Oxidatively pure chiral (salen)Co(III)-X complexes in situ prepared by Lewis acid-promoted electron transfer from chiral (salen)Co(II) to oxygen: Their application in the hydrolytic kinetic resolution of terminal epoxides

Abstract

The Lewis acid (MX 3)-promoted oxidation of chiral (salen)Co(II) complex afforded neither the mononuclear nor dinuclear heterobimetallic chiral (salen)Co complexes, such as [(salen)Co(III)-X]⋯MX 3 or [(salen)Co(III)-X] 2⋯MX 3, as reported in the literatures, but rather the Jacobsen's complexes, chiral (salen)Co(III)-X. The chiral (salen)Co(III)-X complexes in situ generated by the reaction of MX 3 and (salen)Co(II) in molar ratio of 1:3 were shown to have identical catalytic properties in the hydrolytic kinetic resolution (HKR) reactions of racemic epoxides, in terms of activity, enantioselectivity and stability, to those of the purified (salen)Co(III)-X complex. This result strongly indicates that metallic residues of Lewis acids (e.g., metal oxide residues) remained in the reaction mixture did not display any negative influence on the catalytic efficiencies. Thus, the present in situ method for catalyst activation can be applicable for large scale process for HKR reactions of racemic epoxides.