Stamoulis Alexios G, Bruns David L, Stahl Shannon S
Department of Chemistry, University of Wisconsin─Madison, Madison, Wisconsin 53706, United States.
J Am Chem Soc. 2023 Aug 16;145(32):17515-17526. doi: 10.1021/jacs.3c02887. Epub 2023 Aug 3.
Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording HO or HO. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NO/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NO cocatalysts. In addition, we show how the reductive activation of O provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.
分子氧是有机化学合成中典型的氧化剂，但诸多挑战仍在限制其效用和应用范围。广泛的历史研究聚焦于克服由O的基态三重态电子结构带来的动力学挑战，以及与O还原衍生的活性氧物种相关的各种反应性和选择性挑战。本观点将分析催化需氧氧化反应背后的热力学原理，借鉴燃料电池中氧还原反应（ORR）研究的概念。这种分析对于“氧化酶”型液相催化需氧氧化反应尤为重要，该反应通过耦合两个连续的氧化还原半反应的机制进行：（1）底物氧化和（2）氧还原，通常生成HO或HO。这些反应的催化剂具有介于底物氧化和氧还原反应相关电位之间的氧化还原电位，催化剂电位的变化会导致两个半反应的有效过电位发生变化。在低ORR过电位下运行的催化剂为底物氧化步骤保留了更大的热力学驱动力，使O能够用于更具挑战性的氧化反应。而在高ORR过电位下运行的催化剂用于底物氧化的驱动力较小，但相对于高电位催化剂，它们通常表现出不同或更好的化学选择性。这些概念在一系列案例研究中得到阐述，以突出它们对化学合成的影响。例子包括（a）NO/氧鎓铵和Cu/硝酰基催化剂、（b）高电位醌和胺氧化酶仿生醌以及（c）有或没有NO助催化剂的Pd需氧氧化催化剂的比较。此外，我们展示了O的还原活化如何提供一种获得传统氧化酶型机制无法达到的电位的方法。总体而言，这种分析突出了催化剂过电位在指导需氧氧化反应发展中的核心作用。
