EVIDENCE FOR THE LIGATION OF PALLADIUM(0) COMPLEXES BY ACETATE IONS -CONSEQUENCES ON THE MECHANISM OF THEIR OXIDATIVE ADDITION WITH PHENYLIODIDE AND PHPD(OAC)(PPH(3))(2) AS INTERMEDIATE IN THE HECK REACTION

Addition of acetate anions to solutions of Pd-0(PPh(3))(4) results in the formation of anionic species in which the acetate ion coordinates the palladium(0) center, Pd-0(PPh(3))(3)(OAc)(-), which is in equilibr ium with the less ligated complex, Pd-0(PPh(3))(2)(OAc)(-). The latter undergoes oxidative addition with phenyl iodide to afford a mixture o f PhPdI(PPh(3))(2) and PhPd(OAc)-(PPh(3))(2). Acetate ions react with PhPdI(PPh(3))(2) to afford PhPd(OAc)(PPh(3))(2). Mixtures ofPd(OAc)(2) and nPPh(3) (n greater than or equal to 4), commonly used as catalyst s in Heck reactions, afford a palladium(0) complex that is ligated by one acetate ion, yielding the anionic species Pd-0(PPh(3))(3)(OAc)(-) and Pd-0(PPh(3))(2)(OAc)(-). The latter reacts with phenyl iodide. How ever, this reaction does not afford the expected PhPdI(PPh3)2 complex but instead affords PhPd(OAc)(PPh(3))(2). Reaction of PhPd(OAc)(PPh(3) )(2) with styrene results in the formation of stilbene, demonstrating that PhPd(OAc)(PPh(3))2 is an intermediate in the Heck reaction.