Pvp-based deep eutectic solvent polymer: Sustainable Brønsted-Lewis acidic catalyst in the synthesis of α-aminophosphonate and bisindole

Abstract

Polymeric deep eutectic solvents (PDESs) have emerged for augmentation of the functions of deep eutectic solvents (DESs) as a sustainable material class. Considering the common synthetic limitation of PDESs to the in-situ polymerization of deep eutectic monomers, herein, we have implemented linear polyvinylpyrrolidone (PVP) to develop a simple-to-prepare, atom-economical, cost-effective, and eco-friendly Brønsted-Lewis acidic PDES via ZnCl2 and a quaternization procedure. The prepared PDES was characterized by SEM, EDS, elemental mapping, FT-IR, and TGA techniques. This method enables physicochemical tunation of the PDES by altering the molar ratio of ZnCl2 to PVP monomer repeating units. The PDES was used as a heterogeneous catalyst to promote greener approaches towards biologically significant scaffolds via a multicomponent reaction of aldehydes, amines, and tri(alkyl/aryl)phosphites to synthesize secondary and tertiary α-aminophosphonates (APs), in addition to a carbon–carbon bond-forming reaction to synthesizing bis(indolyl)methanes (BIMs) from aldehydes and indolyl compounds. PDESs proved to be highly efficient catalysts for these transformations, providing mild and high-yielding green protocols for an extensive library of both APs and BIMs. The desired compounds from either transformation group were obtained shortly with good to excellent yields (15–120 min for APs and 100–200 min for BIMs). Examination with BIMs revealed that the protocol is amenable to large-scale production without significant loss of efficiency. In addition, the stability of the PDES catalyst was confirmed from five successful recycles, in which no significant decrease in the activity was monitored.