Titania nanomaterials: efficient and recyclable heterogeneous catalysts for the solvent-free synthesis of poly-substituted quinolines via Friedlander hetero-annulation

Abstract

The present communication describes the use of titania (TiO₂) nanomaterials of different sizes (16 nm, 35 nm, 70 nm, 200 nm, and 1000 nm) as heterogeneous catalysts for the preparation of a series of medicinally significant poly-substituted quinoline derivatives via Friedlander hetero-annulation. These TiO₂ nanomaterials exhibited remarkable catalytic activity with a high substrate to catalyst molar ratio (20 : 1) to achieve the synthetic targets in excellent yields ranging from 81–94%. The effect of catalyst particle size on the yield of the quinolines was also investigated. Yields of product were found to decrease when the TiO₂ particle size increased from 16 nm to 1000 nm. GC-MS data indicated that TiO₂ nanoparticles having 16 nm and 35 nm size of anatase phase provided better yields of the target compound, followed by particles of 70 nm in size, whereas 200 nm and 1000 nm size of anatase phase did not show the formation of the product under the same reaction conditions. The use of catalyst in solvent-free reaction conditions and its reusability up to five-cycles with similar catalytic response are the unique features of this heterogeneous catalysis. Furthermore, greater selectivity, cost-efficiency, clean reaction profiles, simple work-up procedure and high yield are the noteworthy features of this eco-friendly green protocol.