Selectivity Under the Action of Microwave Irradiation

Antonio de la Hoz, Angel DIaz-Ortiz, and Andres Moreno 5.1 

Microwaves in Organic Synthesis, Second edition. Edited by A. Loupy Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31452-0 

Instrumentation can also have a significant effect on heating pattern and power densities and, consequently, on the absorption of the energy. A comprehensive survey of microwave instrumentation can be found elsewhere [1, 2 see also Chapters 1 and 2]. 

Microwave irradiation has been successfully applied in chemistry since 1975 and many examples in organic synthesis have been described [3, 4], Several reviews have been published on the application of this technique to solvent-free reactions [5], cycloaddition reactions [6], synthesis of radioisotopes [7], fullerene chemistry [8] and advanced materials [9], polymers [10], heterocyclic chemistry [11], carbohydrates [12], homogeneous [13] and heterogeneous catalysis [14], medicinal and combinatorial chemistry [15], and green chemistry [16]. All these applications are described elsewhere in this book. 

Control of selectivity (chemo, regio, stereo, and enantioselectivity) is among the most important objective in organic synthesis. The efficient use of reaction conditions (temperature, time, solvent, etc.), kinetic or thermodynamic control, protecting or activating groups (for example chiral auxiliaries), and catalysts (including chiral catalysts) have all been used to obtain the desired isomer. 

---

Dealkylation of 2-ethoxyanisole has been described by Loupy [48]. Deethylation was the dominant reaction with potassium tert-butoxide without any change in selectivity under the action of microwave irradiation. Demethylation became the dominant reaction when ethylene glycol was added, however under these conditions no reaction was observed when conventional heating was used. 

Several authors also postulate the occurrence of a non-thermal effect that originates from the polarizing electromagnetic field. In this vay the most polar transition state, i.e. the harder transition state, vill be favored under the action of micro-wave irradiation (Chapter 4). These results have been supported by computational calculations. The experimental and theoretical results described here now need to be supported with further examples. If these results are confirmed, however, such systems could be used as a predictive tool to show which reactions can be improved or have modified selectivity under the action of microwave irradiation and even to predict the result of the reaction. 

---