An efficient Friedel-Crafts alkylation for synthesis of 3-indolyl-3-hydroxy oxindoles and unsymmetrical 3,3-diaryl oxindoles catalyzed by Dabco-base ionic liquids in water
A convenient and rapid method for syntheses of 3-indolyl-3-hydroxy oxindoles and unsymmetrical 3,3-diaryl oxindoles has been developed by using Dabco-base ionic liquid catalysts. The two ionic liquids catalysts, [Dabco-H][BF4] and [Dabco-H][HSO4] were found as 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 isatin and indoles stops at the step of addition of the two components and provided 3-indolyl-3-hydroxy oxindoles. While raising 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 efficient synthesis of unsymmetrical 3,3-diaryl oxindoles has been also developed. The use of water as the reaction medium makes the process environmentally benign. The catalysts can be recycled five times without activity loss.