Key Mechanistic Features in Palladium-Catalyzed Methylcyclopropanation of Norbornenes With Vinyl Bromides: Insights From DFT Calculations.

DFT calculations were performed to elucidate mechanistic details of an unusual palladium-catalyzed methylcyclopropanation from [2 + 1] cycloadditions of ()-2-bromovinylbenzene and endo-N-(p-tolyl)-norbornenesuccinimide. The reaction proceeds via oxidative addition (OA), intermolecular alkene insertion, deprotonation/protonation, intramolecular alkene insertion, β-H elimination and reductive elimination (RE). Protonation is the rate-limiting step and requires an overall barrier of 28.5 kcal/mol. The sources of two protons for protonation and exchange have also been clarified and the calculations agree with experimental observations.