Synthesis of environmentally friendly, efficient and highly recyclable Lewis acid-type calix[4]arene catalysts containing flexible or bulky groups for the Mannich reaction

Abstract

Two new calix[4]arene-based Lewis acid-type catalysts containing flexible and bulky groups (2 and 3) were successfully synthesized. Their structures were characterized using FTIR, ¹H-NMR, COSY-NMR, ¹³C-NMR, APT-NMR, and elemental analysis techniques. Moreover, to provide easily recyclable, environmentally benign, cost effective and reusable catalysts, these calix[4]arene-based Lewis acid-type catalysts were grafted onto epoxyl-functionalized silica-coated Fe₃O₄ magnetic nanoparticles (EPPTMS-MN) to afford their corresponding calixarene-grafted magnetic nanoparticles (TMAC[4]Bu^t-MNP and P(Ph)₃C[4]Bu^t-MNP). FTIR, TGA, TEM, VSM and elemental analysis techniques were performed to confirm their structures. In addition, the catalytic capabilities of four Lewis acid-type calixarene catalysts were evaluated in a three component Mannich reaction of benzaldehyde with aniline and acetophenone in aqueous media without co-solvents. The association of the effects of functional groups (flexible and bulky), molar equivalents, amount of water used, and reaction durations on their catalytic efficacies were also evaluated.