Letter pubs.acs.org/OrgLett
Copper-Catalyzed Enantioselective Henry Reaction of β,γ‑Unsaturated α‑Ketoesters with Nitromethane in Water Yanan Li, Yekai Huang, Yang Gui, Jianan Sun, Jindong Li, Zhenggen Zha, and Zhiyong Wang* Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry and Department of Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *
ABSTRACT: A highly enantioselective Henry reaction of β,γ-unsaturated α-ketoesters with nitromethane in water by virtue of chiral copper complexes has been developed. A series of unsaturated β-nitro-α-hydroxy esters bearing tetrasubstituted carbon stereocenters were obtained exclusively with high yields and excellent enantioselectivities. This method could avoid tedious anaerobic anhydrous manipulation and reduce the environmental pollution caused by organic solvents. 1, e.g., 1).8a Then, only 30−60% ee of unsaturated β-nitro-αhydroxy esters were obtained when β,γ-unsaturated α-ketoesters were employed as substrates (Scheme 1, e.g., 2).8b Besides, the chiral cinchona alkaloids were employed for the Henry reaction of α-ketoesters in CH2Cl2 by the Deng group (Scheme 1, e.g., 3).8d Recently, the Pedro group reported the enantioselective Henry reactions of α-ketoesters with nitromethane catalyzed by chiral copper(II)-iminopyridine complex in moderate to high yields with 48−82% ee (Scheme 1, e.g., 4).8e There remains a great challenge to develop a general method for the Henry reaction of β,γ-unsaturated α-ketoesters, and the enantioselectivities still need to be improved in spite of these impressive contributions, let alone in water. Our group has been devoting effort to study aqueous organic reactions,9 especially the aqueous asymmetric reactions.10a,b Herein, we report a highly enantioselective Henry reaction of β,γ-unsaturated α-ketoesters in water. A variety of chiral unsaturated β-nitro-α-hydroxy esters bearing a quaternary center can be obtained with excellent yields and ee values (Scheme 1, e.g., 5). First of all, the addition of (E)-isopropyl 2-oxo-4-phenylbut-3enoate 1a to nitromethane 2a was chosen as a model reaction in water. Based on our previous efforts on asymmetric reactions,10 the chiral L−Cu complex was utilized to optimize this reaction and Bu4NPF6 as the phase-transfer catalyst (PTC). Initially, different copper salts, which were considered to be crucial for the reactivity and selectivity, were screened for this reaction (Table 1, entries 1−4). It was found that CuBr2 was the best Lewis to give the corresponding product with 67% yield and 89% ee (Table 1, entry 4). In order to improve the efficiency of the catalyst in
A
s we know, most of the organic reactions have been carried out in organic solvents for a long time. However, with the development of green and sustainable chemistry, the reaction solvent needs to be more cost-effective. Water as a safe, harmless, cheap, readily available and environmentally benign solvent has attracted increased attention from organic chemists.1 Aqueous asymmetric reactions have been extensively studied in recent years. In particular, Maruoka et al. and Kobayashi et al. have made great contributions in this area.2 Recently, the Garcι ́a-Á lvarez group developed ruthenium-catalyzed redox isomerization of allylic alcohols followed by the enantioselective enzymatic ketone reduction in aqueous medium.3 Moreover, an impressive work in the Michael reaction of unreactive β,β-disubstituted nitroalkenes with dithiomalonates had been developed by the Song group by using a chiral squaramide catalyst in brine.4 However, there remains a great challenge to develop C−C bond formation in net water, especially for asymmetric transitionmetal catalysis.1i Water as a kind of solvent is rarely reported in asymmetric reactions. However, the enantioselective Henry reaction of aldehydes as substrates with nitromethane has been extensively exploited to obtain a diversity of chiral β-nitroalcohols.5 For instance, the Nagasawa group developed a bifunctional guanidine−thiourea catalyst as an efficient catalyst for the Henry reaction in a biphasic system of water and toluene, which led to the efficient synthesis of β-nitroalcohols.6 Afterward, the Henry reaction was developed by Griengl et al. in a biphasic aqueous system using enzymatic catalysis.7 Compared with aldehyde, the Henry reactions of bicarbonyl dienophiles with nitromethane are relatively fewer.8 The Jørgensen group reported the first enantioselective Henry reaction of α-ketoesters with nitromethane catalyzed by chiral copper(II)-bisoxazoline complex in dry nitromethane (Scheme © 2017 American Chemical Society
Received: October 23, 2017 Published: November 20, 2017 6416
DOI: 10.1021/acs.orglett.7b03299 Org. Lett. 2017, 19, 6416−6419
Letter
Organic Letters
then added to the reaction mixture (Table 2, entries 1−7).5j The results showed that 2-fluorophenol favored this reaction, and the
Scheme 1. Previous Study and This Work on Henry Reaction
Table 2. Optimization of Reaction Conditionsa entry
copper salt
PTC
additive
yieldb (%)
eec (%)
1 2 3 4 5 6 7 8d 9e 10f 11g 12h 13i
CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2
Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4 Et4NClO4
HFIP CF3CH2OH 4-ClC6H4OH 2-ClC6H4OH 2-FC6H4OH 3-FC6H4OH 4-FC6H4OH 2-FC6H4OH 2-FC6H4OH 2-FC6H4OH 2-FC6H4OH 2-FC6H4OH 2-FC6H4OH
74 74 75 77 72 77 71 80 85 90 90 84 85
81 88 87 88 91 89 82 92 92 93 94 64 57
a
Unless otherwise noted, all reactions were performed with 1a (0.25 mmol), 2a (2.5 mmol), L (10 mol %), Cs2CO3 (10 mol %), CuBr2 (10 mol %), Et4NClO4 (10 mol %), and additive (10 mol %) in water (1.0 mL) at 0 °C. bIsolated yield. cDetermined by chiral HPLC analysis. d 2-FC6H4OH (50 mol %). e2-FC6H4OH (100 mol %). fCompound 2a (4.0 mmol). gAddition of 100 μL of CHCl3. hWater (1.0 mL) replaced by CHCl3 (1.0 mL). iWater (1.0 mL) replaced by nitromethane (1.0 mL).
corresponding unsaturated β-nitro-α-hydroxy ester 3a could be obtained in 72% yield and 91% ee (Table 2, entry 5). Then, the equivalent of 2-fluorophenol was screened (Table 2, entries 8− 9). The use of 1.0 equiv of the 2-fluorophenol proved to be the best addition, providing the unsaturated β-nitro-α-hydroxy ester 3a in 85% yield and 92% ee (Table 2, entry 9). When increasing the quantity of nitromethane to 4.0 mmol, however, the best result can be obtained with 93% ee and an increased yield of 90% (Table 2, entry 10). If the 100 μL of CHCl3 was added to the reaction mixture, the ee value was improved slightly to 94%, while the yield kept in 90% (Table 2, entry 11). In order to get insight into the role of water, the water (1.0 mL) was replaced by CHCl3 (1.0 mL) and nitromethane (1.0 mL), respectively, the corresponding ee values were obviously dropped to 64% and 57% in spite of the good yields (Table 2, entries 12−13). The results showed that the water promoted the enantioselectivity of this reaction. As a result, the optimal reaction conditions were as follows: L as the chiral ligand, CuBr2 as the Lewis acid, Cs2CO3 as the base, Et4NClO4 as the PTC, 2-fluorophenol as an additive, water as the solvent, and the reaction being carried out at 0 °C. Under the optimized conditions, the substrate scope of β,γunsaturated α-ketoesters for the Henry reaction was investigated. As presented in Table 3, a variety of substrates including challenging heterocyclic, ring-fused, and aliphatic β,γ-unsaturated α-ketoesters were successfully used in the Henry reaction to afford the corresponding unsaturated β-nitro-α-hydroxy esters in good yields and excellent enantioselectivities (Table 3, entries 1−17). First, the electronic effect of the substrates was explored by changing the para-substituents of R1 (Table 3, entries 2−7). When the substrates bearing the electron-donating groups on the para-position of the phenyl ring were employed, the corresponding products could be obtained with high yields and enantioselectivities (Table 3, entries 2−3). Similarly, the electron-withdrawing groups, such as fluoro-, chloro-, and bromo-groups, were compatible with the reaction system (Table 3, entries 4−6). When the strong electron-withdrawing
Table 1. Optimization of the Reaction Conditionsa
entry
copper salt
PTC
yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Cu(OTf)2 CuCl2·2H2O Cu(OAc)2·H2O CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2 CuBr2
Bu4NPF6 Bu4NPF6 Bu4NPF6 Bu4NPF6 Et4NPF6 Et4NClO4 Bu4NClO4 Bu4NBr Bu4NHSO4 Et4NBr Et4NI Bu4NH2PO4 PVP SDS SLS
69 65 70 67 70 70 66 71 65 70 66 73 62 50 55
78 87 79 89 88 90 86 85 87 89 83 67 87 88 57
a Unless otherwise noted, all reactions were performed with 1a (0.25 mmol), 2a (2.5 mmol), L (10 mol %), Cs2CO3 (10 mol %), PTC (10 mol %), and metal salt (10 mol %) in water (1.0 mL) at 0 °C. b Isolated yield. cDetermined by chiral HPLC analysis. PVP = polyvinylpyrrolidone, SDS = sodium dodecyl sulfate, SLS = sodium laurylsulfonate.
water, various phase transfer catalysts were examined in the absence of any additive (Table 1, entries 4−15). From these results, Et4NClO4 was found to be the best choice for the reaction (Table 1, entry 6). However, a satisfactory result still could not be obtained. In order to further enhance stereoselectivity of this reaction, a series of weak acid additives were 6417
DOI: 10.1021/acs.orglett.7b03299 Org. Lett. 2017, 19, 6416−6419
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Organic Letters Table 3. Scope of β,γ-Unsturated α-Ketoestersa
entry
R1
R2
time (h)
3
yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
C6H5 p-MeC6H4 p-OMeC6H4 p-FC6H4 p-ClC6H4 p-BrC6H4 p-NO2C6H4 m-FC6H4 o-FC6H4 (E)-cinnamyl 2-naphthyl 2-thienyl cyclohexyl C6H5 C6H5 C6H5 C6H5
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr Bn Et Me Et t-Bu Bn
36 36 36 36 36 36 45 36 36 45 36 45 36 36 36 36 40
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p 3q
90 85 88 86 87 85 80 88 85 75 91 79 78 89 87 87 85
93 94 96 93 92 91 83 90 90 94 90 91 93 92 93 91 91
Figure 1. X-ray structure of the product 3a.
Scheme 2. Asymmetric Henry Reaction on a Gram Scalea,b,c
a
Unless otherwise noted, the reaction was performed with 1a (5 mmol), 2a (80 mmol), L (10 mol %), Cs2CO3 (10 mol %), CuBr2 (10 mol %), Et4NClO4 (10 mol %), and 2-FC6H4OH (100 mol %) in water (15 mL) at 0 °C, 48 h. bIsolated yield. cDetermined by chiral HPLC analysis.
a
Unless otherwise noted, all reactions were performed with 1 (0.25 mmol), 2 (4.0 mmol), L (10 mol %), Cs2CO3 (10 mol %), CuBr2 (10 mol %), Et4NClO4 (10 mol %), and 2-FC6H4OH (100 mol %) in water (1.0 mL) at 0 °C. bIsolated yield. cDetermined by chiral HPLC analysis.
nitroethane. However, other dicarbonyl compounds, such as methyl 2-oxo-2-phenylacetate were employed as the reaction substrate. The corresponding β-nitro-α-hydroxy ester product was not obtained either. Only the reaction substrate was recovered, perhaps due to the steric and electronic effect of the methyl 2-oxo-2-phenylacetate. In conclusion, a copper-catalyzed asymmetric Henry reaction of β,γ-unsaturated α-ketoesters with nitromethane in water was developed for the first time under mild reaction conditions. A series of unsaturated β-nitro-α-hydroxy esters bearing tetrasubstituted carbon stereocenters were obtained exclusively with high yields and excellent enantioselectivities. Besides, the reaction in gram scale with water as the solvent could be easily realized to give the excellent result. Further studies on other aqueous asymmetric reactions are ongoing in our group.
1
group was installed on the para-position of R , the Henry reactions could still be carried out smoothly, but a slight negative influence on the ee value was observed (Table 3, entry 7). These results indicated that the substrate with the electron-donating group could afford higher yield and more excellent enantioselectivity than that bearing the strong electron-withdrawing group. Then, the steric effect of the reaction was investigated. Substitution at other positions of the phenyl group was also tolerated well, giving the desired products with excellent yields and enantioselectivities (Table 3, entries 4, 8−9). Unsaturated (E)-cinnamyl groups and ring-fused groups were also successfully employed as substrates in this reaction to afford the corresponding products 3j and 3k in excellent yields and enantioselectivities (Table 3, entries 10−11). Moreover, when a heterocyclic group, such as 2-thienyl group, was examined, excellent enantioselectivity was also achieved (Table 3, entry 12). Notably, the product 3m could also be obtained with excellent enantioselectivity when R1 was changed to the aliphatic group (Table 3, entry 13). Finally, different ester groups (R2) were explored under the standard reaction condition. Little influence was found on the reaction yields and enantioselectivities regardless of the steric and electronic effects (Table 3, entries 1, 14−17). Moreover, the absolute configuration of the product 3a was confirmed by X-ray crystal diffraction (Figure 1). Next, to probe the robust nature and practicability of this novel method, a gram-scale experiment was performed in 15 mL of water (Scheme 2). To our delight, the desired product 3c could be obtained with 1.28 g quantity, 83% yield, and 94% ee in the presence of 10 mol % catalyst. To further examine the substrate scope, nitroethane was employed as substrates in this reaction. However, no desired product was observed, perhaps due to the lower reactivity of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03299. Experimental procedures, characterization data, copies of H NMR, and 13C NMR of new compounds; HPLC profiles and crystallographic data of compound 3a (PDF) 1
Accession Codes
CCDC 1561759 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 6418
DOI: 10.1021/acs.orglett.7b03299 Org. Lett. 2017, 19, 6416−6419
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Organic Letters
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(9) (a) Wang, Y.; Zhu, D.-P.; Tang, L.; Wang, S.-J.; Wang, Z. Y. Angew. Chem., Int. Ed. 2011, 50, 8917. (b) Zhang, L.; Chen, H.; Zha, Z.-G.; Wang, Z.-Y. Chem. Commun. 2012, 48, 6574. (c) Tang, L.; Sun, H.-Y.; Li, Y.-F.; Zha, Z.-G.; Wang, Z.-Y. Green Chem. 2012, 14, 3423. (d) Tang, L.; Guo, X.-F.; Li, Y.-F.; Zhang, S.; Zha, Z.-G.; Wang, Z.-Y. Chem. Commun. 2013, 49, 5213. (e) Guo, X.-F.; Tang, L.; Yang, Y.; Zha, Z.-G.; Wang, Z.Y. Green Chem. 2014, 16, 2443. (f) Tang, L.; Guo, X.-F.; Yang, Y.; Zha, Z.-G.; Wang, Z.-Y. Chem. Commun. 2014, 50, 6145. (g) Yang, Y.; Tang, L.; Zhang, S.; Guo, X.-F.; Zha, Z.-G.; Wang, Z.-Y. Green Chem. 2014, 16, 4106. (h) Yang, Y.; Zhang, S.; Tang, L.; Hu, Y.-B.; Zha, Z.-G.; Wang, Z.Y. Green Chem. 2016, 18, 2609. (i) Yang, Y.; Bao, Y.-J.; Guan, Q.-Q.; Sun, Q.; Zha, Z.-G.; Wang, Z.-Y. Green Chem. 2017, 19, 112. (j) Liu, L.Y.; Wang, Z.-Y. Green Chem. 2017, 19, 2076. (10) (a) Lai, G.-Y.; Guo, F.-F.; Zheng, Y.-Q.; Fang, Y.; Song, H.-G.; Xu, K.; Wang, S.-J.; Zha, Z.-G.; Wang, Z.-Y. Chem. - Eur. J. 2011, 17, 1114. (b) Xu, K.; Lai, G.-Y.; Zha, Z.-G.; Pan, S.-S.; Chen, H.-W.; Wang, Z.-Y. Chem. - Eur. J. 2012, 18, 12357. (c) Zhang, S.; Xu, K.; Guo, F.-F.; Hu, Y.B.; Zha, Z.-G.; Wang, Z.-Y. Chem. - Eur. J. 2014, 20, 979. (d) Hu, Y.-B.; Xu, K.; Zhang, S.; Guo, F.-F.; Zha, Z.-G.; Wang, Z.-Y. Org. Lett. 2014, 16, 3564. (e) Li, L.-J.; Zhang, S.; Hu, Y.-B.; Li, Y.-N.; Li, C.; Zha, Z.-G.; Wang, Z.-Y. Chem. - Eur. J. 2015, 21, 12885. (f) Hu, Y.-B.; Li, Y.-N.; Zhang, S.; Li, C.; Li, L.-J.; Zha, Z.-G.; Wang, Z.-Y. Org. Lett. 2015, 17, 4018. (g) Li, Y.-N.; Hu, Y.-B.; Zhang, S.; Sun, J.-N.; Li, L.-J.; Zha, Z.-G.; Wang, Z.-Y. J. Org. Chem. 2016, 81, 2993.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhiyong Wang: 0000-0002-3400-2851 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Nature Foundation of China (J1310010, 21432009, 21472177, 21672200, 21772185, XDB20000000) is greatly acknowledged.
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DOI: 10.1021/acs.orglett.7b03299 Org. Lett. 2017, 19, 6416−6419
