Unusual Transposition of Allenic Framework in Intramolecular Cyclization of Acetal-Tethered (Allenylmethyl)silanes

Treatment of acetal-tethered (allenylmethyl)silanes, which were obtained from the corresponding 3-bromo-5-silyl-1,3-pentadienes by a Pd-catalyzed reaction with an acetal-tethered malonate, with TiCl4 gave not only vinylcyclohexene derivatives via a standard SE2′ pathway but also unusual allenylcyclopentane species via cyclization at the δ-position. Deuterium-labeling experiments revealed participation of a 1,2-hydride shift in a carbocation intermediate for the formation of the latter products.