Oxidative Addition of Haloheteroarenes to Palladium(0): Concerted versus SNAr-Type Mechanism

The kinetics of the oxidative additions of haloheteroarenes HetX (X=I, Br, Cl) to [Pd-0(PPh3)(2)] (generated from [Pd-0(PPh3)(4)]) have been investigated in THF and DMF and the rate constants have been determined. In contrast to the generally accepted concerted mechanism, Hammett plots obtained for substituted 2-halopyridines and solvent effects reveal a reaction mechanism dependent on the halide X of HetX: an unprecedented SNAr-type mechanism for X=Br or Cl and a classical concerted mechanism for X=I. These results are supported by DFT studies.