Highly Enantioselective Hydrogenation of Cyclic Enamides Catalyzed by a Rh-PennPhos Catalyst.

Chiral amines are often critical components of pharmaceutical agents. For example, about 15-25% of the singleenantiomer products in development contain this unit according to a recent analysis.1 The development of practical methods for the synthesis of enantiomerically pure amines is therefore of great interest. Traditional resolution methods and enzymatic transaminase technology are frequent choices in industry for making chiral amines. However, recent attention has been devoted to asymmetric hydrogenation of imines and enamides as potentially more competitive synthetic routes.2 A variety of transition-metal catalysts have been explored to date for asymmetric hydrogenation of imines, albeit with either low enantioselectivity, limited substrate scope, or low activity.3 Alternatively, asymmetric hydrogenation of simple enamides is a more useful strategy. Several Rh-bisphosphine compounds have been developed as efficient catalysts for asymmetric hydrogenation of acyclic enamides,4 and some can even tolerate Z/E mixtures of â-substituted enamides as substrates.4a,c However, only limited success has been achieved in the hydrogenation of cyclic enamides despite the potential importance of this process for the synthesis of biologically active chiral aminotetralins and aminoindanes.5 Herein we report the highly enantioselective hydrogenation of cyclic enamides catalyzed by a Rh-PennPhos compound. In our continuing work on catalytic asymmetric reactions, we have developed conformationally rigid chiral bisphosphines (e.g., BICP ) bis(diphenylphosphino)dicyclopentane;6 phosphinobicyclo[2.2.1]heptanes;7 PennPhos ) P,P′-1,2phenylenebis(endo-2,5-dialkyl-7-phosphabicyclo[2.2.1]heptane,8 Me-PennPhos, Figure 1) and are exploring their synthetic utility. High enantioselectivities have been achieved for the hydrogenation of acyclic enamides with a Rh-BICP catalyst.4c However, only modest enantioselectivities (6070% ee) were obtained for the hydrogenation of cyclic enamides derived from R-tetralone and R-indanone. Encouraged by the excellent results observed upon asymmetric hydrogenation of simple ketones catalyzed by a Rh-PennPhos complex,8 we have now extended our study of the asymmetric hydrogenation of enamides to include this unique catalytic system. N-(3,4-Dihydro-1-naphthyl)acetamide was chosen as a typical enamide substrate. This enamide can be easily prepared by reduction of the corresponding oxime with iron powder in the presence of acetic anhydride (Scheme 1).9a,b This facile synthesis of enamides9 combined with an efficient asymmetric hydrogenation step provides a practical protocol for the synthesis of chiral amines from ketones. Table 1 lists asymmetric hydrogenation results under different conditions and with a variety of chiral bisphosphine systems. The catalyst was generally prepared in situ by mixing a solution of a Rh precursor and a phosphine ligand. The reaction was carried out under an initial H2 pressure of 40 psi at room temperature with a ratio of substrate/Rh/Me-PennPhos of 100:1:1.1. Different Rh catalytic precursors led to large variations in enantioselectivities (entries 1-3). Higher enantioselectivities were observed with cationic Rh precursors (entry 2, 97% ee; entry 3, 98% ee), while the ee is lower with a neutral Rh system (entry 1, 92% ee). Changing solvents and/or H2 pressure have only a small effect on enantioselectivity, although reaction conversions vary to a large extent. The catalytic hydrogenation goes to completion in methylene chloride, methanol, and 2-propanol under 40 psi of H2 in 20 h, while only 24% conversion was achieved under the same conditions in toluene. Reactions performed under 15 psi H2 and 500 psi H2 in MeOH give similar enantioselectivities (>98% ee) but gave different conversions (6% under 15 psi H2 and 100% under 500 psi H2 after 20 h). Optimal reaction conditions with the Rh-Me-PennPhos catalyst use MeOH as the solvent and an initial H2 pressure of 40 psi. Under these conditions, we have investigated the Rh-catalyzed asymmetric hydrogenation of N-(3,4-dihydro1-naphthyl)acetamide with several commercially available chiral bisphosphines. Compared with the Rh-Me-PennPhos catalyst, significantly lower enantioselectivities (10% ee, entry 4; 24% ee, entry 5) were observed with Rh-DIOP and Rh-BINAP catalysts. The most surprising result is the low enantioselectivity (1% ee, entry 6) achieved with the RhMe-DuPhos complex, which is in sharp contrast to its effective asymmetric hydrogenation of acyclic enamides.4a Our results are in agreement with the investigation by Burk,9b who found that under similar conditions hydrogenation of N-(3,4-dihydro-1-naphthyl)acetamide gave 0% ee with a Rh-Me-DuPhos complex and 69% ee with a Rh-Me-BPE catalyst. However, up to 92% ee was reported with a RhMe-BPE catalyst if the reaction was done at 0 °C.9b To the best of our knowledge, Rh-Me-PennPhos-catalyzed asymmetric hydrogenation of N-(3,4-dihydro-1-naphthyl)acetamide gives the highest enantioselectivity reported to date. † Dedicated to Professor Xiyan Lu on the occasion of his 70th birthday. (1) Cannarsa, M. S. Proceedings of the Chiral USA’97 Symposium; Matrix, 1997; and a related report of Technol. Catal. Int. (2) (a) Ojima, I., Ed. Catalytic Asymmetric Synthesis; VCH: New York, 1993. (b) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New York, 1994. (3) (a) Spinder, F.; Pugin, B.; Blaser, H.-U. Angew. Chem., Int. Ed. Engl. 1990, 29, 558. (b) Chan, Y. Ng C.; Osborn, J. A. J. Am. Chem. Soc. 1990, 112, 9400. (c) Becalski, A. G.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.; Kang, G. J.; Rettig, S. J. Inorg. Chem. 1991, 30, 5002. (d) Lensink, C.; Vries, J. G. Tetrahedron: Asymmetry 1992, 3, 235. (e) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1992, 114, 7562. (f) Willoughby, C. A.; Buchwald, S. L J. Am. Chem. Soc. 1994, 116, 8952. (g) Chan, A. S. C.; Chen, C. C.; Lin, C. W..; Lin, Y. C.; Cheng, M. C. J. Chem. Soc., Chem. Coummn. 1995, 1767. (h) Verdaguer, X.; Jange, U. E. W.; Reding, M. T.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 6784. (i) Togni, A. Angew. Chem., Int. Ed. Engl. 1996, 35, 14575. (j) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916. (k) Schnider, P.; Koch, G.; Pretot, R.; Wang, G.; Bohnen, F. 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