Pham John, Jarczyk Cole E, Reynolds Eamon F, Kelly Sophie E, Kim Thao, He Tianyi, Keith Jason M, Chianese Anthony R
Department of Chemistry, Colgate University 13 Oak Drive, Hamilton New York 13346 USA
Chem Sci. 2021 May 24;12(24):8477-8492. doi: 10.1039/d1sc00703c. eCollection 2021 Jun 23.
We previously demonstrated that Milstein's seminal diethylamino-substituted PNN-pincer-ruthenium catalyst for ester hydrogenation is activated by dehydroalkylation of the pincer ligand, releasing ethane and eventually forming an NHEt-substituted derivative that we proposed is the active catalyst. In this paper, we present a computational and experimental mechanistic study supporting this hypothesis. Our DFT analysis shows that the minimum-energy pathways for hydrogen activation, ester hydrogenolysis, and aldehyde hydrogenation rely on the key involvement of the nascent N-H group. We have isolated and crystallographically characterized two catalytic intermediates, a ruthenium dihydride and a ruthenium hydridoalkoxide, the latter of which is the catalyst resting state. A detailed kinetic study shows that catalytic ester hydrogenation is first-order in ruthenium and hydrogen, shows saturation behavior in ester, and is inhibited by the product alcohol. A global fit of the kinetic data to a simplified model incorporating the hydridoalkoxide and dihydride intermediates and three kinetically relevant transition states showed excellent agreement with the results from DFT.
我们之前证明,米尔斯坦用于酯加氢的开创性二乙氨基取代的PNN钳形钌催化剂是通过钳形配体的脱烷基化而活化的,释放出乙烷并最终形成NHEt取代的衍生物,我们认为该衍生物是活性催化剂。在本文中,我们展示了一项支持该假设的计算和实验机理研究。我们的密度泛函理论(DFT)分析表明,氢活化、酯氢解和醛加氢的最低能量途径依赖于新生N-H基团的关键参与。我们分离并通过晶体学表征了两种催化中间体,一种二氢化钌和一种氢醇钌,后者是催化剂的静止状态。详细的动力学研究表明,催化酯加氢对钌和氢呈一级反应,对酯呈饱和行为,并受到产物醇的抑制。将动力学数据整体拟合到一个包含氢醇盐和二氢化物中间体以及三个动力学相关过渡态的简化模型中,结果与DFT结果显示出极佳的一致性。