Influence of Lewis acids and substituents on carbonyl-ene reactions: A density functional theory study

The influences of Lewis acids and substituents on carbonyl-ene reactions between formaldehydes and propenes have been studied using density functional theory (DFT) calculations. Twelve different Lewis acids including AlCl3, MeAlCl2, Me2AlCl, Me3Al, Al(OMe)3, BF3, CoCl3, SnCl4, TiCl4, YCl3, ZnCl2 and ZrCl4 were chosen as promoters, and CF3, COOCH3, NO2, F, Cl, Br, Ph, CH2CH-, t-Bu, CH3 and OCH3 groups were employed as monosubstituents for formaldehyde or propene. The DFT results show that in the presence of a Lewis acid, the activation energy for carbonyl-ene reactions greatly decreases with the endergonic process becoming exergonic. The frontier molecular orbital energies were also investigated to evaluate the influence of Lewis acids, and AlCl3 was found to be the most efficient for the carbonyl-ene reaction of formaldehyde with propene. In addition, it was found that the electronic, conjugative and steric effects of the substituent have significant influence on the activation and free energy changes for the carbonyl-ene reaction. Moreover, the structures of the transition states and the corresponding bond lengths have been discussed. It is found that the substituent effects are quite different from those of the Diels-Alder reaction.