Key Role of Pd^IV Intermediates in Promoting Pd^II-Catalyzed Dehydrogenative Homocoupling of Two Arenes: A DFT Study

Palladium-catalyzed dehydrogenative homocoupling of two arenes provides a powerful and straightforward method for the synthesis of biaryls. In contrast to the Heck reaction for efficient cross-coupling of arene with alkene, dehydrogenative homocoupling of two arenes is not readily accessible through traditional Pd^II/0/II catalytic cycles, which limits its application to the construction of desired C–C bonds. Herein, we performed DFT studies to explore the detailed mechanisms of the Pd^II-catalyzed homocoupling of benzophenones in the presence of the strong oxidant K₂S₂O₈. Calculation results demonstrated that the favorable reaction pathway is a Pd^II/IV/II catalytic cycle, including four sequential processes: C–H activation at the Pd^II center, oxidation of Pd^II to Pd^IV, C–H activation at the Pd^IV center, and reductive elimination. It was found that C–H activation at the Pd^IV center is the rate-determining process, with a free energy barrier of 26.0 kcal mol^–1. The oxidant K₂S₂O₈ plays an important role in converting Pd^II to Pd^IV and facilitating the second C–H activation step. In contrast, the alternative Pd^II/0/II pathway has been characterized as an inaccessible reaction channel from our calculations, because the second C–H activation is hindered by a free energy barrier of 38.9 kcal mol^–1. In addition, the electronic effect of the spectator ligand on C–H activation has been investigated in terms of molecular orbital theory, which disclosed the origin of the critical role of Pd^IV intermediates in promoting the biaryl synthesis