Chelidamic acid functionalized stimuli-responsive hydrogel supported-palladium catalyst for copper-free Sonogashira reaction in aqueous media

A thermo and pH-responsive hydrogel (PNIPAM-co-PPAP) was synthesized via free radical polymerization of N-isopropylacrylamide (NIPAM) and potassium 4-(acryloyloxy) pyridine-2, 6-dicarboxylate (PAP) in mixed solvents of water and tetrahydrofuran. SEM micrographs revealed that the hydrogel was macroporous and the pores within the matrixes were interconnected. The chelidamic acid functionalized PNIPAM-co-PPAP hydrogel exhibited excellent capacity to anchor palladium (II) and the resulting material showed well catalytic activity for copper-free Sonogashira reaction of aryl halides with phenylacetylene in aqueous media. The reactions proceeded well with 0.1 mol% of Pd (II) catalyst, due to the good dispersion of Pd2+ in the macroporous structure as well as the enrichment of reactants within hydrogel under reaction conditions. Through the reversibly swelling/deswelling, the PNIPAM-co-PPAP hydrogel-immobilized Pd (II) catalyst could be easily recovered by simple method. Furthermore, the PNIPAM-co-PPAP /Pd catalyst exhibited good recyclability and could be recycled six times without remarkable loss in catalytic activity.