N-imines, C-ylides, and anhydrobases derived from 1,2,4-triazole

Abstract

Deprotonation of two N-p-nitroanilino-1,2,4-triazolium salts gave stable triazole-N-imines(triazolio-anilides), which underwent protonation, alkylation, and acetylation at the exocyclic nitrogen atom. Treatment of 3-p-nitrobenzyl- and 3-(2,4-dinitrobenzyl)-1,2,4-triazolium perchlorates with sodium ethoxide yielded deeply coloured triazolium C-ylides (16) of varying stability; a triazolo[1,5-a]pyridinium ylide (18) was prepared similarly. Coloured 1,2,4-triazole anhydrobases (5-methylene-Δ²-1,2,4-triazolines)(23) were obtained by proton abstraction from triazolium salts bearing nitrobenzyl-substituents at C-5. Deprotonation of 3,5-di-p-nitrobenzyl-1,4-diphenyl-1,2,4-triazolium perchlorate (22c) occurs from the 5-methylene group to yield the triazoline (23c). The ylides and anhydrobases react reversibly with water to form colourless triazolium hydroxides. The effect of solvent polarity on the first absorption bands of the two types of compounds was studied. The comparative stabilities of ylides and anhydro-bases were investigated by measuring the rates of deuterium exchange of six C-benzyl-1,2,4-triazolium salts. The anomalous properties of 5-(2,4-dinitrobenzylidene)-1,3,4-triphenyl-Δ²-1,2,4-triazoline (23b) are attributed to a steric effect.