Chemically Modified Chitosan as a Biopolymer Support in Copper‐catalyzedipso‐Hydroxylation of Arylboronic Acids in Water

Abstract

In the past decade, copper complexes have been extensively studied and employed in organic transformations. 1 Among these applications, the copper-catalyzed hydroxylation of boronic acids is considered a facile protocol for the preparation of alcohols. 2 Noting the significant usefulness of copper-catalyzed transformation in organic synthesis, we sought to design an alternative greener pathway utilizing biopolymers and environmentally friendly reaction conditions.To achieve this aim, we were interested in utilizing chitosan generated by the deacetylation of chitin, the second-most abundant natural polysaccaharide after cellulose. Owing to its combination of unique characteristics, chitosan is used in the form of an unmodified polymer or as modified derivatives in various applications. This is especially true in the presence of readily functionalizable primary amino groups that enable easy and effective chemical modification of the material, thereby imparting several desirable chemical and biological properties. 3 In general, the polymer-surface modification through the introduction of new complexation groups may result in the formation of different chelating sites. These sites yield considerable changes in the adsorption capacity and the chemical reactivity of the complexed metal ion in solution.Numerous protocols have been used for chemical modification of chitosan over the last decades. However, the introduction of pyridine derivatives into the chitosan backbone (resulting in Schiff base formation) has recently gained significant attention. One reason for such attention is that this protocol can be applied to metal absorption, antimicrobial activity, gene delivery, sensor applications, and biomedical applications. 4 Presumably, the capacity characterizing imine functionality of chitosan can enhance the metal chelating ability, thereby promoting the catalytic activity of the metal.Considering these aspects, we decided to prepare a modified-chitosan platform with pyridine (L1), and then investigate its applicability in the most robust and useful reaction called ipso-hydroxylation of boronic acids, in conjunction with copper catalyst. In addition, to compare the active role of the pyridine moiety in the chitosan backbone, an alternative chitosan platform (L2) modified with thiophene rings was also prepared and applied to the ipsohydroxylation of boronic acids.A straightforward method for the synthesis of pyridineor thiophene-modified chitosan supports (CTS-Py, L1, and CTS Th, L2, respectively, hereafter) is shown in Scheme 1. Quantifiable yields of modified chitosan derivatives (L1 and L2) were obtained upon the treatment of chitosan with 2-pyridinecarboxaldehyde or 2-thiophenecarboxaldehyde in ethanol at a refluxing temperature. 5 Results of 1 H, 13 C NMR spectra, and IR spectra corresponded closely to the literature values. 6 Strong and broad OH absorption bands observed at 3435 and 3300 cm −1 in the IR spectra of L1 and L2, respectively. The diagnostic peak occurring at 1649 cm −1 corresponded to the conden...

Application of Raw and Chemically Modified Biomasses for Heterogeneous Cu-Catalysed Conversion of Aryl boronic Acids to Phenols Derivatives

“…We started our investigations based on the recent work by Kim et al [5] dealing with hydroxylation of boronic acids using modified chitosan as solid support. In this work, experimental conditions were thoroughly studied and optimized.…”

“…We started our investigations based on the recent work by Kim et al [5] dealing with hydroxylation of boronic acids using modified chitosan as solid support. In this work, As a result, the combined presence of both reactants is mandatory to the ipso-hydroxylation process.…”

“…Due to the necessity to use renewable resources versus fossil resources, materials prepared from biomass, such as chitosan, sugarcane bagasse or cellulose, are of particular interest [1][2][3][4]. Recently, biopolymers such as cellulose have received tremendous global attention as a biodegradable, abundant, easy to modify, non-toxic solid support for metal catalysts [5,6]. In general, the polymer-surface modification through the introduction of new ligand-type fragments may result in the formation of different chelating sites, which can be used to produce new organic matrices [5].…”

“…Recently, biopolymers such as cellulose have received tremendous global attention as a biodegradable, abundant, easy to modify, non-toxic solid support for metal catalysts [5,6]. In general, the polymer-surface modification through the introduction of new ligand-type fragments may result in the formation of different chelating sites, which can be used to produce new organic matrices [5]. If raw and modified cellulose are able to complex many metals as Cu [6], Pt [6,7], Pd [8,9], Co [10], and Mo [10], cellulose-[Cu] materials have garnered attention due to their versatile catalytic properties.…”

“…The access to phenols by ipso-hydroxylation of boronic acids can be achieved in three main ways including photocatalysis [18], the use of strong oxidants [19][20][21][22], and the use of transition metals. Among the latter, Cu-catalysed synthesis of phenols from boronic acid precursors appears as a general and efficient procedure under both homogeneous [23] and heterogeneous catalysis using various solid supports including Cu 2 O nanoparticles and banana pulp extracts [20], carbon nanotube-chitosan nanohybrid films [24], microheterogeneous biphasic systems [25], biogenic Cu 2 O nanoparticles [26], Cu-Phen [23], Cu(OH) x -Clay [27], CuCl 2 -cryptand-[2.2.Benzo] [28], or modified chitosan [5,18,29]. However, these protocols may suffer from drawbacks such as longer reaction time, higher temperature, the use of organic solvents and oxidants, and lack of recyclability.…”