Phosphomolybdic Acid Immobilized on Ionic Liquid-Modified Halloysite: Catalytic Utility for the Synthesis of Pyrazolopyranopyrimidines
Heteropolyacids are non-toxic bifunctional catalysts with high thermal stability which can promote both oxidation and acid-catalyzed reactions. The hybridization of this class of catalysts with other species is of great interest. Despite all outstanding features of heteropolyacids, their solubility in water and most organic solvents limit their applications. Herein, to circumvent this issue and develop an efficient heterogeneous catalyst, halloysite clay was used as catalyst support for the immobilization of heteropolyacids. To improve the immobilization, the halloysite clay was functionalized with ionic liquid. The obtained catalyst was characterized with FTIR, SEM/EDX, and XRD and successfully used for catalyzing the ultrasonic-assisted four-component reaction of barbituric acid, hydrazine hydrate, ethyl acetoacetate and benzaldehyde for the synthesis of pyrazolopyranopyrimidine derivatives in high yield. Notably, the catalyst was reusable and could be reused for four reaction runs.
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