Center for Catalysis Research and Innovation, and Department of Chemistry and Biomolecular Sciences, University of Ottawa , Ottawa K1N 6N5, Canada.
Department of Chemistry, University of Bergen , Allégaten 41, N-5007 Bergen, Norway.
J Am Chem Soc. 2017 Nov 22;139(46):16446-16449. doi: 10.1021/jacs.7b08578. Epub 2017 Nov 10.
Brønsted bases of widely varying strength are shown to decompose the metathesis-active Ru intermediates formed by the second-generation Hoveyda and Grubbs catalysts. Major products, in addition to propenes, are base·HCl and olefin-bound, cyclometalated dimers [RuCl(κ-HIMes-H)(HC═CHR)] Ru-3. These are generated in ca. 90% yield on metathesis of methyl acrylate, styrene, or ethylene in the presence of either DBU, or enolates formed by nucleophilic attack of PCy on methyl acrylate. They also form, in lower proportions, on metathesis in the presence of the weaker base NEt. Labeling studies reveal that the initial site of catalyst deprotonation is not the HIMes ligand, as the cyclometalated structure of Ru-3 might suggest, but the metallacyclobutane (MCB) ring. Computational analysis supports the unexpected acidity of the MCB protons, even for the unsubstituted ring, and by implication, its overlooked role in decomposition of Ru metathesis catalysts.
布朗斯特碱强度差异很大，能够分解由第二代 Hoveyda 和 Grubbs 催化剂形成的复分解活性 Ru 中间体。除丙烯外，主要产物还有碱·HCl 和烯烃配位的、环金属化二聚体[RuCl(κ-HIMes-H)(HC=CHR)]Ru-3。在 DBU 或亲核进攻甲基丙烯酰胺形成的烯醇盐存在下，它们在甲基丙烯酰胺、苯乙烯或乙烯的复分解反应中以约 90%的产率生成。在较弱碱 NEt 的存在下，它们也以较低的比例形成。标记研究表明，催化剂去质子化的初始部位不是 HIMes 配体，正如 Ru-3 的环金属化结构所暗示的那样，而是金属环丁烷（MCB）环。计算分析支持 MCB 质子的出人意料的酸性，即使对于未取代的环也是如此，并暗示其在 Ru 复分解催化剂分解中的被忽视的作用。
