Ionic Liquid: An Efficient and Facile Catalyst for the Synthesis of Trisubstituted Imidazole Derivatives via Multi-Component Pathway Using Green Techniques

Background: A green path for the synthesis of 3-aryl-1-phenyl-4-(4,5-diphenyl-1H-imidazol-2-yl)-1Hpyrazole derivatives using [BMIM][BF4] as a catalyst and green methods such as ultrasound and microwave irradiation is discussed in this paper. The titled compounds were obtained by the multi-component condensation of various 3-aryl-1-phenyl-1H-pyrazole-4-carboxaldehydes, benzil and ammonium acetate. One pot synthesis, simple reaction conditions and quantitative yields illustrate the utility of this green approach. Methods: Conventional Reflux Condition: A mixture of 3-aryl-1-phenyl-1H-pyrazole-4-carboxaldehyde 1 (1 mmol), benzil 2 (1 mmol), ammonium acetate 3 (2 mmol) and catalytic amount of [BMIM][BF4] (15 mmol %) was placed in a round bottom flask containing 10 mL of ethanol. The reaction mixture was refluxed for completion. The course of the reaction was monitored by thin layer chromatography. After completion of the reaction, the mixture was poured over crushed ice. Solid imidazole thus obtained was separated by filtration, dried well, and recrystallized by ethanol. Ultrasound Irradiation Method: A mixture of 3-aryl-1-phenyl-1H-pyrazole-4-carboxaldehyde 1 (1 mmol), benzil 2 (1 mmol), ammonium acetate 3 (2 mmol) and catalytic amount of [BMIM][BF4] (15 mmol %) was placed in a round bottom flask containing 10 mL of ethanol. The round bottom flask was placed in an US bath at 50ºC for 80-90 min. The course of the reaction was monitored by thin layer chromatography. After completion of the reaction, the mixture was poured into crushed ice, solid imidazoles thus obtained were separated by filtration, dried well, and recrystallized by ethanol. Microwave Irradiation Method: A 10 mL round bottom flask was charged with 3-aryl-1-phenyl- 1H-pyrazole-4-carboxaldehyde 1 (1 mmol), benzil 2 (1 mmol), ammonium acetate 3 (2 mmol) and catalytic amount of [BMIM] [BF4] (15 mmol %), and placed under MW irradiation at 240 watts for 7-9 min. The course of the reaction was monitored by thin layer chromatography. After completion of the reaction, the mixture was poured over crushed ice, the solid imidazole thus obtained was separated by filtration, dried well, and recrystallized by ethanol. Results: The 3-aryl-1-phenyl-4-(4,5-diphenyl-1H-imidazol-2-yl)-1H-pyrazole derivatives 4a-i (Scheme 1, Table 2) were synthesized by using [BMIM] [BF4] as a catalyst with good yields under reflux in ethanol (68-70%), US irradiation in ethanol (76-80%) and MW irradiation (80-86%) without solvent. All these methods provided good results with IL [BMIM][BF4]. However, the MW and US irradiation methods give good yield in a short period of time with 3-aryl-1-phenyl-1H-pyrazole-4-carboxaldehyde 1 containing a variety of substituents, whereas the conventional reflux condition gives lower yields and takes longer time as compared with MW and US irradiation. The structures of the synthesized compounds have been confirmed on the basis of spectroscopic techniques such as FTIR, HRMS, LCMS, 1H and 13C NMR. Conclusion: In conclusion, we have synthesized differently substituted imidazoles using [BMIM][BF4] as a catalyst under MW and US irradiation via MC condensation strategy. Under the conventional reflux conditions, we get a lower yield in a longer time, while US and MW assisted synthesis gave better results. Comparatively IL [BMIM][BF4] with US and MW irradiation protocol provides several advantages such as improved reaction speed, shorter reaction times, superior yields and a significant contribution towards sustainability.