Ultrasound and microwave assisted synthesis of isoindolo-1,2-diazine: A comparative study

A comparative study, ultrasound (US) versus microwave (MW) versus conventional thermal heating (TH), for synthesis of isoindolo-1,2-diazine is described. The reaction pathway is fast, efficient and straight applicable, involving a Huisgen [3 + 2] dipolar cycloaddition of cycloimmonium ylides to 1,4-naphthoquinone. A feasible reaction mechanism for the obtaining of the fully aromatized tetra- and penta- cyclic isoindolo-1,2-diazine is presented. Under US irradiation the yields are much higher (sometimes substantially, by almost double), the reaction time decreases substantially, the reaction conditions are milder. The use of a generator with a higher nominal power induces higher yields and short reaction times. Overall the use of US it proved to be more efficient than MW or TH. A feasible explication for US efficiency is presented. (C) 2012 Elsevier B.V. All rights reserved.