Polydonor-substituted 1-oxa- and 1-thia-3,5-diazahexatrienes: Synthesis, structures, ring-chain tautomerism and theoretical calculations

Previously unknown 1-oxa-2,4-diazahexatrienes 1a,b and c, substituted with alkoxy groups, have been prepared by carboxylation of N-alkylideneisoureas 2 with chloroformates 5. Xray analyses of compounds 1b and c show nonplanar , open-chain structures. In contrast, thiocarboxylation of 2 with thiochlor oformate 6 leads to the spiro compounds 7-ring, as verified by spectroscopi c data and X-ray analysis. A tetradonor-substituted 1-oxa-2,4-diazahexatrie ne 1d was obtained from the reaction of the 1-oxa-3-aza-butadiene 10 with t he triply donor-substituted carbenium ion 9. Attempts at further chain elon gation of 1a by an alkylation/condensation sequence failed because of prefe rential N-alkylation (compound 11). Quantum chemical calculations (AM1, PM3 , RHF/3-21G, B3LYP/6-31+G*) on model systems of 1 predict the predominance, in the gas phase, of twisted, nonplanar chain structures for the oxa syste ms, and cyclic structures for the corresponding thia system, as found exper imentally in the condensed or solid phase. In the series of oxa systems, th e barrier towards ring-opening of the cyclic form of the tautomeric equilib rium decreases with increasing number of donor groups; the preference for t he chain structures increases simultaneously.