Copper immobilized onto a triazole functionalized magnetic nanoparticle: a robust magnetically recoverable catalyst for “click” reactions

Abstract

A novel magnetic heterogeneous copper catalyst was synthesized by immobilization of copper ions onto triazole functionalized Fe₃O₄. The catalyst was fully characterized by FT-IR, TGA, CHN, SEM, TEM, EDX and atomic adsorption spectroscopy. The resulting catalyst was used in the synthesis of 1,2,3-triazoles via a one-pot three component reaction of alkynes, alkyl halides, sodium azides under green conditions. The catalyst was reused ten times and no significant loss of activity was observed.

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Introduction

Having a small glance at the recent literature clearly reveals the huge number of investigations related to “click chemistry”.^1,2 Due to the growing demands for pharmaceuticals, chemists have tended to the more challenging task of biologically-oriented syntheses in recent years.³ Conventional procedures used for the total synthesis of drugs and drug-like molecules have some main drawbacks such as being costly, slow and complex, having laborious protection/deprotection and purification steps, etc.⁴ Considering all these facts, finding new, facile and uncomplicated techniques is still a challenge in modern organic chemistry. Click chemistry is one of these new techniques that aims to establish an ideal set of straightforward and highly selective reactions, simplifying compound synthesis and providing faster lead discovery and optimization.^1,2,4a,5 These reactions are simple, modular, stereospecific, wide in scope, result in very high yields and have simple workup and purification steps.^4a,6

A prime example and one of the most investigated click reactions is the Huisgen reaction, the copper-catalyzed 1,3-dipolar cycloaddition of azides to alkynes that results in the formation of a vast number of five-membered heterocycles.⁷ This is a concerted [3 + 2] cycloaddition of type [_π4_s + _π2_s] thermally allowed reactions. Mild reaction conditions (low reaction temperature, minimal workup and purification), very high functional group tolerance, specificity and the water tolerant nature of the reaction are the main advantages.⁸

Since alkynes are poor 1,3-dipole acceptors and also because of the kinetic stability of alkynes and azides, the rate of their reaction is quite slow in the absence of a catalyst and requires elevated temperatures, if it is possible.⁹ Furthermore, the uncatalyzed Huisgen reaction suffers from a main drawback limiting its scope: low regioselectivity in the approaching mode of alkyne to azide.¹⁰ Employing copper-based catalysts in this reaction not only improves the regioselectivity in favor of 1,4-regioisomer but also causes an increase in the rate of the reaction by a factor of 10⁷ and rectifies the need for elevated temperatures.¹¹

The copper(I) required to catalyze this reaction can be originated from several different sources including the direct introduction of Cu(I) salts (usually copper iodide), or in situ reduction of Cu(II) salts using appropriate reducing agents.¹² The latter method, in which sodium ascorbate is usually used as a reducing agent, has been recognized as a very useful method for this purpose. Copper salts such as copper iodide catalyze the Huisgen reaction in a short time with excellent yields under mild conditions. Due to the hard recovery process for homogenous catalysts, the use of homogeneous catalysts could be costly and uneconomical on a large scale. Since trace amounts of metal contamination could have undesirable effects on biological systems, the final products should be completely pure without any copper contamination. Designing heterogeneous catalysts is a useful and applicable method to overcome this problem. Several surfaces such as nanosilica,^13–15 zeolite,^16,17 mesoporous silica,^18,19 guar-gum,²⁰ CuO hollow nanospheres²¹ and charcoal²² have been used as supports for copper immobilization. However, copper immobilization on these surfaces has some disadvantages such as difficult catalyst separation, high copper leaching, low catalyst activity after immobilization and low thermal stability. Magnetic nanoparticles are one of the best solid surfaces for the immobilization of copper.²³ Fe₃O₄ magnetite nanoparticles are well known as the most promising nanomaterials for biomedical and catalytic purposes. Easy separation, high thermal stability, large surface area and low toxicity for biological systems make these nanoparticles an excellent catalyst in organic synthesis even on a large scale.²⁴

Taking into account the high appearance of click chemistry in academic and industrial drug discovery programs and anticipating its growing impact, we intended to report for the first time the synthesis of a new efficient magnetite-base catalytic system, Fe₃O₄–silica-coated@functionalized 3-glycidoxypropyltrimethoxysilane [MNPs@FGly], along with its application in a practical and highly efficient approach to 1,2,3-triazole derivatives. The presented procedure uses phenyl acetylene and 1-heptyne as the alkyne components and various alkyl azides (from the in situ reaction of alkyl halides (tosylates)) as the other components in the presence of sodium ascorbate as a reducing agent to form triazoles. The reaction was carried out in mild and green conditions. The reaction yields were excellent for most of the precursor alkyl halides.

Results and discussions

In this work, the silica-coated magnetite nanoparticles were synthesized according to a procedure reported elsewhere in the literature.²⁵ The magnetic Fe₃O₄ nanoparticles were first prepared by a coprecipitation method and were then used to synthesize [MNPs@FGly][Cl] nanoparticles (Scheme 1) through the procedure explained in the experimental section.

Scheme 1 Immobilization and functionalization of 3-glycidoxypropyltrimethoxysilane on Fe₃O₄@SiO₂.

The FT-IR spectra of Fe₃O₄, Fe₃O₄@SiO₂, [MNPs@FGly] and [MNPs@FGly][Cl] are shown in Fig. 1. The peaks that appear at 650 cm⁻¹ and 1100 cm⁻¹ are attributed to Fe–O and Si–O stretching vibrations of Fe₃O₄ and SiO₂, respectively. The peaks at 2950 and 2930 cm⁻¹ (Fig. 1c and d) are attributed to C–H stretching vibrations and confirm the presence of alkyl silane groups on Fe₃O₄@SiO₂.

Fig. 1 FTIR spectra of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), [MNPs@FGly] (c) and [MNPs@FGly][Cl] (d).

The SEM image of the catalyst shows spherical magnetic nanoparticles (Fig. 2a). The TEM image of the catalyst shows an average size of 5–15 nm. The EDX analysis also shows the presence of copper ions in the catalyst structure (Fig. 2c).

Fig. 2 SEM image (a), TEM image (b) and EDX analysis (c) of [MNPs@FGly][Cl].

The CHN analysis of [MNPs@FGly] revealed that the carbon, nitrogen and hydrogen contents of the catalyst are 19.02, 2.922 and 2.398 wt%, respectively, indicating that the alkyl silane groups were successfully immobilized on the magnetic nanoparticles. Loading calculations based on carbon content showed that the amount of organic groups is 1.13 mmol g⁻¹. The results show that a high level of organosilane has been coated on the magnetic nanoparticles.

The TGA curve of the catalyst (Fig. 3) shows that the decomposition of the organic groups starts at 265 °C and complete degradation occurs up to 720 °C. An organic group content of 30% proved that a high value (1.14 mmol g⁻¹) of triazole groups had been immobilized on the magnetic nanoparticles.

Fig. 3 TGA of [MNPs@FGly].

Atomic absorption spectroscopy (AAS) of the catalyst showed that the amount of copper loaded on the magnetic nanoparticles is about 0.51 mmol g⁻¹. The high yields and selectivity of the products are attributed to high loading of copper ions on the surface of the magnetic nanoparticles.

The activity of the synthesized catalytic system was investigated in the 1,3-dipolar cycloaddition reaction of phenyl acetylene to azides resulting in the formation of various five-membered heterocyclic compounds (Scheme 2).

Scheme 2 Huisgen reaction catalyzed by [MNPs@FGly][Cl].

Initially, the optimized reaction conditions were determined in the reaction between sodium azide, phenyl acetylene and benzylbromide as model substrates using various types of solvents at different temperatures and in the presence of various concentrations of catalyst. The results are listed in Table 1. Firstly, the reaction was carried out at room temperature in the absence of a catalyst. After 24 h, the crude materials remained intact and no significant triazole was detected even when the temperature was increased up to 70 °C (entries 1 and 2).

Table 1 Control experiment for the synthesis of 1,3,5-triazole catalyzed by copper supported [MNPs@FGly][Cl]^a

Entry	Catalyst	Cat. loading (mol%)	Solvent	T (°C)	Time (h)	Yield^b (%)
a Reaction condition: phenyl acetylene (0.5 mmol), benzyl bromide (0.5 mmol), sodium azide (0.5 mmol), sodium ascorbate (10 mol%), solvent (3 mL).b Isolated yield.c The ratio of H2O/t-BuOH is 3/1.d Mainly recovery of the starting materials.e Mixture of regioisomers.f Magnetic catalyst without copper(II).g Without sodium ascorbate.
1	—	—	H2O/t-BuOH^c	r.t	24	<1^d
2	—	—	H2O/t-BuOH	70	8	<3^d
3	[MNPs@FGly]^f	10 mg	H2O/t-BuOH	100	8	<12^d
4	CuSO4	5	H2O/t-BuOH	55	10	28
5	CuSO4	5	H2O/t-BuOH	110	10	87^e
6	[MNPs@FGly][Cl]	1	H2O/t-BuOH	55	2	99
7	[MNPs@FGly][Br]	1	H2O/t-BuOH	55	2	83
8	[MNPs@FGly][NO3]	1	H2O/t-BuOH	55	2	72
9	[MNPs@FGly][OAc]	1	H2O/t-BuOH	55	2	88
10	[MNPs@FGly][Cl]	1	H2O/t-BuOH	r.t	2	80
11	[MNPs@FGly][Cl]	1	H2O/t-BuOH	r.t	9	99
12	[MNPs@FGly][Cl]	0.8	H2O/t-BuOH	55	2	99
13	[MNPs@FGly][Cl]	0.5	H2O/t-BuOH	55	2	99
14	[MNPs@FGly][Cl]	0.5	H2O	55	2	91
15	[MNPs@FGly][Cl]	0.5	t-BuOH	55	4	81
16	[MNPs@FGly][Cl]	0.5	CH3CN	55	4	83
17	[MNPs@FGly][Cl]	0.5	CH3OH	55	4	69
18	[MNPs@FGly][Cl]	0.5	Toluene	55	4	37
19	[MNPs@FGly][Cl]^g	0.5	H2O/t-BuOH	55	5	Trace

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The effects of the type and amount of the catalyst required for the reaction were then examined. Reactions were carried out using different types of catalysts including CuSO₄, [MNPs@FGly][Cl], [MNPs@FGly][Br], [MNPs@FGly][NO₃] and [MNPs@FGly][OAc] (entries 3–9). The effect of different counter ions in the catalyst structure was investigated and as can be seen, the best results were obtained using [MNPs@FGly][Cl] as the catalyst (entry 6). Both polar and non-polar solvents were applied in this reaction in the presence of 0.5 mol% of the catalyst (entries 14–18). In the absence of sodium ascorbate, only a trace of the corresponding triazole was obtained after 5 hours. In order to meet the green chemistry principles, the reaction was also conducted in water as a green solvent (entry 14). The results showed that the reaction was completed in a short time that is comparable with and in many cases is even better than the other organic solvents. However, the yield of the reaction in the mixture of H₂O/t-BuOH (3 : 1) was higher in comparison with each of the solvents separately (entries 14 and 15). Therefore, this mixture was selected as the optimized reaction medium (entry 13).

To optimize the amount of catalyst loaded, the model reaction was repeated in the presence of 1, 0.8 and 0.5 mol% of the catalyst under the optimized reaction conditions (entries 6, 12 and 13). It was demonstrated that using 0.5 mol% of catalyst can effectively catalyze the reaction in a short time with a reasonable yield (entry 13).

Having these optimized reaction conditions in hand, the scope of the reaction between alkynes and alkyl halides was explored in the presence of sodium ascorbate and sodium azide. According to the results shown in Table 2, different alkyl halides (tosylates) can participate effectively in this reaction under the defined conditions. In all cases, the rates of reactions for alkyl chlorides are lower than those of alkyl bromides/tosylates. However, alkyl bromides and tosylates react with almost equal rates, perhaps because of the fact that –Br and –Ts are both good leaving groups and can be substituted by the azide anion in the reaction mechanism, much better than –Cl.

Table 2 Huisgen 1,3-dipolar cycloaddition catalyzed by [MNPs@FGly][Cl]^a

Entry	Alkyne	Alkyl/aryl halide	Product	Time (h)	Yield^b (%)
a Reaction conditions: alkyne (0.5 mmol), alkyl halide (0.5 mmol), NaN3 (0.5 mmol), sodium ascorbate (10 mol%), Cu (0.5 mol%), solvent (H2O/t-BuOH; 3/1), 55 °C.b Isolated yield.
1				2	99
2				3	97
3				2.5	98
4				2	97
5				3	96
6				4.5	99
7				5	95
8				7	92
9				3.5	97
10				2.5	95
11				3	93
12				4	96
13				5	92
14				4	97
15				6	94
16				7	85
17				4.5	89
18				4	95
19				3.5	94
20				4	94
21				5	93
22				3	92
23				3.5	91
24				7	90
25				2	92
26				4.5	89
27				3	93
28				4	93
29				4	94
30				5	93
31				4	91
32				5.5	90
33				4.5	95

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Also, it can be concluded from Table 2 that increasing the steric hindrance on the alkyl halide component causes lower reaction rates as can be realized by comparing entries 2 and 15; 6 and 10; and 25 and 27. In the case of 1,2-dibromoethane as the alkyl halide, the reaction was proceeded from both sides of the chain and both Br atoms participated in the reaction. In general, the yields of the reactions in all cases were excellent and reactions were completed in short times. In the cases of alkynes with electron donor and acceptor groups (entries 31–33), the reactions were conducted with high yields and regioselectivity.

On the other hand, the reaction can be conducted using both linear (1-heptyne) and aromatic (phenyl acetylene) alkynes; however, in most cases better results were obtained by using phenyl acetylene.

According to the results shown in Table 2, all the reactions were proceeded with high regioselectivity and in all cases, only the 1,4-regioisomers were obtained.

To check the reusability of the catalyst, the reaction of phenyl acetylene, benzyl bromide and sodium azide was performed under the aforementioned optimized reaction conditions (Scheme 1). After the first run, the isolated yield of the reaction was measured exactly. Then, the catalysts were separated by using a magnet, washed twice with methanol and dried overnight at room temperature. These recovered nanoparticles were reused in another run under the same reaction conditions. This process was repeated 10 times and each time the isolated yield was calculated exactly. As is shown in Fig. 4 no significant decrease in reaction yield was observed after repeated cycles of the reaction.

Fig. 4 Reuse of [MNPs@FGly][Cl] in the Huisgen reaction between phenyl acetylene and benzyl bromide.

The AAS results indicated that there is no significant metal leaching even after 10 cycles of the reactions. In order to have a reasonable comparison, the reaction times were kept constant in all the experiments and each reaction was stopped after 1 h. The results shown in Scheme 1 clearly indicate that the catalytic efficiency of the catalyst remains almost constant during these cycles. Therefore, according to these observations, the proposed catalytic system can not only catalyze the cycloaddition of phenylacetylene to azides, but can also be easily separated and reused several times without any significant loss of activity.

Conclusion

The synthesis and application of [MNPs@FGly][Cl] in the 1,3-dipolar cycloaddition of phenyl acetylene to azides was examined. This catalyst can catalyze the desired reactions with excellent yields. Furthermore, due to its magnetic characteristics, separation of the catalyst is easy using an external magnetic field; it can be totally removed from the reaction medium and reused in another run. The stability of the nanoparticles to air, moisture and elevated temperatures is very high and none of these parameters can cause their deactivation. In addition, total removal of the copper-based catalyst from the reaction media makes it possible to use the procedure in pharmaceuticals, drug discovery approaches and other sensitive synthetic procedures. Some of the main advantages of the procedure include mild reaction conditions (low reaction temperature, minimal workup and purification), very high functional group tolerance, specificity and the water tolerant nature of the reaction. The reaction under the defined reaction conditions results in the desired triazoles with high regioselectivity in favor of 1,4-regioisomer.

Experimental

1. General data

All the materials used are commercially available and were purchased from Merck and used without any additional purification. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker (Avance DRX-500) spectrometer using CDCl₃ as the solvent at room temperature. Chemical shifts δ were reported in ppm relative to tetramethylsilane as an internal standard. FTIR spectra of the samples were taken using an ABB Bomem MB-100 FTIR spectrophotometer. The morphology of the catalyst was observed by scanning electron microscopy (SEM) using a Philips XL30 scanning electron microscope, transmission electron microscopy (TEM) images were taken using a Philips CM30 electron microscope, and thermogravimetric analysis (TGA) was acquired under a nitrogen atmosphere with a TGA Q 50 thermogravimetric analyzer. CHN analysis was done with a LECO TruSpec analyzer.

2. Synthesis of Fe₃O₄–silica coated nanoparticles

In a three necked flask, a solution of FeCl₃ (66.58 mmol, 10.8 g) and FeCl₂ (31.56 mmol, 4 g) was prepared and diluted with deionized water (50 mL). In order for the pH to reach 10, a solution of ammonium hydroxide (28% v/v) was added dropwise to the solution under the inert atmosphere of argon and the resulting solution was mechanically stirred at room temperature for 20 min. The magnetic nanoparticles were collected by employing an external magnetic field using a magnet and washed with deionized water (30 mL) and ethanol (3 × 30 mL). The synthesized Fe₃O₄ nanoparticles were dispersed in ethanol (80 mL) with the aid of ultrasonic waves for 20 min (ultrasonic power: 100 W). These magnetite nanoparticles were used to synthesize the Fe₃O₄–silica coated nanoparticles through the following procedure. In a round bottom flask 3 mL tetraethyl orthosilicate (TEOS) was slowly added to the solution. Then, 3 mL ammonium hydroxide (28% v/v) was added to the mixture during a 15 min interval while the solution was mechanically stirred. The mixture was stirred for a further 12 h at 40 °C. Finally, the magnetic nanoparticles coated with silica (Fe₃O₄@SiO₂) were collected using an external magnet and washed three times with ethanol (30 mL) and dried under reduced pressure using a rotary evaporator.

3. Immobilization of 3-glycidoxypropyltrimethoxysilane on Fe₃O₄@SiO₂

The synthesized magnetic nanoparticles of Fe₃O₄@SiO₂ (2 g) were added to 10 mL of toluene and sonicated for 30 min. Then, (11.32 mmol, 2.5 mL) of 3-glycidoxypropyltrimethoxysilane was added to the above mixture while stirring and the resulting mixture was refluxed for 48 h. The final product was washed with methanol (3 × 30 mL) and dried under reduced pressure using rotary evaporator.

4. Synthesis of (MNPs@FGly)[Cl]

In a round bottom flask equipped with a magnetic stirring bar, sodium azide (22.7 mmol, 1.5 g), copper(II) chloride (0.74 mmol, 0.1 g) and sodium ascorbate (0.76 mmol, 0.15 g) were dissolved in a mixture of THF/water (80/20). Afterwards, the modified magnetic nanoparticles (1 g) and phenyl acetylene (18.2 mmol, 2 mL) were added. The mixture was stirred and heated at 60 °C for 10 h. The final product was extracted by an external magnet, washed with methanol three times (3 × 20 mL) and dried under reduced pressure.

5. General procedure for the synthesis of triazole

A glass tube was charged with sodium ascorbate (30 mg, 10 mol%), phenyl acetylene (0.5 mmol, 0.055 mmol), benzylbromide (0.5 mmol, 0.06 mL), sodium azide (0.5 mmol, 0.032 g), catalyst (5 mg, 0.5 mol%) and H₂O/t-BuOH with 3/1 ratio (3 mL). The reaction mixture was stirred at 55 °C for 4 h and the completion of the reaction was monitored by TLC (EtOAc/n-hexane, 25 : 75). In each case, after completion, the product was worked up and purified according to the following procedure: the mixture was diluted with ethyl acetate and water. The organic layer was washed with brine, dried over MgSO₄ and concentrated under reduced pressure using a rotary evaporator. The residue was purified by recrystallization from ethyl acetate/n-hexane. In order to reuse the catalyst, the nanomagnetic Cu catalyst was collected using an external magnet, washed with methanol and dried overnight to be ready for the next run.

Acknowledgements

We gratefully acknowledge the funding support received for this project from the Sharif University of Technology (SUT), Islamic Republic of Iran.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13330g

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