Microwave oven technique

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... 

The common microwave oven has been brought into the laboratory. Using special Teflon reaction vessels, components are mixed together, the vessel sealed and put into the microwave oven. Reaction times are greatly accelerated in many reactions, and reactions that took hours to be complete in refluxing solvents are done in minutes. Benzyl alcohol was converted to benzyl bromide, for example, using microwaves (650 W) in only 9 min on a doped Montmorillonite K-10 clay. This is a growing and very useful technique. 

Many organic reactions can be conducted very rapidly under microwave irradiation. Microwave-induced organic reaction enhancement chemistry techniques were used for the rapid formation of an ot-benzyloxy-p-lactam (10 in Fig. 4.2) and the hydrogenolysis of its benzyloxy group on a few-gram scale in 1-5 minutes with HC02NH4 and Pd/C in ethylene glycol as the reaction medium in a domestic microwave oven.243... 

Long DJ, Buggs C. Microwave oven-based technique for immunofluorescent staining of paraffin-embedded tissues. I. Mol. Histol. 2008 39 1-4. 

The objective of this section is to provide the reader with a basic knowledge of micro-wave techniques used in the design and the construction of microwave ovens. 

Thioacetals have been prepared using essentially a similar technique [33]. The active methylene compounds are adsorbed on KF-alumina, admixed with methanesul-fonothioate and are irradiated in microwave oven to produce thioacetals in good yields (Scheme 6.3). 

The same authors also studied recently the preparation of substituted vinyl -lactams 14, with efficient stereocontrol [11], by use of limited amounts of solvent (chlorobenzene) (Scheme 8.7). Microwave oven-induced reaction enhancement (MORE) chemistry techniques have been used to reduce pollution at the source and to increase atom economy. 

These examples of the use of microwave irradiation in the synthesis of heterocyclic compounds show the great versatility of this technique, which can be used under a variety of experimental conditions. Even when there is no improvement of yields, or rates, or specificity, the technique is worthwhile owing to its simplicity. It can be foreseen that microwave ovens are going to be among the basic equipment of research laboratories in the near future. Combinatorial and parallel synthesis under the action of microwave irradiation is becoming a powerful tool for discovery of new molecules and should develop very rapidly. 

Most average homes contain a microwave oven. Researchers have taken that same technology and applied it to sample extraction. Using microwave technology with acidic solutions has become a commonplace replacement for traditional acid digests. The sample is placed in a closed, chemical-resistant vessel and heated in a microwave. This process is much faster than hot-plate techniques and has become widely accepted by such agencies as the US EPA. 

Shi SR, Key ME, Kalra KL (1991) Antigen retrieval in formalin fixed, paraffin embedded tissues an enhancement method for immunohistochemical staining based on microwave oven heating of tissue sections. J Histochem Cytochem 39 741 748 Shi SR, Cote RJ, Taylor CR (2001) Antigen retrieval techniques current perspectives. J Histo chem Cytochem 49 931 938... 

More recently, microwave ovens have been used for sample dissolution. The sample is sealed in a Teflon bottle or a specially designed microwave digestion vessel with a mixture of suitable acids. The high-frequency microwave, temperature (ca. 100-250°C) and increased pressure have a role to play in the success of this technique. An added advantage is the significant reduction in sample dissolution time [25, 26],... 

Fast, solid state, microwave-assisted, synthesis. A short history of reactions carried out in the solid state under the influence of microwaves was reported by Kniep (1993) who presented this technique as characteristic of a fast solid-state chemistry and described the preparation of CuInS2 by using a method first developed by Whittaker and Mingos (1992) and extended by Landry and Barron(1993). Copper, indium and sulphur (all powders between 100 and 300 mesh) in a molar ratio 1 1 2 were thoroughly mixed and loaded into a silica ampoule. This, sealed under vacuum, was placed in a domestic microwave oven and irradiated at 400 W and 2450 MHz. 

Tacha, D. E. and Chen, T. (1994) Modified antigen retrieval procedure calibration technique for microwave ovens. J. Histotechnol. 17, 365. 

A rapid preseparation technique was developed for the extraction of SAL from various chicken tissues using the irradiation of the sample in EtOH-2-PrOH for 9 s in a common household microwave oven. The extract was analyzed without further cleanup and detected via postcolumn reaction with DMABA at 86°C. Recoveries ranged between 87% and 100% (105). 

From these publications, workers interested in exploring the microwave technique perceived it to be simultaneously beneficial through increased rates, yet hazardous in the presence of flammable organic solvents. Subsequently, a vast body of work was carried out with domestic microwave ovens, but under solvent-free conditions and without recourse to sample mixing or temperature measurement. This continued across a broadening front on the laboratory scale. These and other developments in microwave chemistry have been reviewed extensively in journals, book chapters4-20 and in a recent monograph21. 

A mixture of phenyl acetaldehyde 1 (0.6 g, 5 mmol) and morpholine 2a (0.48 g, 5.5 mmol) was placed in a small beaker and irradiated in an unmodified household microwave oven at its full power (900 W) for 2 min. Salicylaldehyde 4a (0.61 g, 5 mmol) and ammonium acetate (0.02 g, 0.25 mmol) were then added to the same reaction vessel, and the reaction mixture was further irradiated in the microwave oven at its 50% power for 5 min using a pulse technique. Upon completion of the reaction, followed by TLC, the reaction mixture was passed through a bed of basic alumina using hexane-ether (9 1, v/v) as an eluent to afford pure 2-aminomorpholinoisoflav-3-enes 6a (yield 80%, mp 103-105 °C). 

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