59]. This efficient catalytic system had some unique advantages: 1) water could be directly used as the reaction medium; 2) a predominately anti-selectivity (mostly above 15/1) was observed in the Henry reaction of higher order nitroalkanes with aldehydes; 3) the facile manipulation procedure and good scalability provided a practical method to access synthetically valuable β-nitroalcohols. To the best of our knowledge, this is first example to reach such high level of anti-selectivity in the copper catalyzed Henry reaction.
On the other hand, the 1,1'-binaphthylazepine derived amino alcohol as a bidentate ligand (L55) was employed in the copper catalyzed Henry reaction (Scheme 30) [60]. Good to excellent enantioselectivities (82%-97% ee) were obtained for both aromatic and aliphatic aldehydes. Extending the substrate scope to other nitroalkanes gave the corresponding syn-selective products with 1.6/1-19/1 dr and >90% ee values. Later, the structurally simplified amino alcohol ligand (L56) was synthesized from commercially available (1S, 2R)-2-amino-1,2diphenylethanol (Fig. 6) [61]. Using in combination with copper acetate, about 38 aromatic β-nitroalcohol products could be easily accessed with 84%-99% ee. It is worth mentioning that this efficient protocol is also compatible with an array of benzofuryl aldehydes affording the corresponding bioactive benzofuryl β-amino alcohols. To further expand the substrate scope to aliphatic aldehydes, the optimal ligand L57 was selected according to the result of testing a library of modified amino alcohols (Fig. 6) [62]. Unsurprisingly, up to 18.6/1 syn-selectivity and above 90% ee values (for syn-isomer) were observed for 18 examples.
Very recently, the extension on the previous catalytic system has drawn considerable interest (Fig. 7). Two types of thiophenecontaining amino alcohol ligands (L58, L59) were developed by Aydin and Xu et al. [63]. The two types ligands were used with different copper salts (Cu(OTf)2/CuCl) and different substrate generalities were presented. Inspired by the work of Lu and Chen[60, 62], another bidentate ligand with a chiral aziridinyl alcohol structure (L60) was reported by Zhao and Liu, although the newly developed ligand resulted in some unsatisfactory results compared with the previous one [64]. 7.
Miscellaneous chiral ligands
Chiral N,N'-dioxides ligands were first developed by Feng et al., which proved to be a powerful chiral ligands in numerous asymmetric transformations [65]. The asymmetric Henry reaction was also explored with the corresponding copper complexes in 2007
[66]. As a continuous work in this respect, the N,N'-dioxides-copper (L61-Cu) catalyzed diastereoselective Henry reaction was unveiled very recently (Scheme 31) [67]. Under the optimal conditions, about 35 examples, including aromatic, heteroaromatic and alkenyl substituted β-amino alcohols, were achieved with moderate to good anti-selectivities and enantioselectivities. It was worth mentioning the diastereoselectivity in the reaction was related with substitutent pattern, the ortho-substitution enhanced the diasteroselectivity (up to 16.7/1). Sparteine-copper catalyzed Henry reaction was first implemented by Maheswaran et al [68]. However, the restricted accessibility of sparteine-like ligands seriously hampers the further development in the modified catalysts. In this respect, Breuning and coworkers developed two analogues of sparteine (L62-L63) via the multi-step synthesis route (Fig. 8) [69]. The corresponding copper complexes were found to be an effective catalyst in Henry reaction delivering the products with >90% ee and moderate diastereoselectivity for the specific examples.
As a longstanding interest in the synthesis and catalytic application of 1,1'-bisisoquinolines, Judeh recently has explored the reactivity and selectivity of various N-alkyl substituented C1-tetrahydro-1,1'-bisisoquinoline ligands in the enantioselective Henry reaction (Fig. 9) [70]. The methylated ligand L64 and unsubstituted ligand L65 were used in combination with copper(I) chloride as the catalysts. Choosing different optimal reaction medium could afford the products with comparable enantioselectivities. Specifically, the methylated chiral ligand was also compatible with other high order nitroalkanes giving the β-nitroalcohol adducts with moderate to good ee values, albeit low dr values.
Cinchona alkaloid as one of the most important privileged organocatalysts has been widely used in numerous asymmetric transformations [71]. By contrast, the cinchona alkaloid derived chiral copper catalyst has received far less attention. Recently, the cinchona-copper L66-Cu catalyzed Henry reaction was first revealed by Skarżewski et al. and a promising result was achieved (Fig. 10) [72]. Later, bipyridine (L67) and phenanthroline (L68) moiety were incorporated into the cinchona alkaloid by the group of Wang and Huang [73]. With this modified catalyst, an obvious enhancement in the enantioselectivity was observed with a broader substrate scope (Fig. 10). 8. Conclusions and perspectives Copper catalysis has recently emerged as a powerful tool in the asymmetric Henry reaction and considerable advances in the diastereo- and enantioselective Henry reaction of aldehydes have been achieved with numerous efficient catalytic system. It is notable that a wide range of nitroalcohol products, including aromatic, heteroaromatic, aliphatic and vinylic adducts, were readily obtained with up to 50/1 dr values and 99% ee values. Despite the great achievements in the Henry reaction of aldehydes, the direct Henry reaction of unactivated ketones with nitroalkanes still remains elusive and underdeveloped. In addition, the scope of nitroalkanes in the well-developed Henry reaction need to be further expanded. Consequently, a novel chiral copper catalyst with lower catalyst loading (<1 mol%), broader substrate scope (ketones and fuctionlized nitroalkanes) and excellent sterocontrol is still in high demand.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Nos. 2127222, 21432009, 21472177, J1310010) and Chinese Academy of Sciences (No. XDB20000000).
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Fig. 1. The application of Henry reaction in natural products.
Fig. 2. Recyclable Schiff-base catalysts in Henry reaction.
Fig. 3. Structurally modified Schiff-base catalyst in Henry reaction.
Fig. 4. A library of macrocyclic salen ligands reported by Kureshy.
Fig. 5. Recyclable salen-based catalyst in Henry reaction.
Fig. 6. Simple chiral amino alcohol-copper catalyzed Henry reaction.
Fig. 7. Newly developed amino alcohol-copper catalysts in Henry reaction.
Fig.8. Sparteine-like ligands in asymmetric Henry reaction.
Fig. 9. 1,1'-Bisisoquinolines-copper catalyzed asym-metric Henry reaction.
Fig. 10. Cinchona alkaloid derived copper catalyst in Henry reaction.
Scheme 1. The versatile building block nitroalcohol.
Scheme 2. The chiral bis(sufonamide)-diamine ligand in the Henry reaction.
Scheme 3. The C1-symmetrical copper catalyzed diastereo-selective Henry reaction.
Scheme 4. Chiral branched diamine ligand in the asymmetric Henry reaction.
Scheme 5. The nonlinear effect in the diamine-copper cataly-zed Henry reaction.
Scheme 6. 1,3-Diamine-copper catalyst in the Henry reaction.
Scheme 7. Sulfonyldiamine in the Henry reaction of branched aldehydes.
Scheme 8. Polymer supported diamine catalyst in Henry reaction
Scheme 9. Chiral diamine-copper catalyst in the Henry reaction of trifluoromethyl ketones.
Scheme 10. The diamine ligands derived from camphor.
Scheme 11. Chiral 2-(pyridin-2-yl)imidazolidin-4-one in the Henry reaction.
Scheme 12. A highly efficient copper catalyst prepared from 2-aminomethyl-5-phenylpyrrolidine in the Henry reaction.
Scheme 13. Spiro diamine-copper catalyzed Henry reaction.
Scheme 14. Pyrrolidine phenylmethanamine ligand in the Henry reaction
Scheme 15. The in-situ generated copper-oxazoline Shiff-base catalyst.
Fig. 1. Recently developed oxazoline-based catalysts.
Scheme 16. The Henry reaction of nitromethane with 2-acylpyridine N-oxides.
Scheme 17. Ionic-tagged bis(oxazoline) catalyst used for Henry reaction.
Scheme 18. Charge transfer interactions in bis(oxazoline) recyclable catalyst
Scheme 19. PS-supported bis(oxazline)-based catalyst in the Henry reaction.
Scheme 20. The release-capture strategy in the recyclable ditopic bisoxazo-line-copper catalyst.
Scheme 21. Fluorous bis(oxazoline)-based catalyst in the Henry reaction
Scheme 22. Cinchona derived Schiff-base catalyst in Henry reaction
Scheme 23. Aminoisoborneol-derived Schiff base catalyst in Henry reaction.
Scheme 24. Chiral sulfoxide-Schiff base catalyst in Henry reaction.
Scheme 25. Strong molecular-recognition effect in Henry reaction.
Scheme 26. cis-2,5-Diaminobicyclo[2.2.2]octane derived chiral tetrahydro-salen-copper catalyst in Henry reaction.
Scheme 27. The effect of N,N-substituents on the salen-copper catalyzed Henry reaction.
Scheme 28. A pyridine-containing amino alcohol catalyst in Henry reaction.
Scheme 29. Highly diastereo- and enantioselective Henry reaction catalyzed by amino alcohol derived copper catalyst.
Scheme 30. Binaphthylazepine derived amino alcohol ligand in copper catalyzed Henry reaction.
Scheme 31. Chiral N,N’-dioxides-copper catalyzed diastereo- and enantio-selective Henry reaction.
S1001-8417(17)30418-7 https://doi.org/10.1016/j.cclet.2017.10.001 CCLET 4274
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Chinese Chemical Letters
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Please cite this article as: Sheng Zhang, Yanan Li, Youguo Xu, Zhiyong Wang, Recent progress in copper catalyzed asymmetric Henry reaction, Chinese Chemical Letters https://doi.org/10.1016/j.cclet.2017.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Review
Recent progress in copper catalyzed asymmetric Henry reaction Sheng Zhang a,b, Yanan Li a, Youguo Xu a, Zhiyong Wanga,* a
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China b College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
Corresponding author.
E-mail address: [email protected] (Z. Wang)
Graphical Abstract Recent progress in copper catalyzed asymmetric Henry reaction Sheng Zhang a,b, Yanan Li a, Youguo Xu a, Zhiyong Wanga a
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China b
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
Henry reaction is one of the most classical reactions to construct synthetically useful product nitro alcohol, which as a privileged skeleton is widely distributed in various pharmaceuticals. This review summarizes the recent progress of copper-catalyzed asymmetric Henry reaction from 2011 to 2016. The significant progress that has been made in this area will be highlighted and some of challenges that the author believes may be hindering further progress will be revealed.
ARTICLE INFO: Article history:Received 8 June 2017 Received in revised form 26 August 2017 Accepted 21 September 2017 Available online ABSTRACT
Henry reaction is one of the most classical reactions to construct synthetically useful product nitro alcohol, which as a privileged skeleton is widely distributed in various pharmaceuticals. This review summarizes the recent progress of copper-catalyzed asymmetric Henry reaction from 2011 to 2016. The significant progress that has been made in this area will be highlighted and some of challenges that the author believes may be hindering further progress will be revealed. Keywords: Henry reaction Asymmetric catalysis Copper catalysis Nitro compounds C-C bond formation
1.
Introduction
The Henry or nitroaldol reaction represents a highly attractive synthetic method, which provides a facile and direct access to versatile building block β-nitroalcohol [1]. The difunctional group of β-nitroalcohol can be readily converted into other functional groups via a variety of transformations (Scheme 1) [2]. For the nitro group, it can be easily reduced to amino group under mild conditions (Path a). Additionally, the Nef reaction of nitroalkanes provides an alternative route to carbonyl compounds (Path b). With a radical condition, the nitro group can also be replaced with a hydrogen atom (Path c). With respect to the hydroxyl group, the dehydration process can be employed to prepare nitroolefin (Path d), which is one of the most popular Michael acceptors.
Due to the versatility of nitroalcohol, the Henry reaction has been widely applied in numerous synthetic ventures with the tremendous development of asymmetric Henry reaction (Fig. 1) [3]. For instance, the Henry reaction has been employed as a key step in the total synthesis of nucleoside antibiotics tunicamycin developed by Suami and coworkers [3a]. The synthesis of natural product tetrodotoxin starting from D-glucose was also achieved by using two-step Henry reactions in Sato group [3b]. Recently, an efficient method to construct marine alkaloid manzamine A, involving Henry and aza-Henry reaction, was reported by Dixon et al. [3c, 3d]. The above facts serve to emphasize that the great utility of Henry reaction in the practical synthesis. Consequently, considerable efforts have been devoted to the asymmetric Henry reaction since the pioneering work of Shibasaki in the first enantioselective Henry reaction [4]. Among the enormous reports, the copper catalyzed Henry reaction has occupied an important place for the advantages of flexible coordination mode, economical accessibility and the good tolerance for air and moisture [5]. Accordingly, in this respect, numerous chiral ligands were developed and tremendous progress was achieved. However, to the best of our knowledge, there was no specific review focused on the copper-catalyzed Henry reaction. Although Velmathi et al. summarized the transition metal catalyzed Henry reaction, covering some relative works on copper catalysis prior to 2011, a number of meaningful works dealing with new catalysts and concepts have recently emerged [6,7]. In this context, the recent advances in the field will be discussed in this digest and some previous papers will be introduced as background when necessary. To better illustrate the progress in the area, this review will be introduced according to the category of chiral ligands. 2.
Chiral diamine ligands
Chiral 1,2-diaminocyclohexane as a privileged skeleton has been widely employed in the synthesis of chiral diamine ligands, which turned out to be effective ligands for many metal-catalyzed asymmetric transformations [8]. For the Henry reaction, this catalytic system was first introduced by Arai in 2006 [9]. Recently, a chiral ligand bis(sulfonamide)-diamine L1 with multiple coordination sites was prepared by Wan and coworkers, which could readily coordinate with Cu(I) to form an active chiral catalyst (Scheme 2) [10]. This chiral copper catalyst was effective not only in the Henry reaction involving nitromethane but also for the reaction of substituted nitroalkanes. In particular, with the assistance of pyridine, the corresponding syn-selective adducts could be obtained with moderate to good diastereoselectivities (Scheme 2). In 2011, a range of C1-symmetrical diamine ligands were reported by Woggon to address the challenging diastereo-selectivity issue in the nitroaldol reaction of aliphatic aldehydes (Scheme 3) [11]. In order to find the optimal conditions for the reaction, a screening was performed using more than 60 chiral diamines-copper complexes. With the optimal ligand L2, various prochiral nitro compounds could react smoothly with 3-phenylpropionaldehyde giving the syn-selective product with 1.9/1-5.7/1 dr values and 90-99% ee values (Scheme 3). However, the shortcoming of this method was that a lower stereoselectivity was observed in the cases of aromatic aldehydes. Further investigation on the chiral diamine ligand structure showed that the alkyl substituent significantly affected the stereoselectivity in the Henry reaction [12]. The study of Gou et al. showed that branched diamine L3 was the optimal ligand in terms of the yield and enantioselectivity (Scheme 4). With this optimal ligand in hand, the generality of the substrate scope was examined in the presence of the corresponding copper complexes. Specifically, better enantioselectivities with 91-94% ee were observed for the aliphatic aldehydes compared with the aromatic counterparts (Scheme 4). Despite the admirable results in the Henry reaction of nitromethane, other nitroalkanes had not been tried in the report with the optimal catalyst. Very recently, the chirality induction mechanism in the diamine-copper catalyzed Henry reaction was investigated by Tanaka (Scheme 5) [13]. Interestingly, a significant positive nonlinear effect was observed when the reaction was performed in EtOAc, while a similar effect could not be detected in MeOH. This solvent-dependent nonlinear effect might originate from the difference solubility between the optically pure catalyst (R,R)-L4-Cu and the racemic one, especially in the aprotic organic solvents (Scheme 5). To the best of our knowledge, the strong solvent-dependent asymmetric amplification in Henry reaction was unprecedented in the literature. Because of the wide application of the vicinal diamine ligand in Henry reaction, the chiral 1,3-diamine L5 derived from cis-2benzamidocyclohexanecarboxylic acid was also explored by Kodama and Hirose in the asymmetric Henry reaction (Scheme 6) [14]. To achieve a satisfactory enantioselectivity, the racemization of the product induced by retro-Henry process was suppressed by using a lower reaction temperature (0 oC). With the optimized conditions, various substituted aromatic aldehydes were well tolerated to provide the Henry products with 83-91% ee, while the aliphatic aldehydes resulted in a dramatic decrease in the yield and ee value.
In continuation of their efforts to develop novel efficient catalysts, Arai and coworkers recently synthesized a series of binaphthylcontaining diamine ligand L6 starting from (R,R)-diphenylethylenediamine [15]. These chiral ligands combined with copper salts could be readily applied in the Henry reactions, regardless of the substituents of nitroalkanes. Remarkably, high levels of asymmetric
induction continued to be observed when chiral branched aldehydes were utilized as electrophile (Scheme 7). This method provided a direct access to a variety of highly functionalized nitroalcohols with multi-stereogenic centers (Scheme 7). To realize the recycling of catalyst, a polystyrene copolymer supported chiral copper catalyst (L7-Cu) was reported by Drabina based on the works of Arai (Scheme 8) [16]. With the high diffusion rate of the swelling pearl-like copolymer styrene, a comparable reaction rate and enantiomeric excess were achieved. Notably, the catalyst could be recycled for five times without losing its effectiveness with regard to the yield and enantioselectivity (Scheme 8).
In 2015, a simple chiral diamine ligand L8 was prepared from commercially available (1R,2R)-1,2-diphenyl-1,2-diaminoethane and tert-butylbromoacetate in Kureshy and Ganguly’s group (Scheme 9) [17]. The in-situ generated copper catalyst, obtained from the ligand L8 and copper triflate, was found to be an effective catalyst for the Henry reaction of trifluoromethyl ketones as well as aldehydes (Scheme 9). To get insight into the reaction mechanism, the DFT calculations were performed with B3LYP and M06-2X functional, which showed the crucial role of steric factors and noncovalent interactions in the stereoselectivity enhancement. The natural product camphor as one of accessible chiral sources has been attempted in chiral ligand design for numerous asymmetric transformations including the Henry reaction [18]. Particularly, an impressive progress in the camphor derived pyridinecontaining diamine ligands has been made by Pedro and Blay in the past decade [19]. Later, Gong et al. also achieved remarkable breakthrough in the Henry reaction with a C1-symmetric chiral secondary diamine (L9), which was derived from L-proline and camphor (Scheme 10) [20]. Taking advantage of the broad substrate generality exhibited by the corresponding catalyst, numerous of synthetically useful skeletons (6-9) were constructed by some cascade reactions. As part of Gong’s ongoing work on the synthesis and application of C1-symmetric ligands, a simplified chiral ligand L10 proved to be an alternative excellent catalyst in combination with CuCl2·2H2O for the nitroaldol reaction. Very recently, based on these reports, a 3D-printed flow reactor equipped with catalyst L11-Cu was developed by Benaglia and Puglisi (Scheme 10) [21]. To highlight potential industrial applications of this method, a multistep continuous synthesis of norephedrine has been readily achieved.
Amino acid derived diamine ligand recently received increasing attention. In 2011, 2-(pyridin-2-yl)imidazolidin-4-one L12 derived from amino amide was utilized as ligand by Drabina (Scheme 11) [22]. It was found that the relative configuration of the chiral ligand substantially impacted on the enantioselectivity. For example, the anti configuration resulted in >90% ee, while the syn configuration could only afford the product with 25% ee. Meanwhile, the geometry of the corresponding copper complex was unambiguously determined by single crystal X-ray diffraction (Scheme 11). The ongoing research in the same group revealed a heterogeneous catalyst, which could be recovered more than ten times without affecting the performance of catalysts [23]. In 2014, a cis-2-aminomethyl-5-phenylpyrrolidine ligand (L14), which could lead to a rigid bicyclic system complex, was found to be a highly efficient chiral ligand for Cu(II)-catalyzed Henry reaction (Scheme 12) [24]. An unprecedented enantiocontrol was observed with 98.5%-99.6% ee values for 36 substrates, including aromatic, heteroaromatic, vinylic, and aliphatic aldehydes (Scheme 12). To illustrate the superb performance of the copper catalyst, a plausible transition state model was proposed. As shown in the Scheme 12, the steric and electronic properties of the diamine ligand were believed to be a decisive factor for the perfect stereoselectivity.
The exploitation of chiral ligands with spiro-backbone has always been a hot topic due to the conformational rigidity and stable environment [25]. In this context, tunable spiro diamine ligands (L15-L16) were recently presented by Kałuża and coworkers (Scheme 13) [26]. Interestingly, both of the Henry product enantiomers could be selectively obtained by varying the substituents in the spiro diamine ligands. In addition, two reasonable transition state models accounted for the reverse enantioinduction were proposed based on the experimental findings (Scheme 13). To test the rigidity of chiral ligand on the catalytic efficiency, several non-rigid analogues were also attempted in the Henry reaction as a continued research of Kałuża (Scheme 14) [27]. The non-rigid pyrrolidine phenylmethanamine (L17) could also promote the Henry reaction when using in combination with copper acetate, albeit with a slightly lower stereocontol. Unexpectedly, an enhancement of enantioselectivity driven by the formation of a novel complex with a dicopper tetraacetate core was observed in the case of ligand L18. 3.
Chiral oxazoline based ligands
The effective asymmetric environments created by bis(oxazoline) ligand make it compatible with a wide range of mechanistically unrelated reactions [28]. These properties perfectly fulfill the requirements for “privileged ligands”, a term coined by Jacobsen [29]. Unexceptionally, bis(oxazoline)-copper catalyst has also been extensively explored in the Henry reaction. Since the pioneering work of Jørgensen and Evans, various structurally modified oxazoline-based ligands have been reported in Henry variants [30].
In 2011, a strategy to in-situ generate chiral copper catalyst was reported by Du et al. [31]. As an extention of previous work, a BINOL dicarboxaldehyde was used to replace achiral salicylaldehyde (in L19) to form a C2-symmetric modular BINOL-oxazoline Schiff-base ligand (L20), which could in-situ coordinate with two equivalents of copper acetate giving an active copper catalyst (Scheme 15). This modification obviously improved the catalytic performance, especially for aliphatic aldehydes (96%-98% ee).
To further expand the types of oxazoline-based catalysts, various chiral building blocks and synthetic methods have been utilized in the newly chiral ligand synthesis [32] (Fig. 1). For instance, the glucoBOX (L21), cyclopropane-based bisoxazoline (L22), and phenoloxazoline ligands (L23) were developed by Reddy, Zhong and Aydin, respectively, based on the structure of cyclopropane-1,2dicarbonyl dichloride, glucosamine and salicylic acid. As for the ligand 1,2,4-triazine-oxazoline (L24, L25), it was prepared through the cross-coupling reactions of oxazolylanilines with 3-halo-1,2,4-triazines. These structurally diverse oxazoline-based catalysts were generally suitable for the Henry reaction of aldehydes with nitromethane affording the product with moderate to excellent ee values, while only a limited substrate scope was observed for the Henry reaction of nitroethane. In 2014, the direct Henry reaction of nitromethane with 2-acylpyridine N-oxides was first disclosed by Pedro and Blay (Scheme 16) [33]. With the previously developed catalyst L26-Cu(OTf)2, a broad spectrum of tertiary nitroaldols bearing a quaternary stereocenter bonded to a pyridine ring were obtained with good to excellent stereoselectivity (Scheme 16). However, the catalytic system does have some limitations on the substrate scope: (1) the aryl substituted 2-acylpyridine N-oxides could not react smoothly to afford the products with satisfactory chemical or optical yields; (2) other nitroalkanes had never been employed in the reaction.
The recovery of chiral catalysts, as an enormous challenge and practical issue, has always been pursued by chemists. To address this pressing issue, the bis(oxazoline)-based catalyst was chosen as an ideal precursor of recyclable catalyst due to its versatile skeleton. In this respect, a great contribution has been made by Zhou and coworkers [34]. A library of chiral ionic liquid catalysts L27-L29-Cu was established by varying the counter anion, the aliphatic chain and the chiral scaffold, as shown in the Scheme 17. All of these catalysts could be recovered for more than six times without any notable loss in enantioselectivity. On the other hand, charge-transfer interaction between the anthracenyl group and trinitrofluorenone (TNF) has emerged as an efficient tool for recycling bis(oxazoline)-based catalysts (L30-Cu-TNF) since the first report of Schulz [35]. The asymmetric Henry reaction has also been attempted to demonstrate the general applicability of this novel concept (Scheme 18) [36]. Following the same recovery concept, a new heterogeneous catalyst (L31-Cu-SiO2TNF) was developed by immobilizing the trinitrofluorenone to a silica support. As a result, the immobilized catalyst could be recycled for 7 times with a maintained enantioselectivity through the reversible, non-covalent interactions. .
Almost at the same time, a polystyrene (PS) supported bis(oxazoline)-based catalyst was synthesized by Maggi via a facile procedure of copolymerizing monofunctionalized ligand (L32) with styrene and divinylbenzene (Scheme 19) [37]. With an appropriate catalyst loading value, the polystyrene supported catalyst (L32-Cu-Ps) could effectively promote the Henry reaction affording the product with high enantiopurity over at least five cycles. To probe the substrate generality, the heterogeneous catalysis was performed for several aldehydes substrates. In 2012, García has revealed an efficient release-capture strategy for the recovery of ditopic bisoxazoline catalysts in the Henry reaction (Scheme 20) [38]. In the non-coordinating solvent, the self-assembly effect of the ditopic bisoxazoline-copper catalysts L33Cu and L34-Cu resulted in an insoluble coordination polymer, whereas the coordination polymer could also release monomeric catalytic complexes through a reversible disassembly process in the coordinating solvent (Scheme 20). Taking advantage of the reversible equilibrium, the rationally designed ditopic catalyst could be recovered and reused for up to 11 reaction cycles keeping good yields and enantioselectivities. As an alternative method for the catalyst recovery, fluorous technology has drawn considerable attention since the first exploration of Curran [39]. Accordingly, the fluorous bis(oxazoline)-based (L35) catalyst recently has been designed for Henry reaction by Cai et al. (Scheme 21) [40]. The good solubility of this fluorous chiral catalyst made it totally different with the common heterogeneous catalyst, which could be easily recovered by fluorous solid-phase extraction (F-SPE). As shown in the report, 4 catalytic cycles had no significant effect on the product yields and ee values. Meanwhile, some aldehydes could be employed in the reaction as well, giving 72-99% ee (Scheme 21). .
4. Chiral Schiff-base ligands Chiral Schiff-base ligand was first developed by Aratani for the practical synthesis of chrysanthemic acid via an asymmetric cyclopropanation [41]. Since then, the applicability of the Schiff-base in various transformations has been widely explored [42].
Among these transformations, the Schiff-base catalyzed Henry reaction has caught growing attention since the groundbreaking research was realized in Wang’s group [43]. Encouraged by the promising result, the Schiff-base catalysts L36/L37-Cu derived from cinchona alkaloids were reported as efficient catalysts in Henry reaction by He and Zhang (Scheme 22) [44]. In the presence of copper-Schiff-base catalysts, a large number of aldehydes could smoothly reacted with nitromethane as well as nitroethane, although only a moderate dr value were detected. Moreover, the transition-state model accounting for the high stereocontrol was proposed in the report.
As shown in the aforementioned examples, the development of a robust Schiff-base catalyst, with lower catalyst loading and good substrate tolerance, is highly desirable. To meet this demand, Chen et al. reported a sterically bulky Schiff-base catalyst [L38-Cu]2 derived from aminoisoborneol (Scheme 23) [45]. As expected, the reaction was conducted with as little as 1 mol% catalyst loading and afforded 39 β-nitroalcohol products with good to excellent enantioselectivity (76%-98% ee). When this protocol extended to other nitroalkanes, the enantioselectivity was maintained while a lower dr value was observed. In 2012, a new class of chiral sulfoxide-Schiff base ligands (L39-L40) was developed by introducing an additional binding site chiral sulfoxide to the Schiff base scaffold (Scheme 24) [46]. The benzyl substituted sulfoxide-Schiff base (L39) was evaluated as the optimal ligand with respect to the yield and enantioselectivity. To demonstrate the generality of the rationally designed catalyst, numerous aldehydes, including aromatic, heteroaromatic, aliphatic and alkenyl aldehydes, were employed in the reaction giving the nitroalcohol products with uniformly high stereoselectivity. Unfortunately, expanding the substrate scope to nitroethane led to an unsatisfactory stereoinduction. To further explore the potential of the ligands in chiral heterocycle synthesis, a sequential one-pot Henry reaction and iodocyclization was discovered for tetrahydrofuran synthesis [47]. This novel methodology enabled a direct route to enantioenriched 2,5-polysubstituted tetrahydrofuran. Despite unappealing results in the diastereoslectivity, the following reduction dramatically improved the diastereopurity of the final amine product. Inspired by the great success in the homogeneous catalysis, the recyclable Schiff-base catalyst caught more and more attention. An array of ditopic Schiff-base ligands (L41) and polymer-supported ligands (L42) were reported as recyclable catalysts by Kureshy and Shen, repectively (Fig. 2) [48]. As previously mentioned, the ditopic Schiff-base ligands (L41) could be recovered via the equilibrium of assembly and disassembly. After 5 catalytic runs, a complete retention of activity and enantioselectivity was detected. While in the Shen’s work, the recovery of polymer-supported Schiff base catalyst (L42-Cu) was realized by a simple filtration and 8 catalytic recycles maintained the catalytic efficiency with 90% yield and 86% ee.
To further improve the catalyst performance as high as possible, the structurally modifying on the previously reported ligands was explored extensively (Fig. 3) [49]. Among these exploitations, the halogen atom substituted chiral ligands were prepared and screened. It was found that the iodine substituted Schiff-base ligands (L43, L44) were the better chiral ligands compared with previous results. While in the Xu’s work, a camphor-derived Schiff base ligand L45 was developed and applied in the Henry reaction. 5.
Chiral salen based ligands
The chiral salen-cobalt complex as an effective catalyst in Henry reaction was first revealed by Yamada [50]. In this context, it was anticipated that the modified chiral salen ligands would coordinate with copper to promote the asymmetric Henry reaction. Recently, Kureshy et al. reported a simply modified macrocylic salen ligand as a recyclable catalyst in copper catalyzed Henry reaction (Fig. 4) [51]. Interestingly, two types of macrocyclic ligands (L46, L47) could be selectively obtained by varying the reaction medium from the same reactants. With different chiral scaffolds and linked salicylaldehydes (L48, L49), a library of macrocylic salen ligands was achieved [52]. To evaluate the generality of these chiral ligands, a wide range of aldehydes were employed in the reaction. As expected, more than 90% ee values were observed for the most examples with an excellent recyclability (8 catalytic runs). In 2012, a chiral copper-salen catalyst with a rotatable aromatic π-wall was disclosed by Xu, which could selectively recognize simple aromatic aldehydes in the Henry reaction (Scheme 25) [53]. The further UV/vis spectroscopic study clearly illustrates the molecular interaction between chiral catalyst (L50-Cu) and benzaldehyde. This unique molecular recognition effect caused an excellent enantioselectivity for benzaldehyde and halide-containing aromatic aldehydes, and a diminished yield and selectivity for aliphatic and other aromatic aldehydes. In 2012, the chiral rigid scaffold cis-2,5-diaminobicyclo-[2.2.2]octane was employed by the group of White in a novel chiral salen ligand synthesis [54]. A high catalytic performance of the chiral ligand-copper complex in Henry reaction was detected (Scheme 26). The evaluation of reaction parameters showed that 10 mol% tetrahydrosalen ligand (L51) and 1 mol% copper (I) triflate was the optimal catalyst ratio, delivering the nitroalcohol products with 91%-98% ee for most examples. Notably, this efficient catalytic system could be directly used in the construction of the vicinal amino alcohol structure. Additionally, the syn-selective products could be
obtained with >20/1 dr value for the reaction of nitropropane with specific aldehydes. To rationalize the sense of the stereoinduction, the authors proposed a plausible transition-state model (Scheme 26).
To better understand the effect of N,N-substituents on the salen-copper catalyzed Henry reaction, a series of N-substituted tetrahydrosalen and dihydrosalen ligands was prepared via a concise approach [55]. As shown in the Scheme 27, the N,N-methylated tetrahydrosalen-copper (L52-Cu) catalyst proved to be the most efficient catalyst in terms of reaction yield and stereoselectivity. The further investigation on the substrate scope illustrated that this catalytic protocol was well compatible with aryl, heteroaryl, and alkyl aldehydes giving the corresponding products with moderate to good ee values.
To expand the category of the recyclable catalysts, some novel strategies were successfully applied in the salen-based catalysts, such as, MCM-41 supported salen catalyst (cat.1), salen oligomer catalyst (cat.2), zeolite-Y encapsulated salen catalyst (cat.3) (Fig. 5) [56]. Among these catalysts, cat.1 and cat.2 exhibited an excellent recyclability in Henry reaction with 5 and 10 catalytic runs, respectively. While for the encapsulated catalyst cat.3, the recovery of catalyst had not been studied. It is noteworthy that the encapsulated copper complex produced an obvious enhancement in enantioselectivity compared with the homogeneous counterpart. 6. Chiral amino alcohol ligands Chiral amino alcohol ligands were generally used in combination with ZnEt 2 to promote a broad range of asymmetric transformations. For the nitroaldol reaction, a dinuclear zinc-amino alcohol catalyst was first introduced by Trost as a powerful catalyst [57]. To avoid using the high reactive zinc reagent, copper salt was considered as an alternative Lewis acid to combine with amino alcohol. In 2011, a chiral amino acohol ligand (L53) containing an additional pyridine coordination site was prepared by Reddy and coworkers [58]. The activity of the catalyst L53-Cu in Henry reaction was next investigated with various substituted aldehydes. As shown in the Scheme 28, both aromatic and aliphatic aldehydes could react smoothly with nitromethane affording the corresponding adducts with 50%-86% ee. With respect to the mechanism, a reasonable transition-state model was proposed by the authors. To further improve the catalytic efficiency, an elegant amino alcohol-derived ligand (L54) was disclosed by Wang et al. and an ultimate success in Henry reaction was achieved with broad substrate scope and excellent stereoselectivity (Scheme 29) [59]. This efficient catalytic system had some unique advantages: 1) water could be directly used as the reaction medium; 2) a predominately anti-selectivity (mostly above 15/1) was observed in the Henry reaction of higher order nitroalkanes with aldehydes; 3) the facile manipulation procedure and good scalability provided a practical method to access synthetically valuable β-nitroalcohols. To the best of our knowledge, this is first example to reach such high level of anti-selectivity in the copper catalyzed Henry reaction.
On the other hand, the 1,1'-binaphthylazepine derived amino alcohol as a bidentate ligand (L55) was employed in the copper catalyzed Henry reaction (Scheme 30) [60]. Good to excellent enantioselectivities (82%-97% ee) were obtained for both aromatic and aliphatic aldehydes. Extending the substrate scope to other nitroalkanes gave the corresponding syn-selective products with 1.6/1-19/1 dr and >90% ee values. Later, the structurally simplified amino alcohol ligand (L56) was synthesized from commercially available (1S, 2R)-2-amino-1,2diphenylethanol (Fig. 6) [61]. Using in combination with copper acetate, about 38 aromatic β-nitroalcohol products could be easily accessed with 84%-99% ee. It is worth mentioning that this efficient protocol is also compatible with an array of benzofuryl aldehydes affording the corresponding bioactive benzofuryl β-amino alcohols. To further expand the substrate scope to aliphatic aldehydes, the optimal ligand L57 was selected according to the result of testing a library of modified amino alcohols (Fig. 6) [62]. Unsurprisingly, up to 18.6/1 syn-selectivity and above 90% ee values (for syn-isomer) were observed for 18 examples.
Very recently, the extension on the previous catalytic system has drawn considerable interest (Fig. 7). Two types of thiophenecontaining amino alcohol ligands (L58, L59) were developed by Aydin and Xu et al. [63]. The two types ligands were used with different copper salts (Cu(OTf)2/CuCl) and different substrate generalities were presented. Inspired by the work of Lu and Chen[60, 62], another bidentate ligand with a chiral aziridinyl alcohol structure (L60) was reported by Zhao and Liu, although the newly developed ligand resulted in some unsatisfactory results compared with the previous one [64]. 7.
Miscellaneous chiral ligands
Chiral N,N'-dioxides ligands were first developed by Feng et al., which proved to be a powerful chiral ligands in numerous asymmetric transformations [65]. The asymmetric Henry reaction was also explored with the corresponding copper complexes in 2007
[66]. As a continuous work in this respect, the N,N'-dioxides-copper (L61-Cu) catalyzed diastereoselective Henry reaction was unveiled very recently (Scheme 31) [67]. Under the optimal conditions, about 35 examples, including aromatic, heteroaromatic and alkenyl substituted β-amino alcohols, were achieved with moderate to good anti-selectivities and enantioselectivities. It was worth mentioning the diastereoselectivity in the reaction was related with substitutent pattern, the ortho-substitution enhanced the diasteroselectivity (up to 16.7/1). Sparteine-copper catalyzed Henry reaction was first implemented by Maheswaran et al [68]. However, the restricted accessibility of sparteine-like ligands seriously hampers the further development in the modified catalysts. In this respect, Breuning and coworkers developed two analogues of sparteine (L62-L63) via the multi-step synthesis route (Fig. 8) [69]. The corresponding copper complexes were found to be an effective catalyst in Henry reaction delivering the products with >90% ee and moderate diastereoselectivity for the specific examples.
As a longstanding interest in the synthesis and catalytic application of 1,1'-bisisoquinolines, Judeh recently has explored the reactivity and selectivity of various N-alkyl substituented C1-tetrahydro-1,1'-bisisoquinoline ligands in the enantioselective Henry reaction (Fig. 9) [70]. The methylated ligand L64 and unsubstituted ligand L65 were used in combination with copper(I) chloride as the catalysts. Choosing different optimal reaction medium could afford the products with comparable enantioselectivities. Specifically, the methylated chiral ligand was also compatible with other high order nitroalkanes giving the β-nitroalcohol adducts with moderate to good ee values, albeit low dr values.
Cinchona alkaloid as one of the most important privileged organocatalysts has been widely used in numerous asymmetric transformations [71]. By contrast, the cinchona alkaloid derived chiral copper catalyst has received far less attention. Recently, the cinchona-copper L66-Cu catalyzed Henry reaction was first revealed by Skarżewski et al. and a promising result was achieved (Fig. 10) [72]. Later, bipyridine (L67) and phenanthroline (L68) moiety were incorporated into the cinchona alkaloid by the group of Wang and Huang [73]. With this modified catalyst, an obvious enhancement in the enantioselectivity was observed with a broader substrate scope (Fig. 10). 8. Conclusions and perspectives Copper catalysis has recently emerged as a powerful tool in the asymmetric Henry reaction and considerable advances in the diastereo- and enantioselective Henry reaction of aldehydes have been achieved with numerous efficient catalytic system. It is notable that a wide range of nitroalcohol products, including aromatic, heteroaromatic, aliphatic and vinylic adducts, were readily obtained with up to 50/1 dr values and 99% ee values. Despite the great achievements in the Henry reaction of aldehydes, the direct Henry reaction of unactivated ketones with nitroalkanes still remains elusive and underdeveloped. In addition, the scope of nitroalkanes in the well-developed Henry reaction need to be further expanded. Consequently, a novel chiral copper catalyst with lower catalyst loading (<1 mol%), broader substrate scope (ketones and fuctionlized nitroalkanes) and excellent sterocontrol is still in high demand.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Nos. 2127222, 21432009, 21472177, J1310010) and Chinese Academy of Sciences (No. XDB20000000).
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Fig. 1. The application of Henry reaction in natural products.
Fig. 2. Recyclable Schiff-base catalysts in Henry reaction.
Fig. 3. Structurally modified Schiff-base catalyst in Henry reaction.
Fig. 4. A library of macrocyclic salen ligands reported by Kureshy.
Fig. 5. Recyclable salen-based catalyst in Henry reaction.
Fig. 6. Simple chiral amino alcohol-copper catalyzed Henry reaction.
Fig. 7. Newly developed amino alcohol-copper catalysts in Henry reaction.
Fig.8. Sparteine-like ligands in asymmetric Henry reaction.
Fig. 9. 1,1'-Bisisoquinolines-copper catalyzed asym-metric Henry reaction.
Fig. 10. Cinchona alkaloid derived copper catalyst in Henry reaction.
Scheme 1. The versatile building block nitroalcohol.
Scheme 2. The chiral bis(sufonamide)-diamine ligand in the Henry reaction.
Scheme 3. The C1-symmetrical copper catalyzed diastereo-selective Henry reaction.
Scheme 4. Chiral branched diamine ligand in the asymmetric Henry reaction.
Scheme 5. The nonlinear effect in the diamine-copper cataly-zed Henry reaction.
Scheme 6. 1,3-Diamine-copper catalyst in the Henry reaction.
Scheme 7. Sulfonyldiamine in the Henry reaction of branched aldehydes.
Scheme 8. Polymer supported diamine catalyst in Henry reaction
Scheme 9. Chiral diamine-copper catalyst in the Henry reaction of trifluoromethyl ketones.
Scheme 10. The diamine ligands derived from camphor.
Scheme 11. Chiral 2-(pyridin-2-yl)imidazolidin-4-one in the Henry reaction.
Scheme 12. A highly efficient copper catalyst prepared from 2-aminomethyl-5-phenylpyrrolidine in the Henry reaction.
Scheme 13. Spiro diamine-copper catalyzed Henry reaction.
Scheme 14. Pyrrolidine phenylmethanamine ligand in the Henry reaction
Scheme 15. The in-situ generated copper-oxazoline Shiff-base catalyst.
Fig. 1. Recently developed oxazoline-based catalysts.
Scheme 16. The Henry reaction of nitromethane with 2-acylpyridine N-oxides.
Scheme 17. Ionic-tagged bis(oxazoline) catalyst used for Henry reaction.
Scheme 18. Charge transfer interactions in bis(oxazoline) recyclable catalyst
Scheme 19. PS-supported bis(oxazline)-based catalyst in the Henry reaction.
Scheme 20. The release-capture strategy in the recyclable ditopic bisoxazo-line-copper catalyst.
Scheme 21. Fluorous bis(oxazoline)-based catalyst in the Henry reaction
Scheme 22. Cinchona derived Schiff-base catalyst in Henry reaction
Scheme 23. Aminoisoborneol-derived Schiff base catalyst in Henry reaction.
Scheme 24. Chiral sulfoxide-Schiff base catalyst in Henry reaction.
Scheme 25. Strong molecular-recognition effect in Henry reaction.
Scheme 26. cis-2,5-Diaminobicyclo[2.2.2]octane derived chiral tetrahydro-salen-copper catalyst in Henry reaction.
Scheme 27. The effect of N,N-substituents on the salen-copper catalyzed Henry reaction.
Scheme 28. A pyridine-containing amino alcohol catalyst in Henry reaction.
Scheme 29. Highly diastereo- and enantioselective Henry reaction catalyzed by amino alcohol derived copper catalyst.
Scheme 30. Binaphthylazepine derived amino alcohol ligand in copper catalyzed Henry reaction.
Scheme 31. Chiral N,N’-dioxides-copper catalyzed diastereo- and enantio-selective Henry reaction.