N,N′-Difluoro-1,4-diazoniabicyclo[2.2.2]octane Salts, Highly Reactive and Easy-to-Handle Electrophilic Fluorinating Agents

Article abstract of DOI:10.1246/bcsj.69.2287

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 F₂ diluted with N₂ 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 ¹⁹FNMR. An assessment was made of the usefulness of N,N′-difluoro-1,4-diazoniabicyclo[2.2.2]octane bis(triflate), bis(HSO₂), 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.