QUANTUM CHEMICAL MODELING OF CHIRAL CATALYSIS .6. ON THE RELATIVE STABILITY OF DIMERS OF CHIRAL OXAZABOROLIDINES USED IN THE CATALYTIC ENANTIOSELECTIVE REDUCTION OF KETONES

Dimerization of oxazaborolidines functioning as chiral catalysts was investigated by means of ab initio molecular orbital methods. The most stable form of the oxazaborolidine dimers studied was found to be structurally analogous to cyclodiborazane (H2B-NH2)2. The heat of dimerization of H2B-NH2 was predicted at 298 K to be -22 kJ mol-1 (6-31G*//6-31G* energy corrected with respect to vibrational and temperature effects). Heats of dimerization of oxazaborolidines were predicted to be less negative than -22 kJ mol-1. In a Lewis basic solvent the dimerization energy was predicted to be about 20 kJ mol-1 more positive than in nonpolar solvents.