State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis , Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences , 345 Lingling Road , Shanghai 200032 , China.
J Am Chem Soc. 2018 Mar 28;140(12):4417-4429. doi: 10.1021/jacs.8b01038. Epub 2018 Mar 14.
The first general catalytic approach to effecting transfer hydrogenation (TH) of unactivated alkenes using ethanol as the hydrogen source is described. A new NCP-type pincer iridium complex (-NCP)IrHCl containing a rigid benzoquinoline backbone has been developed for efficient, mild TH of unactivated C-C multiple bonds with ethanol, forming ethyl acetate as the sole byproduct. A wide variety of alkenes, including multisubstituted alkyl alkenes, aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal alkynes, are suitable substrates. Importantly, the (-NCP)Ir/EtOH system exhibits high chemoselectivity for alkene hydrogenation in the presence of reactive functional groups, such as ketones and carboxylic acids. Furthermore, the reaction with CDOD provides a convenient route to deuterium-labeled compounds. Detailed kinetic and mechanistic studies have revealed that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic difference. The OH group of ethanol displays a normal kinetic isotope effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene or 1-octene with relatively strong binding affinity to the Ir(I) center has (-NCP)Ir(alkene) adduct as an off-cycle catalyst resting state, and the rate law shows a positive order in EtOH, inverse first-order in styrene, and first-order in the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst resting state of (-NCP)Ir(H)[O(Et)···HO(Et)···HOEt] that features a six-membered iridacycle consisting of two hydrogen-bonds between one EtO ligand and two EtOH molecules, one of which is coordinated to the Ir(III) center. The rate law shows a negative order in EtOH, zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium isotope effect for the pre-equilibrium formation of (-NCP)Ir(H)(OEt) from the catalyst resting state via ethanol dissociation. Regardless of the substrate, ethanol dehydrogenation is the slow segment of the catalytic cycle, while alkene hydrogenation occurs readily following the rate-determining step, that is, β-hydride elimination of (-NCP)Ir(H)(OEt) to form (-NCP)Ir(H) and acetaldehyde. The latter is effectively converted to innocent ethyl acetate under the catalytic conditions, thus avoiding the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.
本文描述了首例使用乙醇作为氢源实现非活化烯烃转移氢化（TH）的通用催化方法。开发了一种新型的 NCP 型钳式铱配合物（-NCP）IrHCl，它含有刚性苯醌骨架，可用于高效、温和地实现非活化 C-C 多重键与乙醇的 TH，形成乙酸乙酯作为唯一的副产物。各种烯烃，包括多取代烷基烯烃、芳基烯烃和杂原子取代烯烃，以及含有 O 或 N 的杂芳环和内部炔烃，都是合适的底物。重要的是，（-NCP）Ir/EtOH 体系在存在反应性官能团（如酮和羧酸）时对烯烃氢化具有高化学选择性。此外，与 CDOD 的反应为氘标记化合物提供了一种方便的途径。详细的动力学和机理研究表明，单取代烯烃（如 1-辛烯、苯乙烯）和多取代烯烃（如环辛烯（COE））表现出基本的机理差异。乙醇中的 OH 基团在苯乙烯的反应中表现出正常的动力学同位素效应（KIE），但在 COE 的情况下则表现出显著的反 KIE。与 Ir(I) 中心具有较强结合亲和力的苯乙烯或 1-辛烯的催化作用具有（-NCP）Ir（烯烃）加合物作为非循环催化剂的静止状态，速率定律在 EtOH 中呈正序，在苯乙烯中呈逆一级，在催化剂中呈一级。相比之下，COE 的催化作用具有（-NCP）Ir（H）[O(Et)···HO(Et)···HOEt]的非循环催化剂静止状态，其特征是由一个 EtO 配体和两个 EtOH 分子之间的两个氢键组成的六元桥环 iridacycle，其中一个与 Ir(III) 中心配位。速率定律在 EtOH 中呈负序，在 COE 中呈零级，在催化剂中呈一级。观察到的反 KIE 对应于催化剂静止状态通过乙醇解离形成（-NCP）Ir(H)(OEt)的预平衡形成的反平衡同位素效应。无论底物如何，乙醇脱氢都是催化循环的缓慢部分，而烯烃氢化在速率决定步骤之后很容易发生，即（-NCP）Ir(H)(OEt)的β-氢化物消除形成（-NCP）Ir(H)和乙醛。后者在催化条件下有效地转化为无害的乙酸乙酯，从而避免了通过铱介导的乙醛脱羰使催化剂中毒。
