Stable and easy-to-handle zwitterions containing carbanion and pyridinium moieties were synthesized, and their structural studies by both X-ray crystallography and theoretical methods revealed the stereoelectronic effect in the zwitterionic ‘C−–C–N+’ system.
2-(Pyridinium-1-yl)-1,1-bis(triflyl)ethanides have been used as 1,2-dipole precursors in a metal-free direct [2+2] cycloaddition reaction of alkynes. Remarkably, this mild and facile uncatalyzed protocol requires neither irradiation nor heating.
2-(Pyridinium-1-yl)-1,1-bis(perfluoroalkylsulfonyl)ethan-1-ide: A Practical Reagent for Synthesis of Strongly Acidic 1,1-Bis(perfluoroalkylsulfonyl)alkanes
By mixing (RfSO2)2CH2 (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 (RfSO2)2C=CH2/2-fluoropyridine
Carbazoles possessing Tf2CHCH2 groups were obtained by the reaction of 1-(indol-2-yl)but-3-yn-1-ols with in situ-generated Tf2C[double bond, length as m-dash]CH2 through vicinal difunctionalisation of the alkyne moiety, where the vinyl-type carbocation intermediate was selectively attacked by the indole moiety and not by the carbanion moiety.