Syntheses, Structures, and Catalytic Activities of Hemilabile Thioether-Functionalized NHC Complexes

Four imidazolium (5a/b) and benzimidazolium (6a/b) salts with hemilabile alkyl-aryl thioether functions have been prepared via a straightforward and modular pathway in order to compare their reactivities toward palladation. Reaction of 5a/b with Pd(OAc)2 gave complex product mixtures, whereas 6a/b afforded the desired bis(benzimidazolin-2-ylidene) complexes 8a/b in good yields. The difference in reactivities of benzimidazole versus imidazole derivatives was attributed to the presence of additional acidic protons at C4/5 positions of the imidazolium ring, leading to competing and unselective deprotonation reactions. The milder Ag-NHC transfer reaction, on the other hand, provided either mono- or bis(imidazolin-2-ylidene) complexes (9 or 7a/b) in good yields depending on the ligand:metal ratio. The interesting hemilability in monocarbene complex 9 was investigated by spectroscopy and thioether displacement reaction with PPh3, yielding the mixed NHC-PPh3 complex 10 in high yields. An initial comparative catalytic study also reveals that the mixed-donor complex 10 exhibits the highest activity among the complexes tested.

本研究通过简便且模块化的合成路径，制备了四种带有半配位性烷基芳基硫醚官能团的咪唑鎓（imidazolium，5a/b）与苯并咪唑鎓（benzimidazolium，6a/b）盐，旨在对比其在钯化反应中的反应活性。将5a/b与乙酸钯（Pd(OAc)₂）反应时得到复杂的产物混合物，而6a/b则以良好收率获得目标双(苯并咪唑啉-2-亚基)配合物8a/b。咪唑鎓衍生物与苯并咪唑鎓衍生物的反应活性差异，可归因于咪唑鎓环C4/5位点存在额外的酸性质子，进而引发竞争性且无选择性的去质子化副反应。相较而言，更为温和的银-氮杂环卡宾（Ag-NHC）转移反应可根据配体与金属的投料比，以良好收率得到单(咪唑啉-2-亚基)配合物9或双(咪唑啉-2-亚基)配合物7a/b。研究人员通过光谱学手段以及与三苯基膦（PPh₃）的硫醚置换反应，对单卡宾配合物9中有趣的半配位特性展开了探究，最终以高收率得到混合氮杂环卡宾-三苯基膦配合物10。初步对比催化研究显示，在所测试的所有配合物中，该混合供体配合物10展现出最高的催化活性。