Catalytic activity of η2-(olefin)palladium(0) complexes with iminophosphine ligands in the Suzuki–Miyaura reaction. Role of the olefin in the catalyst stabilization

The catalytic activity of η2-(olefin)palladium(0)(iminophosphine) complexes in the Suzuki–Miyaura coupling is strongly dependent on the reaction conditions and on the nature of the ligands. The reaction is at the best carried out in aromatic solvents in the presence of K2CO3 at 90–110 °C. Higher reaction rates are obtained when the R substituent on the N-imino group is an aromatic group of low steric hindrance and the olefin is a moderate π-accepting ligand such as dimethyl fumarate. At temperatures lower than 90 °C, a self-catalyzed process leading to catalyst deactivation becomes predominant. Preliminary mechanistic investigations indicate that the oxidative addition of the aryl bromide to a Pd(0) species is the rate determining step in the catalytic cycle and that the olefin plays a key role in catalyst stabilization. Systems in situ prepared by mixing Pd(OAc)2 or Pd(dba)2 with 1 equiv of iminophosphine appear substantially less active than the preformed catalysts.