Unraveling the Mechanism of Palladium-Catalyzed Base-Free Cross-Coupling of Vinyl Carboxylates: Dual Role of Arylboronic Acids as a Reducing Agent and a Coupling Partner

There is continued interest in developing new Pd-catalyzed cross-coupling reactions. This density functional theory (DFT) study explores the detailed workings of a notable base-free cross-coupling reaction of vinyl carboxylates with arylboronic acids at ambient conditions enabled by Pd(OAc)2 along with a phosphine ligand. Extensive DFT calculations have been performed on the proposed Pd(II)-only mechanism and other possibilities, suggesting that the reaction preferably proceeds by a Pd(0)/Pd(II) pathway with an intricate Pd(0)-generating process. Two consecutive transmetalations with phenylboronic acid lead to a diphenyl-Pd(II) phosphine intermediate, which would undergo a phenyl-phenyl reductive elimination rather than a redox-neutral carbopalladation. The resulting Pd(0) phosphine species introduces a Pd(0)/Pd(II) catalytic cycle involving the key elementary steps of oxidative addition, transmetalation, and reductive elimination. The oxidative addition of the vinyl carboxylate to Pd(0) via R-OAc bond cleavage is the rate-determining step. The dual role of an arylboronic acid as a reducing agent and coupling partner in Pd-catalyzed cross-coupling reactions has been elucidated for the first time. This and other mechanistic insights gained can have implications for new reaction development.