| The reaction of C6F5H and H2C=CHSiMe 3 with catalytic amounts of the Ni(0) complex, [iPr 2Im]Ni(eta2-H2C=CHSiMe3) 2, forms the C--H silylation product C6F5SiMe 3 exclusively, with ethylene as a byproduct [(iPr 2Im) = 1,3-(diisopropyl)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. The analogous reaction with [IPr]Ni(eta2-H2C=CHSiMe3) 2 provided only the alkene hydroarylation product C6F 5CH2CH2SiMe3 [(IPr) = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene]. Mechanistic studies show that the C--H activation and beta-Si elimination steps are reversible under catalytic conditions with both Ni(0) catalysts, and tuning steric bulk on the ancillary carbene ligand plays a major role in reactivity of the catalysts.;The IBn and IMes carbene ligands have similar electronic parameters to IPr and iPr2Im, but varied % Vbur [(IBn) = 1,3-dibenzyl-1,3-dihydro-2H-imidazol-2-ylidene], [(IMes) = 1,3-bis[2,4,6-trimethylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene]. Studies were performed by reacting C6F5H and H 2C=CHER3 (ER3 = SnBu3, SnPh3, GePh3, SiMe3) with catalytic amounts of Ni(COD) 2 and the carbene ligands IPr, IMes, IBn, and iPr2Im. Catalytic C--H stannylation was facile with all ligands except for one anomaly. The correspondingly more difficult C--H germylation and C--H silylation reactions could form selective germylation and silylation products by using the small IBn and iPr2Im carbene ligands. Using the larger IPr or IMes carbenes resulted in either a mixture of germylation/silylation and hydroarylation products, or exclusive conversion to the hydroarylation product. |
