Key Role of PdIV Intermediates in Promoting PdII-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 PdII/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 PdII-catalyzed homocoupling of benzophenones in the presence of the strong oxidant K2S2O8. Calculation results demonstrated that the favorable reaction pathway is a PdII/IV/II catalytic cycle, including four sequential processes: C–H activation at the PdII center, oxidation of PdII to PdIV, C–H activation at the PdIV center, and reductive elimination. It was found that C–H activation at the PdIV 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 PdII to PdIV and facilitating the second C–H activation step. In contrast, the alternative PdII/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 PdIV intermediates in promoting the biaryl synthesis.