Practical and efficient enantioselective borohydride reduction of aromatic ketones catalyzed by optically active cobalt(II) complexes using pre-modified borohydride

A catalytic enantioselective borohydride reduction of aromatic ketones in presence of 0.1-1 mol% of optically active aldiminato cobalt(II) complex catalysts at 0 °C was successfully achieved using precisely pre-modified borohydride with one equivalent each of tetrahydrofurfuryl alcohol, ethanol and NaBH4. The corresponding secondary alcohols were produced in less than 45 min with high enantiomeric excesses (up to 98% ee). The above method was successfully applied to kinetic and dynamic kinetic resolutions of aromatic ketones.