Effects of Ligands and Additive Alcohols on Enantioselection in Highly Efficient Asymmetric Borohydride Reduction of Ketones Catalyzed by Optically Active Aldiminato Cobalt(II) Complexes

Highly enantioselective reduction of ketones with sodium borohydride was achieved in the presence of a catalytic amount of optically active aldiminato cobalt(II) complexes to afford the corresponding secondary alcohols in quantitative yields with high enantiomeric excesses. The effects of ligands of the cobalt(II) complexes and of the additive alcohol pairs used for modification of the borohydride on the present reduction were described. The enantiofacial discrimination in the present asymmetric reduction was discussed based on the conformation of the cobalt complex determined by X-ray analysis.