A Chiral Bimetallic Lewis Acid as a Reaction Template. Asymmetric Reduction of Unsymmetric Ketones with LiBH4

A new series of chiral bimetallic Lewis acids, 5,5′-bis[(R)-2-aryl-1,3,2-dioxaborolan-4-one], have been prepared from L-(+)-tartaric acid and arylboronic acids. In the presence of these Lewis acids, unsymmetric ketones were reduced quantitatively by LiBH4, in moderate to good ee’s (up to 99% ee). The counter cation of BH4− significantly affects the ee’s of the products. The reaction is applicable to aryl alkyl ketones, dialkyl ketones, and diaryl ketones.