Microwave dielectric effect

The MW-enhanced chemistry is based on the efficiency of the interaction of molecules with electromagnetic waves generated by a microwave dielectric effect . This process mainly depends on the abihty of a specific... 

Another reason for interest in microwaves in chemical technology involves the fields of dielectric spectrometry, electron spin resonance (esr), or nuclear magnetic resonance (nmr) (see Magnetic spin resonance). AppHcations in chemical technology relating to microwave quantum effects are of a diagnostic nature and are not reviewed herein. 

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... 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

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

Figure 1.4 Differences in the temperature-time profiles for conventional and microwave dielectric heating. Particularly noteworthy are the far higher heating and cooling rates and the greater reaction temperatures achieved. Since at these higher temperatures the rate of reaction is much larger, it is not necessary to hold the reaction at this temperature for an extended period of time. Since the rate of reaction depends exponentially on temperature the translation of these profiles into product yields as a function of time will magnify these effects.

Baghurst, D.J., Mingos, D.M.P. and Watson, M.J., Application of microwave dielectric loss heating effects for the rapid and convenient synthesis of orga no metallic compounds, /. Organomet. Chem., 1989, 368, C43. 

Mingos, D.M.P. and Baghurst, D.R., Applications of microwave dielectric heating effects to synthetic problems in chemistry, Chem. Soc. Rev., 1991, 20, 1, and references therein. 

Baghurst, D.R. and Mingos, D.M.P., Super heating effects associated with microwave dielectric heating, /. Chem. Soc., Chem. Commun., 1992, 674. 

Zhang, X., Hayward, D.O. and Mingos, D.M.P., Effects of microwave dielectric heating on heterogeneous catalysis, Catal. Lett., 2003, 88, 33. 

Sengwa, R.J., Solvent effects on microwave dielectric relaxation in poly(ethylene glycols), Polym. Int., 1998, 45, 43. 

The combination of microwave-assisted chemistry and solid-phase synthesis applications is a logical consequence of the increased speed and effectiveness offered by microwave dielectric heating. While this technology is heavily used in the pharmaceutical and agrochemical research laboratories already, a further increase in the use of microwave-assisted solid-phase synthesis both in industry and in academic laboratories can be expected. This will depend also on the availability of modern microwave instrumentation specifically designed for solid-phase chemistry, involving for example dedicated vessels for bottom filtration techniques. 

MAOS is mainly based on the efficient heating of materials by the microwave dielectric heating effect [15] mediated by dipolar polarization and ionic conduction. When irradiated at microwave frequencies, the dipoles (e.g., the polar solvent... 

Steroidal, alicyclic or aromatic annulated pyridines were prepared via a microwave-assisted, base-catalyzed Henry reaction of /1-formyl enamides and nitromethane on an alumina support [97]. Highly substituted tri- and tetrasubstituted pyridines were synthesized in a Bohlmann-Rahtz reaction from ethyl /3-amino crotonate and various alkynones. The reaction involved a Michael addition-cyclodehydration sequence and was effected in a single synthetic step under microwave heating conditions [98]. An alternative approach towards polysubstituted pyridines was based on a reaction sequence involving an inverse electron-demand Diels-Alder reaction between various enamines 45 and 1,2,4-triazines 44 (Sect. 3.6), followed by loss of nitrogen and subsequent elimination-aromatization. Enamines 45 were formed in situ from various ketones and piperidine under one-pot microwave dielectric heating conditions [99]. Furthermore, a remarkable acceleration of the reaction speed (from hours and days to minutes) was observed in a microwave-assisted cycloaddition. Unsymmetrically substituted enamines 45 afforded mixtures of regioisomers (Scheme 35). 

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