Ruthenium Olefin Metathesis Catalysts with Frozen NHC Ligand Conformations

The catalytic behavior of Grubbs and Hoveyda–Grubbs II type ruthenium complexes bearing N-heterocyclic carbene (NHC) ligands with syn-phenyl groups on the backbone and syn- or anti-oriented o-tolyl N-substituents was evaluated in a series of olefin metathesis transformations. Further advance in the synthesis of the best-performing syn catalysts and a deeper investigation into the solution-state structure of the Hoveyda–Grubbs type II complex with anti N-tolyl groups by 2D-NMR and DFT studies are also reported. Of particular interest, syn complexes emerged among the best-performing catalysts in all of the explored metathesis reactions, especially in the ring-closing metathesis (RCM) of hindered olefins, allowing also for the difficult formation of macrocyclic trisubstituted alkenes. An important unexpected result was obtained in the RCM of linalool, where both syn and anti catalysts appeared to be involved in the dehydration reaction of the cyclization product (1-methylcylopent-2-en-1-ol). This process allowed for the formation of well-defined mixtures of methylcyclopentadiene isomers, which represent valuable precursors for the synthesis of renewable high-density fuels.本研究评估了骨架带有顺式苯基、且N取代基为顺式或反式取向邻甲苯基的氮杂环卡宾（N-heterocyclic carbene, NHC）配体修饰的格拉布（Grubbs）与霍维达-格拉布II型钌配合物的催化性能，测试体系涵盖一系列烯烃复分解转化反应。本研究还报道了高性能顺式催化剂合成的进一步进展，以及通过二维核磁共振（2D-NMR）与密度泛函理论（DFT）研究，对带有反式N-甲苯基的霍维达-格拉布II型配合物的溶液态结构开展的深入探究。尤为值得关注的是，在所有测试的复分解反应中，顺式配合物均跻身性能最优催化剂之列，尤其在位阻烯烃的关环复分解（ring-closing metathesis, RCM）反应中表现突出，甚至可实现此前极具挑战性的大环三取代烯烃的构建。在芳樟醇的关环复分解反应中，我们得到了一项重要的意外结果：顺式与反式催化剂均参与了环化产物（1-甲基环戊-2-烯-1-醇）的脱水反应。该过程可生成组分明确的甲基环戊二烯异构体混合物，此类异构体是合成可再生高密度燃料的重要前驱体。