High chemoselectivity of CS dipolarophile in 1,3‐dipolar cycloaddition of nitrilimines and 1,2,4‐triazepin‐5‐one derivatives: experimental, theoretical and X‐ray study

Substituted 2,7-dimethyl-3-thioxo-3,4,5,6-tetrahydro-2H-[1,2,4]triazepin-5-one reacts as a dipolarophile with several N-aryl-C-ethoxycarbonylnitrilimines, in equimolar quantities, to give, in all cases, two types of products: diethyl 3-(p-aryl)-2-[N′-(p-aryl)-N′-(2′,7′-dimethyl-5′-oxo-5′,6′-dihydro-2H-[1,2,4]triazepin-3′-yl)-hydrazino]-2,3-dihydro[1,3,4]thiadiazole-2,5-dicarboxylate (3a–3c in 20–25% yield) and ethyl 4-(p-aryl)-5-imino-1,4-dihydro[1,3,4]thiadiazole carboxylate (4a–4c in 45–50% yield). When 1:2 stoichiometry was used, the formation of product 3 (50%) was favoured. The reaction is entirely chemo- and regioselective. The structures of the compounds obtained, where aryl stands for p-chloro-phenyl (3b) in the first type and for tolyl (4a) in the second type, were determined by X-ray crystallography and analysed by spectral methods (NMR and mass spectroscopy). The global and local electrophilicity/nucleophilicity have been analysed to rationalize the chemical reactivity of the reactants. Copyright © 2005 John Wiley & Sons, Ltd.