Redox Couple Involving NO in Aerobic Pd-Catalyzed Oxidation of sp-C-H Bonds: Direct Evidence for Pd-NO/NO Interactions Involved in Oxidation and Reductive Elimination.

NaNO is used in oxidative Pd-catalyzed processes as a complementary co-catalyst to common oxidants, e.g., Cu salts, in C-H bond activation and Wacker oxidation processes. NaNO and NaNO (with air or O) assist the sp-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 NO anions is examined in this investigation. Evidence for an NO anion interaction at Pd is presented. Palladacyclic complexes containing NO anions are competent catalysts for acetoxylation of 8-methylquinoline, with and without exogenous NaNO. The oxidation of 8-methylquinoline to the corresponding carboxylic acid has also been noted at Pd. O-Labeling studies indicate that oxygen derived from nitrate appears in the acetoxylation product, the transfer of which can only occur by interaction of O 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 °C) and without Pd (110 °C). A catalytically competent palladacyclic dimer intermediate has been identified. Head-space analysis measurements show that NO and NO gases are formed within minutes on heating catalytic mixtures to 110 °C from room temperature. Measurements by in situ infrared spectroscopy show that NO is formed in sp-C-H acetoxylation reactions at 80 °C. Studies confirm that cyclopalladated NO complexes are rapidly oxidized to the corresponding NO adducts on exposure to NO(g). The investigation shows that NO anions act as participating ligands at Pd in aerobic sp-C-H bond acetoxylation processes and are involved in redox processes.