Rate and mechanism of the reversible formation of cationic (eta(3)-allyl)palladium complexes in the oxidative addition of allylic acetate to palladium(0) complexes ligated by diphosphanes

The oxidative addition of the allylic acetate, CH2=CH-CH2-OAc, to the palla dium(0) complex [Pd-0(P,P)], generated from the reaction of [Pd(dba)(2)] wi th one equivalent of P,P (P,P = dppb = 1,4-bis(diphenylphosphanyl)butane, a nd P,P = dppf = 1,1'-bis(diphe nylphosphanyl)ferrocene), gives a cationic ( eta (3)-allyl)palladium(II) complex, [(eta (3)-C3H5)Pd(P,P)(+)], with AcO- as the counter anion, This reaction is reversible and proceeds through two successive equilibria. The overall equilibrium constants have been determin ed in DMF. Compared with PPh3, the overall equilibrium lies more in favor o f the cationic (eta (3)-allyl)palladium(II) complex when bidentate P,P liga nds are considered in the order: dppb>dppf> PPh3. The reaction proceeds via a neutral intermediate complex [(eta (2) -CH2=CH-CH2-OAc)Pd-0(P,P)], which has been kinetically detected. The rate constants of the successive steps have been determined in DPMF by UV spectroscopy and conductivity measuremen ts. The overall complexation step of the Pd-0 by the allylic acetate C=C bo nd is faster than the oxidative addition/ionization step which gives the ca tionic (eta (3)-allyl)palladium(II) complex.