Hydrophobicity-enhanced magnetic solid sulfonic acid: A simple approach to improve the mass transfer of reaction partners on the surface of the heterogeneous catalyst in water-generating reactions

• A new generation of green, magnetic, reusable, and cost effective nano-catalyst is introduced.
• Two heterogeneous, solid sulfonic acids, 1 and 2, are synthesized and compared in catalytic water-generating reactions.
• Catalyst 2 was superior due to its bifunctionality, higher water-tolerance and hydrophobicity.

Two novel and environmentally benign organosulfonic acid-functionalized silica-coated magnetic nanoparticle catalysts 1 (Fe3O4@SiO2@Et-PhSO3H) and 2 (Fe3O4@SiO2@Me&Et-PhSO3H) were prepared and their surface hydrophobicity was assessed by esterification reaction of fatty alcohols. Also, their water-toleration in two water-generating reactions was investigated, namely: synthesis of bis(indolyl)methane and the Biginelli reaction of benzaldehyde, methyl acetoacetate, and urea under solvent-free conditions. In these reactions, the catalyst 2 that was more hydrophobic and water-resistant showed higher catalytic activity and was characterized extensively by various techniques. This catalyst was successfully used in the Biginelli reaction of a series of aldehydes, β-keto esters, and urea/thiourea for the synthesis of a diverse range of pharmacologically active 3,4-dihydropyrimidin-2-one/thione (DHPM) derivatives. The catalyst 2 was easily separated by an external magnet and the recovered catalyst was reused in five cycles without any significant loss of activity.

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