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 halid es, omicron -ZCH(2)-C6H4-X [X = I, Br; Z = OMe, NEt2, N(CH2)(5)] to Pd-0(PP h3)(4) or to Pd-0(dba)(dppp) generated from Pd-0(dba)(2) and 1 equiv. of dp pp is determined. The oxidative addition is slower for ortho-substituted ar yl halides than for the corresponding nonsubstituted or meta-substituted ar yl halides. The ortho substituents investigated here do not participate in the oxidative addition by a preliminary complexation of the active Pd-0(PPh 3)(2) or Pd-0(dppp) complex. The observed decelerating effect induced by th e ortho substituents is due to steric hindrance and electronic donor effect s and is less important for aryl bromides than for aryl iodides; ortho-subs tituted aryl iodides remain more reactive than ortho-substituted aryl bromi des for both ligands PPh3 and dppp. The cis-(omicron -ZCH(2)-C6H4)PdX(dppp) complexes are formed in the oxidative addition whereas an equilibrium take s place between trans-(omicron -ZCH(2)-C6H4)PdX(PPh3)(2) and (omicron -ZCH( 2)-C6H4)PdX(PPh3)(Z-Pd) complexes.