The mechanism of acceptorless dehydrogenative coupling reaction (ADC) of alcohols to esters catalyzed by aliphatic pincer PHNP ruthenium complexes was experimentally studied. Relevant intermediate species involved in the catalytic cycle were isolated and structurally characterized by single-crystal X-ray diffraction studies, and their reactivity (including toward substrates related to the catalytic process) was probed. VT NMR studies unveiled several chemical exchanges connecting the Ru amido hydride, the Ru alkoxide, and the alcohol substrate. Under catalytic conditions, in situ IR spectroscopy monitoring demonstrated the production of ester via aldehyde as intermediate. A Tishchenko-like pathway is proposed as the main path for the production of ester from aldehyde, involving alkoxide and hemiacetaloxide Ru species (the latter being identified in the reaction mixture by NMR). Catalytic system deactivation under base-free conditions was found to be related to water traces in the reaction medium (either as impurity or derived from aldol reactions) that lead to the formation of catalytically inactive acetato Ru complexes. These react with alkali metal alkoxides to afford catalytically active Ru species. In line with this observation, running the ADC reaction in the presence of water scavengers or alkoxides allows maintaining sustained catalytic activity.

中文翻译：

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深入了解Ru夹钳介导的无受体脱氢醇偶联反应：交换，中间体和失活物种

脂肪族钳子P H催化醇与酯的无受体脱氢偶联反应（ADC）的机理对NP钌配合物进行了实验研究。通过单晶X射线衍射研究分离了涉及催化循环的相关中间物种，并对其结构进行了表征，并探查了它们的反应性（包括对与催化过程相关的底物的反应性）。VT NMR研究揭示了连接Ru酰氨基氢化物，Ru醇盐和醇底物的几种化学交换。在催化条件下，原位红外光谱监测表明通过醛作为中间体生产了酯。提出了类似提申科科的途径作为由醛生产酯的主要途径，涉及醇盐和半缩醛Ru物种（后者在反应混合物中通过NMR鉴定）。发现在无碱条件下的催化体系失活与反应介质中的痕量水（作为杂质或源自醛醇缩合反应）有关，导致形成催化惰性的乙酰丙酮钌配合物。这些与碱金属醇盐反应，得到催化活性的Ru物质。与该观察结果一致，在水清除剂或醇盐的存在下进行ADC反应可以保持持续的催化活性。