Regarding a Persisting Puzzle in Olefin Metathesis with Ru Complexes: Why are Transformations of Alkenes with a Small Substituent <i>Z</i>-Selective?
作者:Sebastian Torker、Ming Joo Koh、R. Kashif M. Khan、Amir H. Hoveyda
DOI:10.1021/acs.organomet.5b00970
日期:2016.2.22
An enduring question in olefin metathesis is that reactions carried out with widely accessible Ru dichloro complexes, which typically favor E alkenes, generate Z isomers preferentially when substrates bearing a smaller substituent are used; Z enol ethers, alkenyl sulfides, 1,3-enynes, alkenyl halides, or alkenyl cyanides can be prepared reliably with reasonable efficiency and selectivity. Transformations thus proceed via the more hindered syn-substituted metallacyclobutanes, which is mystifying because catalyst features implemented in the more recently developed and broadly applicable Z-selective catalysts are absent in the Ru dichloro systems. Herein, we describe experimental and computational investigations that offer a plausible rationale for these puzzling selectivity trends. The following will be demonstrated. (1) Kinetic Z selectivity depends on the relative barrier for olefin association/dissociation versus metallacyclobutane formation/cleavage. There can be appreciable stereochemical control when metallacyclobutane formation/breakage is turnover-limiting. (2) Stereoelectronic-not purely steric-effects are central: achieving the p-orbital overlap needed for alkene formation while minimizing steric repulsion between the incipient olefin substituent and a catalyst's anionic ligand during the cycloreversion step is crucial. We show that similar stereoelectronic factors are probably operative in the more recently introduced Z-selective (and enantioselective) olefin metathesis transformations promoted by stereogenic-at-Ru complexes containing a bidentate aryloxide ligand.
<i>Z</i>- and Enantioselective Ring-Opening/Cross-Metathesis with Enol Ethers Catalyzed by Stereogenic-at-Ru Carbenes: Reactivity, Selectivity, and Curtin–Hammett Kinetics
作者:R. Kashif M. Khan、Robert V. O’Brien、Sebastian Torker、Bo Li、Amir H. Hoveyda
DOI:10.1021/ja304827a
日期:2012.8.1
The first instances of Z- and enantioselective Ru-catalyzed olefin metathesis are presented. Ring-opening/cross-metathesis (ROCM) reactions of oxabicyclic alkenes and enol ethers and a phenyl vinyl sulfide are promoted by 0.5-5.0 mol % of enantiomerically pure stereogenic-at-Ru complexes with an aryloxy chelate tethered to the N-heterocyclic carbene. Products are formed efficiently and with exceptional
介绍了 Z 和对映选择性 Ru 催化的烯烃复分解的第一个实例。0.5-5.0 mol % 的对映异构纯立体 Ru 配合物与连接在 N-杂环卡宾上的芳氧基螯合物促进了氧杂双环烯烃和烯醇醚与苯基乙烯基硫化物的开环/交叉复分解 (ROCM) 反应. 产品可高效形成且具有出色的对映选择性(> 98:2 对映异构体比)。令人惊讶的是,对映选择性 ROCM 反应以高 Z 选择性(高达 98% Z)进行。此外,反应以与芳基烯烃相反的对映选择性进行,芳基烯烃仅提供 E 异构体。提供了支持 Curtin-Hammett 动力学的初步 DFT 计算以及解释立体选择性水平和趋势的初始模型。