Microwaves in Cycloadditions

Abstract

The first examples of microwave-induced reactions were cycloadditions and pericyclic reactions performed under pressure, because of the use of domestic ovens, to avoid the risk of igniting flammable solvents (Scheme 9.1) [11,12].Reactions were performed in sealed thick-walled glass tubes or in Teflon acid-digestion vessels, in domestic microwave ovens [13]. Teflon vessels can be used at pressures up to 14 atm, at temperatures below 250 8C, and are resistant to most commonly used chemicals, although they deform at temperatures >250 8C.The lack of temperature and pressure control in these systems could lead to safety problems, because of overpressure resulting from the heating rate caused by microwaves. These problems can be reduced by using solvent-free methodology, which also enables the use of larger quantities of reagents. Yields have been greatly improved, and reaction times reduced, in comparison with conventional procedures in solvents under reflux.These reactions were next revisited by Strauss and it was shown that, under identical conditions in xylene, the reaction rates were strictly identical within experimental error [14]. Reactions under RefluxReactions involving a solvent under reflux require the use of modified commercial microwave ovens. In these modified systems the oven is perforated on the top to accommodate a reflux condenser and a 10 cm pipe is used to avoid microwave leakage; the turnable dish is replaced by a magnetic stirrer or by monomode reactors especially designed for chemical synthesis [15].The advantage of reactions involving solvent is that the reaction temperature is controlled by the reflux temperature of the solvent [16]. It should, however, be re-296 9 Microwaves in Cycloadditions CO 2 Me MeO 2 C MeO 2 C CO 2 Me MICROWAVE 10 min p-xylene, 87% 325º<361 ºC CONTROL 4 h p-xylene, 67% 138 ºC 1 2 3 Scheme 9.1 297 9.3 Solvent-free Conditions 5 6 7 8 Scheme 9.2 silica gel [29] and alumina [30]have also been used to generate 1,3-dipoles and, in this way, heterocyclic compounds can be obtained in a one-pot procedure. In these reactions changes in the regioselectivity of the reaction have not been observed under the action of microwave irradiation. Reactions without SupportAn alternative to the use of supported reagents is the use of uncatalyzed ªneat reactionsº. Under these conditions the radiation is absorbed directly by the reagents and this results in spectacular acceleration, higher yields and purity of the reaction products, a simple workup procedure, and sometimes changes in the selectivity of the cycloaddition. Diels±Alder reactions [31] and 1,3-dipolar cycloadditions [32,33] have been performed by use of this methodology. For example, DÌaz-Ortiz described the hetero-Diels±Alder and 1,3-dipolar cycloaddition reactions of ketene acetals. The reactions were improved and products were isolated directly from the crude reaction mixture without polymerization of the ketene acetals [34].When the enantiomerically pure ketene acetal 16 was used, cycloaddition with N,a-diphenylnitrone (11) or ...

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