A novel water-tolerant organosulfonic acid-functionalized silica-coated magnetic nanoparticles as a hydrophobic, recyclable and magnetically separable catalyst for the solvent-free Biginelli reaction

Two novel and environmentally benign organosulfonic acid-functionalized silica-coated magnetic nanoparticle catalysts 1a (Fe₃O₄@SiO₂@Me&Et-PhSO₃H) and 1b (Fe₃O₄@SiO₂@Et-PhSO₃H) have been prepared and their hydrophobicity and acidity were investigated, and tested for the three-component Biginelli reaction of benzaldehyde, methylacetoacetate, and urea under solvent-free conditions. 1a, the solid catalyst which was more hydrophobic, showed higher catalytic activity and was characterized extensively by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis, H₂O-sorption analysis; thermal gravimetric analysis (TGA), vibrating sample magnetometer (VSM) and acid/base titration. This catalyst was used in the reactions of a series of aldehydes, β-keto esters, and urea/thiourea. The catalyst was easily separated by an external magnet and the recovered catalyst was reused in four cycles without significant loss of activity. This green and reusable catalyst accompanied by simple experimental procedure and product isolation could be considered as an alternative protocol which will hopefully develop a clean and eco-friendly strategy for the synthesis of 3,4-dihydropyrimidin-2-ones/thiones (DHPMs) derivatives.