<i>Z</i>- and Enantioselective Ring-Opening/Cross-Metathesis with Enol Ethers Catalyzed by Stereogenic-at-Ru Carbenes: Reactivity, Selectivity, and Curtin–Hammett Kinetics

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 enantioselectivity (>98:2 enantiomer ratio). Surprisingly, the enantioselective ROCM reactions proceed with high Z selectivity (up to 98% Z). Moreover, reactions proceed with the opposite sense of enantioselectivity versus aryl olefins, which afford E isomers exclusively. Preliminary DFT calculations in support of Curtin–Hammett kinetics as well as initial models that account for the stereoselectivity levels and trends are provided.本文报道了首例兼具Z构型选择性与对映选择性的钌（Ru）催化烯烃复分解反应。使用负载有与氮杂环卡宾（N-heterocyclic carbene, NHC）相连的芳氧基螯合结构的对映纯中心手性钌配合物作为催化剂，催化剂用量为0.5–5.0 mol%，可实现氧双环烯烃、烯醇醚与苯基乙烯基硫醚的开环/交叉复分解（Ring-opening/cross-metathesis, ROCM）反应。目标产物可高效获得，且具有极高的对映选择性（对映体比例>98:2）。令人意外的是，该对映选择性开环/交叉复分解反应同时表现出优异的Z构型选择性，最高可达98%的Z构型产物占比。此外，该反应与芳基烯烃参与的同类反应的对映选择性趋势截然相反：芳基烯烃参与的此类反应仅生成E构型异构体。本文同时提供了支持柯廷-哈姆顿（Curtin–Hammett）动力学的初步密度泛函理论（DFT）计算结果，以及能够解释该立体选择性水平与变化趋势的初始模型。