Nonthermal Effects of Microwaves in Organic Synthesis

Perreux, Laurence; Loupy, André

doi:10.1002/3527601775.ch3

Cited by 24 publications

(18 citation statements)

References 57 publications

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“…These results are in agreement with the qualitative theory proposed by Perreux and Loupy [32,33]: ''If the polarity of a system is enhanced from the GS to the TS, it can result in acceleration due to an increase in material-wave interactions during the course of the reaction.'' Kappe and co-workers [23] re-evaluated this reaction, introducing a multiple fiber-optic probe system as an accurate temperature measurement device in both the MW and the conventionally heated reactors and assuring efficient stirring/agitation of the mixture.…”

Section: Computational Calculationssupporting

confidence: 80%

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Elucidation of Microwave Effects: Methods, Theories, and Predictive Models

Hoz

Díaz‐Ortiz

Gómez

et al. 2012

Microwaves in Organic Synthesis

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IntroductionThe impact of microwave (MW) irradiation in organic synthesis has increased over the years and especially since the development of new dedicated and reliable MW instruments [1].MW-assisted organic synthesis has been characterized by the spectacular accelerations produced in many reactions as a consequence of a heating rate which, in most cases, cannot be reproduced by classical heating. This effect is particularly important in (i) the preparation of isotopically labeled drugs for positron emission tomography (PET) that have a short half-life ( 11 C, t1 / 2 = 20 min; 122 I, t1 / 2 = 3.6 min; 18 F, t1 / 2 = 100 min) [2], (ii) high-throughput chemistry (combinatorial and parallel chemistry) where a rapid synthesis may increase the efficiency [3], (iii) catalysis where the short reaction time protects the catalyst from decomposition and increases the catalyst efficiency [4], and (iv) the synthesis of unstable or sensitive compounds, for example, natural products, where the short reaction time prevent the decomposition that occurs with long reaction times [5].The occurrence of results that cannot be explained exclusively by rapid heating led several authors to postulate the existence of a so-called ''microwave effect.'' Hence acceleration or modifications of the selectivity and reactivity could be explained by an effect of the electromagnetic radiation and not merely by the heating effect. In this way, in combination with the rapid heating, specific thermal effects and non-thermal effect could be the responsible for the improvement of many chemical processes.The occurrence of MW effects has been the subject of much controversy. In this chapter, we describe the state-of-the-art of this subject and we present many examples in favor of and against the presence of MW effects with a critical appraisal.Microwaves in Organic Synthesis, Third Edition.

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Section: Computational Calculationssupporting

confidence: 80%

“…The nature of the MW effect was carefully studied, analyzed, and classified considering the reaction medium and the reaction mechanism [33]. The main conclusions of this seminal paper are collected and extended in Chapter 4 of this edition.…”

Section: Non-thermal Effectsmentioning

confidence: 99%

Elucidation of Microwave Effects: Methods, Theories, and Predictive Models

Hoz

Díaz‐Ortiz

Gómez

et al. 2012

Microwaves in Organic Synthesis

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“…Loupy et al [55] observed an increase in yield and purity of the diazepine 36, in the reaction of ethyl acetoacetate with o-phenylenediamine using monomode MW reactor with focused MW heating, when compared with conventional heating with the same temperature profile.…”

Section: Reactions In Nonpolar Solventsmentioning

confidence: 99%

Organic Synthesis Using Microwaves in Homogeneous Media

Gedye

2002

Microwaves in Organic Synthesis

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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

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“…Amorphous carbon and graphite, in their powdered form, if irradiated at 2.45 GHz, rapidly (within 1 min) reach very high temperatures ([1,300 K) and this property has been used to explain MW-assisted syntheses of inorganic solids. In these syntheses carbon is a secondary suscepter which assists the initial heating but does not react with other reactants [10].…”

Section: Introductionmentioning

confidence: 99%