Organic Synthesis using Microwaves in Homogeneous Media

Organic Synthesis using Microwaves in Homogeneous Media 

Although the ability of microwaves (MW) to heat water and other polar materials has been known for half a century or more, it was not until 1986 that two groups of researchers independently reported the application of MW heating to organic synthesis. Gedye et al. [1] found that several organic reactions in polar solvents could be performed rapidly and conveniently in closed Teflon vessels in a domestic MW oven. These reactions included the hydrolysis of amides and esters to carboxylic acids, esterification of carboxylic acids with alcohols, oxidation of alkyl benzenes to aromatic carboxylic acids and the conversion of alkyl halides to ethers. 

Most of these reactions occurred much faster using MW irradiation (at increased pressure and temperature) than under conventional heating (ambient pressure, reflux), with rate enhancements ranging from 5 to 1200 times. 

Shortly after these results were published, Giguere and coworkers reported dramatic reductions in reaction times in other MW-assisted syntheses, including Diels-Alder, Claisen rearrangements and ene reactions [2]. These reactions were also performed at elevated pressures, but sealed glass vessels (inside a bath packed with ver-miculite) were used rather than Teflon. 

It is to be noted that the reactions mentioned so far were performed under homogeneous conditions and, in most cases, using polar solvents, which are efficient absorbers of MW energy. Rate enhancements were attributed to the superheating of the solvent due to the elevated pressures generated in the closed vessels. 

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I 4 Organic Synthesis using Microwaves in Homogeneous Media... 

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