Flash Vacuum Pyrolysis of Isoxazoles, Pyrazoles and Related Compounds

Flash vacuum pyrolysis is an interesting methodology for studying reaction mechanisms as well as synthesis. Its characteristics of submitting the molecule to high temperature (350 - 900ºC) in a vacuum system (∼ 10-2 Torr) for a short time (∼ 10-2s) makes possible to isolate kinetically controlled products and, in some cases, to avoid consecutive reactions. Studies of reaction mechanisms may include kinetic and non-kinetic methods, and of course, the knowledge of reaction mechanisms and the influence of substituents in it, makes possible the synthetic application of these reactions. In this review reactions of different nitrogen heterocycles will be divided by type of reaction, such as isomerizations, radical reactions, nitrogen extrusion, eliminations, and reactions of the side chain. Special attention will be dedicated to kinetic measurements because it is not a common way to study flash vacuum pyrolysis reactions but they give important information about reaction mechanisms. An improvement of flash vacuum pyrolysis reactions with the use of solid catalysts will also be discussed. These catalysts have two effects, in some cases they lower the reaction temperature and in some others there is a change of the reaction with formation of different products. These results extend the possibilities of flash vacuum pyrolysis of a single substrate.