Quantum chemical study on enantioselective reduction of aromatic ketones catalyzed by chiral cyclic sulfur-containing oxazaborolidines. Part 2. Structures of catalyst-borane-ketone adducts

In the present paper, the ab initio molecular orbital method is employed to study the structures of the adducts of borane and aromatic ketone to chira l cyclic sulfur-containing oxazaborolidine used as a catalyst in the enanti oselective reduction of aromatic ketone. The catalyst-borane-ketone adducts have four different structures. All die structures are optimized completel y by means of the Hartree-Fock method at 6-31g* basis sets. The structure w hich is of the greatest advantage to a hydride transfer from the borane moi ety to the carbonyl carbon of aromatic ketone is the one with the next lowe st formation energy, and the plausible transition state for the hydride tra nsfer is predicted to be of a twisted boat structure. (C) 2000 John Wiley & Sons, Inc.