Preparation of some indole derivatives in the presence of modified magnetic nanoparticles with acidic ionic liquid as new catalyst under thermal and microwave conditions

Abstract

The indole ring system is probably the most ubiquitous heterocycle that represents an important structural component in many pharmacologically active compounds, agrochemistry and materials science1 . Although many methods have been developed for the synthesis of indoles2 , compared with other methodologies for indole synthesis, the Fischer indole synthesis through the reaction of ketones with aryhydrazines remains an extremely useful and important method3 . Since its discovery in the 1880s by Emil Fischer, various catalysts have been used to effect the cyclization of enolizable N-arylhydrazones derived from ketones and arylhydrazones. As the catalysts for the Fischer indole synthesis, Bronsed acids (H2SO4, HCl, PPA, AcOH, and TsOH) 4 , Lewis acids (ZnCl2 and TiCl4) 5 , and solid acids (zeolite and montmorillonite clay) 6 , have been reported. The reported Br ّ nsted and Lewis acids are environmentally unfriendly, hazardous, difficult to reuse, and usually required large amounts. Modified magnetic nanoparticles with acidic ionic liquid (Fe3O4@SiO2@IL) have been used as a heterogeneous, recyclable, and efficient catalyst for synthesis of some indole derivatives in good yields under microwave irradiation. Ketone was mixed with Fe3O4@SiO2.TBAPS.SO3H in toluene, and phenylhydrazine was added. The mixture was then stirred at 120°C for about 15 min under microwave irradiation. Reaction progress was monitored by GC–MS. After completion, the reaction mixture was cooled to room temperature, the Fe3O4@SiO2@IL were removed by use of an external magnetic field. The solvent was evaporated under reduced pressure. The solid products obtained were collected by filtration (scheme 1).