Microwave-assisted Fischer Indole Synthesis of 1,2,3,4-tetrahydrocarbazole Catalyzed by 1,3-dialkylimidazolium bromide Ionic Liquids

Abstract

The research and application of green chemistry principles have led to the development of cleaner processes In this sense, ionic liquids (ILs) are being researched intensively as greener alternatives to volatile organic media in a wide range of chemical processes Ionic liquids are salts with melting points less than 100 oC and consist of a large organic cation combined with an organic or inorganic anion Thus, a series of 1,3-dialkylimidazolium bromide ionic liquids were synthesized from commercially available 1-methylimidazole (1-MIM) and alkyl bromides (RBr) with R being n-C4, sec-C4, tert-C4, n-C6, n-C10 and benzyl Synthesis was done by exposing the neat mixture of reactants to microwave irradiations with conditions set at 100W, 300W, and 600W Moreover, the utilization of ionic liquid as greener catalyst for the one-pot Fischer indole synthesis of 1,2,3,4-tetrahydrocarbazole (THC) was examined The efficiency of ionic liquids as catalyst was evaluated in terms of product yield Using these parameters, Fischer indole synthesis of 1,2,3,4-tetrahydrocarbazole catalyzed by ionic liquids was compared with other catalysts such as ionic liquids with ZnCl2 (IL+ ZnCl2) and with ZnCl2 only Results show pronounced catalytic activities were obtained in the combination of ionic liquids and ZnCl2 (IL+ ZnCl2) Among the catalysts used, the 1-decyl-3-methylimidazolium bromide ionic liquid with ZnCl2 gave an excellent yield of 94% THC Furthermore, the structure of the synthesized compounds was elucidated on the basis of spectral evidences such as Fourier Transform Infrared (FT-IR), 1H-NMR and 2D-NMR spectroscopic experiments.