Microwave irradiation technique

Sharifi, A., Mojtahedi, M.M. and Saidi, M.R., Microwave irradiation techniques for the Cannizzaro reaction, Tetrahedron Lett., 1999,40, 1179-1180. 

Wu and Sun [29] applied the single-mode microwave irradiation technique for the synthesis of specifically functionalized bis-benzimidazole (xxii) for potential DNA minor groove recognition study. 

The method also allows the successful synthesis of 11,3 azaphospholo 5,1 -a isoquinolines (57) (Scheme 19) [66], The latter could be prepared in a much shorter time using a microwave irradiation technique [67], The method, however, failed for the anellated [l,3]azaphospholo[l,5-a]quinolines. 

Tiwari and Prabaharan (2010) develop amphiphilic guar gum grafted with poly (epsilon-caprolactone) using microwave irradiation technique. Under the influence of microwave, guar gum with high poly (epsilon-caprolactone) grafting... 

Tetraphettylporphyrin was synthesized in propionic acid from pyrrol and benzalde-hyde by microwave irradiation technique [5] with yield 39%. 

Microwave irradiation technique to enhance protein fibre properties. Autex... 

Hollow CU2O submicron spheres have been successfully prepared in an O/ W/O multiple emulsion, employing the microwave irradiation technique (Liu et al., 2004). Heptane and lauryl alcohol polyoxyethylene were used as the oil... 

The Wolf rearrangement of N -Boc-/Z-protect aminodiazoketones in the presence of silver benzoate under microwave irradiation has been described. The reaction is found to be rapid, efficient and complete. Verma (2002) observed that the microwave irradiation technique has been used to decrease the time neeessary to carry out cleaner reaction with higher selectivity and easier work up. A sample, rapid, and efficient route for the preparation of several amino acid benzyl ester p-toluenesul-fonate and hydrochloride salts under microwave irradiation was also demonstrated by Patil et al. (2002). 

The reaction required 12 h in presence of Lewis acid under conventional heating, but it is completed within 0.5 to 2.0 min by solvent-free microwave irradiation technique (Banks, 1999). 

Calcium oxide (CaO) is an important inorganic compoimd, which can be used across various industries as catalyst, toxic-waste remediation agent, adsorbent, etc. CaO nanoparticles were obtained by the microwave irradiation technique (Roy and Bhattachaiya, 2011). It is a simple and efficient method to produce CaO nanopar-... 

There is sfill some dispufe about how microwave irradiation accelerates reactions. Besides the generally accepted thermal effects, one beheves that there are some specific (but also thermal) microwave effects, such as the formation of hot spots . There is still some controversy about the existence of non-thermal (athermal) microwave effects. At the present time, new techniques such as coohng while heating are being investigated and the problem of upscahng... 

In the 1990s the technique of solid-phase organic synthesis (SPOS) became generally popular, but especially in the medicinal chemistry community, for lead detection and lead optimization via combinatorial techniques. The combination with microwave irradiation brought an elegant solution for the problem of the notoriously slower reactions compared to those in solution phase. 

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 use of microwaves to heat organic reactions has attracted considerable interest in the last 15 years. This technique allows to reduce the time of chemical transformations and, consequently, the formation of by-products is reduced, often with improved yields and purity of the products. Practically every kind of transformation has been tested imder microwave irradiation, in many instances giving better results than conventional heating [ 1 ]. 

It was also shown that the commercially available Wang resin is well suited for the reaction, but a tailor-made resin (based on syringealdehyde and Mer-rifield resin) offers milder cleavage conditions. By comparing conventional heating versus MAOS it was found that similar yields for the cycloaddition step were obtained, but the cleavage from the resin can be performed under much milder conditions when microwave irradiation is used, and in some cases the procedure only works using the microwave technique (Table 1). 

This technique has also been applied in the synthesis of carbostyril analogues 15 and as in the previous example also this reaction is favored by an electron-rich group in the aniline-ring and an electron-poor group attached to the electrophihc specie (b. Scheme 6) [55]. The use of microwave irradiation can reduce the reaction times from 18-58 h to 80 min and the products are generally isolated in high yields and purities. 

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... 

Most examples of microwave-assisted chemistry published to date and presented in this book (see Chapters 6 and 7) were performed on a scale of less than 1 g (typically 1-5 mL reaction volume). This is in part a consequence of the recent availability of single-mode microwave reactors that allow the safe processing of small reaction volumes under sealed-vessel conditions by microwave irradiation (see Chapter 3). While these instruments have been very successful for small-scale organic synthesis, it is clear that for microwave-assisted synthesis to become a fully accepted technology in the future there is a need to develop larger scale MAOS techniques that can ultimately routinely provide products on a multi kg (or even higher) scale. 

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... 

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