Mechanistic and kinetic studies of palladium catalytic systems

It is established that new reactive anionic palladium(0) complexes species are formed in which palladium(0) is ligated by either chloride ions: Pd(0)(PPh3)(2)Cl- (when generated by reduction of PdCl2(PPh3)(2)) or by acetate ions: Pd(0)(PPh3)(2)(OAc)(-) (when generated in situ in mixtures of Pd(OAc), and PPh,). The reactivity of such anionic palladium(0) complexes in oxidative addition to aryl iodides strongly depends on the anion born by the palladium(0). The structure of the arylpalladium(II) complexes formed in the oxidative addition also depends on the anion. Indeed, intermediate anionic pentacoordinated arylpanadium(II) complexes are formed: ArPdI(Cl)(PPh3)(2)(-) and ArPdI(OAc)(PPh3)(2)(-) respectively, whose stability depends on the chloride or acetate anion brought by the palladium(0). ArPdI(Cl)(PPh3)(2)(-) is rather stable and affords trans ArPdI(PPh3)(2) at long times via a neutral pentacoordinated solvated species ArPdI(S)(PPh3)(2) involved in an equilibrium with the chloride ion. ArPdI(OAc)(PPh3)(2)(-) is quite unstable and rapidly affords the stable trans ArPd(OAc)(PPh3)(2) complex. Consequently, the mechanism of the PdCl2(PPh3)(2)-catalyzed cross-coupling of aryl halides and nucleophiles has been revisited. The nucleophilic attack does not proceed on the trans ArPdI(PPh3)(2) complexes as usually postulated but on the intermediate neutral pentacoordinated species ArPdI(S)(PPh3)(2) to afford a pentacoordinated anionic complex ArPdI(Nu)(PPh3)(2)(-) in which the aryl group and the nucleophile are adjacent, a favorable position for the reductive elimination which provides the coupling product Ar-Nu. The mechanism of the Heck reactions catalyzed by mixtures of Pd(OAc)(2) and PPh3 has also been revisited. The nucleophilic attack of the olefin proceeds on ArPd(OAc)(PPh3)(2) and not on the expected trans ArPdI(PPh3)(2) complex which is never formed when the oxidative addition is performed from Pd(0)(PPh3)(2)(OAc)(-). This work emphasizes the crucial role played by the anions born by the precursors of palladium(0) complexes and rationalize empirical findings dispersed in literature concerning the specificity of palladium catalytic systems. (C) 1999 Elsevier Science S.A. All rights reserved.