Pyridine-Assisted Chlorinations and Oxidations by Palladium(IV)

The reactivity of the bis-NHC complex (LPdCl4)-Cl-IV (L = kappa(2)-[R-NHCCH2NHC-R] with R = C14H29) in chlorinations and oxidations of organic substrates was considerably increased in the presence of pyridine. For alkene chlorinations, this effect was due to the in situ formation of highly reactive (LPdCl3)-Cl-IV(py)(+), which was able to transfer Cl+ to the C=C bond in a ligand-mediated process (devoid of pi complexation), which did not require py dissociation. The enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild conditions in a reaction where py served as a base, broadening the known scope of reactivity for Pd-IV complexes. (LPdCl3)-Cl-IV(py)(+) could be formed from Cl-/py exchange or from the oxidation of (LPdCl)-Cl-II(py)(+) by Cl-2. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both Pd-II and Pd-IV complexes allowed for the catalytic chlorination of styrene with (LPdCl4)-Cl-IV as a sacrificial oxidant, thereby establishing the principal feasibility of Pd-II/Pd-IV catalyses that obviates Pd-II activations of the substrate.