VINYL CATIONS IN ORGANIC-SYNTHESIS .2. A NOVEL SYNTHESIS OF METHYLENE-1H-INDENES (BENZOFULVENES) BY CYCLIZATION OF PHENYL-SUBSTITUTED BUT-1-EN-3-YNES

A series of phenyl-substituted but-1 -en-3-ynes has been prepared and their reactivity toward a acid-catalysed cycloisomerisation investigat ed. In boiling dichloromethane or 1,2-dibromoethane, in the presence o f a catalytic amount of methanesulfonic acid, 1,1,2,4-tetraphenylbuten yne, 1,1,4-triphenylbutenyne, and (Z)-1,2,4-triphenylbutenyne afforded 1-benzylidene-2,3-diphenyl-1H-indene, (E)- and (Z)-l-benzylidene-3-ph enyl-1H-indene and (E)- and (Z)-l-benzylidene-2-phenyl-1H-indene, resp ectively, in fair to good yields. Under the same conditions, (E)-1,2,4 -triphenylbutenyne and (E)- and (Z)-1,4-diphenylbutenyne did not affor d cyclisation products and were recovered unchanged. The cycloisomeris ation reaction is rationalised in terms of formation of vinyl cation i ntermediates by exclusive protonation at the 4-position of the butenyn e chain followed by intramolecular electrophilic attack on a suitably located and orientated phenyl nucleus in the 1-position.