Quantum chemical study on enantioselective reduction of aromatic ketones catalyzed by chiral cyclic sulfur-containing oxazaborolidines. Part 1. Structures and properties of catalysts

Abstract

ABSTRACT:The ab initio molecular orbital method is employed to study the structures and properties of chiral cyclic sulfur-containing oxazaborolidine, as a catalyst, and its borane adducts. All the structures are optimized completely by means of the Hartree-Fock method at 6-31g * basis sets. The catalyst is a twisted chair structure and reacts with borane to form four plausible catalyst-borane adducts. Borane-sulfur adducts may be formed, but they barely react with aromatic ketone to form catalyst-borane-ketone adducts, because they are repulsed greatly by the atoms arising from the chair rear of the catalyst with a twisted chair structure. Borane-N adduct has the largest formation energy and is predicted to react easily with aromatic ketone to form catalyst-borane-ketone adducts. The formation of the catalyst-borane adducts causes the B BH3 -H BH3 bond lengths of the BH 3 moiety to be increased and thus enhances the activity of the enantioselective catalytic reduction. The borane-N adduct is of great advantage to hydride transfer.