Direct Observation of the Microscopic Reverse of the Ubiquitous Concerted Metalation Deprotonation Step in C-H Bond Activation Catalysis
Direct Observation of the Microscopic Reverse of the Ubiquitous Concerted Metalation Deprotonation Step in C-H Bond Activation Catalysis
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DOI：
10.1021/jacs.0c10409
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发表时间：
2021-01-11
影响因子：
15
通讯作者：
Lynam, Jason M.
中科院分区：
文献类型：
--
作者：
Hammarback, L. Anders;Aucott, Benjamin J.;Lynam, Jason M.
羧酸酯基团促进有机化合物中C-H键直接官能化的能力无疑是现代化学合成中最重要的发现之一。广泛的计算研究已经表明，该过程通过碱性羧酸盐对金属配位的C-H键的去质子化进行，然而这些预测的机理途径的实验验证是有限的并且充满困难，主要是因为快速质子转移经常在多步反应中的系综测量中被掩盖（即，由几个步骤组成的催化循环）。在本文中，我们描述了一种策略，实验观察的关键C-H键的活化步骤支撑功能化过程（即M-C键质子化）的微观逆转。这已经通过利用热稳定前体[Mn（ppy）（CO）（4）]（ppy =金属化的2-苯基吡啶）在纯乙酸中的光化学活化来实现。皮秒-毫秒时间尺度上的时间分辨红外光谱允许直接观察从乙酸到环化配体的质子转移所涉及的状态，为计算预测的反应途径提供直接的实验证据。这种方法的权力，以探测过渡金属催化反应的机理途径，通过实验证明在甲苯溶液中PhC 2 H和HOAc的存在下进行。这些允许观察到金属结合的溶剂被炔的顺序置换，通过插入Mn-C键形成C-C键，以及HOAc的较慢质子化步骤以产生Mn（I）催化的C-H键官能化反应的产物。
The ability of carboxylate groups to promote the direct functionalization of C-H bonds in organic compounds is unquestionably one of the most important discoveries in modern chemical synthesis. Extensive computational studies have indicated that this process proceeds through the deprotonation of a metal-coordinated C-H bond by the basic carboxylate, yet experimental validation of these predicted mechanistic pathways is limited and fraught with difficulty, mainly as rapid proton transfer is frequently obscured in ensemble measures in multistep reactions (i.e., a catalytic cycle consisting of several steps). In this paper, we describe a strategy to experimentally observe the microscopic reverse of the key C-H bond activation step underpinning functionalization processes (viz. M-C bond protonation). This has been achieved by utilizing photochemical activation of the thermally robust precursor [Mn(ppy)(CO)(4)] (ppy = metalated 2-phenylpyridine) in neat acetic acid. Time-resolved infrared spectroscopy on the picosecond-millisecond time scale allows direct observation of the states involved in the proton transfer from the acetic acid to the cyclometalated ligand, providing direct experimental evidence for the computationally predicted reaction pathways. The power of this approach to probe the mechanistic pathways in transition-metal-catalyzed reactions is demonstrated through experiments performed in toluene solution in the presence of PhC2H and HOAc. These allowed for the observation of sequential displacement of the metal-bound solvent by the alkyne, C-C bond formation though insertion in the Mn-C bond, and a slower protonation step by HOAc to generate the product of a Mn(I)-catalyzed C-H bond functionalization reaction.