Dub Pavel A, Tkachenko Nikolay V
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
J Phys Chem A. 2021 Jul 8;125(26):5726-5737. doi: 10.1021/acs.jpca.1c02516. Epub 2021 Jun 29.
The mechanism of ketones homogeneous hydrogenation with -BuOK in -butanol is currently portrayed as the one proceeding via a six-membered [2 + 2 + 2] cyclic transition state involving the H molecule, the base, and a ketone. However, the concerted nature of the reaction is inconsistent with a number of experimental observations. Here we reanalyze available experimental data and revise the mechanism of this paradigmatic reaction based on the static and dynamic density functional theory (DFT) calculations in solution phase. In contrast to the gas-phase profile where the overall reaction occurs in two elementary steps, there are three consecutive steps in solution: cleavage of the H-H bond in basic -butanol to afford potassium hydride, addition of potassium hydride across the C═O bond of a ketone through the rate-determining transition state, and rapid product formation through K/H exchange. Potassium hydride is therefore an important intermediate of the catalytic process. The free energy profile for the prophetic ester homogeneous hydrogenation with -BuOK in -butanol is also computed herein. The reaction seems to be kinetically possible, but slightly harsher conditions need to be applied, consistent with rate-determining nature of the potassium hydride addition.
在丁醇中，用叔丁醇钾（-BuOK）使酮进行均相氢化的机理目前被描述为通过一个六元的[2 + 2 + 2]环状过渡态进行，该过渡态涉及氢分子、碱和酮。然而，该反应的协同性质与一些实验观察结果不一致。在此，我们重新分析了现有的实验数据，并基于溶液相中的静态和动态密度泛函理论（DFT）计算，对这个典型反应的机理进行了修正。与气相反应历程不同，在气相中整个反应分两个基元步骤进行，而在溶液中存在三个连续步骤：在碱性丁醇中氢氢键的断裂生成氢化钾，氢化钾通过速率决定过渡态加成到酮的碳氧双键上，以及通过钾/氢交换快速形成产物。因此，氢化钾是催化过程中的一个重要中间体。本文还计算了在丁醇中用叔丁醇钾使丙酸酯进行均相氢化的自由能曲线。该反应在动力学上似乎是可行的，但需要应用稍微更苛刻的条件，这与氢化钾加成的速率决定性质是一致的。
