Energies of formation and structural parameters of formaldehyde and acetaldehyde complexes of a model system of borane adducts of oxazaborolidine type of chiral reduction catalysts (CBS reduction) were calculated by using ab initio molecular orbital methods (6-31G*//6-31G*). The energetic preference was determined for the formation of complexes in which the Lewis acidic boron of the borane adduct of an oxazaborolidine would coordinate either syn or anti to the methyl group of acetaldehyde. The formation of anti complex was favored by 15.2 kJ mo −1 which corresponds to a relative anti: syn abundance ratio of 461:1 and an enantiomeric excess of 99.8 %.