Pd Nanocatalyst Adorned on Magnetic Chitosan@N-Heterocyclic Carbene: Eco-Compatible Suzuki Cross-Coupling Reaction.

Roya Sedghi,Pourya Zarshenas,Hatef Shahmohamadi,Bahareh Heidari,Rajender S. Varma

doi:10.3390/molecules24173048

Abstract

A novel magnetic-functionalized-multi-walled carbon nanotubes@chitosan N-heterocyclic carbene-palladium (M-f-MWCNTs@chitosan-NHC-Pd) nanocatalyst is developed in two steps. The first step entails the fabrication of a three-component cross-linking of chitosan utilizing the Debus–Radziszewski imidazole approach. The second step comprised the covalent grafting of prepared cross-linked chitosan to the outer walls of magnetically functionalized MWCNTs (M-f-MWCNTs) followed by introducing PdCl2 to generate the m-f-MWCNTs@cross-linked chitosan with a novel NHC ligand. The repeated units of the amino group in the chitosan polymer chain provide the synthesis of several imidazole units which also increase the number of Pd linkers thus leading to higher catalyst efficiency. The evaluation of catalytic activity was examined in the expeditious synthesis of biaryl compounds using the Suzuki cross-coupling reaction of various aryl halides and aryl boronic acids; ensuing results show the general applicability of nanocatalyst with superior conversion reaction yields, high turnover frequencies (TOFs) and turnover numbers (TON). Meanwhile, nanocatalyst showed admirable potential in reusability tests, being recycled for five runs without losing significant activities under optimum reaction conditions. The successfully synthesis of catalyst and its characterization was confirmed using the Fourier transform infrared spectrometer (FT-IR), spectrometer transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photo-electron spectroscopy (XPS) and thermogravimetric analysis (TGA).

Highlights

IntroductionThe Suzuki–Miyaura cross-coupling reaction plays a significant industrial scale role for the production of biaryl compounds which are widely used for a variety of industrial applications, such as the synthesis of natural products, herbicides, pharmaceuticals, polymers and agrochemicals [1,2,3,4].Commonly, bulky phosphine ligands are employed to facilitate the Suzuki cross-coupling reaction; critical setbacks for this ligand are toxicity, air sensitivity and typical susceptibility to metal-ligandMolecules 2019, 24, 3048; doi:10.3390/molecules24173048 www.mdpi.com/journal/moleculesMolecules 2019, 24, 3048 degradation [5,6,7], posing significant limitations on their uses [8]
The main purpose of this study has been to develop robust Pd NPs stabilized onto a novel supported polymer which is abundant, renewable and biodegradable; such a decorated Pd nanocatalyst on m-f-multi-walled carbon nanotubes (MWCNTs) with larger surface area coupled with ease of its separation using just an external magnet bodes well for its numerous Pd-catalyzed reactions
Our investigations demonstrate that the immobilization of imidazolium cross-linked chitosan to m-f-MWCNTs support, followed by decorating with palladium, produces a well-defined and superior catalyst for the Suzuki cross-coupling reactions

Summary

Introduction

The Suzuki–Miyaura cross-coupling reaction plays a significant industrial scale role for the production of biaryl compounds which are widely used for a variety of industrial applications, such as the synthesis of natural products, herbicides, pharmaceuticals, polymers and agrochemicals [1,2,3,4].Commonly, bulky phosphine ligands are employed to facilitate the Suzuki cross-coupling reaction; critical setbacks for this ligand are toxicity, air sensitivity and typical susceptibility to metal-ligandMolecules 2019, 24, 3048; doi:10.3390/molecules24173048 www.mdpi.com/journal/moleculesMolecules 2019, 24, 3048 degradation [5,6,7], posing significant limitations on their uses [8]. NHC appear to be a superior catalyst to achieve excellent catalytic activities in cross-coupling reactions. Recent advances in ‘green’ Suzuki cross-coupling reactions have focused on designing recoverable catalysts, simplifying product separation and adhering to sustainable green chemistry [17]. The catalysts have been immobilized on a wide variety of supports such as Fe3 O4 [18], SiO2 [19], TiO2 nanoparticles (NPs) [20] and mesoporous silica [21]; potential supports should have high stability and large surface area. In the last few years, techniques based on magnetic materials have attracted much attention to use in the wide variety of fields such as immunoassay [21], targeted drug delivery [23], and environmental detection [24]. Since the Chitosan is soluble in aqueous acidic media, it can be processed, and a wide variety of shapes are known including membranes, fibers or spheres [30,31,32]

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