Effect of borane source on the enantioselectivity in the enantiopure oxazaborolidine-catalyzed asymmetric borane reduction of ketones

The effect of borane source on enantioselectivity in the enantiopure oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been investigated by using (S)-3,1,2-oxazaborobicyclo[3.3.0]octane and (S)-7,3,1,2-thiaxazaborobicyclo[3.3.0]octane as catalysts. The results indicate that the enantioselective order of different borane sources is borane-dimethyl sulfide < borane-N,N-diethylaniline < borane-THF for the asymmetric reduction of a ketone under the same conditions.