Abstract-N-Picolinoyl-(2S)-(diphenylhydroxymethyl)pyrrolidine was found to work as an organic activator in the reduction of aromatic imines to the corresponding amines by Cl 3 SiH. The highest selectivity was 80%ee. This is the first data showing that N-formyl group is not always essential as N-protecting group of pyrrolidine derivatives for the reduction of imines by Cl 3 SiH.Enantioselective reduction of ketones 1 and imines 2 has been one of recent topics in asymmetric synthesis. 3 A variety of reducing reagents have been used in the reductions but it is still worthwhile to exploit new methods which can be carried out using inexpensive reducing reagents under mild conditions. One of such reagents may be trichlorosilane (Cl 3 SiH), a liquid material easily available from silicon industry, 4 though some activator is necessary for Cl 3 SiH to efficiently reduce ketones and imines. 5 We already reported chiral N-formylpyrrolidine derivatives 1 as organic activators in the enantioselective reduction of ketones 6 and imines 7 with Cl 3 SiH. The reduction proceeds smoothly at room temperature with good yields and enantioselectivity of up to 43%eefor the reduction of ketones and 66%ee for the reduction of imines. Recently, a new activator 2 for Cl 3 SiH in reducing imines with high enantioselectivity (up to 92%ee) was reported. 8 The noticeable point in those reductions was that the presence of N-formyl substituent was essential for those reductions. In our continuing effort to exploit new chiral organic compounds in place of 1 to activate Cl 3 SiH, 9 we found N-picolinoylpyrrolidine derivatives 3a-f to also work as organic activators in the reduction of aromatic imine 4 to amine 5 (Eq 1). This is the first data showing that N-formyl group is not always essential in the structure of organic activators for Cl 3 SiH.10,11

[reaction: see text] Aryl ketones were reduced to the corresponding alcohols with excellent enantioselectivity (up to 99.7% ee) by Cl3SiH in the presence of a catalytic amount of N-formyl-alpha'-(2,4,6-triethylphenyl)-L-proline as an activator. Both carboxyl group at the alpha-position of the activator and 2,4,6-triethylphenyl group at the alpha'-position were critical for the high enantioselectivity.

Abstract-N-Picolinoyl-(2S)-(diphenylhydroxymethyl)pyrrolidine was found to work as an organic activator in the reduction of aromatic imines to the corresponding amines by Cl 3 SiH. The highest selectivity was 80%ee. This is the first data showing that N-formyl group is not always essential as N-protecting group of pyrrolidine derivatives for the reduction of imines by Cl 3 SiH.Enantioselective reduction of ketones 1 and imines 2 has been one of recent topics in asymmetric synthesis. 3 A variety of reducing reagents have been used in the reductions but it is still worthwhile to exploit new methods which can be carried out using inexpensive reducing reagents under mild conditions. One of such reagents may be trichlorosilane (Cl 3 SiH), a liquid material easily available from silicon industry, 4 though some activator is necessary for Cl 3 SiH to efficiently reduce ketones and imines. 5 We already reported chiral N-formylpyrrolidine derivatives 1 as organic activators in the enantioselective reduction of ketones 6 and imines 7 with Cl 3 SiH. The reduction proceeds smoothly at room temperature with good yields and enantioselectivity of up to 43%eefor the reduction of ketones and 66%ee for the reduction of imines. Recently, a new activator 2 for Cl 3 SiH in reducing imines with high enantioselectivity (up to 92%ee) was reported. 8 The noticeable point in those reductions was that the presence of N-formyl substituent was essential for those reductions. In our continuing effort to exploit new chiral organic compounds in place of 1 to activate Cl 3 SiH, 9 we found N-picolinoylpyrrolidine derivatives 3a-f to also work as organic activators in the reduction of aromatic imine 4 to amine 5 (Eq 1). This is the first data showing that N-formyl group is not always essential in the structure of organic activators for Cl 3 SiH.10,11

by Trichlorosilane. -A novel activator (I) derived from L-proline allows the preparation of a variety of secondary alcohols in high yield and enantioselectivities up to >99%. -(MATSUMURA*, Y.; OGURA, K.; KOUCHI, Y.; IWASAKI, F.; ONOMURA, O.; Org. Lett. 8 (2006) 17, 3789-3792; Grad. Sch. Biomed. Sci., Nagasaki Univ., Nagasaki 852, Japan; Eng.) -R. Steudel 51-026

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