Dramatic Effect of Ancillary NHC Ligand in the Highly Selective Catalytic Oxidative Carbon–Carbon Multiple Bond Cleavage

Selective and controlled oxidation of olefins to aldehydes is a commonly used important transformation in chemistry. However, chemists still use the dangerous and inconvenient ozonolysis method or the less selective, low-yielding Lemieux–Johnson protocol. In a program of developing effective catalysts for this important reaction, we disclose here that an ancillary ligand can play a dramatic role in the above catalytic phenomenon, depending on the design of the ligand precursor chosen. Proof-of-principle is demonstrated with the help of two newly designed [LnRuII-NHC] precatalysts (NHC = an imidazolydene-based NHC, Im-NHC, or a triazolydene-based NHC, Trz-NHC; Ln = para-cymene) for catalytic selective oxidation of olefins/alkynes to carbonyl compounds. With the electron-deficient Trz-NHC ligand, [(para-cymene)­RuII(Im-Trz)]+ precatalyst was found to be an order of magnitude more efficient than the [(para-cymene)­RuII(Im-NHC)]+ precatalyst.将烯烃选择性可控氧化为醛类，是化学领域中一类常用且重要的转化反应。然而当前化学家仍需使用危险且操作繁琐的臭氧分解法（ozonolysis），或是选择性欠佳、产率较低的勒米厄-约翰逊（Lemieux–Johnson）反应体系。在为本类重要反应开发高效催化剂的研究项目中，我们在此报道：辅助配体可对上述催化过程产生显著调控作用，其效果取决于所选配体前驱体的分子设计。本研究通过两款全新设计的[LnRuII-NHC]前催化剂（N-杂环卡宾（NHC）包括咪唑啉叉基型NHC（Im-NHC）与三唑啉叉基型NHC（Trz-NHC）；Ln为对伞花烃（para-cymene）），实现了烯烃/炔烃选择性催化氧化制备羰基化合物的原理验证。相较于带有缺电子Trz-NHC配体的[(对伞花烃)RuII(Im-NHC)]+前催化剂，[(对伞花烃)RuII(Im-Trz)]+前催化剂的催化效率提升了一个数量级。