Redox Couple Involving NOx in Aerobic Pd-Catalyzed Oxidation of sp3-C-H Bonds: Direct Evidence for Pd-NO3-/NO2- Interactions Involved in Oxidation and Reductive Elimination

NaNO3 is used in oxidative Pd-catalyzed processes as a complementary co-catalyst to common oxidants, e.g., Cu-II salts, in C-H bond activation and Wacker oxidation processes. NaNO3 and NaNO2 (with air or O-2) assist the sp(3)-C-H bond acetoxylation of substrates bearing an N-directing group. It has been proposed previously that a redox couple is operative. The role played by NOx anions is examined in this investigation. Evidence for an NOx anion interaction at Pd-II is presented. Palladacyclic complexes containing NOx anions are competent catalysts for acetoxylation of 8-methylquinoline, with and without exogenous NaNO3. The oxidation of 8-methylquinoline to the corresponding carboxylic acid has also been noted at Pd-II. O-18-Labeling studies indicate that oxygen derived from nitrate appears in the acetoxylation product, the transfer of which can only occur by interaction of O-18 at Pd with a coordinating-acetate ligand. Nitrated organic intermediates are formed under catalytic conditions, which are converted to acetoxylation products, a process that occurs with (50 degrees C) and without Pd (110 degrees C). A catalytically competent palladacyclic dimer intermediate has been identified. Head-space analysis measurements show that NO and NO2 gases are formed within minutes on heating catalytic mixtures to 110 degrees C from room temperature. Measurements by in situ infrared spectroscopy show that N2O is formed in sp(3)-C-H acetoxylation reactions at 80 degrees C. Studies confirm that cyclopalladated NO2 complexes are rapidly oxidized to the corresponding NO3 adducts on exposure to NO2(g). The investigation shows that NOx anions act as participating ligands at Pd-II in aerobic sp(3)-C-H bond acetoxylation processes and are involved in redox processes.