Vinyl cations in organic synthesis. Part 2. A novel synthesis of methylene-1H-indenes (benzofulvenes) by cyclisation of phenyl-substituted but-1-en-3-ynes

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A series of phenyl-substituted but-1-en-3-ynes has been prepared and their reactivity toward acid catalysed cycloisomerisation investigated. In boiling dichloromethane or 1,2-dibromoethane, in the presence of a catalytic amount of methanesulfonic acid, 1,1,2,4-tetraphenylbutenyne, 1,1,4-triphenylbutenyne, and (Z)-1,2,4-triphenylbutenyne afforded 1-benzylidene-2,3-diphenyl-1H-indene, (E)- and (Z)-1-benzylidene-3-phenyl-1H-indene and (E)- and (Z)-1-benzylidene-2-phenyl-1H-indene, respectively, in fair to good yields. Under the same conditions, (E)-1,2,4-triphenylbutenyne and (E)- and (Z)-1,4-diphenylbutenyne did not afford cyclisation products and were recovered unchanged. The cycloisomerisation reaction is rationalised in terms of formation of vinyl cation intermediates by exclusive protonation at the 4-position of the butenyne chain followed by intramolecular electrophilic attack on a suitably located and orientated phenyl nucleus in the 1-position.