Microwave irradiation oxidation-reduction reactions

Dipolar cycloaddition of nitrile oxides to olefins and acetylenes is among the most widely exploited synthetic routes towards isoxazoles and isoxazolines. It is well-known that microwave irradiation in cycloaddition reactions considerably reduces reaction times. Indeed, the use of dielectric heating (microwave-heated reactions were performed in a flask with a reflux condenser mounted outside the apparatus) allowed for a remarkable reduction of the cycloaddition reaction time from 6-12 hours to merely 3 minutes [69]. Simple aqueous workup provided the target isoxazoles and isoxazolines. The requisite nitrile oxides for the cycloaddition reaction were generated in situ from the corresponding nitroalkanes, 4-(4,6-dimethoxy [1,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) and 4-dimethylaminopyridine (DMAP) (Scheme 22). 

Table 5.4), prepared from reduction of Pd(II) salts with potassium graphite. The results suggested that this catalyst was not very active. However, some years later Jikei and Kakimoto [73] prepared a more active Pd/CGr based on a smaller crystallite size. In 2002, Kohler et al. [74] studied a variety of Pd/C catalysts with different properhes (Pd dispersion, oxidation state, water content, conditions of catalysts preparation etc.) in the Heck reaction of aryl bromides with olefins (entry 4, Table 5.4). The authors pointed out the hypothesis that the leached Pd from the support is the active species and the solid Pd/C catalyst acts as a reservoir that delivers catalytically active Pd species into solution. All catalysts were obtained by wet impregnation (5% Pd loading). The Heck reaction can also be conducted in ionic liquids through promotion by microwave irradiation. Moreover the reaction of iodobenzene with methylacrylate in NMP was reported to be accelerated by ultrasound [75]. The ionic liquid containing the catalyst system was used five consecutive times with only a slight loss of activity (entry 5, Table 5.4) [76]. Perosa [77] reported the addition of a phase transfer catalyst to an ionic liquid as a method to accelerate the C-C coupling reaction. As far as we know, only by using ionic liquids has Pd on carbon been recovered and reused with success.

The condensation of co-propargylic benzaldehydes with methyl sarcosi-nate or ethyl N-benzylglycinate followed by in sitn oxidation was also more efficient under microwave-assisted conditions for the synthesis of benzopyranopyrroles 80 [94]. Oxidation of the hexahydrochromeno[4,3-b]pyrrole cycloadducts could be facilitated by the addition of sulfur and further irradiation for 10 min at 130 °C in a one-pot process to give pyrroles 80 in 70-90% yield (Scheme 35). Although there was little difference between reactions carried out under microwave irradiation and those using classical conductive heating methodology, in terms of product yield, the microwave-assisted reactions had considerably shorter reaction times. Furthermore, a more convenient one-pot procedure, in which a mixture of the aldehyde, a-amino ester, sulfur and a minimal amount of xylene was irradiated for 15 min at 130 °C, provided the pyrrole product without any reduction in yield (90%) in a one-pot MCR. 

Synthesis of exfoliated graphite as well as the reduction of graphite oxide was carried out by Zhu et al. (2010). A short-time direct exposure to microwave irradiation was used to bring about these reactions. 

Also, comparison of preparation method on the catalytic behavior of perovskite oxides for liquid reaction was investigated. Kulkami et al [56, 57] prepared a series of Lai-xSrxFeOs catalyst for reduction of nitrobenzene to aniline by microwave irradiation method, finding that the activity is similar to that prepared by conventional methods. The advantage in using microwave method is that it does not involve intermittent grindings and calcinations at elevated temperatures. 

A number of low-grade transition metal ores (for example, minerals containing nickel oxides) can be used as catalysts. Smuda has demonstrated that microwave or radiofrequency irradiation of a mixture of such ores with a carbon source initiates reduction of the oxide to metal. With this approach, poisoning the active sites of the catalyst will not be critical for the process since there will be a constant supply and generation of active catalyst with the feed material. In addition to well-known catalytic properties of nickel in organic reactions, it was also shown that Ni on carbon and other supports, catalyzes hydrodechlorination and dehydrochlorination of chlorinated organic waste streams [22-24],... 

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