Novel base-initiated reactions of N-substituted pyridinium salts

Article abstract of DOI:10.1016/0022-1139(96)03402-1

Reaction of N-fluoropyridinium triflate with a base in dichloromethane gave 2-chloropyridine as the major product along with 2-pyridyl triflate and 2-fluoropyridine, regardless of the nature of the base. This base-initiated reaction was also shown to take place similarly in other halogenated alkanes, ethers, a nitrile, aromatics, a ketone, vinyl ethers, alcohols and trimethylsilyl acetate as solvents to give pyridine derivatives substituted with a solvent molecule (s) at the 2-position. N-(Trifluoromethanesulfonyloxy)-and (benzenesulfonyloxy) pyridinium salts were found to undergo the same base-initiated reaction. These reactions may be explained by a postulated singlet carbene (canonical formula 11b) produced through proton abstraction of N-substituted pyridinium salts. A similar carbene reaction may thus likely occur in the thermal decomposition of thiatriazole 10. Ab initio MO calculations revealed the structure and properties of the labile deprotonated N-fluoropyridinium cation and supported the carbene intermediate reaction mechanism rather than a pyridynium or pyridyl cation mechanism. Quarroz's reports on the reactions of picolinic acid N-oxide and the reported reactions of pyridines with F₂, CH₃COOF or CsSO₄F in solvents may be explained by this carbene mechanism.