Effect of the leaving group on the rate and mechanism of the palladium-catalyzed isomerization of cyclic allylic benzoates in allylic substitutions

In chloroform, the reaction of cis-5-phenylcyclohex-2-enyl 4-Z-benzoate (cis-1(Z), Z = NO2, Cl, H, Me, MeO) with Pd-0 complexes ligated to PPh3 is reversible and proceeds with isomerization at the atlylic position. The rate of isomerization of cis-1(Z) to trans-1(Z) depends on the catalytic precursor: Pd-0(PPh3)(4) > {Pd-0(dba)(2) + 2PPh(3)} in agreement with an S(N)2 mechanism in the rate-determining isomerization of the cationic (eta(3)-allyl)palladium complexes formed in the oxidative addition. For a given precursor, the rate of isomerization of cis-1(Z) to trans-1(Z) also depends on the substituent Z, i.e., on the leaving group. The isomerization rate follows the same order as the leaving group properties: 4-NO2-C6H4-CO2- > 4-Cl-C6H4-CO2- > C6H5-CO2- > 4-Me-C6H4-CO2- > 4MeO-C6H4-CO2-. The same tendency is found for the equilibrium constant between the neutral cis-1(Z) and the cationic (ill-allyl)palladium complex in DMF. The higher the concentration of the cationic (eta(3)-allyl)palladium complex, the faster the isomerization of cis-1(Z) to trans-1(Z) is. The isomerization of cis-1(Z) to trans-1(Z) and that of the cationic (eta 3-allyl)palladium complexes are at the origin of the lack of stereospecificity observed in catalytic nucleophilic allylic substitutions. These isomerizations are affected by both the leaving groups and the Pd-0 precursors, which therefore are not "innocent" but may play an important role in palladium-catalyzed nucleophilic substitutions. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).