Organic Synthesis Using Microwaves in Homogeneous Media

Abstract

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 vermiculite) 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.During the 15 years since these reports, a large number of synthetic applications of MW in organic. synthesis have been reported and these have been summarized in a number of reviews [3±10].The use of dry media (solvent-free) conditions, in which the reactants are absorbed on inert solid supports, in MW-heated reactions, has received a considerable amount of attention recently and has been used in the synthesis of a wide range of compounds [11±16]. These reactions generally occur rapidly and the method avoids hazards, such as explosions, associated with reactions in solvents in sealed vessels in which high pressures may be generated. Also the removal of 115 Microwaves in Organic Synthesis. Edited by Andr Loupy

Photocatalytic Semiconductors

“…Microwave (MW) radiation has been widely explored as a means of activating energy to promote several organic reactions (Geyde 2002). In view of the advantages offered by this technique in the field of organic chemistry, such as reaction rate enhancement, increased yields, cleaner conditions (i.e., solvent-free reactions), and easier workup, the use of this approach has been extended to the production of several inorganic materials (Kitchen et al 2014).…”

Water-borne Polyimides via Microwave-assisted Polymerization

“…Microwave irradiation appeared in the last decade as a very efficient way to speed up a wide range of organic reactions [20,21], which can be performed either in organic solvents [22], in solid state [23][24][25], or even in aqueous media [13,[26][27][28][29][30][31]. This technique is also considered as a very promising tool for the synthesis of polymers [32][33][34][35][36][37], because the reaction times are drastically reduced, and the selectivity of the reactions is high.…”