Role of dba in the reactivity of palladium(0) complexes generated in situ from mixtures of Pd(dba)2 and phosphines

The ligand dba plays a crucial role both on the structure and on the reactivity of palladium(0) complexes generated in situ in mixtures of Pd(dba)(2) and phosphine ligands, Whatever the ligand, the major complex is always Pd(dba)L-2 where I, is a monodentate phosphine ligand or Pd(dba)(L-L) where L-L is a bidentate phosphine ligand. In all cases, the most reactive species in the oxidative addition with phenyl iodide is the lowest ligated complex PdL2 or Pd(L-L) in equilibrium with the major complex and dba. However, Pd(dba)(L-L) also reacts with phenyl iodide. The presence of the major complexes ligated by dba diminishes the concentration of the more reactive species PdL2, or Pd(L-L) and consequently controls the rate of the overall reaction. The overall reactivity is governed both by the intrinsic reactivity of the reactive species and its concentration, two factors which can be antagonistic and a non-linear Hammett correlation of the oxidative addition with the basicity of the phosphine is observed for monodentate ligands. (C) 1998 Elsevier Science S.A. All rights reserved.