Asymmetric Reduction of Ketones Under Mild Conditions Using NaBH4 and TarB‐NO2: An Efficient and Unusual Chiral Acyloxyborohydride Reducing System

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Abstract
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)

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