1,2,3-Triazolium-1-oxide, -1-imide and 1-methanide hetero-1,3,5-triene equilibrium: Ab initio calculations. A new base induced ring expansion of 1-alkyl-1,2,3-triazolium salts to 2,3-dihydro-1,2,4-triazines and 1-amino-imidazoles via the 1,2,5-triazahexa-1,3,5-triene system. Azolium 1,3-dipoles

Treatment of substituted 1-methyl- and 1-trimethylsilylmethyl-1,2,3-triazolium salts with ethoxide and caesium fluoride respectively gave substituted 2,3-dihydro-1,2,4-triazines and 1-aminoimidazoles via 1,2,5-triazahexa-1,3,5-triene intermediates. Expected isomeric 1,2,3-triazolium-1-methanide 1,3-dipoles were not detected and ab initio calculations suggested that they are thermodynamically unfavoured. Ab initio calculations on the equilibrium between the acyclic hetero-1,3,5-triene system and the cyclic forms of 1,2,3-triazolium-1-oxide, -1-imide and -1-methanide are reported. X-ray crystal structures are described for 2,5,6-triphenyl-2,3-dihydro-1,2,4-triazine 6a and 1-(p-nitro-anilino)-4,5-diphenylimidazole 11c.