Cremer Till, Siler Cassandra G F, Rodríguez-Reyes Juan Carlos F, Friend Cynthia M, Madix Robert J
†Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States.
‡School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States.
J Phys Chem Lett. 2014 Apr 3;5(7):1126-30. doi: 10.1021/jz500192k. Epub 2014 Mar 18.
Selective oxidative reactions promoted by gold depend critically on controlling the coverage and stability of adsorbed intermediates, as well as promoting specific bond activations of those intermediates. We demonstrate that acetate, a common intermediate in the oxidation of olefins, aldehydes, and alcohols, is destabilized by 7-10 kcal/mol by coadsorbed oxygen relative to its stability on the clean gold surface. The amount of destabilization depends on the oxygen coverage. Peak temperatures of products indicative of oxygen-assisted and clean-surface bond activation differ by up to 130 K. Experiments with d3-acetate show a kinetic isotope effect of 6.9 at 400 K, indicating that the rate-limiting step of the low temperature oxygen-assisted reaction is γ-CH bond breaking. This clearly demonstrates that coadsorbed oxygen activates γ-CH bonds on gold and suggests that an oxygen-assisted activation may also occur for β-CH bonds crucial in oxygen-assisted alcohol coupling on metallic gold catalysts, as predicted by theory.
金催化的选择性氧化反应关键取决于对吸附中间体的覆盖度和稳定性的控制，以及促进这些中间体的特定键活化。我们证明，相对于其在清洁金表面的稳定性，共吸附的氧会使烯烃、醛和醇氧化过程中的常见中间体乙酸的稳定性降低7-10千卡/摩尔。去稳定化的程度取决于氧的覆盖度。指示氧辅助和清洁表面键活化的产物的峰值温度相差高达130K。用d3-乙酸进行的实验表明，在400K时动力学同位素效应为6.9，这表明低温氧辅助反应的速率限制步骤是γ-CH键的断裂。这清楚地表明共吸附的氧会活化金表面的γ-CH键，并表明如理论所预测的，对于金属金催化剂上氧辅助醇偶联中至关重要的β-CH键，也可能发生氧辅助活化。
