Hydroxyalkylation of indole with cyclic carbonates catalyzed by ionic liquids

A simple and eco-friendly procedure has been developed for the synthesis of hydroxyalkyl indoles from indole and cyclic carbonates in the presence of a catalytic amount of imidazolium based ionic liquids under solvent-free conditions. The effects of reaction time, the catalyst amount, temperature, and the ratio of the reactants were investigated to develop the optimum conditions for the transformation. Under the optimized reaction conditions, indole reacted with ethylene carbonate or propylene carbonate to give 1-(2-hydroxyethyl) indole or 1-(2-hydroxypropyl) indole, as well as the corresponding sequential derivatives, in high yields. The catalytic activity of the ionic liquids was affected by the anions of the ionic liquids in the order of BF₄^- < Br^- < Cl^- < OAc^-, which was consistent with the hydrogen bond basicity of the anions of the ionic liquids.