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.

Automated summary

This study investigates the detailed working mechanism of a notable base-free cross-coupling reaction using density functional theory. It reveals that the reaction proceeds through a complex Pd(0)/Pd(II) pathway and elucidates the dual role of an arylboronic acid as a reducing agent and coupling partner. These mechanistic insights have important implications for new reaction development.