Asymmetric Addition of Dimethylzinc to Benzaldehyde Catalyzed by (2<i>S</i>)-3-<i>e</i><i>xo</i>-(Dimethylamino)isobornenol. A Theoretical Study on the Origin of Enantioselection

The asymmetric reaction of dimethylzinc and benzaldehyde in the presence of (2S)-3-exo-(dimethylamino)isobornenol ((2S)-DAIB) proceeds via a methylzinc aminoalkoxide/dimethylzinc/benzaldehyde mixed-ligand complex. Molecular orbital and density functional calculations revealed that the asymmetric bias is generated in the intramolecular methyl-transfer step by differences in the stabilities of the four diastereomeric transition structures. The anti-configured 5/4/4 tricyclic structures are favored over the syn tricyclic transition states that exhibit electrostatic repulsion between the 1,3-syn-oriented Zn−CH3 groups. The steric origins of enantioselectivity are (1) the high stereodifferentiating ability of the bornane skeleton of DAIB and (2) nonbonded repulsion between the unreactive Zn−CH3 spectator and the aldehyde phenyl group (anti structure) or between the DAIB N-CH3 group and the migrating methyl group and/or carbonyl substituent (syn structure). The ground-state Zn complexes and transition states involved in this organometallic reaction are characterized by flexible frameworks arising from the electrostatic nature of the Zn−heteroatom interaction. The calculated transition states explain many experimental findings, including the substituent effects on the rate and enantioselectivity in methylation of p-substituted benzaldehydes.