Asymmetric Reduction of Ketones under Mild Conditions Using NaBH4 and TarB-NO2: An Efficient and Unusual Chiral Acyloxyborohydride Reducing System.

High enantioselectivities are obtained for the reduction of a series of ketones using the inexpensive and mild reducing agent NaBH4 with TarB-NO2 (1). This easily prepared tartaric acid-based reagent combines a Lewis acid with carboxylic acids in a single bifunctional reagent. When combined with NaBH4, the resulting chiral acyloxyborohydride mediates the reduction of aromatic ketones to provide the product alcohols in enantiomeric excesses of 93–98 %. Several aliphatic ketones were also reduced with moderate to excellent enantioselectivity. A unique mechanism is provided with supporting calculations for the proposed active species and transition state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)