Kinetics of the oxidative addition of ortho-substituted aryl halides to Palladium(0) complexes

The rate constant of the oxidative addition of ortho-substituted aryl halides, omicron -ZCH(2)-C6H4-X [X = I, Br; Z = OMe, NEt2, N(CH2)(5)] to Pd-0(PPh3)(4) or to Pd-0(dba)(dppp) generated from Pd-0(dba)(2) and 1 equiv. of dppp is determined. The oxidative addition is slower for ortho-substituted aryl halides than for the corresponding nonsubstituted or meta-substituted aryl halides. The ortho substituents investigated here do not participate in the oxidative addition by a preliminary complexation of the active Pd-0(PPh3)(2) or Pd-0(dppp) complex. The observed decelerating effect induced by the ortho substituents is due to steric hindrance and electronic donor effects and is less important for aryl bromides than for aryl iodides; ortho-substituted aryl iodides remain more reactive than ortho-substituted aryl bromides for both ligands PPh3 and dppp. The cis-(omicron -ZCH(2)-C6H4)PdX(dppp) complexes are formed in the oxidative addition whereas an equilibrium takes place between trans-(omicron -ZCH(2)-C6H4)PdX(PPh3)(2) and (omicron -ZCH(2)-C6H4)PdX(PPh3)(Z-Pd) complexes.