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Chemistry and Biochemistry
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The reaction of C6F5H and H2C=CHSiMe3 with catalytic amounts of the Ni(0) complex, [iPr2Im]Ni(η2-H2C=CHSiMe3)2, forms the C–H silylation product C6F5SiMe3 exclusively, with ethylene as a byproduct [(iPr2Im) = 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(η2-H2C=CHSiMe3)2 provided only the alkene hydroarylation product C6F5CH2CH2SiMe3 [(IPr) = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene]. Mechanistic studies show that the C–H activation and β-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 H2C=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.
Elsby, Matthew Ryan, "Catalytic C–H Bond Functionalization by Nickel(0) N-Heterocyclic Carbene Complexes: Influence of Ligand Sterics on Selective Silylation" (2017). Electronic Theses and Dissertations. 7253.