Department of Chemistry , Yale University , 225 Prospect Street , New Haven , Connecticut 06520 , United States.
J Am Chem Soc. 2018 Nov 21;140(46):15641-15645. doi: 10.1021/jacs.8b10461. Epub 2018 Nov 8.
Developing new strategies to activate and cleave C-H bonds is important for a broad range of applications. Recently a new approach for C-H bond activation using multi-site concerted proton-coupled electron transfer (PCET) involving intermolecular electron transfer to an oxidant coupled to intramolecular proton transfer was reported. For a series of oxidants reacting with 2-(9 H-fluoren-9-yl)benzoate, experimental studies revealed an atypical Brønsted α, defined as the slope of the logarithm of the PCET rate constant versus the logarithm of the equilibrium constant or the scaled driving force. Herein this reaction is modeled with a vibronically nonadiabatic PCET theory. Hydrogen tunneling, thermal sampling of the proton donor-acceptor mode, solute and solvent reorganization, and contributions from excited vibronic states are found to play important roles. The calculations qualitatively reproduce the experimental observation of a Brønsted α significantly less than 0.5 and explain this shallow slope in terms of exoergic processes between pairs of electron-proton vibronic states. These fundamental mechanistic insights may guide the design of more effective strategies for C-H bond activation and cleavage.
开发新的策略来激活和裂解 C-H 键对于广泛的应用非常重要。最近，一种新的方法被报道用于使用涉及分子间电子转移到氧化剂与分子内质子转移偶联的多位点协同质子耦合电子转移 (PCET) 来激活 C-H 键。对于一系列与 2-(9H-芴-9-基)苯甲酸酯反应的氧化剂，实验研究揭示了一种非典型的 Brønsted α，定义为 PCET 速率常数的对数与平衡常数或标度驱动力的对数的斜率。在此，该反应通过非绝热 PCET 理论进行建模。发现氢隧穿、质子给体-受体模式的热采样、溶质和溶剂重组以及激发的振动态的贡献起着重要作用。计算结果定性地再现了实验观察到的 Brønsted α 显著小于 0.5，并且根据电子-质子振动态对之间的放能过程解释了这种浅斜率。这些基本的机制见解可能指导更有效的 C-H 键激活和裂解策略的设计。
