N,N'difluoro-1,4-diazoniabicyclo[2.2.2]octane salts, highly reactive and easy-to-handle electrophilic fluorinating agents

A series of N,N'-difluoro-1,4-diazoniabicyclo[2.2.2]octane salts were synthesized in a pure form by the fluorination of 1,4-diazabicyclo[2.2.2]octane with F-2 diluted with N-2 in the presence of a Bronsted acid in fluoro alcohol or acetonitrile or by fluorination followed by a treatment with a different Bronsted or Lewis acid. Their complete structural assignment was made based on spectral and elemental analyses of the isolated crystals. A great through-bond interaction between the two N-F's of the salt was observed on (19)FNMR. An assessment was made of the usefulness of N,N'-difluoro-1,4-diazoniabicyclo[2.2.2]octane bis(triflate), bis(HSO4), bis(BF4), and bis(SbF6) salts for electrophilic fluorination; the bis(BF4) salt was demonstrated to be widely applicable as a highly reactive and easy-to-handle fluorinating agent. Thus, the bis(BF4) salt readily fluorinated activated aromatics, active methylene compounds or their salts, substituted styrenes, and vinyl acetates under mild conditions. It was shown that one N-F of the salt was effective for fluorination and that the other N-F played a role to activate fluorination through the bonds. Thus, the reaction mechanism was discussed, and fluorination followed by an immediate intramolecular one-electron transfer was suggested.