An efficient Friedel–Crafts alkylation for the synthesis of 3-indolyl-3-hydroxy oxindoles and unsymmetrical 3,3-diaryl oxindoles catalyzed by Dabco-based ionic liquids in water†
A convenient and rapid method for the syntheses of 3-indolyl-3-hydroxy oxindoles and unsymmetrical 3,3-diaryl oxindoles has been developed by using Dabco-based ionic liquid catalysts. The two ionic liquid catalysts, [Dabco-H][BF4] and [Dabco-H][HSO4] were found to be highly efficient catalysts for controlled 3-indolylation of isatins in water. When [Dabco-H][BF4] was employed as the catalyst in water at 55 °C, the reaction between isatins and indoles stops at the step of addition of the two components and afforded 3-indolyl-3-hydroxy oxindoles. While increasing the reaction temperature to 90 °C, the catalyst [Dabco-H][HSO4] could drive the reaction further and afford symmetrical 3,3-diindolyl oxindoles. By using the two kinds of ionic liquids, a two-step protocol for the efficient synthesis of unsymmetrical 3,3-diaryl oxindoles has also been developed. The use of water as the reaction medium makes the process environmentally benign. The catalysts can be recycled five times without activity loss.