Investigation of the mechanisms of ene reactions of carbonyl enophiles by intermolecular and intramolecular hydrogen-deuterium isotope effects: partitioning of reaction intermediates

Abstract

The mechanisms of ene reactions of selected carbonyl enophiles with methylenecyclohexane (1) and 1-methylenetetralin (2) have been investigated by determinations of intermolecular and intramolecular hydrogen-deuterium isotope effects. In the thermal ene reactions of 1 with dimethyl dioxosuccinate and of 1 with diethyl oxomalonate (8), both of the isotope effects are primary and substantial, consistent with a concerted mechanism for each reaction. In the ene reactions of 1 with 8 catalyzed by tin tetrachloride and of 1 with acetylium hexachloroantimonate, both the intermolecular and intramolecular isotope effects are small, results that are inconsistent with either a stepwise reaction via an equilibrating intermediate or a concerted mechanism. The ene reaction of 2 with acetic anhydride catalyzed by zinc chloride has a small intermolecular and a large intramolecular isotope effect, consistent with rate-determining formation of an equilibrated intermediate