Pyridinium-based dual acidic ionic liquid supported on the pectin for efficient synthesis of pyrazoles

Abstract

In this work, we developed for the first time a pectin-supported dual acidic pyridinium ionic liquid with rich sulfonic acid groups as an efficient catalyst for the synthesis of pyrazole derivatives. Pectin (Pec) as a biopolymer is initially modified with 2,6-diamino pyridine (DAP) to obtain the Pec-DAP nanoparticles. The produced Pec-DAP reacts with 1,4-butane sultone (BS), and followed by treatment with a H2SO4 solution to afford the Pec-DAP-BS catalyst. The structure of the catalyst is characterized by different techniques. This catalyst presented a remarkable activity as a metal-free catalytic system for the three-component synthesis of pyrazoles from aldehydes, phenylhydrazines, and malononitrile due to the cooperative effect of dual acidic sites. Under optimized conditions, pyrazole derivatives are obtained with high turnover frequencies (TOF) up to 184.7 h−1 and excellent yields (92–96%) in short reaction times (15–50 min) in aqueous ethanol as a green solvent. Moreover, the catalyst is used in at least five cycles without significant loss of activity, and its heterogeneous nature is confirmed by the hot-filtration test.