Friedel-Crafts cyclization of 1,1-difluoroalk-1-enes:: Synthesis of benzene-fused cyclic ketones via α-fluorocarbocations

1,1-Difluoroalk-1-enes bearing a phenyl group at the C-3, -4, or -5 position, readily obtained from 2,2,2-trifluoroethyl p-toluenesulfonate, are treated with FSO3H.SbF5 to undergo Friedel-Crafts cyclization in (CF3)(2)CHOH. The cyclization takes place via alpha-fluorocarbocations. followed by spontaneous hydrolysis of the C-F bond to afford bicyclic ketones including a five, six, or seven-membered ring in good yield.