Detection of Palladium(I) in Aerobic Oxidation Catalysis

Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric Pd^I species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C-H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric Pd^I complex, [Pd^I (μ-DAF)(OAc)]₂ , which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this Pd^I complex and its implications for catalytic reactivity are discussed.