Microwave accelerate chemical reactions

Microwave irradiation has been proven useful in accelerating chemical reactions. A unique approach to multicomponent reactions - the combination of microwave irradiation and microreactors - was developed by Organ and Bremner [25]. The three-component coupling reaction of amino pyrazole with an aldehyde and diketone in a glass capillary tube microflow system (1180 pm i.d.) under microwave irradiation (170 W) proceeded smoothly to give the desired quinolinone in high yield (Scheme 4.16). Without microwave irradiation, the reaction efficiency was very low. 

The first reliable device for generating fixed-frequency MW radiation was designed by Randall and Booth at the University of Birmingham, UK, during World War II. MW radiation is a part of the broader electromagnetic spectrum and it was well known that infrared and visible light accelerated chemical reactions, and therefore when it was first observed that microwave radiation was able to heat foodstuffs it did not come as a complete surprise. The first patent for MW dielectric heating was filed by the Raytheon Company in 1946 and commercial MW ovens became available in 1947. 

The higher efficiency of microwave ovens to permit the warming of the samples allows drying, favors the digestion with acid, and accelerates chemical reactions that are controlled by a slow thermal stage. 

Since 1986, when the very first reports on the use of microwave heating to chemical transformations appeared [147,148], microwave-assisted synthesis has been shown to accelerate most solution-phase chemical reactions [24-27,32,35]. The first application of microwave irradiation for the acceleration of reaction rate of a substrate attached to a solid support (SPPS) was performed in 1992 [36]. Despite the promising results, microwave-assisted soHd-phase synthesis was not pursued following its initial appearance, most probably as a result of the lack of suitable instriunentation. Reproducing reaction conditions was nearly impossible because of the differences between domestic microwave ovens and the difficulties associated with temperature measurement. The technique became a Sleeping Beauty interest awoke almost a decade later with the publication of several microwave-assisted SPOS protocols [37,38,73,139,144]. There has been an extensive... 

One of the few theoretical papers trying to explain acceleration under the action of microwaves has recently been published by A. Miklavc [18]. He stated that large increases in the rates of chemical reactions occur because of the effects of rotational excitation on collision geometry. This could be cautiously considered when one has knowledge of the quasi-nil energy involved by microwave interaction according to Planck s law [E = hc/X = 0.3 cal/mol]. 

In chemical syntheses under the action of microwave irradiation the most successful applications are necessarily found to be the use of solvent-free systems [6], In these systems, microwaves interact directly with the reagents and can, therefore, drive chemical reactions more efficiently. The possible acceleration of such reactions might be optimum, because they are not moderated or impeded by solvents. Reactions on solid mineral supports and, in turn, the interaction of microwaves with the reagents on the solid phase boundary, which can substantially increase the rate of the reactions, are of particular interest [7]. 

A related development that had profound impact on heterogeneous reactions is the use of microwave (MW) irradiation techniques for the acceleration of organic reactions. Since the appearance of initial reports on the application of microwaves for chemical synthesis in polar solvents [11], the approach has blossomed into a useful... 

In many cases, the comparison of a reaction accelerated by microwave irradiation has been made with the same reaction in an oil bath at the same bulk temperature. Unfortunately, there have been quite a few reports in the chemical literature that have not been conducted with such proper control of conditions and consequently a fair comparison is not possible. Nevertheless, using this MW approach, the problems associated with waste disposal of solvents that are used several fold in chemical reactions, and excess usage of chemicals are avoided or minimized. The discussion pertaining to the preparation of supported reagents or catalysts has not been included in this chapter because numerous review articles are available on this theme [14—22],... 

Microwave technology—chemical synthesis applications, 16 538-594 microwave-accelerated solvent-free organic reactions, 16 555-584 microwave-assisted organic reactions in the liquid phase, 16 540-555 Microwave technology, 16 509-537. See also Microwave power Microwave technology— chemical synthesis applications... 

It should be stressed that the majority of literature reports on the acceleration of chemical reactions by microwave irradiation (even more than 1000 fold) come from the initial period of the application of microwaves in organic synthesis (i.e. from late 1980 s and early 1980 s). At this time there were no dedicated microwave scientific reactors available on the market, and most of theses reactions were conducted in household microwave ovens. Recently, applying modern microwave reactors, scientists have verified a number of these reports, and it turns out that the claimed acceleration of the majority chemical reactions were attributed to difficulties with proper temperature measuments rather than to non-thermal micowave effects. Sometimes it was found that these effects were results of faster delivering of energy to the reaction systems [35,38]. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

Microwave (MW) irradiation as a nonconventional reaction condition [703] has been applied in various areas of chemistry and technology to produce or destroy diverse materials and chemical compounds, as well as to accelerate chemical processes. The advantages of its use are the following [704] ... 

The substances or materials have different capacity to be heated by microwave irradiation, which depends on the substance nature and its temperature. Generally, chemical reactions are accelerated in microwave fields [704], as well as those by ultrasonic (US) treatment, although the nature of these two techniques is completely distinct. 

Wisen S, Androsavich J, Evans CG et al (2008) Chemical modulators of heat shock protein 70 (Hsp70) by sequential, microwave-accelerated reactions on solid phase. Bioorg Med Chem Lett 18 60-65... 

In addition to their utility in increasing chemiluminescence intensity, silver nanoparticles, in combination with low power microwaves, have also been shown to kinetically accelerate the chemical reactions that produce chemiluminescence[14]. 

As pointed out already in [62], the mechanism in question could also have a crucial role in reactions in condensed phases where it should arise due to large amplitude oscillatoiy motions, like internal rotations or librations, of molecules or reactive atomic groups. Such oscillatoiy motion could be excited, e.g., by microwaves and the mechanism in question should thus be of great importance also for understanding microwave acceleration of chemical reactions in condensed phases. 

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