Allylic alcohols of unexpected configuration by oxazaborolidine-catalysed reduction of α,β-unsaturated ketones. An explanation based on MO calculations
While the reduction of most alpha,beta-unsaturated ketones with BH3:SMe(2) in the presence of (R)B-methyl-4,5,5-triphenyl-1,3,2-oxazaborolidine [(R)-2] affords allylic alcohols of the S configuration, that of alpha,beta-unsaturated ketones branched at both the alpha and alpha' positions gives alcohols of the R configuration. Theoretical calculations on complexes of representative enones with BH3 (6-31G*) or with BH3:(R)-2 (AMI) may account for the apparent changes in the steric requirements on either side of the CO group.