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

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• 作者：Margot N. Wenzel ; Philippa K. Owens ; Joshua T. W. BrayJason M. Lynam ; Pedro M. Aguiar ; Christopher Reed ; James D. Lee ; Jacqueline F. Hamilton ; Adrian C. Whitwood ; Ian J. S. Fairlamb
• 刊名：Journal of the American Chemical Society
• 出版年：2017
• 出版时间：January 25, 2017
• 年：2017
• 卷：139
• 期：3
• 页码：1177-1190
• 全文大小：867K
• ISSN：1520-5126

文摘

NaNO₃ is used in oxidative Pd-catalyzed processes as a complementary co-catalyst to common oxidants, e.g., Cup>IIp> salts, in C–H bond activation and Wacker oxidation processes. NaNO₃ and NaNO₂ (with air or O₂) assist the spp>3p>-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_x anions is examined in this investigation. Evidence for an NO_x anion interaction at Pdp>IIp> is presented. Palladacyclic complexes containing NO_x 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 Pdp>IIp>. p>18p>O-Labeling studies indicate that oxygen derived from nitrate appears in the acetoxylation product, the transfer of which can only occur by interaction of p>18p>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 NO₂ gases are formed within minutes on heating catalytic mixtures to 110 °C from room temperature. Measurements by in situ infrared spectroscopy show that N₂O is formed in spp>3p>-C–H acetoxylation reactions at 80 °C. Studies confirm that cyclopalladated NO₂ complexes are rapidly oxidized to the corresponding NO₃ adducts on exposure to NO₂(g). The investigation shows that NO_x anions act as participating ligands at Pdp>IIp> in aerobic spp>3p>-C–H bond acetoxylation processes and are involved in redox processes.