Microwave radiation, Diels-Alder reaction

FIGURE 12.25. Diels-Alder reaction with microwave radiation. (Source Niicher, 2004). 

The Diels-Alder reaction between fumaric diethyl ester and anthracene can be done in 30 minutes at 250°C with microwave radiation, whereas its conventional counterpart in p-xylene requires several hours. See Figure 12.25. 

Solvent-free conditions were preferable [77] due to several benefits. The reaction runs safely at atmospheric pressures with increased yield percentage. Here, the radiations are directly absorbed by the reactants thus increasing the yield as well as the purity of the product. The Diels-Alder cycloaddition of vinylpyrazoles under microwave irradiation was carried out by Diaz-Ortiz et al. [78]. It was observed that poor reactive dienophiles like phenyl propiolate do not react when heated conventionally but were found to react under microwave heating (Scheme 11.25). 

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