Mechanism of oxidative allyl transfer from allylic ammonium cations to palladium(0) alpha-diimine complexes

The palladium(0) complex [Pd(eta(2)-fn)(N-N')] (1, fn = fumaronitrile; N-N' = C5H4N-2-CH=NC(6)H(4)OMe-4) reacts slowly and reversibly with A-CH2-CH=CH2 (2a, A = NEt(3); 2b, A = C5H5N) to yield the cationic eta(3)-allylpalladium(II) derivative [Pd(eta(3)-C3H5)(N-N')](+) (3) the free amine A and fn. The equilibrium constant K-e is (2.6 +/- 0.1) X 10(-3) for 2a and 1.0 +/- 0.4 for 2b. Kinetic studies of these oxidative allyl-transfer reactions show that the rates increase with increasing concentration of 2 and with decreasing concentration of fn. A stepwise mechanism is proposed which involves slow and reversible displacement of fn by 2 to give a labile intermediate [Pd(eta(2)-CH2=CH-CH2-A)(N-N')]. This undergoes slow and reversible intramolecular allyl transfer through nucleophilic attack by the palladium(0) metal centre on the nitrogen-bound allyl carbon. The kinetic parameters evaluated by a steady-state treatment satisfactorily generate the observed equilibrium constant K-e. The rate of formation of the intermediate and the relative rate of its decay to the starting reactants and final products are virtually independent of the nature of the amine A.