Computational Mechanistic Analysis of Olefin Formation in Ketyl Radical-Mediated Photoexcited Palladium Catalysis.

Abstract

Olefin formation in palladium-catalyzed reactions is commonly rationalized by β-hydride elimination. However, under photoexcited conditions, palladium intermediates can differ substantially from those formed under thermal conditions, raising questions about the operative hydrogen removal pathway. In this study, we performed a computational mechanistic analysis of olefin formation in a ketyl radical-mediated photoexcited palladium-catalyzed reaction. Density functional theory calculations were carried out starting from a common intermediate to compare β-hydride elimination with hydrogen atom transfer (HAT). The lower-energy β-hydride elimination pathway requires ligand dissociation and the formation of an agostic intermediate before Cβ-H bond cleavage, whereas the HAT pathway proceeds more directly while retaining the phosphine coordination environment. Although the HAT pathway was consistently calculated to be energetically favored in the present system, β-hydride elimination can still become competitive when the preceding elementary steps are taken into account. These findings indicate that the operative hydrogen removal pathway in photoexcited palladium catalysis cannot be evaluated solely from the Cβ-H bond-cleavage step but must be considered together with the elementary steps required to reach each transition state.