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Enantioselective Henry and Aza-Henry Reaction in the Synthesis of (R)-Tembamide Using Efficient, Recyclable Polymeric Cu [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Chiral copper(II) polymeric [H4]salen (salen=bis[(salicylidene)ethylenediaminato]) complexes CuII -1-3 were generated in situ and used as efficient catalysts in the asymmetric Henry and aza-Henry reaction of various aromatic and aliphatic aldehydes and N-tosylimines in the presence of various nitroalkanes at room temperature (27±2°C) for 20 hours. This group of polymeric [H4]salen complexes demonstrated excellent performance (product yield and enantiomeric excess ee up to 98%) in the formation of ß-nitroalcohol by using nitromethane with low catalyst loading of 1 mol% (with respect to the monomeric salen unit) and high enantioinduction (ee 94%) in the aza-Henry product; ß-nitroamines were obtained with moderate yield (78 %). This catalytic system also worked well with nitroethane and 1-nitropropane in the case of Henry reaction, to furnish the corresponding products in high yields and enantioselectivities for syn diastereomers. The CuII -2 complex retained its performance at the gram level and was expediently recycled eight times with no significant loss in its performance. The kinetic study with CuII -2 complex for the enantioselective aza-Henry reaction of N-Ts-benzylimine (Ts=tosyl) revealed a first-order dependence on catalyst and nitromethane concentration and was zero order with respect to the substrate. The product obtained was transformed straightforwardly to the pharmaceutically important enantiomerically pure (R)-tembamide (ß-adrenergic agonist) drug in good yield by the asymmetric nitroaldol reaction of 4-methoxybenzaldehyde in three successive steps. Keywords : asymmetric catalysis · enantioselectivity · Henry reaction · kinetics · polymeric ligands Introduction The high demand for enantiomerically pure drugs[1] has gained a surge of interest owing to the inherent therapeutic behavior of the individual enantiomers of the drug in the biological system. Chiral b-amino alcohols containing enantiopure drugs have received much attention and several routes have been re- ported for their synthesis. Among them, catalytic enantioselec- tive addition of nitroalkanes to carbonyl compounds provides optically active b-nitroalkanols,[2] useful intermediates in the asymmetric synthesis of the b-receptor agonists (^)-denopa- mine[3] and (^)-arbutamine,[3] b-blockers (S)-metoprolol,[4] (S)- propanolol,[5] and (S)-pindolol,[6] and pharmacologically impor- tant b-amino alcohol derivatives, such as chloramphenicol,[7] ephedrine,[7] sphingosine,[8] and so forth. On the other hand, the addition of nitroalkanes to imines (aza-Henry reaction) is [a] [b] a powerful and efficient carbon-carbon bond forming reac- tion[9] in which the resulting products of the aza-Henry reac- tion can easily be converted to 1,2-diamines[10] under reductive conditions or oxidatively cleaved to afford a-amino acids.[11] Shibasaki and co-workers demonstrated a series of heterobi- metallic catalysts that proved to be effective for asymmetric Henry reactions.[12] Starting from these promising results, in recent years lots of different organocatalytic[13] and chiral metal-based catalytic systems[14] have been reported to accom- plish the enantioselective Henry and aza-Henry reactions. Among them, copper-catalyzed Henry/aza-Henry reactions with bis[(salicylidene)ethylenediaminato] (salen)-type C2-sym- metric ligands[14d-f, 15] have shown better performance over or- ganocatalytic systems in terms of low temperature, high cata- lyst loading, long reaction time, and use of organic and inor- ganic bases as additives, but with no recyclability data. As chiral catalysts are quite expensive, their recyclability is an im- portant aspect in offsetting the catalyst cost. Herein, we report a couple of modifications in the catalyst design. First, to improve the efficiency of the catalyst, a poly- meric ligand with about eight repeating catalytic sites was in- corporated, thereby increasing the molecular weight of the catalyst to facilitate its recyclability. Second, incorporation of basic sites in the catalyst might assist abstraction of the proton from nitromethane to generate a nitronate ion, which func- tions as a nucleophile and provides excellent results with alde- hydes/imines. With this knowledge, and in the quest for the development of recyclable chiral catalysts, we have previously reported Cu-catalyzed Henry/aza-Henry reactions of aldehydes/ imines with various nitroalkanes as active nucleophiles in the presence of monomeric and dimeric macrocyclic salen and [H4]salen ligands with the trigol, piperazine, 1,1'-bi-2-naphthol (BINOL), and homopiperazine motif.[14d-f] Herein, we report the synthesis of a polymeric [H4]salen ligand having inbuilt base functionality (melamine-piperazine) as linker that can facilitate the abstraction of protons from nitroalkane to form the nitro- nate ion, which works as a nucleophile in the Henry and aza- Henry reactions. The complex of [H4]salen ligand with Cu metal ion was generated in situ and used for asymmetric Henry and aza-Henry reactions with very low catalyst loading (1 mol % with respect to the monomeric salen unit, the lowest so far reported in the literature) and effective recyclability (eight times reuse) of the active catalyst with preservation of its catalytic activity and selectivity. Results and Discussion According to the mechanism of the Henry and aza-Henry reac- tions, it is known that designing an efficient catalyst requires the presence of cooperative activation of two reactive sites (acidic and basic sites) for achieving the corresponding prod- ucts in higher yield and enantioselectivity. The acidic site helps to amplify the electrophilic nature of the carbonyl compounds by binding with the lone pair of electrons of the carbonyl group in the case of the Henry reaction, and with nitrogen atoms in the case of the aza-Henry reaction. On the other hand, basic sites are required for abstracting the proton from the nitroalkane to generate the active nucleophile. The poly- meric ligands, because of their high molecular weight, have lower solubility in nonpolar solvent, and hence provide the re- cyclable character of the catalyst as well as reducing the cata- lyst loading. Accordingly, the polymeric salen ligands 1-3 were synthesized by the interaction of (melamine-piperazine) tri- meric aldehyde A with diamines of different bulkiness at the chiral collar, namely (1R,2R)-(^)-1,2-diaminocyclohexane, (1R,2R)-(+)-1,2-diphenyl-1,2-diaminoethane, and (R)-(+)-1,1'-bi- naphthyl-2,2'-diamine, respectively, followed by reduction with NaBH4 (Scheme 1). Initial screening was conducted with all these ligands, which possess different bulkiness in the diamine collar, in combina- tion with Cu(OAc)2·H2O as metal source for the asymmetric Henry reaction of 3,4-dimethoxybenzaldehyde and nitrome- thane as model substrate by using ethanol and dichlorome- thane solvent mixture at room temperature for 20 hours. Among them, CuII-2 featuring the diphenyldiamine collar was proved to be the best with respect to both yield and selectivi- ty of the corresponding nitroalcohol (Table 1, entry 2). Encouraged by the preliminary result with the optimized catalyst and to ascertain an effective metal complex for the catalytic reaction, we performed systematic screening of vari- ous copper salts (both monovalent and divalent) with the chiral ligand 2 (the most active and enantioselective). The ra- tionale behind this is based on the concept that copper sour- ces with different counterions are well known to form structur- ally different complexes with a given ligand, and hence strong- ly influence the catalytic activity. Among all the copper salts screened it was found that Cu(OAc)2·H2O with the chiral [H4]salen ligand 2 could generate an active catalyst to promote the enantioselective addition of nitromethane to the 3,4-dime- thoxybenzaldehyde giving an effective yield and selectivity in 20 hours (Figure 1). These results are in agreement with our previous reports.[14d,f] The solvent is known to influence the activity and enantiose- lectivity of a catalytic protocol. Therefore, a group of solvents was screened for the asymmetric Henry reaction of 3,4-dime- thoxybenzaldehyde and nitromethane as model substrate (Table 2, entries 1-7). Aprotic solvents, such as THF, nitrome- thane, diethyl ether, dimethylformamide (DMF), and acetoni- trile (Table 2, entries 1, 2, 4-6), halogenated solvents such as di- chloromethane (Table 2, entry 3), and mixed solvent (ethanol/ dichloromethane, 1:1; Table 2, entry 7) were used. Gratifyingly, THF proved to be the most suitable solvent for this reaction in view of the enantioselectivity and yield of the corresponding b-nitroalcohol in 20 hours (Table 2, entry 1). Temperature had a significant effect on the enantioselectivity of the product. Lowering the temperature from room temperature (RT) to 0 and ^10 8C showed a decreased yield of the product b-nitroal- cohol without any increase in enantioselectivity (Table 2, en- tries 8 and 9). On the other hand, upon increasing the tem- perature from room temperature to 40 8C the yield of the corre- sponding b-nitroalcohol was little enhanced but there was a detrimental effect on its enan- tioselectivity (Table 2, entry 10). Hence, room temperature was the optimized temperature for giving an excellent yield (91 %) of the corresponding b-nitroal- cohol with 95 % enantioinduc- tion (Table 2, entry 1). For its consequences in poten- tial industrial processes, the cata- lyst loading was further assessed for a wide range of values from 0.5 to 2 mol % with respect to the representative substrate 3,4- dimethoxybenzaldehyde. It was found that 1 mol % of the cata- lyst was sufficient to provide ex- cellent yield (85 %) and enantio- selectivity (95 %) in 20 hours (Figure 2), whereas 2 mol % of the catalyst was able to produce the desired product in high yield (89 %) in less reaction time but with a decrease in enantiomeric excess ee (81 %). Hence, 1 mol% catalyst loading is considered as optimum. These results are su- perior in terms of the lowest cat- alyst loading reported so far for the systems described in the lit- erature, and our own system containing inbuilt base functiona- lity.[14d] Inspired by the above observations under optimized reac- tion conditions, next we evaluated the nitroaldol reaction pro- tocol for various aldehydes having aromatic, aliphatic, or a,b- unsaturated substituents with nitromethane and the results are summarized in Figure 3. All the aldehydes used in the pres- ent study underwent nitroaldol reaction to give the corre- sponding b-nitroalcohol in good to excellent yields of isolated products (68-90 %) and with moderate to excellent enantiose- lectivity (ee, 64-98 %) depending on the substituents on the substrate. The highest ee value of up to 98 % accompanied by an 88 % yield was furnished for 4-methoxybenzaldehyde. To further investigate this reaction protocol, several less ex- plored nitroalkanes (nitroethane and 1-nitropropane) other than nitromethane were then evaluated with various alde- hydes (Table 3). High yields (95 %) with excellent enantioselectivities (98 %) and moderate to good diastereoselectivities (up to 90 :10, favoring the syn product) were achieved. It was ob- served that the electronic properties and the steric hindrance of the aromatic aldehydes have a limited effect on the diaste- reoselectivity and enantioselectivity of the corresponding prod- uct. A probable working model of the catalytic system for fa- voring the syn diastereomer was proposed by the Newman projection in Figure 4. To further explore the catalytic application of the present system, this protocol was extended for the asymmetric aza- Henry reaction of various N-tosylimines with nitromethane. To begin with the optimized reaction conditions, we conducted the aza-Henry reaction using N-Ts-benzylimine (Ts = tosyl) as model substrate with nitromethane in the presence of CuII-2 as active complex in different solvents, namely acetonitrile, THF, and mixtures of solvents (EtOH + CH2Cl2 and MeOH + CH2Cl2), for which THF was the solvent of choice (Table 4, entry 6). Next, the loading of the catalyst was varied from 0.5 to 2 mol % (Table 4, entries 4-7) and the reaction temperature from ^10 to 40 8C (Table 4, entries 8-10) in the presence of THF. It is evident from the results that 1.5 mol % catalyst load- ing at room temperature is optimum in THF as solvent (Table 4, entry 6). With these optimized reaction conditions, a series of N-tosy- limines having different substituents at the aromatic ring were tested with nitromethane to find the generality of substrate scope in the aza-Henry reaction (Scheme 2). A moderate yield (78 %) of b-nitroamines with high enantioinduction (up to ee 94 %) was achieved at room temperature in THF. However, no clear trend was observed for the electronic effect of the substituents on the yield as well as on selectivity. These results of the aza-Henry reaction using the active complex CuII-2 are quite comparable to those of the reaction with aromatic alde- hydes. Kinetics of the reaction To understand the dependence of the reaction on catalyst load- ing, substrate concentration, and nitromethane concentration and to find out a probable reaction mechanism of the asymmetric aza-Henry reaction, we studied the kinetics of the CuII-2 complex generated in situ from ligand 2 and Cu(OAc)2·H2O as catalysts for the aza-Henry reac- tion of the N-Ts-benzylimine as substrate with nitromethane at room temperature. Effect of concentration of catalyst on reaction rate To screen the effect of catalyst concentration on the rate of re- action (kobs), we performed the aza-Henry reaction of N-Ts-ben- zylimine with nitromethane at different concentrations of the catalyst CuII-2 (0.69-3.75 mm) keeping all other parameters identical. The kobs at each concentration of the catalyst was ob- tained by plotting the log [product] against the time at three different time intervals (60, 180, and 300 min) at that particular concentration of the catalyst, and from the slope of the plot the kobs values were obtained. The plot of log kobs versus log [ catalyst] (Figure 5) gives a straight line with slope of approxi- mately 1, which indicates the first order with respect to the catalyst concentration. Effect of concentration of nitromethane on reaction rate To find out the effect of nitromethane (nucleophile) concentra- tion on reaction rate, we performed the aza-Henry reaction with fixed concentrations of N-Ts-benzylimine and catalyst with variable concentration of nitromethane (0.5-2.5 m). The kobs values were found at the different concentrations of nitrome- thane in a similar way to the catalyst concentration variation. In this case also, the plot of log kobs against log [nitromethane] (Figure 6) gave a straight line with a slope of almost unity, thus indicating the first-order dependency of the reaction on nitro- methane concentration. Variation of imine concentration For further evaluating order with respect to N-Ts-benzylimine, the aza-Henry reaction of N-Ts-benzylimine was performed with substrate concentrations ranging from 0.1 to 0.35 mm. The plot of log kobs for different concentrations of the imine versus log [imine] gave zero-order dependence of the rate on the substrate concentration (Figure 7). To support the mechanism (Scheme 3) of the catalytic aza- Henry reaction, a stepwise UV-visible spectral study was per- formed with N-Ts-benzylimine and nitromethane in THF as sol- vent at room temperature (Figure 8). After addition of sub- strate to the solution of the complex, both the d-d and ligand-to-metal charge transfer (LMCT) bands showed a blue- shift with isosbestic points at approximately 420 and 595 nm. These observations confirm the direct coordination of the sub- strate to the copper complex through the lone pair of the ni- trogen atom of the N-Ts-benzylimine. The generation of the isosbestic points may result from the changes in the geometry of the copper complex on complexation of substrate to the copper. On further addition of nitromethane to the reaction mixture, we did not observe a considerable blueshift without an isosbestic point ( ^ 437 nm) confirming further changing of the geometry of the copper complex. To perform a recycling study of the catalytic protocol using CuII-2 as catalyst, benzaldehyde with nitroethane were chosen as the model substrates with 1 mmol aldehyde in THF at room temperature. After eight successive cycles of the catalytic system, there was no significant change in the activity, enantioselectivity of the syn and anti products, or diastereomeric excess (de) of the b-nitroalcohol (Figure 9). To confirm the sta- bility of the recovered catalyst during the course of the asym- metric nitroaldol reaction, the IR spectra (Figure 10) were re- corded for both virgin catalyst and recovered catalyst. The spectra matched well, which suggests that no major structural changes had taken place during the course of the post-catalyt- ic workup procedure. As a synthetic relevance of this asymmetric catalytic proce- dure, (R)-tembamide, a naturally occurring hydroxyamide (hy- poglycemic agent)[16] isolated from various members of the Ru- taceae family,[17] was synthesized from 4-methoxybenzaldehyde with nitromethane to give the corresponding nitroalcohol (A) in 88 % yield and 98 % ee on the 50 mmol scale (Scheme 4). Catalytic hydrogenation of A in the presence of 10 % Pd/C in methanol gave the corresponding amino alcohol (B)in92% yield followed by condensation with benzoyl chloride ; the final product was obtained in good yield and enantioselectivity. Conclusion We have developed a new, recyclable, polymeric chiral CuII- [H4]salen catalyst for both the asymmetric Henry and aza- Henry reactions with good substrate generality. The catalytic system works well with very low catalyst loading (1 mol %) to produce the corresponding products in moderate to high yields with excellent enantioselectivities as well as diastereose- lectivities in the case of nitroethane and 1-nitropropane, using THF as solvent at room temperature. The catalytic system was recycled and reused for eight consecutive catalytic runs with virtually the same activity and selectivity. The valuable drug (R)-tembamide (hypoglycemic agent) was synthesized starting from 4-methoxybenzaldehyde with good yield and enantiose- lectivity. Based on our knowledge this catalytic protocol worked well at a very low catalyst loading, the lowest so far re- ported for both Henry and aza-Henry reactions. Experimental Section Preparation of chiral polymeric [H4]salen ligands 1-3 A solution of trialdehyde A (2 mmol) in THF (10 mL) was added to a solution of (1R,2R)-(^)-1,2-diaminocyclohexane/(1R,2R)-(+)-1,2-di- phenyl-1,2-diaminoethane and (R)-(+)-1,1'-binaphthyl-2,2'-diamine (3.2 mmol) in THF (10 mL). The resulting mass was heated at reflux for 3 h (checked by TLC). The resulting dark yellow solution was cooled to room temperature followed by reduction with NaBH4 to obtain the polymeric ligands 1-3.[18] (Data are given in the Sup- porting Information.) Acknowledgements CSMCRI Communication No. 035/2014. A.D. and R.I.K. are grateful to DST and CSIR-Indus Magic Project CSC0123 for financial assis- tance. A.D. is grateful to UGC for awarding SRF and to AcSIR for Ph.D. registration. We are also grateful to the Analytical Disci- pline and Centralized Instrument Facility of CSMCRI for providing instrumental facilities. [1] For examples, see : a) R. 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Tech- nol. 2013, 3, 2661 - 2667. Received : March 24, 2014 Published online on May 21, 2014 Anjan Das,[a, b] Manoj K. Choudhary,[a, b] Rukhsana I. Kureshy,*[a, b] Tamal Roy,[a] Noor-ul H. Khan,[a,b] Sayed H. R. Abdi,[a,b] and Hari C. Bajaj[a,b] [a] A. Das, M. K. Choudhary, Dr. R. I. Kureshy, T. Roy, Dr. N.-u. H. Khan, Dr. S. H. R. Abdi, Dr. H. C. Bajaj Discipline of Inorganic Materials and Catalysis Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI) Bhavnagar 364021, Gujarat (India) Fax: (+ 91) 0278-2566970 E-mail : [email protected] [b] A. Das, M. K. Choudhary, Dr. R. I. Kureshy, Dr. N.-u. H. Khan, Dr. S. H. R. Abdi, Dr. H. C. Bajaj Academy of Scientific and Innovative Research (AcSIR) CSIR-CSMCRI Bhavnagar 364021, Gujarat (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402078. (c) 2014 Blackwell Publishing Ltd. |