ALLYLIC ALCOHOLS OF UNEXPECTED CONFIGURATION BY OXAZABOROLIDINE-CATALYZED REDUCTION OF ALPHA,BETA-UNSATURATED KETONES - AN EXPLANATION-BASED ON MO CALCULATIONS

While the reduction of most alpha,beta-unsaturated ketones with BH3:SM e(2) in the presence of (R)B-methyl-4,5,5-triphenyl-1,3,2-oxazaborolid ine [(R)-2] affords allylic alcohols of the S configuration, that of a lpha,beta-unsaturated ketones branched at both the alpha and alpha' po sitions gives alcohols of the R configuration. Theoretical calculation s on complexes of representative enones with BH3 (6-31G) or with BH3: (R)-2 (AMI) may account for the apparent changes in the steric require ments on either side of the CO group.