Effect of the nature of the starting aromatic ring on the cyclization of o-nitroaryl azides: kinetic and thermodynamic studies of the conversion of two azido(methoxycarbonyl)nitrothiophenes into methoxycarbonylthienofurazan oxides

Abstract

The cyclization of 2-azido-5-methoxycarbonyl-3-nitro-(8) and of 3-azido-5-methoxycarbonyl-2-nitro-thiophene (9) which give mixtures of 5-methoxycarbonylthieno[3,2-c- and 5-methoxycarbonylthieno]2,3-c]-furazan oxides (10) and (11) has been studied in NN-dimethylformamide, dioxane, methanol, and acetonitrile at various temperatures. The equilibrium (10)⇌(11) has been studied in deuteriochloroform by dynamic ¹H n.m.r. spectroscopy. The kinetic and thermodynamic data obtained agree with an electrocyclic mechanism for the conversion characterized by a late transition state, as indicated by the effect on the reactivity of the nature of the starting aromatic ring.