Abstract
Intermolecular kinetic isotope effects with 2,3-dimethyl-2-butene and methylenecyclohexane and intramolecular isotope effects with gem-, trans-, and cis-2,3-dimethyl-2-butene-if6 establish that the Lewis acid catalyzed ene reactions of
methyl propiolate, formaldehyde, and diethyl oxomalonate proceed through a stepwise reaction with rate-determining formation
of either (1) a three-membered ring intermediate lacking the geometrical rigidity of perepoxides and related intermediates,
(2) a pair of rapidly equilibrating zwitterions, or (3) a -complex between the ene component and enophile-Lewis acid complex. The intermolecular isotope effects are small and the intramolecular isotope effects with 2 and 3 are large, indicating that the reaction is stepwise. The large intramolecular isotope effects with 1 require that the reaction proceeds through an intermediate that can abstract a hydrogen from either end of the ene component, i.e., one of the three possibilities mentioned above.