Anionic Pd(0) and Pd(II) Intermediates in Palladium-Catalyzed Heck and Cross-Coupling Reactions

The anions of PdCl(2)L(2) and Pd(OAc)(2), precursors of palladium(0) used in cross-coupling and Heck reactions, play a crucial role in these reactions. Tricoordinated anionic complexes Pd(0)L(2)Cl(-) and Pd(0)L(2)(OAc)(-) are the effective catalysts instead of the usually postulated Pd(0)L(2) complex. The anion ligated to the palladium(0) affects the kinetics of the oxidative addition to ArI as well as the structure and reactivity of the arylpalladium(II) complexes produced in this reaction. Thus, pentacoordinated anionic complexes are formed, ArPdI(Cl)L(2)(-) or ArPdI(OAc)L(2)(-), the precursor of neutral trans-ArPd(OAc)L(2), instead of the usually postulated trans-ArPdIL(2) complex (L = PPh(3)).