Enantioselective Reduction of Electrophilic CC Bonds with sodium tetrahydroborate and ‘semicorrin’ cobalt catalysts

‘Semicorrin’ cobalt complexes, prepared in situ from cobalt(II) chloride and the corresponding ligands, are efficient catalysts for the enantioselective reduction of electrophilic CC bonds with NaBH 4 . The best selectivities (> 90% ee) are achieved with α,β‐unsaturated carboxamides and carboxylates. Analogous α,β‐unsaturated nitriles, sulfones, and phosphonates afford enantiomeric excesses of 50–70%. Interestingly, in the reduction of α,β‐unsaturated sulfones, the highest enantioselectivities were obtained with unsymmetrical ‘semicorrins’, whereas in all other cases C 2 ‐symmetric ‘semicorrins’ proved to be superior.