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, o-ZCH2−C6H4−X [X = I, Br; Z = OMe, NEt2, N(CH2)5] to Pd0(PPh3)4 or to Pd0(dba)(dppp) generated from Pd0(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 Pd0(PPh3)2 or Pd0(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-(o-ZCH2−C6H4)PdX(dppp) complexes are formed in the oxidative addition whereas an equilibrium takes place between trans-(o-ZCH2−C6H4)PdX(PPh3)2 and (o-ZCH2−C6H4)PdX(PPh3)(Z−Pd) complexes.