Can Donor Ligands Make Pd(OAc) a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria.

Palladium(II)-catalyzed oxidation reactions represent an important class of methods for selective modification and functionalization of organic molecules. This field has benefitted greatly from the discovery of ancillary ligands that expand the scope, reactivity, and selectivity in these reactions; however, ancillary ligands also commonly poison these reactions. The different influences of ligands in these reactions remain poorly understood. For example, over the 60-year history of this field, the Pd redox potentials for catalytically relevant Pd complexes have never been determined. Here, we report the unexpected discovery of (L)Pd(OAc)-mediated oxidation of hydroquinones, the microscopic reverse of quinone-mediated oxidation of Pd commonly employed in Pd-catalyzed oxidation reactions. Analysis of redox equilibria arising from the reaction of (L)Pd(OAc) and hydroquinones (L = bathocuproine, 4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones, provides the basis for determination of (L)Pd(OAc) reduction potentials. Experimental results are complemented by density functional theory calculations to show how a series of nitrogen-based ligands modulate the (L)Pd(OAc) reduction potential, thereby tuning the ability of Pd to serve as an effective oxidant of organic molecules in catalytic reactions.