Synthesis, reactions and theoretical studies of[1,2,4]triazolo[4,3-c]pyrimidinium- and[1,2,4]triazolo[4,3-a]pyrazinium-3-aminides

[1,2,4]Triazolo[4,3-c]pyrimidinium-3-aminides 5a–h have been synthesised by treating pyrimidin-6-yl thiosemicarbazide derivatives 7f–n with dicyclohexylcarbodiimide (DCC). The above aminides 5a–h were slowly hydrolysed in water but very rapidly hydrolysed in 5 M aqueous hydrochloric acid to give substituted 1,2,4-triazole derivatives (e.g. 5a,d,g → 8e,a,f, respectively); related nucleophilic ring-opening reactions occurred when the aminides (cf. 5a–h) were treated with (separately) methanol and ethanol (e.g. 5d → 8c and 8d, respectively). A series of analogous [1,2,4]triazolo[4,3-a]pyrazinium-3-aminides 6a–e was prepared following the procedures described above. The pyrazinium aminides 6 are stable in aq. 2 M HCl, and a stable hydrochloride salt 13 was formed from one such substrate 6a.The structure and electronic properties of condensed triazolium betaines 1, 5d, 6a and 15 have been studied using the semi-empirical PM3/COSMO method and with the ab initio 6-31 basis set; the implications of these results in respect of the potential of such betaines for molecular rearrangement are discussed.