Microwave-Enabled Synthesis of Dihydropyrazolo[4',3':5,6]pyrano[2,3-d]pyrimidine Derivatives Utilizing Hf based Magnetic–Metal Organic Frameworks (MMOFs) - Fe3O4@SiO2@UiO-66 (Hf) as Heterogeneous Catalysts

Abstract

A novel, efficient, and green synthetic protocol for dihydropyrazolo[4',3':5,6]pyrano[2,3-d]pyrimidine derivatives was developed using microwave irradiation and a magnetic–metal organic framework (MMOF) catalyst, Fe₃O₄@SiO₂@UiO-66(Hf). The catalyst, synthesized via a stepwise approach, combines the magnetic properties of Fe₃O₄ with the robust, porous UiO-66 (Hf) framework, enabling facile separation and reuse. Characterization techniques including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and EDX analysis confirmed the successful fabrication of the MMOF. The identification of the product compounds was achieved through spectral analysis of dihydropyrazolo[4',3':5,6]pyrano[2,3-d]pyrimidine derivatives, employing ¹H NMR, ¹³C NMR, and IR spectroscopy. The catalytic system demonstrated high efficiency under microwave irradiation, affording excellent yields (85–94%) within very short reaction times (6 min) under solvent-free conditions. Optimization studies revealed the optimal catalyst loading and microwave power for maximum conversion. The catalyst exhibited remarkable recyclability, retaining activity over five consecutive cycles with minimal loss. Comparative studies highlighted superior performance over conventional catalysts in terms of reaction rate and environmental sustainability. Mechanistic investigations suggest a cooperative activation of substrates via Lewis acidic Hf sites and microwave-induced heating, promoting rapid cyclization. This heterogeneous catalytic approach aligns with principles of green chemistry, offering an expedient, recyclable, and environmentally benign route to valuable heterocyclic scaffolds with potential applications in medicinal chemistry.