Mechanistic studies of the oxidation of reduced palladium complexes by benzoquinone and molecular oxygen

Pd-catalyzed oxidation reactions have shown to be efficient in achieving a wide variety of oxidative transformations of organic molecules. Because irreversible decomposition of the Pd catalyst remains one of the challenges in this field, the mechanism of catalyst reoxidation has been a topic of significant interest and debate in recent years. This work describes fundamental mechanistic studies on the reactions of reduced palladium species with two commonly used stoichiometric oxidants, benzoquinone (BQ) and molecular oxygen (O2). Although the general picture of these reagents as oxidants is the same, their uses in catalytic reaction are not necessarily interchangeable. The origins of the differences in the reactivities of BQ and O2 were investigated by direct comparisons between BQ and O2 in the reaction with well-defined Pd(II)-hydride and Pd(0) species. The oxidation of Pd(II)-hydride species by O2 was further investigated in the context of catalytic relevance. The results from these studies have provided insights into the catalyst reoxidation step. This knowledge will hopefully provide a foundation for rational catalyst design and method development that will result in an improvement in catalyst stability and efficient catalytic turnover.