Evidence for a Concerted Mechanism in a Palladium Trimethylenemethane Cycloaddition

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