Palladium(IV) in directed carbon-hydrogen bond oxidations: Synthetic and mechanistic investigations

A new catalytic method for chelate-directed oxidative functionalization of C--H bonds was developed. The scope and limitations of catalytic sp² C--H bond acetoxylation, bromination, and chlorination were explored, and the stoichiometric amination of C--H bonds was demonstrated. Aromatic compounds containing basic nitrogen atoms as directing groups, which form 5-membered chelates upon cyclometalation, were found to be the best substrates for these transformations.; Platinum(II) complexes designed to model the catalytic transformation were synthesized, and their reactivity with a variety of oxidants to provide oxygenated platinum(III) dimers and platinum(IV) monomers was studied in detail. The products obtained from oxidation of these complexes in alcohol solvents displayed striking similarities to those formed in related catalytic processes. However, these high oxidation state platinum complexes did not react further by reductive elimination to form oxygenated organic products in the absence of external nucleophiles.; As a model even more closely related to the catalytic system, a series of palladium(II) complexes containing two identical chelating C∼N ligands were synthesized. Upon oxidation with iodobenzene dicarboxylate reagents, stable oxygenated palladium(IV) compounds bearing two carboxylate ligands were obtained. When thermolyzed, most underwent clean C--O bond-forming reductive elimination to form oxidized organic products identical to those obtained under catalytic conditions.; The mechanism of this C--O bond-forming reductive elimination was studied in detail using solvent effects, transition state analysis, and Hammett plots. Additionally, the effect of rigidity of the C∼N chelating ligand on the rate of reductive elimination was examined. Based on all of these experiments, the mechanism was proposed to involve initial dissociation of the nitrogen atom of the chelating ligand, followed by carbon--oxygen bond-forming reductive elimination from this neutral, five-coordinate intermediate.; For palladium(IV) complexes containing two benzo[h]quinoline ligands, C--O bond formation was not the only process observed upon thermolysis, and C--C reductive elimination also became competitive. The mechanism of this reaction was also probed using solvent effects and transition state analysis, and preliminary progress toward Hammett data was achieved. Yet this transformation was found to be quite complex, and further experimentation will be necessary to fully elucidate the reaction mechanism.