Ultrasound-assisted high-yield multicomponent synthesis of triazolo[1,2-a]indazole-triones using silica-coated ZnO nanoparticles as a heterogeneous catalyst†
We report here comprehensively detailed systematic investigations into a new method of an ultrasound-assisted synthesis of 4(a–m) triazolo[1,2-a]indazole-triones (TAITs) using the multicomponent condensation reaction among dimedone, 4-phenylurazole and various individual aryl aldehydes in the presence/absence of SiO2, ZnO and SiO2-coated ZnO (ZnO@SiO2) nanoparticles (NPs) or nanorods as heterogeneous catalysts in deionized water (DIW) at considerably low temperature (∼60 °C). For this, the nanorods are synthesized in an efficient, facile and scalable wet chemical route in DIW at 27 °C. Oxygen-excessive ZnO nanorods of ∼21 nm diameter with up to nearly 259 nm length become oxygen-deficient ZnO@SiO2 of about 48 nm diameter with rather complicated internal structures and a smooth-looking surface. Remarkably, attributed to oxygen deficiency, ZnO@SiO2 NPs exhibit excellent catalytic activities with good to high yields of TAIT products, brilliant recyclability up to six runs, over −30 mV zeta potential (ζ) values, the smallest hydrodynamic diameter (HD) of 752 nm, best dispersion stability and largest specific surface area of 97.5 m2 g−1. Therefore, the greenest reaction medium for the syntheses of both these catalysts as well as TAITs was realized, making it an environmentally friendly doubly green reaction with a wide application potential due to its high efficiency, ease of handling and cost effectiveness.