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

Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsil anes 3b-c was catalyzed dramatically by Lewis acids such as EtAlCl2 to give the corresponding six-membered silacycles 4b-c in high yields. The reactio n proceeded via an exo-mode cyclization. In addition to the vinylsilylation , the intramolecular trans-arylsilylations using carbon-tethered alkynylary lsilanes 8a-b were also catalyzed by Lewis acids such as HfCl4 to give six- and seven-membered cyclic (E)-vinylsilanes 9a-b, respectively. The cycliza tion of silicon-tethered substrates 13a-d afforded five- and six-membered s ilacycle, products 14a-d in low to high yields. All arylsilylation reaction s proceeded via an exo-mode fashion exclusively.