Green Three-component One-pot Synthesis of Benzo[b]pyran and Pyrano[c]chromene Derivatives Using Nanosilica-supported Dendritic Polymer Encapsulated Phosphotungstic acid (Dendrimer-PWAn) Nanoparticles in Aqueous Media under Ultrasound Irradiation

An efficient and green approach was developed for the synthesis of various derivatives of tetrahydrobenzo[b]pyran and 3,4-dihydropyrano[c]chromene in the presence of Dendrimer-PWAn as nanocatalyst at ambient temperature through a one-pot three-component reaction between aromatic aldehydes, malononitrile, and dimedone or 4-hydroxycoumarin in an aqueous media under ultrasound irradiation. In order to prepare this catalyst, H3PW12O40 (PWAn) nanoparticles provided by a solvo-thermal method were encapsulated into dendritic polymer-supported on nanosilica. The catalyst is mesoporous and has a particle size of 40 nm. The mean pore diameter, BET surface area and pore volume are 6.1 nm, 479 m2/g and 0.6 cm3, respectively. The amounts of PWAn immobilized on the nanocatalyst was found to be 39.7 wt% (0.14 mmol/g). The developed method is not only suitable for aromatic aldehydes but also successfully used for acid sensitive heteroaromatic aldehydes, and the corresponding products were synthesized with high purity in high yield, and short reaction times. This new method has significant advantages such as operational simplicity, high efficiency, short reaction time and lack of elaborate workup or tedious purification procedures. In addition, excellent yields in a mild environment, thermal stability, and simple catalyst separation make it a suitable alternative to other heterogeneous catalysts. Also, this catalyst can be easily recycled through simple filtration or centrifugation and used for six consecutive periods without significant reduction in activity.