For the enantioselective borohydride reduction of aliphatic ketones, the optically active ketoiminatocobalt(II) catalysts was successfully designed based on their axial ligand. Instead of chloroform for the aryl ketone reduction, various axial ligand precursors were examined for the aliphatic ketone. Consequently, 1, 1, 1-trichloroethane was found to be the most effective activator of the cobalt(II) complexes to generate the corresponding 1-chlorovinyl cobalt(III) derivatives as the reactive intermediate. Several aliphatic ketones were successfully reduced to afford the corresponding secondary alcohols with high enantioselectivities.
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