Identification of the Effective Palladium(0) Catalytic Species Generated in Situ from Mixtures of Pd(dba)2 and Bidentate Phosphine Ligands. Determination of Their Rates and Mechanism in Oxidative Addition

Mixtures of Pd(dba)2 + 2L-L (where L-L is a bidentate ligand such as dppm, dppe, dppp, dppb, dppf, and DIOP) lead to the formation of Pd(L-L)2 complexes which do not undergo an oxidative addition with phenyl iodide. Mixtures of Pd(dba)2 + 2 BINAP do not afford Pd(BINAP)2 but Pd(dba)(BINAP). Mixtures of Pd(dba)2 + 1L-L (L-L = dppm, dppe, dppp, dppb, dppf, DIOP, and BINAP) lead to Pd(dba)(L-L) complexes via the formation, at short time, of the complex Pd(L-L)2, except for dppf and BINAP where the complex Pd(dba)(L-L) is directly formed. Pd(dba)(L-L) is the main complex in solution but is involved in an endergonic equilibrium with the less ligated complex Pd(L-L) and dba. Pd(L-L) is the more reactive species in the oxidative addition with phenyl iodide. However, Pd(dba)(L-L) also reacts in parallel with phenyl iodide. From the kinetic data concerning the reactivity of these different catalytic systems in the oxidative addition with phenyl iodide, one observes the following order of reactivity: Pd(dba)2 + 1DIOP > Pd(dba)2 + 1dppf ≫ Pd(dba)2 + 1BINAP. All these systems associated to one bidentate ligand are less reactive than Pd(dba)2 + 2PPh3.