CoFe₂O₄/Cu(OH)₂ magnetic nanocomposite: an efficient and reusable heterogeneous catalyst for one-pot synthesis of β-hydroxy-1,4-disubstituted-1,2,3-triazoles from epoxides

Abstract

A magnetically separable CoFe₂O₄/Cu(OH)₂ nanocomposite was prepared and characterized by various techniques such as FESEM, EDS, TEM, XRD, VSM and FT-IR. This novel composite was used as a heterogeneous catalyst for the regioselective synthesis of β-hydroxy-1,4-disubstituted-1,2,3-triazoles from sodium azide, terminal alkynes and structurally different epoxides in water at 60 °C. The formation of the product proceeds in one pot through a mechanism that involves an in situ generated organic azide intermediate, followed by rapid ring closure with the alkyne component. The simple procedure, short reaction times, perfect regioselectivity, high product yields, and use of a benign solvent and nontoxic catalyst are among the considerable advantages of this protocol. Furthermore, the catalyst was easily separated using an external magnet and reused several times without any significant loss of catalytic activity or magnetic properties.

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1. Introduction

1,2,3-Triazoles are significant heterocyclic intermediates for the manufacture of important compounds such as pharmaceuticals, agrochemicals, dyes and corrosion retarding agents.^1–4 They also show vital biological activities such as anticancer,⁵ anti HIV,⁶ antiviral, antiallergic,⁷ antifungal,⁸ antibacterial,⁹ antitubercular,¹⁰ anti-inflammatory,¹¹ antimalarial,¹² and antioxidant activities.¹³

The Huisgen 1,3-dipolar cycloaddition of alkynes and organic azides has been developed as the principal method for synthesis of 1,2,3-triazoles.^14–22 Recently, multicomponent one-pot synthesis of β-hydroxy-1,2,3-triazoles has been carried out through in situ click reaction of azides, alkynes and epoxides in the presence of heterogeneous copper catalysts such as carbon supported copper nanoparticles,^23,24 porphyrinatocopper nanoparticles,²⁵ CuSO₄·5H₂O/sodium ascorbate,²⁶ copper(I)-zeolite,²⁷ Cu[N²,N⁶-bis(2-hydroxyphenyl)pyridine-2,6-dicarboxamidate]/ascorbic acid,²⁸ copper ferrite nanoparticles,²⁹ copper(I)@phosphorated SiO₂,³⁰ Cu(OAc)₂·H₂O,³¹ (Cu/Cu₂O) nanoparticles,³² Cu(II)-azide complexes,³³ Cu(II)-DA@nano AlPO₄,³⁴ 1,4-dihydroxyanthraquinone-copper(II) nanoparticles immobilized on silica gel,³⁵ 4′-phenyl-2,2′:6′,2′′-terpyridine copper(II) complex immobilized on activated multiwalled carbon nanotubes,³⁶ CuI³⁷ and NiFe₂O₄-glutamate-Cu.³⁸ Ramachary–Bressy–Wang cycloaddition of enolate intermediates and organic azides is the other basic method for one-pot preparation of 1,4-disubstituted-1,2,3-triazoles.^39–43

The reduction of work-up procedures and purification steps, high purity of desired products, short reaction times, high to excellent yields are among the significant benefits of multicomponent one-pot reactions.^44,45 Furthermore, the reaction involving ring opening of epoxides with organic azides in water solvent occurs with perfect regioselectivity and takes advantage of green and benign conditions.

In recent years, magnetic nanoparticles (MNPs) due to their facile preparation, easy separation using an external simple magnet leading to avoid traditional filtration processes, recoverability and reusability,^46,47 high catalytic activity,⁴⁸ thermal and chemical stability^49,50 have been widely utilized as green and efficient nanocatalysts in the synthesis of organic compounds. Nevertheless, the nano-ferrites often suffer from high tendency of self-aggregation and low quantity of functional groups on the surface.^51,52 The coating of nano-ferrites with a functionalized shell, not only reduces the aggregation of the nanoparticles in the solution, but also improves the efficiency of the catalyst by increasing the amount of functional groups on the surface.⁵³ Recently, these coated nanostructures have received special attention as air-stable materials and easy separable catalysts in organic transformations, drug targeting, and magnetic cell separation.^54–57

In connection with our recent works on nano-ferrites,^58–63 herein we wish to report an efficient one-pot three-component protocol for the synthesis of β-hydroxy-1,2,3-triazoles from sodium azide, terminal alkynes, and various epoxides in the presence of CoFe₂O₄/Cu(OH)₂ magnetic nanoparticles as a novel and green heterogeneous catalyst in water (Scheme 1).

Scheme 1 One-pot synthesis of triazoles from epoxides catalyzed by CoFe₂O₄/Cu(OH)₂ nanoparticles.

2. Experimental

2.1. Instruments and materials

All materials were purchased from the Merck and Aldrich Chemical Companies with the best quality and they were used without further purification. The synthesized nanocatalyst was characterized by XRD on a Bruker D8-Advanced diffractometer with graphite-monochromatized Cu Kα radiation (λ = 1.54056 A°) at room temperature. Magnetic property of synthesized nanocatalyst was measured using a VSM (Meghnatis Daghigh Kavir Co., Kashan Kavir, Iran) at room temperature. TEM image was recorded using an EM10C-100 kV series microscope from the Zeiss Company, Germany. FESEM images were determined using FESEM-TESCAN. The energy dispersive X-ray spectrometer (EDS) analysis was taken on a MIRA3 FE-SEM microscope (TESCAN, Czech Republic) equipped with an EDS detector (Oxford Instruments, UK). IR and ¹H/¹³C NMR spectra were recorded on Thermo Nicolet Nexus 670 FT-IR and 400 MHz Bruker Avance spectrometers, respectively. The products were characterized by their spectra data and comparison with the reported data in literature. All yields refer to isolated pure products.

2.2. Preparation of CoFe₂O₄ nanoparticles

CoFe₂O₄ nanoparticles were prepared by a solid-state procedure according to our previous research.⁵⁸ Briefly, in a mortar, CoCl₂, Fe(NO₃)₃·9H₂O, NaOH, and NaCl were mixed with the desired molar ratio (1 : 2 : 8 : 2) and ground together for 55 min. The reaction started quickly along with the release of heat. The mixture color changed from yellow to chocolate brown after 4 min. Next, in order to remove the additional salts, the produced mixture was washed with distilled water for several times. The product was dried at 80 °C for 2 h and it was then calcined at 900 °C for 2 h to obtain the final cobalt nanoferrite as a dark brown powder.

2.3. Synthesis of CoFe₂O₄/Cu(OH)₂ nanocomposite

In a round-bottomed flask equipped with a magnetic stirrer, a solution of CuCl₂·2H₂O (0.8 g, 4.7 mmol) in distilled water (30 mL) was prepared and then CoFe₂O₄ (1.01 g, 4.31 mmol) was added. The mixture was stirred vigorously for 10 min at room temperature and followed by dropping of NaOH solution (6 mL, 1 M) in order to basify the mixture up to pH ∼ 13. The stirring of alkali mixture was continued at room temperature for 24 h. The black nanoparticles of CoFe₂O₄/Cu(OH)₂ were separated using a magnet, washed with distilled water and then dried under air atmosphere within two days.

2.4. One-pot synthesis of β-hydroxytriazoles from epoxides catalyzed by CoFe₂O₄/Cu(OH)₂ in water: a general procedure

In a round-bottomed flask equipped with a magnetic stirrer and condenser, a solution of the epoxide (1 mmol), alkyne (1 mmol) and sodium azide (0.078 g, 1.2 mmol) in H₂O (5 mL) was prepared. CoFe₂O₄/Cu(OH)₂ nanocomposite (0.032 g, 0.1 mmol) was then added to the solution and the resulting mixture was stirred magnetically for 3–6.5 h at 60 °C. The progress of the reaction was monitored by TLC using CCl₄ : Et₂O (10 : 2) as an eluent. After completion of the reaction, the magnetic nanocatalyst was separated using an external magnet and collected for the next run. The reaction mixture was extracted with EtOAc (3 × 10 mL) and then dried over anhydrous Na₂SO₄. After evaporating the organic solvent, the crude β-hydroxytriazoles were obtained. Removal of the solvent under vacuum, followed by recrystallization with EtOH/H₂O (3 : 1) or purification on silica gel using CCl₄ : Et₂O (10 : 2) as the eluent afforded the pure β-hydroxytriazoles derivatives in 75–95% yield (Table 2). The collected CoFe₂O₄/Cu(OH)₂ nanoparticles were washed with distilled water and dried for the next cycle. All products are known compounds and were characterized by comparison of their spectra (FT-IR, ¹H NMR and ¹³C NMR) with those of valid samples;^{23,25,26,29,30,38,45} these data are given in the ESI.†

3. Results and discussion

3.1. Synthesis and characterization of CoFe₂O₄/Cu(OH)₂ magnetic nanocatalyst

The coating of CoFe₂O₄ nanoparticles was carried out with Cu(OH)₂ in order to reduce the self-aggregation and also increase the amount of hydroxyl functional groups on the surface of nano-ferrite which makes the catalyst more active than the bare cobalt ferrite. The use of CoFe₂O₄ nanoparticles as catalyst support not only donated sufficient magnetic property for the easy separation of nanocatalyst, but also enhanced the efficiency of the catalyst by increasing the recoverability and reusability of the novel nanocomposite.

The magnetic nanoparticles of CoFe₂O₄/Cu(OH)₂ was synthesized in a two-step process. CoFe₂O₄ was prepared using solid-state reaction of CoCl₂, Fe(NO₃)₃·9H₂O, NaCl and NaOH in an agate mortar (Scheme 2). After calcination of crude powder at 900 °C, the CoFe₂O₄ nanoparticles were obtained with high crystallinity, phase purity and increased saturation magnetization (M_s). The calcination process at high temperatures improves the efficiency of CoFe₂O₄, and also leads to decompose the excess salts used in the preparation of these nanoparticles.⁶⁴ Afterwards, the collected pure CoFe₂O₄ nanoparticles were added to an aqueous solution of CuCl₂ and followed by dropping of the NaOH solution under intense stirring. Eventually, the black precipitate was accumulated through magnetic separation, washed with deionized water, and dried at room temperature (Scheme 3). The prepared CoFe₂O₄/Cu(OH)₂ nanocatalyst was characterized by vibration sample magnetometer (VSM), FT-IR, X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), and energy dispersive X-ray spectrometer (EDS) techniques.

Scheme 2 Synthesis of CoFe₂O₄ nanoparticles.

Scheme 3 Synthesis of CoFe₂O₄/Cu(OH)₂ nanocomposite.

3.1.1. Vibration sample magnetometer (VSM). The hysteresis loops, saturation magnetization (M_s) and switching field (H_c) of the prepared magnetic nanoparticles were measured at room temperature using vibrating sample magnetometer. The hysteresis loops of CoFe₂O₄ and CoFe₂O₄/Cu(OH)₂ nanoparticles are shown in Fig. 1. The wide cycles and the hysteresis loops show the behavior of hard ferromagnetic particles with high coercivity. CoFe₂O₄/Cu(OH)₂ nanoparticles show lower magnetization saturation (18 emu g⁻¹) than the uncoated cobalt ferrite nanoparticles (128 emu g⁻¹). This is due to the effect of Cu(OH)₂ shell coating where each particle was separated from its neighbors by the coated layer leading to decrease the magnetostatic coupling between the particles.

Fig. 1 Magnetization curves of (a) CoFe₂O₄ (b) CoFe₂O₄/Cu(OH)₂ nanoparticles.

3.1.2. Fourier transform infra-red (FT-IR) spectrum. The FT-IR spectrum of synthesized CoFe₂O₄, Cu(OH)₂ and CoFe₂O₄/Cu(OH)₂ nanoparticles are shown in Fig. 2. A strong absorption band at 596 cm⁻¹ is corresponded to the stretching vibration of metal–oxygen (M–O) indicating the formation of spinel ferrite structure. The absorption peaks at 1608 cm⁻¹ and 3390 cm⁻¹ are related to bending and stretching vibrations of O–H group in surface absorbed H₂O in the sample, respectively.⁶⁵ A sharp and intense absorption band at 3568 cm⁻¹ is related to the O–H stretching vibrations in crystal structure of Cu(OH)₂ and the peak at 941 cm⁻¹ indicates Cu–O–H bending vibrations.⁶⁶

Fig. 2 FT-IR (KBr) spectrum of (a) Cu(OH)₂, (b) CoFe₂O₄ and (c) CoFe₂O₄/Cu(OH)₂.

3.1.3. X-ray diffraction (XRD). Fig. 3 displays the X-ray diffraction (XRD) patterns of CoFe₂O₄, Cu(OH)₂ and CoFe₂O₄/Cu(OH)₂ nanoparticles. In the XRD pattern of CoFe₂O₄/Cu(OH)₂, all the peaks of CoFe₂O₄ and Cu(OH)₂ are detectable. The lines (111), (220), (311), (222), (400), (422), (511), (440), (620), (533) and (622) related to 2θ = 18.36°, 30.18°, 35.54°, 37.18°, 43.17°, 53.55°, 57.09°, 62.68°, 71.13°, 74.13° and 75.20° respectively, are assigned to the diffraction of CoFe₂O₄ crystals and indicate that the synthesized CoFe₂O₄ nanoparticles are pure and high crystalline. These peaks are consistent with the standard data (JCPDS card no. 22-1086).⁶⁷ The peaks at 2θ = 16.81°, 23.89°, 34.09°, 38.24°, and 39.77° correspond to orthorhombic Cu(OH)₂ which are in good agreement with JCPDS no. (13-420).⁶⁸ The peak at 53.52° for copper hydroxide overlaps with the 53.55° peak of CoFe₂O₄. The average crystallite size of CoFe₂O₄/Cu(OH)₂ nanoparticles is calculated using the Scherrer's formula (40 nm).

Fig. 3 The X-ray diffraction patterns of (a) nano-Cu(OH)₂, (b) nano-CoFe₂O₄ and (c) CoFe₂O₄/Cu(OH)₂ nanocomposite.

3.1.4. TEM, FESEM and EDS of CoFe₂O₄/Cu(OH)₂ nanocomposite. The morphology and size distribution of the synthesized nanocatalyst have been investigated by TEM and FESEM techniques. Fig. 4 shows the TEM images of the CoFe₂O₄/Cu(OH)₂ nanocomposite. The TEM images with different magnifications clearly reveal the wire shape of the Cu(OH)₂ nanoparticles, and also show that many nanowires come together to form a bundle-like morphology. The CoFe₂O₄ nanoparticles are spherical and have been uniformly surrounded by Cu(OH)₂ nanowires, indicating the successful coating of CoFe₂O₄ surface with Cu(OH)₂. The images also demonstrated that the agglomeration of CoFe₂O₄ nanoparticles has not occurred. The cobalt ferrite particles are clearly distinguishable as black spherical segments with diameter 40 nm. The gray particles of Cu(OH)₂ are also obviously detectable in TEM images.

Fig. 4 TEM images of CoFe₂O₄/Cu(OH)₂.

The FESEM images confirm the nanowire structure of the catalyst containing the spherical cobalt ferrite nanoparticles, and show that the diameter of wires lies in the range of 28 to 40 nm. Each nanobelt with an average diameter of 32 nm is composed of tiny nanostructured grains that were created during the reaction in solution but has a uniform shape (Fig. 5). The obtained results are in good agreement with TEM and XRD data.

Fig. 5 FESEM images of CoFe₂O₄/Cu(OH)₂.

The chemical composition of CoFe₂O₄/Cu(OH)₂ was approved with EDS analysis. In this analysis, Cu, Co, Fe, and O signals are detectable (Fig. 6).

Fig. 6 EDS of CoFe₂O₄/Cu(OH)₂.

3.1.5. Structure of CoFe₂O₄/Cu(OH)₂ nanocomposite. The possible structure of CoFe₂O₄/Cu(OH)₂ nanocomposite was proposed based on the characterization results (Scheme 4). In the presented catalytic system, copper plays the key role in the [3 + 2] cycloaddition between alkyne and azide components, and the use of nano-CoFe₂O₄ as a magnetic support facilitates the separation, accumulation and recycling of nanoparticles.

Scheme 4 Structure of CoFe₂O₄/Cu(OH)₂ as a surface modified nanocatalyst.

3.2. Catalytic activity of CoFe₂O₄/Cu(OH)₂ for the synthesis of 1,4-disubstituted β-hydroxy-1,2,3-triazoles

The optimization of reaction conditions was conducted with styrene oxide, phenylacetylene and sodium azide as a model reaction. The quantity of catalyst, temperature, time and solvents were investigated as the main optimization factors (Table 1). The desired result was obtained using styrene oxide (1 mmol), phenylacetylene (1 mmol) and sodium azide (1.2 mmol) in the presence of CoFe₂O₄/Cu(OH)₂ (0.032 g, 0.10 mmol, 10 mol%) in water at 60 °C (entry 5). As can be seen, the use of catalytic amount of nanocatalyst was essential to perform the reaction and in the absence of catalyst, no cyclization product was generated after 10 h (entry 1). The model reaction was tested using the various amounts of CoFe₂O₄/Cu(OH)₂ (0.01, 0.02, 0.025, 0.032 and 0.045 g), and the best result was achieved using (0.032 g) of catalyst (entries 2–6). The enhancement of catalyst amount from 0.01 to 0.032 g, not only increased the reaction speed, but also led to raise the product yield significantly. Applying the more amounts of nanocatalyst did not ameliorate the product yield (entry 6).

Table 1 Nano-CoFe₂O₄/Cu(OH)₂-catalysed reaction of styrene oxide with phenylacetylene and sodium azide under different conditions^a

Entry	CoFe2O4/Cu(OH)2 (g)	Solvent	Time (h)	Temperature (°C)	Yield^b (%)
a All reactions were carried out with styrene oxide (1 mmol), phenylacetylene (1 mmol) and sodium azide (1.2 mmol).b Isolated yields.c Catalysed by CoFe2O4.d Catalysed by Cu(OH)2.
1	—	H2O	10	60	0
2	0.01	H2O	8	60	25
3	0.02	H2O	8	60	55
4	0.025	H2O	6	60	85
5	0.032	H2O	3.3	60	94
6	0.04	H2O	3	60	94
7	0.032	EtOH	4	78	50
8	0.032	CH3CN	4	82	35
9	0.032	EtOAc	4	77	60
10	0.032	MeOH	4	65	50
11	0.032	DMF	4	100	45
12	0.032	THF	24	60	0
13	0.032	n-Hexane	24	68	0
14	0.032	H2O	8	25	50
15	0.032	H2O	5	40	65
16^c	0.032	H2O	5	100	Trace
17^d	0.032	H2O	3.5	60	90

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The effect of solvents was also studied by performing the cyclization reaction in various solvents. It was observed that the reaction was carried out in polar solvents, such as water, ethanol, acetonitrile, ethyl acetate, methanol and dimethylformamide whereas no reaction took place in nonpolar solvents (entries 7–13). The reaction was accomplished efficiently in H₂O and in comparison with water, the product yields were lower in all other solvents; hence, this solvent was selected as a green and eco-friendly option for synthesis of triazoles.

In order to determine the effect of temperature, the reaction was repeated at different temperatures (25 °C, 40 °C and 60 °C). At room temperature (25 °C), the reaction was carried out in 8 h affording the moderate yield of product (entry 14). But increasing the temperature encouragingly raised both the product yield and reaction speed, and the corresponding triazole was prepared in 65% yield after 5 h at 40 °C (entry 15). With further increase in temperature to 60 °C the yield increased to 94% after 3.3 h (entry 5).

Performing the mentioned reaction under the optimized conditions but in the presence of CoFe₂O₄ (without the copper hydroxide) produced a trace of the corresponding triazole after 5 h (entry 16). Cobalt ferrite without Cu(OH)₂ was not capable of conducting the reaction. The reaction was also investigated using bare Cu(OH)₂ nanoparticles under the same conditions (entry 17). The reaction was carried out within 3.5 h giving 90% yield of product. The presence of copper hydroxide significantly enhanced the catalytic capability. These results approved the essential catalytic role of copper hydroxide particles. It also showed that the interactions between the Cu(OH)₂ and CoFe₂O₄ nanoparticles may increase the catalytic activity of CoFe₂O₄/Cu(OH)₂ composite. Cobalt ferrite plays the role of a magnetic support that enhances the effectiveness of the catalyst by increasing the contact surface and easy separability.

In order to exhibit the diversity of the CoFe₂O₄/Cu(OH)₂ catalyst, the optimized conditions were applied to epoxides containing either electron-donating or -withdrawing substituents, and cyclic epoxides as shown in Table 2. In addition, the reaction of aliphatic terminal alkynes with styrene oxide was also considered under mentioned conditions (entries 18–21). All reactions were carried out successfully within 3–6.5 h to give 1,2,3-triazoles in 75–95% yields.

Table 2 One-pot synthesis of β-hydroxy-1,2,3-triazoles from epoxides catalyzed by nano-CoFe₂O₄/Cu(OH)₂^a

Entry	Epoxide (a)	Alkyne	Triazole (b)	Time (h)	Yield^b (%)
a All reactions were carried out with 1 mmol of epoxide in the presence of alkyne (1 mmol), sodium azide (1.2 mmol) and nano-CoFe2O4/Cu(OH)2 (0.032 g, 0.1 mmol, 10 mol%) in water at 60 °C.b Yields refer to isolated pure products.
1				3.3	94
2				3	95
3				3.3	90
4				4.3	88
5				4.5	85
6				4.5	87
7				5.5	75
8				4.3	80
9				4.2	85
10				6.3	84
11				5	82
12				6	85
13				6	87
14				6.5	88
15				5	80
16				5	90
17				6	92
18				5.5	85
19				4	90
20				5	88
21				6.5	84

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3.3. Recycling of CoFe₂O₄/Cu(OH)₂ nanoparticles

The recyclability of nano-CoFe₂O₄/Cu(OH)₂ was investigated in the synthesis of 3-chloro-1-(4-phenyl-1H-1,2,3-triazol-1-yl)propan-2-ol as halogen containing sample (Table 2, entry 15) and 2-hydroxy-3-(4-phenyl-1H-1,2,3-triazol-1-yl)propyl methacrylate with the lowest yield in the first run (Table 2, entry 7). After completion of the reaction, the reaction mixture was cooled to room temperature and the nanoparticles were easily collected by an external magnet, washed several times with distilled water and ethanol and dried at 70 °C for 1 h. The recovered catalyst was reused 6 times without any significant loss of activity, under the same conditions (Fig. 7). The structure of the recovered CoFe₂O₄/Cu(OH)₂ nanocatalyst was confirmed using VSM, XRD, FESEM and TEM analyses after five runs (Fig. 8). Comparison of the results obtained from the recycled catalyst and the freshly prepared nanoparticles revealed that the magnetic properties, original structure and morphology of the catalyst remained unchanged after several reuses.

Fig. 7 Recycling of nano-CoFe₂O₄/Cu(OH)₂ in the synthesis of 3-chloro-1-(4-phenyl-1H-1,2,3-triazol-1-yl)propan-2-ol and 2-hydroxy-3-(4-phenyl-1H-1,2,3-triazol-1-yl)propyl methacrylate.

Fig. 8 (a) Magnetization curve, (b) XRD pattern, (c) FESEM and (d) TEM images of CoFe₂O₄/Cu(OH)₂ after five runs in the synthesis of 3-chloro-1-(4-phenyl-1H-1,2,3-triazol-1-yl)propan-2-ol.

The hot filtration test was used for confirming the heterogeneity of as-prepared catalyst. Accordingly, the filtration of the catalyst was carried out after 30 min at 80 °C and the filtrate was allowed to react for additional 2 hours, but the reaction due to the absence of copper did not take place, and no cycloaddition reaction was occurred.

3.4. Comparison of CoFe₂O₄/Cu(OH)₂ catalytic activity with other catalysts

The comparison of the catalytic activity of the prepared CoFe₂O₄/Cu(OH)₂ nanocomposite with various catalysts in the reaction of styrene oxide, phenylacetylene and sodium azide is summarized in Table 3. As can be seen, our proposed procedure is more efficient than the other methods in terms of product yield, and it is also comparable to the most of them from viewpoints of reaction time and temperature. The study of reported literature shows that magnetic nanocatalysts have been rarely used for one-pot synthesis of 1,2,3-triazoles from epoxides (entries 2 and 3), and the overwhelming majority of the reported cases have been carried out in the presence of non-magnetic catalysts (entries 4–11). On the other hand, in some cases, the use of acid or base alongside the catalyst is essential for the reaction progress (entries 10 and 11). Therefore, our method, due to the use of ferromagnetic nanocatalyst, takes advantage of both heterogeneity benefits and magnetic properties. In addition, performing of reaction in a green solvent under neutral conditions is really important privilege. Easy preparation and utilization of surface modified nanoparticles, simple separation just by an external magnet, convenient recovering, reusability after several cycles without loss of catalytic activity or magnetic property and subsequently facilitating the achievement to the goals of green chemistry are among the remarkable advantages of CoFe₂O₄/Cu(OH)₂ catalytic system, which highlight its importance in comparison with the most reported catalysts. Furthermore, the one-pot reaction due to the reduction of work-up procedures and purification steps required compared to a more stepwise approach was used in order to conduct synthesis in a greener fashion.

Table 3 Comparison of CoFe₂O₄/Cu(OH)₂ catalytic activity with various catalysts reported for the one-pot synthesis of 2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethanol from styrene oxide, phenylacetylene and sodium azide^a

Entry	Catalyst system	NaN3 (mmol)	Conditions	Time (h)	Yield (%)	Ref.
a All reactions were carried out with styrene oxide (1 mmol) and phenylacetylene (1 mmol).b bhppda = N^2,N^6-bis(2-hydroxyphenyl)pyridine-2,6-dicarboxamidato dianion.
1	CoFe2O4/Cu(OH)2 (10 mol%)	1.2	H2O/60 °C	3.3	94	This work
2	Nano CuFe2O4 (10 mol%)	1.1	H2O/60 °C	6	87	29
3	Nano Fe2O3 (10 mol%)	1.2	H2O/100 °C	5	81	45
4	Cu nanoparticles/C (0.5 mol%)	1.1	H2O/70 °C	8	93	23
5	Cu(I)@phosphorated SiO2 (0.64 mol%)	1.1	H2O/60 °C	1	93	30
6	Copper(I)–zeolite (8 mol%)	1.2	H2O/r.t.	20	77	27
7	CuI (5 mol%)	1.1	PEG-400/r.t.	16	83	37
8	CuCl2 (10 mol%)	1	H2O/r.t.	5	35	31
9	Cu(NO3)2·3H2O (10 mol%)	1	H2O/r.t.	5	36.8	31
10	[Cu(bhppda)H2O]^b (5 mol%)	1.2	Ascorbic acid/H2O/r.t.	2	90	28
11	CuSO4·5H2O (10 mol%)	1.2	Sodium ascorbate/H2O/r.t.	4	92	26

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3.5. The proposed mechanism for synthesis of 1,4-disubstituted β-hydroxy-1,2,3-triazoles catalyzed by CoFe₂O₄/Cu(OH)₂ nanocomposite

The proposed mechanism for the formation of β-hydroxy-1,4-disubstituted-1,2,3-triazoles includes two pathways (A and B) in which CoFe₂O₄/Cu(OH)₂ catalyzes both ring opening and 1,3-dipolar cycloaddition reactions.²⁸ The pathway A demonstrates the formation of metal azide which is followed by epoxide activation via CoFe₂O₄/Cu(OH)₂ catalyst. Ring opening of epoxide takes place through azide delivery from the catalyst and leads to the formation of 2-azido-2-arylethanol. The regioselective cleavage of oxiranes carrying alkyl and allyl substituents by azide ion is strongly favored from less hindered carbon of the epoxide ring via S_N2 type of mechanism (β-cleavage); however, aryl-substituted epoxides prefer to be opened from the more sterically hindered position via S_N1 type of mechanism (α-cleavage).

The pathway B displays the insertion of copper catalyst to the C–H bond of terminal alkyne in order to activate the phenylacetylene and produce the intermediate (I), which facilitates the [3 + 2] cycloaddition between carbon–carbon triple bond of the 2-azido-2-arylethanol and in situ generated intermediate (II), to give the Cu–C-triazole (IV). The alkyne consumption and also the generation and the disappearance of the 2-azido-2-alkylethanol intermediate, were monitored by gas chromatography (GC) analysis and thin layer chromatography (TLC) runs of the reaction mixture. Finally, protonolysis of the Cu–C bond of intermediate (IV) by aqueous media affords the corresponding β-hydroxy-1,2,3-triazole (V) (Scheme 5).

Scheme 5 Reaction mechanism proposed for the one-pot synthesis of 1,4-disubstituted β-hydroxy-1,2,3-triazole catalyzed by CoFe₂O₄/Cu(OH)₂ bifunctional catalyst.

4. Conclusions

In summary, the magnetic nanocatalyst of CoFe₂O₄/Cu(OH)₂ has been successfully prepared through a simple method and it was then characterized using various techniques such as FESEM, EDS, TEM, XRD, VSM and FT-IR. This novel composite has been demonstrated to be an efficient catalyst for one-pot synthesis of a wide variety of 1,4-disubstituted-1,2,3-triazoles via three component reactions of sodium azide, terminal alkyne, and different epoxides in water. The simple procedure, short reaction times, perfect regioselectivity, high product yields, the use of benign solvent, easy separation of catalyst using an external magnet and efficient recycling are remarkable aspects of this proposed protocol that make it to be an important addition to the existing methodologies for synthesis of 1,4-disubstituted-1,2,3-triazoles.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support of this work by the Research Council of Payame Noor University is gratefully acknowledged.

References

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Footnote

† Electronic supplementary information (ESI) available: Spectral data for β-hydroxy-1,2,3-triazoles. See DOI: 10.1039/c9ra06038c

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