Copper(II) nanodots stabilized on Cassia fistula galactomannan: preparation and catalytic application towards fast solvent-free Biginelli reactions.

New Cu(II) nanodots have been developed using biopolymeric polysaccharide galactomannan. The nanocatalyst Cu(II)NDs@CFG has been developed through a one-step clean and sustainable reaction of Cassia fistula galactomannan and CuSO4·5H2O in an aqueous medium. The catalyst Cu(II)NDs@CFG is well characterized by FT-IR, FE-SEM, EDS, ICP-MS, HR-TEM, XPS, XRD, TGA and BET analysis. This is the first example of preparing copper nanodots by using polysaccharide galactomannan as a supporting template to form copper nanodots in water. Moreover, the copper nanodots act as a potential nanocatalyst for multicomponent Biginelli reactions. A simple, one pot, efficient and environmentally benign synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones has been achieved with wide variety of aldehydes, ß-dicarbonyl compounds and urea or thiourea indicating the good tolerance of the catalyst towards various functionalities. The presented work has several merits in terms of economy which include easy operation, complete avoidance of toxic organic solvents and expensive catalysts, simple work-up, less reaction time, and excellent yields.

Cassia fistula galactomannan stabilized copper nanocatalyst as an efficient, recyclable heterogeneous catalyst for the fast clickable [3+2] Huisgen cycloadditions in water.

In this study, we have first time investigated the synthesis of copper nanocatalyst by using biopolymer galactomannan, naturally extracted from Cassia fistula pods. The methodology involved for the preparation of copper nanocatalyst is economical, efficient, environment friendly, and did not involve further processing for stabilization or reduction of copper nanoparticles. The morphology and structural characterization of the nanocatalyst was performed by using different techniques such as FT-IR, 1H NMR, SEM, EDX, HR-TEM, XRD, XPS, ICP-MS, BET, and TGA analysis. The prepared copper nanocatalyst is applied for the click [3+2] Huisgen cycloadditions of various azides and alkynes, employing water as environmentally benign solvent. In comparison to earlier reported methods, our method requires lowest catalyst loading, less reaction time, excellent yields and have wide substrate scope. Additionally, the catalyst was easily recovered by simple filtration and recycled at least ten consecutive times without any appreciable loss of efficiency and selectivity. The effect of mannose and galactose (Man/Gal) ratio of Cassia fistula galactomannan on the catalytic activity were also investigated.