INTIMATE MECHANISM OF OXIDATIVE ADDITION TO ZEROVALENT PALLADIUM COMPLEXES IN THE PRESENCE OF HALIDE-IONS AND ITS RELEVANCE TO THE MECHANISM OF PALLADIUM-CATALYZED NUCLEOPHILIC SUBSTITUTIONS

The mechanism of oxidative addition of aryl halides to low-ligated zerovalent palladium species obtained by reduction of their divalent precursor complexes, PdIICl2(PR3)2, was investigated on the basis of P-31 NMR and electrochemistry. In strong contrast to usual expectations, it is shown that the reaction proceeds via a complex sequence of steps: (i) fast addition of the aryl halide to a halide ligated zerovalent palladium center, so as to afford a pentacoordinated anionic arylpalladium(II) center; (ii) a fast but reversible uphill elimination from the pentacoordinated anionic arylpalladium(II) center of a halide ion ligand, possibly through its substitution by a solvent ligand; (iii) rearrangement of this second short-lived intermediate into the thermodynamically stable trans-arylpalladium(II) product of the reaction. On the basis of this detailed mechanism, a new mechanism is proposed for the catalysis of nucleophilic substitutions by palladium complexes. In contrast to the catalytic cycles that are usually considered, this new catalytic cycle accounts for the well-used effects of halide ions as well as that of small metal cations and rationalizes their role in the overall efficiency of palladium-catalyzed nucleophilic substitutions.