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Novel Non‐toxic and Non‐hazardous Solvent Systems for the Chemistry of Indoles: Use of a Sulfone‐containing Brønsted Acid Ionic Liquid Catalyst in Butyl Acetate

Dipolar and aprotic solvents are often required in acid‐catalyzed reactions, however, many of them are listed in solvent selection guides as not‐recommended due to their toxicity or explosivity. A catalytically functionalized ionic liquid, namely a sulfone‐containing imidazolium‐based Brønsted acid ionic liquid, was synthesized. It alleviates the detrimental effect of classical dipolar aprotic solvents because this ionic liquid integrates the function of a dipolar aprotic solvent able to stabilize carbocation intermediates and the activating effect of a strong Brønsted acid on electrophiles. The use of this tailor‐made ionic liquid was exemplified in some transformation reactions of indoles, which proceeded with high yield and selectivity using a green and an industrially acceptable solvent, butyl acetate. The recyclability of the ionic liquid catalyst was also demonstrated.