Evidence for the ligation of palladium(0) complexes by acetate ions: Consequences on the mechanism of their oxidative addition with phenyl iodide and PhPd(OAc) (PPh3)2 as intermediate in the heck reaction

Article abstract of DOI:10.1021/om00012a029

Addition of acetate anions to solutions of Pd⁰(PPh₃)₄ results in the formation of anionic species in which the acetate ion coordinates the palladium(0) center, Pd⁰(PPh₃)₃(OAc)^-, which is in equilibrium with the less ligated complex, Pd⁰(PPh₃)₂(OAc)^-. The latter undergoes oxidative addition with phenyl iodide to afford a mixture of PhPdI(PPh₃)₂ and PhPd(OAc)-(PPh₃)₂. Acetate ions react with PhPdI(PPh₃)₂ to afford PhPd(OAc)(PPh₃)₂. Mixtures of Pd-(OAc)₂ and nPPh₃ (n ≥ 4), commonly used as catalysts in Heck reactions, afford a palladium(0) complex that is ligated by one acetate ion, yielding the anionic species Pd⁰(PPh₃)₃(OAc)^- and Pd⁰(PPh₃)₂(OAc)^-. The latter reacts with phenyl iodide. However, this reaction does not afford the expected PhPdI(PPh₃)₂ complex but instead affords PhPd(OAc)(PPh₃)₂. Reaction of PhPd(OAc)(PPh₃)₂ with styrene results in the formation of stilbene, demonstrating that PhPd(OAc)(PPh₃)₂ is an intermediate in the Heck reaction.