High chemoselectivity of CS 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‐2 H ‐[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‐2 H ‐[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.