Novel multiple-acidic ionic liquids: Green and efficient catalysts for the synthesis of bis-indolylmethanes under solvent-free conditions

• Design and synthesis of four novel multiple-acidic ionic liquids.
• Development of protocol for the efficient and green preparation of bis-indolylmethanes catalyzed by the novel triethanolamine (TEA) based ILs.
• Fully discussed the proposed mechanism and verified it with experiment.

Four novel multiple-acidic ionic liquids based on triethanolamine (TEA) were prepared and used as efficient catalysts to synthesize bis-indolylmethanes at room temperature without any organic solvent. [TEOA][HSO4] showed the best catalytic performance. The optimal amount of catalyst was 10 mol%. Various aldehydes/ketones reacted with indole/substituted indole smoothly and afforded to corresponding products in 70–99% yields within minutes. Additionally, the ionic liquid could be reused up to five times with only a slight decrease in catalytic activity. Finally, a possible reaction mechanism was given. Techniques of acidity test and NMR were introduced to verify the proposed mechanism.

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