Are pyridinium ylides radicals?

Abstract

Pyridinium ylides are usually considered nucleophiles that can undergo various reactions involving electron pairs. However, it was found that ylides resulting from deprotonation of N-alkyl-substituted pyridinium salts exhibit radical characters, with no discernable NMR signals but decent EPR spectra in both solution and the solid state. An observed correlation between lowered π* energy level of the pyridinium ring and increased EPR activity indicates that thermally induced electron-transfer processes could be involved, where the variable-temperature EPR spectrum indicates a singlet ground state and a thermally activated triplet state for the ylide. The fact that the high-resolution mass spectrum confirms the presence of oligomers of a less sterically hindered pyridinium ylide further points to a radical mechanism.