A quantum mechanics/molecular mechanics study of the highly enantioselective addition of diethylzinc to benzaldehyde promoted by (R)-2-piperidino-1,1,2-triphenylethrunol

The enantioselective addition of diethylzinc to benzaldehyde mediated by (R )-2-piperidino-1,1,2-triphenylethanol has been studied with the IMOMM proce dure, using a RHF description for the quantum mechanical part of the calcul ation and the mm3(92) force field for the molecular mechanics part. Both th e sign and the value of the observed enantioselectivity are well reproduced by the calculations, the relative energies of the four possible transition states being as follows: anti-Si (0.0 kcal.mol(-1)), syn-Re (2.7 kcal.mol( -1)), anti-Re (4.7 kcal.mol(-1)), syn-Si (5.9 kcal.mol(-1)). These energies show a perfect correlation with the bond distance in the transition state between zinc and the methylene unit of the ethyl group being transferred.