Convenient, efficient, and green method for synthesis of bis(indolyl)methanes with nano SIO2 under ultrasonic irradiation

A green and efficient procedure for the preparation of bis(indolyl)methanes via the condensation of indoles with various carbonyl compounds in the presence of catalytic amount of nanosilica gel under ultrasonic irradiation in good to excellent yields (83% to 94% ) in solvent-free condition is described. Short reaction time, easy and quick isolation of the products, environmentally friendly procedure, excellent chemoselectivity, and excellent yields are the main advantages of this procedure. Native nanosilica gel was used as an inexpensive and readily available catalyst without any support of reagent or modification on it. The reaction is carried out in solvent-free condition in the absence of ultrasonic irradiation at 80°C, too. This methodology offers significant improvements for the synthesis of bis(indolyl)methanes with regard to the yield of products, simplicity in operation, and green aspects by avoiding toxic catalysts and solvents. Recovery and recycling of the silica nanoparticle catalyst were also described.