Chitosan Capped Copper Oxide Nanocomposite: Efficient, Recyclable, Heterogeneous Base Catalyst for Synthesis of Nitroolefins

Abstract

In this article, chitosan copper oxide nanocomposite was synthesized by the solution casting method under microwave irradiation. The nanocomposite solution was microwave irradiated at 300 watt for 3 min under optimal irradiation conditions. By suppressing particle agglomeration, the chitosan matrix was successfully used as a metal oxide stabilizer. The goal of this research was to create, characterize, and test the catalytic potency of these hybrid nanocomposites in a number of well-known organic processes. The prepared CS-CuO nanocomposites were analyzed by different techniques, including Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Moreover, energy-dispersive X-ray spectroscopy (EDS) was used to measure the copper content in the prepared nanocomposite film. The finger-print peaks in the FTIR spectrum at around 632–502 cm⁻¹ confirmed the existence of the CuO phase. The CS-CuO nanocomposite has been shown to be an efficient base promoter for nitroolefin synthesis via the nitroaldol reaction (Henry reaction) in high yields. The reaction variables were studied to improve the catalytic approach. Higher reaction yields, shorter reaction times, and milder reaction conditions are all advantages of the technique, as is the catalyst’s reusability for several uses.

1. Introduction

Due to their unique features and vast range of applications as potent catalytic agents in various organic transformations, nano catalysis has attracted the interest of many researchers [1,2,3]. Nano catalysts have emerged as a low-cost, long-term alternative to hazardous commercial catalysts since they contain nanosized particles with a large, exposed surface area, which increases catalytic activity and regioselectivity in a variety of chemical processes [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19].

For the above-mentioned reasons, and because of their vast range of applications in numerous disciplines, many efforts have been made to synthesize new hybrid materials, including metal oxide nanoparticles. In particular, these hybrid nanocomposites confer a considerable synergistic effect, as compared to their constituents, that increases their potential use in many applications, especially in the field of heterogenous catalysis [14,15,16,17,18,19,20,21]. Solid catalysts have unique characteristics such as a large surface area, a high atom economy, and the ability to be easily removed from the product, recovered, and reused multiple times without losing catalytic activity. Chitosan, a natural bio-stabilizer, has recently been employed as a good template for fabricating metal oxide nanoparticles [20,21,22,23,24,25]. The existence of many binding sites, notably NH₂ and OH groups along the chains, is responsible for chitosan’s high adsorption capability. That explains why it is so good at forming stable complexes with metal ions and metal oxides [15,16,17,18,20,21]. On the other hand, among all metal oxides, copper oxide nanoparticles (CuONPs) have gained more attention due to their unique properties, as an inexpensive and efficient heterogeneous catalyst, and various applications [26].

Copper oxide nanoparticles’ catalytic activity allowed them to be used as a potent base catalyst in aromatic nucleophilic substitution processes [6,7] and in various heterocyclic synthesis methods [8,9,10,11,12,13,14,15,16]. In nucleophilic substitutions, Jammi et al. described a simple, general, and efficient procedure for the cross-coupling of nitrogen, oxygen, and sulfur nucleophiles with aryl iodides using CuO nanoparticles under ligand-free conditions [6]. On the other hand, Kidwai et al. have developed an economical method for the synthesis of 3-arylpentane-2,4-diones and diethyl 2-aryl-malonates has been developed using CuO-nanoparticles as the catalyst [7]. Satish et al. have developed a nanocrystalline CuO catalyzed coupling of 2-iodoaniline, carbon disulfide, and nitrogen nucleophile under ligand-free conditions in good yields [9].

Shaterian et al. have synthesized an efficient one-pot quantitative procedure for the synthesis of benzo[a]pyrano[2,3-c]phenazine derivatives in the presence of nano CuO as the catalyst [11].

Nitroolefins are useful organic compounds that are used as intermediates both in synthetic chemistry and in industrial chemistry. They are commonly employed in various carbon–carbon bond formations, including Diels–Alder cycloaddition, Michael reaction, and Morita–Baylis–Hillman reaction. Furthermore, the nitro group may readily be transformed into other useful functionalities. They are widely used in agrochemicals, materials, medicines, and other industries. As a result, researchers are interested in developing efficient procedures for the synthesis of nitroolefin. Recently, several useful approaches have been established, mainly including nitration of vinylic C–H bonds, nitro-decarboxylation of α, β-unsaturated aromatic carboxylic acids, and the traditional approach, particularly the Henry reaction [27,28].

The Henry reaction is a classical name reaction known for more than a century. It depends on base-mediated aldehyde or ketone condensation using nitroalkanes. The extensive studies performed by organic chemists and the commercial availability of the relatively low-cost starting materials make it a versatile and widely used reaction. Indeed, the Henry reaction is one of the most simple and economical sets of reaction conditions reported in the literature.

As a result of the aforementioned findings, we introduce an environmentally friendly nitroolefin synthesis method using chitosan-CuO nanocomposites as an efficient heterogeneous base catalyst.

2. Materials and Methods

2.1. Apparatus and Instrumentations

Melting points were recorded on Gallen Kamp apparatus and are reported uncorrected. The Fourier Transform Infrared (FTIR) spectra were recorded in KBr pellets on a JASCO FT-IR-6300 system at a resolution of 4 cm⁻¹, ranging from 400 to 4000 cm⁻¹. Field Emission Scanning Electron Microscopy (FESEM) was carried out using a model Leo (Zeiss) Remotely Operational Variable Pressure Field Emission SEM. The X-ray Diffraction (XRD) measurements were performed at room temperature on a Siemens diffractometer model D500 (Germany) operating in the reflection mode. Cu-Kα radiation (35 kV, 30 mA) was used for the analysis. Microwave experiments were carried out using a CEM Discover Labmate microwave apparatus (Discover, SP, NC, USA, 300 W). Sodium hydroxide was purchased from Sigma-Aldrich (St. Louis, MO, USA). Triple distilled water was used in all solution preparations. Chitosan of medium molecular weight and with 85% deacetylation was purchased from Sigma-Aldrich. Copper oxide (nanopowder, <50 nm particle size (TEM), 544868) was purchased from Sigma-Aldrich.

2.2. Preparation of Chitosan-Based CuO Nanocomposite Films

Chitosan solution 2 w/v% was made by stirring chitosan (CS) in a 1 v/v% acetic acid solution for 12 h at room temperature on a magnetic stirrer. The pH of the resultant CS solution was adjusted to the range of 6–7 by adding a calculated amount of 1 M NaOH solution under continuous stirring after full dissolution. Then, under continuous stirring, a suspension of 0.5 g CuO nanopowder in a small amount of double-distilled water was added portion by portion to the CS solution. The mixture was then subjected to microwave irradiation at a power of 300, 400, and 500 watts for 3, 4, and 5 min, then the solution was cast into a 100 mm Petri dish and dried overnight at 70 °C in order to remove any traces of acetic acid. The CS-CuO nanocomposite film was detached after complete drying, rinsed with distilled water, and then dried for 1 h at 70 °C. Finally, characterizations were performed on the formed films.

2.3. Synthesis of Nitroolefin

Method A: In a conical flask, 10 mmol of aldehyde and 5 mmol of ammonium acetate were dissolved in 5 mL of nitromethane. The mixture was refluxed with stirring for 4–5 h and the consumption of the aldehyde was checked by TLC. The reaction mixture was evaporated under vacuum and the residue was dissolved in methylene chloride and washed with water. The organic layer was dried and finally purification over a short silica column with EtOAc: hexane as eluent afforded the nitroolefin [29,30].

Method B: The same procedure as Method A, using a chitosan-CuO nanocomposite (15 wt.%) (0.1 g) instead of ammonium acetate and ethanol-water (70% solution), was used as solvent in this reaction. After 3–4 h of refluxing the reaction mixture, the hot solution was filtered to remove chitosan-CuO nanocomposite and the filtrate was evaporated. The residue was treated with DCM and washed with water. The organic layer was dried and finally purified over a short silica column with EtOAc: hexane as eluent to give authentic samples of nitroolefin (m.p, mixed m.p, IR, and TLC). The catalyst film was removed from the reaction mixture and thoroughly washed with ethanol before being dried and put to use in additional reactions.

3. Results and Discussion

3.1. Preparation of Chitosan-CuO Nanocomposite

In fact, in our previous work, we used the simple solution casting method to prepare the chitosan-copper oxide nanocomposite, but in this paper, we attempted to modify this methodology by using microwave irradiation to achieve a more homogeneous distribution of the copper oxide nanoparticles within the bulk of the polymer matrix. Thus, in this article, the (CS-CuO) nanocomposite was prepared using a modified microwave assisted simple solution casting process [15,16], as indicated in Scheme 1.

3.1.1. FTIR Characterization

Infrared spectroscopy provides more structural information by the presence or absence of the main characteristic absorption bands that help to detect the main functional groups in the compounds. Herein, the FTIR spectra were measured for chitosan (A), CuO nanoparticles (B), and chitosan-CuO nanocomposites (C), and they are shown in Figure 1. The spectrum of pure CS [15,16,17,18,31] (A) revealed a broad band at 3426 cm⁻¹ due to the intermolecular H-bonding of the O-H stretching and NH₂ stretching bands, that are overlapped together in the same region. Furthermore, the amide characteristic bands were clearly visible at υ = 1655 and 1604 cm⁻¹ in the spectrum, whereas the CH bands throughout the CS chain were clearly visible at υ = 2915, 2876, and 1370 cm⁻¹. In Figure 1B, for CuO nanoparticles, two peaks, one at 3418 cm⁻¹ of the O-H stretching and the other at 1622 cm⁻¹ of the O-H bending, appeared due to the presence of a trace amount of water in the copper oxide nanoparticles. There are two peaks at 1381 cm^-1 and 1060 cm⁻¹ linked to the asymmetric C-O in the copper oxide structure. Additionally, the CuO spectra showed the presence of two characteristic bands, at υ = 606 and 456 cm⁻¹ that correspond to the expected Cu-O stretching vibrations, in the range of 620–420 cm⁻¹ as previously reported [32,33], which are attributed to the monoclinic phase of CuO nanoparticles. Figure 1C, for the hybrid chitosan-CuO nanocomposite, showed the noisy shape with the presence of the broad band at 3438 cm⁻¹ and the two characteristic bands at 2115 and 2362 cm⁻¹ that are strong evidence for the coordination of CuO molecules within the binding sites (NH₂ and OH groups) along the chitosan backbone. Finally, the visible shift in the chitosan fingerprint region, especially at 632 and 502 cm⁻¹, is strong evidence for structural changes caused by the incorporation of copper oxide nanoparticles.

3.1.2. X-ray Diffraction (XRD)

The structural features of native chitosan (A), pure CuO nanoparticles (B) chitosan-CuO nanocomposites (15 wt.%), and (C) were analyzed using the XRD technique as shown in Figure 2. The chitosan (A) displayed the fundamental typical peak at 2θ = 20° (17–23°), which was typical of the hydrated crystalline structure of chitosan reported earlier [15,16,17,18]. In Figure 2B, the same characteristic peaks of both chitosan and CuO molecules, 2θ of chitosan at 20° and for CuO nanoparticles at 35.6° and 38.8° [34], were interfered with a clear shift, which confirms the coordination between CuO molecules and the chitosan chain. Furthermore, the Debye–Scherrer formula was used to calculate the average grain size from the XRD patterns [35]:

$$D\left(nm\right)=\frac{-0.9\times \lambda }{\mathsf{\beta }·\cos \mathsf{\theta }}$$

where, D (nm): represents the crystalline size in nm, λ is the wavelength of Cu-Kα₁ = 1.54060 A°, β can be calculated for the most intense peak for the CS-CuO nanocomposite pattern. Applying the equation, the average particle size was found to be 35.2 nm.

3.1.3. SEM and Morphological Changes

SEM images were used to investigate the morphological alterations of CS-CuO nanocomposites in comparison to unmodified chitosan. As shown in Figure 3, the nonporous, smooth membranous phase of chitosan (A) contained dome-shaped orifices, microfibrils, and crystallites, as shown in the SEM image [15,16,17,18]. On the other hand, the image of the CS-CuO nanocomposite (B) exhibited distinct modifications due to coordination with CuO molecules that are uniformly dispersed over the chitosan surface.

3.1.4. Energy-Dispersive X-ray Spectroscopy (EDS) and Estimation of Copper

Figure 4 shows an EDS graph of chitosan-CuO nanocomposites that was used to quantify the copper content within the chitosan. The hybrid material’s EDS revealed the appearance of the standard Cu signals, which indicated the amount of copper in the chitosan matrix. As shown in Figure 4, the Cu content in the prepared sample was about 13.62 (wt.%).

3.2. CS-CuO Nanocomposite Film as Basic Heterogeneous Catalyst in Synthesis of Nitroolefin

3.2.1. Optimization of the Reaction Conditions

Compound 1a was selected as a model compound for the optimization of the experimental conditions. CS-CuO was selected as the catalyst for the optimization reactions. The basic character of this catalyst and the catalytic activity of copper oxide nanoparticles promoted us to use it as a powerful catalyst in comparison to ammonium acetate (experimental Section 2.3) in the nitroaldol condensation reaction (Scheme 2)

The nature of the solvent and catalyst loading have both been optimized and the results obtained are reported in

Table 1. Henry reaction of benzaldehyde and nitromethane catalyzed by CS-CuO: effect of the solvent ^a.

Entry	Catalyst (mol%)	Solvent	Time (h)	Temperature (°C)	Yield (%)
1	10	THF	3	120	8
2	10	Toluene	3	120	13
3	10	DMF	3	120	20
4	10	CH3CN	3	120	25
5	10	EtOH	3	120	14
6	10	EtOH-H2O (1:1)	3	120	58
7	10	EtOH-H2O (1:3)	3	120	65
8 a	10	EtOH-H2O (1:5)	3	120	80

^a Experimental conditions: catalyst CS-CuO (wt.% = 10), T = 120 °C, and reaction time = 3 h.

and Figure 5.

Nature of Solvent

In the screening of the solvents (Table 1), different solvents were studied, including nonpolar, polar protic, and aprotic solvents. THF and toluene exhibited the lowest catalytic results. This could be explained by the fact that the charged intermediate formed during the reaction can’t be stabilized either in an aprotic polar or in a nonpolar solvent. Ethanol showed very poor yield to the desired nitroolefin. However, ethanol-water mixture as solvent showed the best result which suggests that water molecules are essential for enhancing the basicity of CuO nanoparticles. The other three tested polar aprotic solvents showed poor results. Thus, we selected an ethanol-water mixture as the best choice for this reaction.

Amount of Catalyst (Catalyst Loading)

We initially set the catalyst/benzaldehyde weight percent (wt.%) to be 5. The nitroolefin 1a was successfully isolated from the reaction mixture and its structure has been fully identified by NMR, mass spectroscopy, and elemental analysis.

As shown in Figure 5, the catalyst loading of 15 wt.%. was found to be the optimal quantity, as this percentage of catalyst provided the highest yield of product 1a (90%) in 3 h. After the reaction was complete, the used catalyst film was detached, purified with hot ethanol, and recovered to be usefully utilized four more times with minor loss in catalytic activity (Figure 6).

By summing up the data from the optimization, the best result was obtained by employing CS-CuO (15 wt.%), at 120 °C for 3 h in (1:5) EtOH-H₂O as the solvent.

3.2.2. Scope of the Reaction

The scope of the reaction has been explored (

Table 2. Scope of the reaction.

Entry	R1	R2	Product	Yield (%)
1	4-Cl	CH3		65
2	4-F	CH3		72
3	4-NO2	CH3-CH2		68
4	3-Cl	CH3		74
5	3-Br	CH3		73
6	4-OCH3	CH3		60
7	4-CH3	CH3		66
8	4-NMe2	CH3		49

Experimental conditions: catalyst CS-CuO (15 wt.%), solvent = EtOH-H₂O, T = 120 °C, and reaction time = 3 h.

). Different substituted benzaldehydes and nitroalkanes were subjected to our optimum conditions. It is interesting to note that the reaction was found to be tolerant to both electron donating (NMe₂, OCH₃, and CH₃) and electron attracting groups (F, Cl, Br, and NO₂) that are present in the phenyl ring of benzaldehyde.

4. Conclusions

The present study used a simple solution casting approach to efficiently prepare chitosan-copper oxide (CS-CuO) nanocomposite. The produced nanocomposite film was thoroughly examined using FTIR, XRD, FESEM, and EDS measurements. The existence of copper oxide nanoparticles within the chitosan matrix was confirmed by all the tools’ data. The existence of CuO molecules was demonstrated by FTIR spectra that exhibited a noticeable alteration in the fingerprint region. In addition, both chitosan and CuO characteristic peaks were visible in the XRD pattern. Furthermore, the SEM picture of the CS-CuO nanocomposite revealed a clear uniform surface modification due to CuO molecule coordination. This CS-CuO hybrid nanocomposite was effectively employed as an environmentally friendly heterogeneous basic catalyst in the manufacture of a high-impact nitroolefin. The created catalyst is a promising catalyst not only because of its non-toxic nature but also because of its economic impact, which means it might be employed in the industrial manufacturing of the compounds mentioned. Finally, we conclude that the nanocomposite base catalyst can be used to efficiently synthesize a variety of heterocycles that have previously been generated via non-green methods.