Lewis acid catalyzed stereoselective carbosilylation. Intramolecular trans-vinylsilylation and trans-arylsilylation of unactivated alkynes.

Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsilanes 3b-c was catalyzed dramatically by Lewis acids such as EtAlCl(2) to give the corresponding six-membered silacycles 4b-c in high yields. The reaction proceeded via an exo-mode cyclization. In addition to the vinylsilylation, the intramolecular trans-arylsilylations using carbon-tethered alkynylarylsilanes 8a-b were also catalyzed by Lewis acids such as HfCl(4) to give six- and seven-membered cyclic (E)-vinylsilanes 9a-b, respectively. The cyclization of silicon-tethered substrates 13a-d afforded five- and six-membered silacycle products 14a-d in low to high yields. All arylsilylation reactions proceeded via an exo-mode fashion exclusively.