Asymmetric Reductions Involving Borohydrides:  A Practical Asymmetric Reduction of Ketones Mediated by (l)-TarB−NO₂:  A Chiral Lewis Acid

The asymmetric reduction of prochiral ketones has been achieved through a myriad of different methods. Early reductions gave low to moderate enantiomeric excesses (ee's), but more modern reagents have led to dramatic increases in enantioselectivity. The development of (l)-TarB-NO2 boronic ester is extensively reviewed in the context of other well-known asymmetric reducing reagents. In combination with NaBH4, the chiral intermediate is able to reduce prochiral ketones to optically active secondary alcohols in ee's as high as 99% and can be easily recovered under basic conditions.