Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis

详细信息    查看全文

• 作者：Colin W. Anson ; Soumya Ghosh ; Sharon Hammes-Schiffer ; Shannon S. Stahl
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：March 30, 2016
• 年：2016
• 卷：138
• 期：12
• 页码：4186-4193
• 全文大小：568K
• ISSN：1520-5126

文摘

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Co^II(salophen) complex, a Co^III-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the Co^III–O₂ species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co^III-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H₂O₂ intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O₂ reduction and generate the reactive p-benzoquinone co-catalyst.