Department of Chemistry and Biochemistry, University of Windsor , 401 Sunset Avenue, Windsor, Ontario N9B 3P4, Canada.
J Am Chem Soc. 2017 Jul 12;139(27):9401-9407. doi: 10.1021/jacs.7b05574. Epub 2017 Jun 23.
The reaction of CFH and HC═CHSiMe with catalytic [PrIm]Ni(η-HC═CHSiMe) (1b) exclusively forms the C-H silylation product CFSiMe with ethylene as a byproduct ([PrIm] = 1,3-di(isopropyl)imidazole-2-ylidene). Catalytic C-H bond silylation is facile with partially fluorinated aromatic substrates containing two ortho fluorine substituents adjacent to the C-H bond and 1,2,3,4-tetrafluorobenzene. Less fluorinated substrates react slower. Under the same reaction conditions, catalytic [IPr]Ni(η-HC═CHSiMe) (1a) ([IPr] = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene) provided only the alkene hydroarylation product CFCHCHSiMe. Mechanistic studies reveal that the C-H activation and β-Si elimination steps are reversible under catalytic conditions with both catalysts 1a and 1b. With catalytic 1a, reversible ethylene loss after β-Si elimination was also observed despite its inability to catalyze C-H silylation; the reductive elimination step to form the silylation product is much slower than reductive elimination to form the alkene hydroarylation product. Reversible ethylene loss was not observed with 1b, which suggests that the rate-limiting step in the reaction is neither C-H activation nor β-Si elimination but either ethylene loss or reductive elimination of cis-disposed aryl and SiMe moieties.
CFH 和 HC═CHSiMe 与催化 [PrIm]Ni(η-HC═CHSiMe)（1b）反应仅形成 CFSiMe，其副产物为乙烯（[PrIm] = 1,3-二异丙基咪唑-2-亚基）。对于含有两个邻位氟取代基且与 C-H 键相邻的部分氟化芳基底物以及 1,2,3,4-四氟苯，催化 C-H 键硅烷化反应很容易进行。氟取代程度较低的底物反应较慢。在相同的反应条件下，催化 [IPr]Ni(η-HC═CHSiMe)（1a）（[IPr] = 1,3-双[2,6-二异丙基苯基]-1,3-二氢-2H-咪唑-2-亚基）仅提供烯烃氢芳基化产物 CFCHCHSiMe。机理研究表明，在催化剂 1a 和 1b 存在的情况下，C-H 活化和 β-Si 消除步骤在催化条件下是可逆的。对于催化 1a，尽管其不能催化 C-H 硅烷化，但在 β-Si 消除后也观察到可逆的乙烯损失；形成硅烷化产物的还原消除步骤比形成烯烃氢芳基化产物的还原消除步骤慢得多。对于 1b，没有观察到可逆的乙烯损失，这表明反应的限速步骤既不是 C-H 活化也不是 β-Si 消除，而是乙烯损失或顺式芳基和 SiMe 部分的还原消除。
