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 i ons: 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 oxidat ive addition to aryl iodides strongly depends on the anion born by the pall adium(0). The structure of the arylpalladium(II) complexes formed in the ox idative addition also depends on the anion. Indeed, intermediate anionic pe ntacoordinated arylpanadium(II) complexes are formed: ArPdI(Cl)(PPh3)(2)(-) and ArPdI(OAc)(PPh3)(2)(-) respectively, whose stability depends on the ch loride 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 neutr al pentacoordinated solvated species ArPdI(S)(PPh3)(2) involved in an equil ibrium 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 halide s and nucleophiles has been revisited. The nucleophilic attack does not pro ceed 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 reduct ive 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 b een 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 neve r 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 dis persed in literature concerning the specificity of palladium catalytic syst ems. (C) 1999 Elsevier Science S.A. All rights reserved.