Microwave-accelerated thermal

Table 6.4 Scope of selectivity of amidoxime addition to DMAD and hydroxypyri midi none formation via microwave-accelerated thermal rearrangement. 

The most common synthetic method towards quinazolin-4-ones is the Niementowski reaction, a cyclocondensation of anthranilic acid with for-mamide which requires high temperatures (130-150 °C) and long reaction times (6 hours). It is noteworthy that a remarkable reduction of the reaction time (20 min) was achieved under microwave heating conditions (150 °C) [123]. Moreover, microwave-accelerated reactions were cleaner and afforded higher yields than those under conventional thermal conditions (Scheme 48). 

Previte, M. J., Zhang, Y., Aslan, K., and Geddes, C. D. (2007). Real-time Thermal Im ng of Microwave Accelerated Metal-Enhanced Fluorescence (MAMEF) Based Assays on Sapphire Plates Journal of fluorescence 17 639-642. 

Microwave irradiation has received significant research attention to accelerate thermal reactions.In conventional methods, reactants are heated by an external heat source such as a hot-water bath or an oil bath. Heat is driven through the walls of a reaction flask however, this... 

The microwave acceleration for the direct synthesis of N-phenylmaleimide from maleic anhydride and aniline based on specific microwave absorption of the ionic intermediates has been reported (Bezdushna and Ritter, 2005). Fischer et al. (2005) investigated the free radical polymerization of N-alkylacrylamides with 3-mercap-topropionic acid as chain transfer agent in methanol with thermal heating at ambient pressure and under superheated conditions as well as under microwave irradiation. While the chain transfer polymerization could be accelerated from 5 h to 1 h when going to superheated conditions with thermal heating, it was further accelerated down to several seconds under microwave irradiation. 

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... 

Since the early days of microwave synthesis, the observed rate accelerations and sometimes altered product distributions compared to oil-bath experiments have led to speculation on the existence of so-called specific or non-thermal microwave effects. Historically, such effects were claimed when the outcome of a synthesis per-... 

Microwave heating is often applied to already known conventional thermal reactions in order to accelerate the reaction and therefore to reduce the overall process time. When developing completely new reactions, the initial experiments should preferably be performed only on a small scale applying moderately enhanced temperatures to avoid exceeding the operational limits of the instrument (temperature, pressure). Thus, single-mode reactors are highly applicable for method development and reaction optimization. 

In general, most reactions that can be carried out under thermal heating can be performed and accelerated by microwave irradiation. As discussed in Section 2.2, the efficiency of the microwave heating is highly dependent on the dielectric properties of the reaction mixture. Most results suggesting rate enhancements and improved yields can be explained in terms of simple thermal effects. However, for two main reasons, some reactions may not be suitable for performance in micro-wave reactors ... 

The acceleration of reactions by exposure to microwaves results from material-wave interactions leading to thermal effects (which can easily be estimated by temperature measurement) and specific (not purely thermal) effects. Clearly, a combination of these two contributions can be responsible for the effects observed. 

Significant rate accelerations and higher loadings are observed when the micro-wave-assisted and conventional thermal procedures are compared. Reactions times are reduced from 12-48 h with conventional heating at 80 °C to 5-15 min with microwave flash heating in NMP at temperatures up to 200 °C. Finally, kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) rather than to a specific nonthermal microwave effect [17]. 

The existence of results that cannot be explained solely by the effect of rapid heating has led authors to postulate the existence of a so-called microwave effect . Hence, acceleration or changes in reactivity and selectivity could be explained by a specific radiation effect and not merely by a thermal effect. Several reviews have collected synthetic results that have been attributed to the microwave effect [5 b, 38, 39]. 

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