Nickel-Catalyzed C–H Silylation of Arenes with Vinylsilanes: Rapid and Reversible β‑Si Elimination

The
reaction of C6F5H and H2CCHSiMe3 with catalytic [iPr2Im]­Ni­(η2-H2CCHSiMe3)2 (1b) exclusively forms the C–H
silylation product C6F5SiMe3 with
ethylene as a byproduct ([iPr2Im] = 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(η2-H2CCHSiMe3)2 (1a) ([IPr] = 1,3-bis­[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene)
provided only the alkene hydroarylation product C6F5CH2CH2SiMe3. 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 SiMe3 moieties.