2-(Pyridinium-1-yl)-1,1-bis(perfluoroalkylsulfonyl)ethan-1-ide: A Practical Reagent for Synthesis of Strongly Acidic 1,1-Bis(perfluoroalkylsulfonyl)alkanes.

On mixing (Rf SO2 )2 CH2 (Rf =perfluoroalkyl), paraformaldehyde, and substituted pyridines, a three-component reaction proceeded smoothly to give unusual zwitterions bearing both pyridinium and stabilized carbanion moieties in good to excellent yields. Of these, 2-fluoropyridinium derivatives rapidly dissociated in acetonitrile to give equilibrium mixtures of the zwitterions and (Rf SO2 )2 C=CH2 /2-fluoropyridine, as confirmed by detailed variable-temperature NMR studies. The dynamic behavior of such 2-fluoropyridinium compounds allows them to be used as shelf-stable, easy-to-handle sources of (Rf SO2 )2 C=CH2 . With these reagents, strongly acidic carbon acids (Rf SO2 )2 CHR were synthesized, which served as a new type of acid catalysts. Moreover, C-C bond-forming reactions with a ketene silyl acetal proceeded efficiently with Tf2 C=CH2 generated in situ.