Microwave Irradiation Effect on Friedel-Crafts Type Cyclization Reaction

Background: Isoquinolones are widely used compounds as bioactive substances and antibiotics. Therefore, it is important to establish their synthetic process. One possibility for this purpose is the use of Curtius rearrangement. Such reactions are applied to produce heterocycles containing nitrogen atom. In this study, shortening of the synthetic process is discussed using the Friedel-Crafts type cyclization reaction. Furthermore, high yields and highly selective reactions are required during the microwave irradiation. Methods: The reactions were carried out under microwave irradiation conditions as well as conventional heating conditions. The microwave irradiation effect was investigated by comparing their yields. Results: The reactions occurred in several nonpolar solvents. Especially, in decalin, the yields under the microwave irradiation conditions were higher than under the conventional heating conditions. This would be due to selective absorption of microwaves by the substrates. For the improvement of the yields, the Friedel-Crafts type cyclization reactions were carried out in the presence of FeCl3. The addition of FeCl3 at a higher temperature would not be effective for this Friedel-Crafts type cyclization reaction. The use of FeCl3 enable to proceed the reaction at lower temperature. Conclusion: The microwave accelerating effect was observed in a nonpolar solvent. In this solvent, microwaves would be selectively absorbed not by the solvent, but by the substrate. Furthermore, by the use of FeCl3 as a catalyst, the reaction was able to proceed at a lower temperature. This would result in the adduct formation between the substrate and FeCl3. These results show that the microwave irradiation can be a potential tool to proceed the Friedel-Crafts type cyclization reaction, but further optimization of the reaction conditions would be required.