Microwave methodology

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck-type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinyl positions. The first report on the application of microwave methodology to this type of reaction was published by Hallberg et al. in 1996 [86], Recently, the palladium catalyzed Heck coupling reaction induced by microwave irradiation was reported under solventless liquid-liquid phase-transfer catalytic conditions in the presence of potassium carbonate and a small amount of [Pd(PPh3)2Cl2]-TBAB as a catalyst [87]. The arylation of alkenes with aryl iodides proceeded smoothly to afford exclusively trans product in high yields (86-93%) (Eq. 61). 

Lee and colleagues studied the ionic-liquid-mediated Beckmann rearrangement under microwaves (equation 96). They screened several ionic liquids and investigated the amount of sulfuric acid needed as catalyst. The microwave methodology was found to be superior to analogous conventional heating. All the ionic liquids tested worked well in the microwave and only 5 mol% sulfuric acid was needed. The BMIM ionic liquids ([BMIM]PFg, [BMIM]Bp4, [BMIMJSbFg and [BMIMJOTf) were found to be thermally stable. 

While studying the chemistry of 4,5-dichloro-l,2,3-dithiazolium chloride (Appel s salt) and its derivatives, Besson reported the synthesis of various benzothiazoles from N-arylimino-l,2,3-dithiazoles, which could be synthesised from commercially available aromatic amines28. In this work, the authors explored a variety of strategies to construct the benzothiazole ring and demonstrated that in all cases the focused microwave methodologies were more productive and under well-defined conditions provided convenient methods for scale-up (Scheme 3.17)28. Comparisons were also made between reactions performed under solvent-free conditions and in the presence of solvent29. It is noteworthy that there is no general rule and some reactions performed in the presence of solvent may sometimes be more convenient than the same dry media reaction. 

In 2002 Raboisson et al. reported a method for synthesis of 4,7-dihydro-lH-imidazo[4,5-e]-l,2,4-triazepin-8-one derivatives by use of microwave methodology. The reaction involves aminoimidazole carboxhydrazide and ortho esters, and the subsequent alkylation of the l,2,4-triazepin-8-one obtained occurs regioselectively at Cl [120]. 

Another basic aspect related to the microwave heating process in polymeric systems is the choice of an adequate microwave heating system. As mentioned before, it is crucial to select the best microwave methodology for each specific application and component size. 

Trans-4-(4-fluorophenyl)-3-chloromethyl-l-methylpiperidine was subjected to elimination reaction on alumina or KF-alumina imder solvent-free conditions and microwave irradiation. A solvent-free supported microwave methodology for dehydrochlorination of compound, trans-4-(4-fluorophenyl)-3-chloromethyl-l-methylpi-peridine, provided significantly better yields in comparison with conventional heating in the presence of organic base and solvent (Navratilova et al., 2004). 

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