Microwave Assisted Reactions in Organic Solvents

This section includes those microwave induced reactions in which one or both the reactants (if liquid) act as a solvent and also those reactions in which organic solvent is used to assist the reaction. 

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These early results led us to investigate the feasibility of carrying out Suzuki coupling reactions on alumina in the absence of solvents. We investigated both thermal and microwave-assisted reactions, eq 4, Microwave irradiation of organic reactions has gained in popularity in recent years since it was found to accelerate a wide variety of transformations (26,27)... 

One of the benefits of microwave-assisted synthesis is the remarkable acceleration of reactions conducted in unconventional reaction media. The use of water is an economical and nonflammable option to traditional organic solvents. It is important as the high dielectric constant of the water allows efficient absorption of microwave irradiation. Tu et al. (2009) developed a microwave-assisted reaction under aqueous conditions for an efficient synthesis of benzo[e][l,4]thiazepin-2-ones via a multi-component reaction between benzaldehyde, an aromatic amine and mercaptoacetic acid at 110 °C in 7-10 min with 94% yield using water as the solvent. 

This timescale of these microwave-assisted reactions is in minutes and it enables a facile and rapid scoping of reaction conditions, for example, time, temperature, reagents and solvents. This rapid optimization can be used to rapidly identify routes for the synthesis of novel chemical entities. Microwave-assisted organic synthesis is no longer a curiosity now, it is a rapidly growing technology, but it has not been used with full potential. 

Triazoles have been obtained via microwave-assisted [3-i-2] cycloaddition, under solvent-free conditions [54], starting from organic azides and acetylenic amides at 55 °C for 30 min (Scheme 23). The complete conversion of the reagents into AT-substituted-1,2,3-triazoles 69 was achieved without decomposition and side products. A control reaction carried out at the same temperature in an oil bath did not give the cycHc products, not even after 24 h of reaction time. 

Alternatively, microwave-assisted synthesis has been carried out using standard organic solvents under open-vessel conditions. If solvents are heated by microwave irradiation at atmospheric pressure in an open vessel, the boiling point of the solvent typically limits the reaction temperature that can be achieved. In order to none-... 

In modern microwave synthesis, a variety of different processing techniques can be utilized, aided by the availability of diverse types of dedicated microwave reactors. While in the past much interest was focused on, for example, solvent-free reactions under open-vessel conditions [1], it appears that nowadays most of the published examples in the area of controlled microwave-assisted organic synthesis (MAOS) involve the use of organic solvents under sealed-vessel conditions [2] (see Chapters 6 and 7). Despite this fact, a brief summary of alternative processing techniques is presented in the following sections. 

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