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 PdIV monohydrocarbyl hydroxo complexes has been carried out, and the PdIV 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 PdII 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 PdIV monohydrocarbyl and the second is unreactive in this type of transformation. All the monohydrocarbyl PdIV 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 °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.