Evidence for a Concerted Mechanism in a Palladium Trimethylenemethane Cycloaddition

Abstract

Carbon kinetic isotope effects were determined at natural abundance for the palladium trimethylenemethane cycloaddition of ester−amide 5. Substantial intermolecular 13C KIEs were observed for both olefinic carbons of 5. In contrast, a Michael addition to 5 exhibits a significant 13C KIE only at the carbon β to the ester group. Intramolecular KIEs determined for the product-determining step(s) for reaction of the Pd−TMM intermediate would require a surprising isotope-dependent selection between diastereomeric ring closures. These results cannot be reconciled with a stepwise cycloaddition mechanism but are readily interpreted in terms of a concerted cycloaddition.