Room temperature catalytic synthesis of biaryl and benzimidazole derivatives – Triggered by nano-sized Pd-loaded-modified chitosan polyurethane foams

Abstract

Though Pd-based catalysts have been the most efficient in catalytic C-C coupling reactions, but the leaching of Pd is one of the serious limitations. To shed more light on the Pd-based catalysts to overcome the existing disadvantages, four different new functionalized polyurethane foams, namely, PUCS-Pd, PUCS-CA-Pd, PUCS-TA-Pd and PUCS-GA-Pd (where PU = polyurethane; CS = chitosan, CA = citric acid, TA = tartaric acid and GA = gallic acid) were prepared. Further, these materials were employed as an effective catalytic system in C-C coupling reactions yielding a series of biaryl products (80–98%) and benzimidazole derivatives (80–98%) at room temperature. The structures and compositions of these catalysts were well characterized to understand the morphology and physicochemical properties. The BET surface area of all these chitosan-based PU foams can be altered and was achieved as high as 12.156 m2g−1. The TEM data demonstrate the presence of 3 to 10 nm sized nano-Pd species grafted on the surface of polymer support. The catalyst, PUCS-CA-Pd facilitated the cross-coupling reactions even at a low Pd-loading of 0.624 mg in methanol-water. Because of the pore structure toward the captured Pd species, PUCS-CA-Pd displayed exceptional stability, recyclability and reusability at least 8 times without loss of activity.