Stable vinyl cations. Direct spectroscopic observation of vinyl-substituted vinyl cations

Abstract

Vinyl cations, although well established as reactive interme-diatesIa and predicted to be rather stable thermodynamical1y,lb have been rather elusive toward direct spectroscopic observation.Attempts to generate vinyl cations via SbF5-assisted heterolysis of a-arylvinyl halides resulted in attack of SbF5 at the alkene A system and formation of CT complexes.2Protonation of alkynes under strongly acidic conditions also failed to give stable vinyl cation solutions;' rapid inter-4 and intramolecular5 sequential reactions took place instead.Hitherto the only spectroscopic evidence comes from treatment of a-alkynyl alcohols with superacids, leading to alkynylcarbenium ions, which may be regarded as vinyl cations if the allenic resonance structure is important.6