Effect of the leaving group and the allylic structure on the kinetics andthermodynamics of the reaction of allylic carboxylates with palladium(0)complexes

Abstract

The reaction of allylic carboxylates (allyl-OZ, OZ: acetate, chloroacetate, trifluoroacetate, substituted benzoates, carbonate) with Pd0 complexes ligated by monodentate (PPh3) or bidentate (dppb, dppf) ligands is a reversible multistep reaction, which eventually gives in DMF a cationic (η3-allyl)PdII(P P)+ complex with ZO- as the counter anion (free ions). The formation of an intermediate neutral complex (η2-allyl-OZ)Pd0(P P) where the Pd0 is ligated to the C=C bond of the allylic carboxylate (complexation step) has been evidenced kinetically in the case of moderate OZ leaving groups (acetate, benzoates) for monodentate and bidentate phosphine ligands. The overall equilibrium constants and the rate constants of the complexation and oxidative addition-ionization steps (when not too fast) have been determined in DMF. With very good leaving groups (carbonate, trifluoroacetate), the oxidative addition-ionization step is faster than the complexation step whereas the oxidative addition-ionization step is the slowest step for less good leaving groups (acetate, benzoate). The forward rate constant of the equilibrium in which the active Pd0(PPh3) 2 is formed from Pd0(dba)(PPh3)2 has been determined.