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

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 alpha-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 I and 2 were, respectively, 12.27 +/- 1.44 and 9.07 +/- 1.20, while the values of the Arrhenius activation energy (E-a/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 N-2 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/N-2 fragments (Series I and 2). (C) 2003 Elsevier Ltd. All rights reserved.