A Quantum Mechanics/Molecular Mechanics Study of the Highly Enantioselective Addition of Diethylzinc to Benzaldehyde Promoted by (R)-2-Piperidino-1,1,2-triphenylethanol

The enantioselective addition of diethylzinc to benzaldehyde mediated by (R)-2-piperidino-1,1,2-triphenylethanol has been studied with the IMOMM procedure, using a RHF description for the quantum mechanical part of the calculation and the mm3(92) force field for the molecular mechanics part. Both the 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.