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.