Synthesis, Properties, and Reactivity of N,N‘-Difluorobipyridinium and Related Salts and Their Applications as Reactive and Easy-To-Handle Electrophilic Fluorinating Agents with High Effective Fluorine Content¹

N,N ‘-Difluoro-2,2‘-, -2,4‘-, -3,3‘-, -4,4‘-bipyridinium and substituted N,N ‘-difluoro-2,2‘-bipyridinium bis(triflates), bis(tetrafluoroborates), bis(hexafluorophosphates), and bis(hexafluoroantimonates) 1 − 9 were synthesized in high yields by the direct fluorination of a mixture of a bipyridyl and a Lewis acid, a Brønsted acid, or the alkali metal salt of an acid. The higher homologues, trimer 10 and polymer 11, were also synthesized. Unsubstituted or electron-donating group-substituted N,N ‘-difluorobipyridinium salts are stable nonhygroscopic crystals, while the electron-withdrawing group-substituted N,N ‘-diflurobipyridinium salts 3, 5, and 6 are moisture-sensitive crystals. Hydrolysis of 1b in boiling water gave 3,3‘-dihydroxy-2,2‘-bipyridyl. The reactivity determination indicated that the fluorinating capability decreased in the order 2,2‘- ≫ 2,4‘ > 3,3‘- ≈ 4,4‘-isomer ≫ N -fluoropyridinium salt and that the two N−F moieties in a molecule were effective for fluorination. This fluorination occurred in a step-by-step manner, and the reactivity difference between the first and second fluorinations was very small. N,N ‘-Difluoro-2,2‘-bipyridinium bis(tetrafluoroborate) ( 1b ) is thus shown to be a highly reactive and easy-to-handle electrophilic fluorinating agent with the high effective fluorine content (103.3 g/kg) for preparing many fluoro organic compounds.