Palladium(IV) Monohydrocarbyls: Mechanistic Study of the Ligand-Enabled Oxidation of Palladium(II) Complexes with H2O2 in Water

A detailed mechanistic study of the di(2-pyridyl)ketone (dpk)-enabled oxidation with H2O2 in water of a series of monohydrocarbylpalladium(II) complexes derived from cyclopalladated 2-(3-R-benzoyl)pyridines (R = H, Me) and 2-(p-R'-phenyl)pyridines (R' = H, Me, MeO, F) to produce corresponding Pd-IV monohydrocarbyl hydroxo complexes has been carried out, and the Pd-IV hydrocarbyls have been characterized in detail. The study involves kinetics, isotopic labeling experiments, and the DFT calculations. A reaction mechanism has been proposed for the oxidation of dpk-supported Pd-II complexes in water that includes elimination of water from the hydrated dpk ligand of the monohydrocarbylpalladium(II) species as the rate-limiting step. Subsequent reversible addition of H2O2 across the resulting ketone C=O bond leads to the formation of two diastereomeric hydroperoxoketals, one of which can rapidly produce a Pd-IV monohydrocarbyl and the second is unreactive in this type of transformation. All the monohydrocarbyl Pd-IV complexes undergo clean C-O reductive elimination to form the corresponding phenols or derived palladium(II) phenoxides. The kinetics of the C-O reductive elimination of the Pd(IV) monohydrocarbyls derived from cyclopalladated 2-(p-R-phenyl)pyridines was studied at 22 degrees C; the corresponding first-order rate constants were found to be only weakly dependent on the nature of the substituent R (H, Me, OMe, F). To account for these observations, a detailed DFT analysis of plausible C-O reductive elimination mechanisms in water was carried out. A direct elimination mechanism of six-coordinate complexes resulting from the oxidation above was proposed to be operational that involves an "early" C-O coupling transition state whose structure varies insignificantly among the substrates studied.