QUANTUM-CHEMICAL MODELING OF CHIRAL CATALYSIS .11. ISOMERISM IN BORANE ADDUCTS OF CHIRAL OXAZABOROLIDINES USED AS CATALYSTS IN THE ENANTIOSELECTIVE REDUCTION OF KETONES

The relative stability of isomers of borane adducts of 1,3,2-oxazaborolidines was investigated by means of ab initio molecular orbital methods. One open-chain isomer was found to be more stable than the borane N-adduct proposed to be one of the key-intermediates of the catalytic enantioselective reduction of ketones. Borane N-adduct of 1,3,2-oxazaborolidine was found to be 19 kJ mol-1 (6-31G*//6-31G*) more stable than the corresponding O-adduct. Geometry of the ring oxygen of the O-adduct appeared out to be practically planar.