1,2,3-TRIAZOLIUM-1-OXIDE, -1-IMIDE AND 1-METHANIDE HETERO-1,3,5-TRIENE EQUILIBRIUM - ABINITIO 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-AMINOIMIDAZOLES 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-nitroanilino)-4,5-diphenylimidazole 11c.