Deprotonation of N-phenacyl- and N-acetonyl-4-cyanopyridinium halides with 1,4-diazabicyclo[2,2,2]octane

Abstract

In N-phenacyl- and N-acetonyl-4-cyanopyridinium halides ( 1a· Br and 1b· Cl) the N +CH 2 hydrogens are replaced by deuterium in D 2O or CH 3OD solutions, similarly as in β-diketones, proving a prototropic equilibrium 1⇌ 2. The rate constants have been measured for deprotonation of 1a· Br and 1b· Cl with 1,4-diazabicyclo[2,2,2]octane (DABCO) in solutions. UV, FTIR, 1H and 13C NMR spectra are consistent with the ylide structure of the deprotonated species. N-phenacyl-4-cyanopyridinium ylide ( 3a) is relatively stable in the solid state and unstable in solutions. The observed parallel changes in Δ S ≠ and the time of disappearance of the ylide absorption band suggest that solvation is responsible for the ylide stability. The UV–Vis spectra of 1a· Br and 1b· Cl with DABCO show solvent dependent isosbestic points, which suggest that the decomposition reactions of ylides are very probably uniform. The PM3 and B3LYP calculations were carried out to investigate the structures of the cations, ylides and their complexes with ammonia and DABCO. Ylides are formed by abstraction of hydroxy proton from enol form ( 2) via complex 8.