QUANTUM CHEMICAL MODELING OF CHIRAL CATALYSIS .5. ON THE ROLE OF ALKOXYBORANES IN THE CATALYTIC ENANTIOSELECTIVE REDUCTION OF CARBONYL-COMPOUNDS BY THE CBS METHOD

Energies of the formation, relative stabilities and structural parameters of alkoxyborane N-adducts of oxazaborolidine type of chiral reduction catalysts were evaluated by means of ab initio molecular orbital calculations. Three models of the oxazaborolidine system were used. Stability of the alkoxyborane adducts and the nature of borane - catalyst interactions was found to depend strongly on the conformation of the alkoxy group. Two different mechanisms for the formation of the alkoxyborane adducts, a direct coordination of an alkoxyborane to the nitrogen of an oxazaborolidine ring and an intramolecular rearrangement of an oxazadiboretane inter-mediate. are discussed. Properties of borane and alkoxyborane adducts are compared.