Ou Xinrui, Occhipinti Giovanni, Boisvert Eliza-Jayne Y, Jensen Vidar R, Fogg Deryn E
Center for Catalysis Research & Innovation, and Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
Department of Chemistry, University of Bergen, Allégaten 41, N-5007 Bergen, Norway.
ACS Catal. 2023 Apr 5;13(8):5315-5325. doi: 10.1021/acscatal.2c03828. eCollection 2023 Apr 21.
Ruthenium catalysts bearing cyclic (alkyl)(amino)carbene (CAAC) ligands can attain very high productivities in olefin metathesis, owing to their resistance to unimolecular decomposition. Because the propagating methylidene species RuCl(CAAC)(=CH) is extremely susceptible to bimolecular decomposition, however, turnover numbers in the metathesis of terminal olefins are highly sensitive to catalyst concentration, and hence loadings. Understanding how, why, and how rapidly the CAAC complexes partition between the precatalyst and the active species is thus critical. Examined in a dual experimental-computational study are the rates and basis of initiation for phosphine-free catalysts containing the leading CAAC ligand , in which a CMePh group α to the carbene carbon helps retard degradation. The Hoveyda-class complex (RuCl(L)(=CHAr), where L = , Ar = CH-2-O Pr-5-R; R = H) is compared with its nitro-Grela analogue ( ; R = NO) and the classic Hoveyda catalyst (L = HIMes; R = H). -Butyl vinyl ether (BuVE) was employed as substrate, to probe the reactivity of these catalysts toward olefins of realistic bulk. Initiation is ca. 100× slower for than in CD, or 44× slower in CDCl. The rate-limiting step for the CAAC catalyst is cycloaddition; for , it is BuVE binding. Initiation is 10-13× faster for than in either solvent. DFT analysis reveals that this rate acceleration originates in an overlooked role of the nitro group. Rather than weakening the Ru-ether bond, as widely presumed, the NO group accelerates the ensuing, rate-limiting cycloaddition step. Faster reaction is caused by long-range mesomeric effects that modulate key bond orders and Ru-ligand distances, and thereby reduce the trans effect between the carbene and the trans-bound alkene in the transition state for cycloaddition. Mesomeric acceleration may plausibly be introduced via any of the ligands present, and hence offers a powerful, tunable control element for catalyst design.
带有环状（烷基）（氨基）卡宾（CAAC）配体的钌催化剂在烯烃复分解反应中能够实现非常高的生产率，这归因于它们对单分子分解的抗性。然而，由于增长的亚甲基物种RuCl(CAAC)(=CH)极易发生双分子分解，端烯烃复分解反应中的周转数对催化剂浓度以及负载量高度敏感。因此，了解CAAC配合物如何、为何以及多快地在预催化剂和活性物种之间分配至关重要。在一项实验与计算相结合的研究中，对含有主要CAAC配体的无膦催化剂的引发速率和引发基础进行了研究，其中卡宾碳α位的CMePh基团有助于减缓降解。将霍维达类配合物（RuCl(L)(=CHAr)，其中L = ，Ar = CH-2-O Pr-5-R；R = H）与其硝基-格雷拉类似物（ ；R = NO）以及经典的霍维达催化剂 （L = HIMes；R = H）进行了比较。使用叔丁基乙烯基醚（BuVE）作为底物，以探究这些催化剂对实际体积烯烃的反应活性。在CD中， 的引发速度比 慢约100倍，在CDCl中慢44倍。CAAC催化剂的限速步骤是环加成；对于 ，限速步骤是BuVE配位。在任何一种溶剂中， 的引发速度都比 快10 - 13倍。密度泛函理论（DFT）分析表明，这种速率加速源于硝基被忽视的作用。与广泛推测相反，NO基团并非削弱Ru - 醚键，而是加速了随后的限速环加成步骤。更快的反应是由远程中介效应引起的，这种效应调节了关键键级和Ru - 配体距离，从而降低了环加成过渡态中卡宾与反式配位烯烃之间的反式效应。中介加速可能通过存在的任何一种配体合理引入，因此为催化剂设计提供了一个强大的、可调节的控制元素。
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