Transition states of amine-catalyzed aldol reactions involving enamine intermediates: Theoretical studies of mechanism, reactivity, and stereoselectivity

The mechanisms, transition states, relative rates, and stereochemistries of amine-catalyzed aldol reactions involving enamine intermediates have been explored with density functional theory (B3LYP/6-31G*) and CPCM solvation models. Primary enamine-mediated aldol reactions involve half-chair transition states with hydrogen bonding leading to proton transfer. This leads to charge stabilization and low activation energies as compared to secondary enamine-mediated aldol reactions. Oxetane intermediates can be formed when C-C bond formation occurs without H-transfer in the transition state. The stereoselectivities of reactions of ketone enamines with aldehydes, including the facial stereoselectivity involving chiral aldehydes, were modeled and compared with experimental results. Transition states for the intramolecular aldol reactions leading to the formation of hydrindanone-beta -ketol and decalone-beta -ketol aldol products showed a preference for the formation of the cis-fused rings, in agreement with experimental results.