https://doi.org/10.15255/KUI.2005.001
Published: Kem. Ind. 54 (11) (2005) 469–476
Paper reference number: KUI-01/2005
Paper type: Review
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Microwave Assisted Organic Synthesis

I. Zrinski and M. Eckert Maksić

Abstract

Many chemists have been attracted by high-speed synthesis with microwaves in recent years. More than 2000 articles have been published in the area of microwave assisted organic synthesis (MAOS) since the first reports on the use of microwave heating to accelerate organic chemical reactions by Gedye at el in 1986.2 One of the advantages of MW over classical heating is reduction of reaction times from hours to minutes, but it is also known to reduce side reactions, increase yields, and improve reproducibility. Microwave-enhanced chemistry is based on the efficient heating of materials by “microwave dielectric heating” effects. This phenomenon is dependent on ability of a specific material (solvent or reagent) to absorb microwave energy and convert it into heat. Traditionally, organic reactions have been heated using an external heat source (oil baths, sand baths and heating jackets). These heating techniques are, however, rather slow and a temperature gradient can develop within the sample. In additional, local overheating can lead to product, substrate and reagent decomposition. In contrast, in microwave dielectric heating, the microwave energy is introduced into the chemical reactor remotely and direct access by the energy source to the reaction vessel is obtained. The microwave radiation passes through the walls of the vessel (borosilicate glass, quartz or teflon) and heats only the reactants and solvent, not the reaction vessel itself. If the apparaturs is properly designed, the temperature increase will be uniform throughout the sample, which can lead to less by-products and/or decomposition products. In pressurized systems, it is possible to rapidly increase the temperature far above the conventional boiling point of the solvent used. Since the introduction of microwave assisted organic synthesis in 1986, the main debate has dealt with the question of what actually alters the outcome of the synthesis. Is it an effect of the thermal heat generated by the microwaves or is it an effect specific for microwave heating? Today, it is thought that in the most cases the observed rate enhancements is purely thermal/kinetic effect due to the high instantaneous heating of the substance(s) above the normal bulk temperature when irradiating polar materials in a microwave field. The early stage experiments in MAOS were carried out in domestic, sometimes modified, kitchen microwave ovens, while the current trend is to use dedicated instruments which have only become available in the last few years for chemical synthesis. MAOS can be performed in standard organic solvent either under open- or sealed vessel conditions or as solventless (“dry-media”) procedures where the reagents are preadsorbed onto either a more or less microwave transparent (silica, aluminia, or clay) or strongly absorbing (graphite) inorganic support, which can additionally be doped with a catalyst or reagent. The solvent-free approach was very popular particularly in the early days of MAOS since it allowed the safe use of domestic household microwave ovens and standard open-vessel technology. Many academic and industrial research groups are already using MAOS as a forefront technology for rapid optimization of reactions, for the efficient synthesis of new chemical entities, and for discovering and probing new chemical reactivity.

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Keywords

microwave theory, microwave effects, microwave assisted organic synthesis, synthetic methods