A novel and sustainable synthesis-functionalization strategy for the oxidative cyclization towards 2,5-disubstituted-1,3,4-oxadiazoles using bentonite-supported copper(i) iodide as a green catalyst

Abstract

This study developed a novel, sustainable approach for synthesizing 2,5-disubstituted-1,3,4-oxadiazoles via oxidative cyclization of hydrazones using a heterogeneous bentonite-supported copper(I) iodide (CuI/Bent-NH₂) as a catalyst. CuI/Bent-NH₂ was prepared through a two-step process: functionalization of natural bentonite clay with (3-aminopropyl)triethoxysilane (APTES) and subsequent immobilization of CuI. The structure of CuI/Bent-NH₂ was determined by methods such as FT-IR spectroscopy, Raman spectroscopy, XRD analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS), which confirmed successful surface modification, metal incorporation, and desirable structural and thermal properties of the material. The catalytic system efficiently promoted the oxidative cyclization of hydrazone derivatives in the presence of potassium persulfate as a green oxidant, and potassium carbonate as a base agent under optimized conditions (100 °C, DMSO, 5 h). A broad range of hydrazones bearing both electron-donating and electron-withdrawing substituents were converted into their corresponding oxadiazole products in moderate to good yields (up to 67%). Mechanistic studies proposed a radical-mediated pathway facilitated by Cu(II)/Cu(I) redox cycling and stabilized by π-conjugation and phenyl substituents. The CuI/Bent-NH₂ catalyst exhibited good recyclability, retaining substantial activity over four cycles, although a slight decline in product yield was observed due to potential catalyst deactivation or product adsorption. Comparative analysis demonstrated the superiority of this method over previously reported homogeneous systems, especially in terms of environmental friendliness, simplicity, and reusability.