Dielectric heating, microwave

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

In recent years, parallel to the emergence of SPOS, microwave-mediated organic synthesis has come to hght and has developed into a popular field [24-31]. The main advantage of microwave dielectric heating compared to other conventional methods, such as hot plate, oil bath or isomantle, is the tremendous rate enhancement generally observed under microwave irradiation conditions. Various theories have been proposed to explain the source of the rapidity of microwave chemistry [32,33]. However, the gener-... 

Investigation of the microwave-assisted attachment of Fmoc-protected amino acids onto 2-chlorotrityl chloride resin indicated higher loadings and increased rates compared to standard room temperature procedures [146]. In this comparative study standard procedures yielded 0.37 mmol/g loading after 1 hour, whereas at 110 °C using microwave dielectric heating, a similar result (0.38 mmol/g) was obtained after only 15 min (Fig. 7). 

Microwave (dielectric) heating in solution may distinguish three situations, which determine heating characteristics ... 

In addition to the above mentioned thermal/kinetic effects, microwave effects that are caused by the unique nature of the microwave dielectric heating mechanisms (see Section 2.2) must also be considered. These effects should be termed specific... 

D. M. P. Mingos, A. G. Whittaker, Microwave Dielectric Heating Effects in Chemical Synthesis in Chemistry under Extreme or non-Classical Conditions,... 

Zhang X, Hayward DO (2006) Applications of microwave dielectric heating in environment-related heterogeneous gas-phase catalytic systems. Inorg Chim Acta 359 3421... 

Increasing the interface by rapid stirring, use of ultrasound, microwave dielectric heating, etc. 

Mingos, D.M.P. and Baghurst, D.R. (1991) Applications of microwave dielectric heating eflFects to synthetic problems in chemistry. Chem. Soc. Rev., 20, 1-47. 

Cresswell, S.L., Parsonage, J.R., Riby, P.G., and Thomas, M.J.K. (1995) Rapid synthesis of magnesium aluminophos-phate-5 by microwave dielectric heating. J. Chem. Soc. Dalton Trans., 2315-2316. 

Wessjohann s group reported a one-pot synthesis of tryptophane DKPs [37]. This combination of two biologically highly active moieties was achieved in a one-pot variation of the UDC protocol (Scheme 4). It was shown that, assisted by microwave irradiation, the reaction times can be reduced significantly and that the formation of DKPs occurs even without the necessity to activate the ester. By employing microwave (dielectric) heating, the reaction times were reduced substantially. 

Bazureau s group reported extensive work with TSILs for synthesis of various 4-thiazolidinones, 1,4-dihydropyridines, 3,4-dihydropyrimidin-2(lH)-ones and polyhydroquinones. They functionalized imidazole compounds with esters, which were then reacted with reagents to afford the target compound. They report that microwave dielectric heating is a useful aid to rapidly produce the desired product in high yields. 

In summary, it is important to emphasise that microwave dielectric heating is not a quantum mechanical phenomenon localised at one molecular centre, but is a collective property that occurs in a semi-classical maimer and involves aggregates of molecules. Energy transfer is rapid between these molecules and this limits the extent of localisation of the heating. 

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