Department of Chemistry , Princeton University , Princeton , New Jersey 08544 , United States.
J Am Chem Soc. 2019 Feb 13;141(6):2721-2730. doi: 10.1021/jacs.8b13451. Epub 2019 Feb 1.
Here we present a detailed kinetic study of the multisite proton-coupled electron transfer (MS-PCET) activations of aryl ketones using a variety of Brønsted acids and excited-state Ir(III)-based electron donors. A simple method is described for simultaneously extracting both the hydrogen-bonding equilibrium constants and the rate constants for the PCET event from deconvolution of the luminescence quenching data. These experiments confirm that these activations occur in a concerted fashion, wherein the proton and electron are transferred to the ketone substrate in a single elementary step. The rates constants for the PCET events were linearly correlated with their driving forces over a range of nearly 19 kcal/mol. However, the slope of the rate-driving force relationship deviated significantly from expectations based on Marcus theory. A rationalization for this observation is proposed based on the principle of non-perfect synchronization, wherein factors that serve to stabilize the product are only partially realized at the transition state. A discussion of the relevance of these findings to the applications of MS-PCET in organic synthesis is also presented.
在这里，我们使用各种布朗斯特酸和激发态基于铱(III)的电子供体，对芳基酮的多位点质子耦合电子转移 (MS-PCET) 激活进行了详细的动力学研究。描述了一种简单的方法，可以从荧光猝灭数据的解卷积中同时提取氢键平衡常数和 PCET 事件的速率常数。这些实验证实，这些激活以协同方式发生，其中质子和电子在单个基本步骤中转移到酮底物。在近 19 kcal/mol 的范围内，PCET 事件的速率常数与其驱动力呈线性相关。然而，速率-驱动力关系的斜率与基于马库斯理论的预期有很大偏差。基于非完美同步原理，提出了一种对这种观察结果的合理化解释，其中有助于稳定产物的因素仅在过渡态部分得到实现。还讨论了这些发现与 MS-PCET 在有机合成中的应用的相关性。
