Enantioselective borohydride reduction catalyzed by cobalt complexes: Discovery, analysis, and design for a halogen-free system

On the basis of a study on the enantioselective version of the oxidation-reduction hydration catalyzed by bis(1,3-diketonato)cobalt complexes, the highly enantioselective borohydride reduction of carbonyl compounds was developed in the presence of a catalytic amount of an optically active cobalt(II) complex catalyst. The experimental and theoretical studies on the mechanistic insight of this reaction revealed that the key reactive intermediate of borohydride reduction catalyzed by ketoiminatocobalt(II) complexes would be the dichloromethyl-cobalt hydride with a sodium cation, which was generated from chloroform and sodium borohydride. The theoretical simulation of various axial groups in active cobalt complex catalysts predicted that the cobalt-carbene complexes could be employed as efficient catalysts. The newly designed complexes generated from cobalt complex and methyl diazoacetate made it possible to catalyze the enantioselective borohydride reduction in a completely halogen-free solvent.