Gas‐Phase Thermolysis of Benzotriazole Derivatives. Part 2. Synthesis of Benzimidazo[1,2‐b]cinnolines, a Novel Heterocyclic Ring System, by Pyrolysis of Benzotriazole Derivatives. Kinetic and Mechanistic Study.

Abstract

Abstract Gas-phase pyrolysis of benzotriazolyl ketones and their arylhydrazones gave indole, benzimidazole and benzimidazo[1,2-b]cinnoline derivatives via interesting novel routes. The homogeneous first-order gas-phase pyrolysis of 10 arylhydrazono derivatives of α-benzotriazol-1-yl ketones was investigated over the temperature range 420–530 K. Five of these substrates were 2-(arylhydrazono)-2-(benzotriazol-1-yl)-1-phenylethanone derivatives (Series 1), and the remaining five were the corresponding acetone analogues (Series 2). The values of the Arrhenius frequency factor (log A/s−1) for the pyrolysis of the compounds of Series 1 and 2 were, respectively, 12.27±1.44 and 9.07±1.20, while the values of the Arrhenius activation energy (Ea/kJ mol−1) were 132.8±8.4 and 123.2±23.0, respectively. Besides, reaction pathways are offered to rationalize the kinetic results and to account for the products of pyrolysis of the substrates under study, namely: (i) extrusion of N2 and formation of a 1,3-biradical reactive intermediate leading to substituted imidazoles (Series 1); (ii) intramolecular nucleophilic addition, cyclization and subsequent fragmentation with or without loss of H2O/N2 fragments (Series 1 and 2).