The ruthenium-catalyzed silylation of aromatic C-H bonds with triethylsilane

The Ru₃(CO)₁₂-catalyzed reaction of aromatic compounds such as arylazoles, arylimines, and arylpyridines with triethylsilane (2) gave silylation products in good to excellent yields. Amide and ester groups could also be used as a directing group. In nearly all cases, C-Si bond formation took place at a position ortho to the directing groups. This coupling reaction is tolerant of both electron-donating and -withdrawing groups such as methoxy, fluoro, and trifluoromethyl groups. We expected that the use of 2-naphthyl-3-methylpyridine would not result in the formation of a coupling product, because the severe steric repulsion between the methyl group and the peri-hydrogen at the 8-position would strongly inhibit the attainment of a co-planar geometry. Interestingly, however, the reaction of 2-naphthyl-3-methylpyridine afforded the corresponding silylation product in quantitative yield. This observation indicates that the formation of metalacycle intermediate is not essential for the catalytic reaction to proceed. When a deuterium-labeling experiment using phenyloxazoline-d₅ was carried out in the absence of olefin, partial H/D scrambling occurred between the ortho positions of the phenyloxazoline and the Si-H of triethylsilane. This labeling experiment indicates that C-H bond cleavage is not the rate-determining step and that a rapid equilibrium prior to C-Si bond formation (i.e. the reductive elimination step) occurs in this catalytic reaction.