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Copper Dithiol #Complex Supported on Silica #Nanoparticles: A Sustainable, Efficient and Eco-#friendly #Catalyst for Multi-component Click Reaction mahnaz tavassoli, Amir Landarani-Isfahani, Majid Moghadam, Shahram Tangestaninejad, Valiollah Mirkhani, and Iraj Mohammadpoor-Baltork ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01432 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 4, 2016

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ACS Sustainable Chemistry & Engineering

Copper Dithiol Complex Supported on Silica Nanoparticles: A Sustainable, Efficient and Eco-friendly Catalyst for Multicomponent Click Reaction Mahnaz Tavassoli, Amir Landarani-Isfahani, Majid Moghadam,* Shahram Tangestaninejad,* Valiollah Mirkhani, Iraj Mohammadpoor-Baltork Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran Corresponding Author Majid Moghadam Tel:

[email protected]

+98-31-37934920

KEYWORDS: copper(II) triflate, ionic liquid, silica nanoparticle, 1,2,3-triazole, water-in-oil microemulsion.

ABSTRACT

Silica nanoparticles supported copper containing ionic liquid (SNIL-Cu(II)) provided a highly stable, active, reusable, spherical, and solid-phase catalyst for click chemistry. The SNIL-Cu(II) catalyst was readily prepared from 1,2-bis(4-pyridylthio)ethane immobilized on silica nanoparticles modified with 3-chloropropyltrimethoxysilane and Cu(OTf)2, and the morphology, structure and properties of nanoparticles were investigated through different analytical tools. This catalytic system showed high activity in a one-pot synthesis of 1,4-disubstituted 1,2,3triazoles by click reactions between variety of alkynes, organic halides, and sodium azide at

ACS Paragon Plus Environment

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room temperature in aqueous poly ethylene glycol as a green media with high turnover frequency (up to 7920 h−1). Moreover, the SNIL-Cu(II) was also used as an efficient catalyst for the preparation of a series of multi-fold 1,4-disubstituted 1,2,3-triazoles. Also, this unique catalyst was readily reused without any decrease in its catalytic activity to give the corresponding triazoles quantitatively. INTRODUCTION Over the past decade, ionic liquids (ILs) have received a great deal of attention in a wide range of different areas reaching from material synthesis to separation science as well as alternative reaction media. In fact, their fascinating properties such as low vapor pressure, wide liquid range, good conductivity, thermal and chemical stability, and modification of the chemical structures of their ions offer a basis for new and improved technologies.1-5 Recently, the heterogeneous immobilized ILs have attracted considerable interest due to the extended advantages such as easy recovery and reusability, low cost and operational simplicity.610

In this regard, nanostructure solid supports exhibit higher activity and selectivity than their

corresponding bulk materials, because of their high surface area, catalyst loading capacity and good dispersion.11-12 So, design and preparation of novel and efficient series of immobilized nano catalysts derived from metal ion containing ILs, may afford an opportunity to achieve new catalytic materials with practical high performance engineering applications.13-17 Among transition metals, Cu(II) complexes have long been found as efficient catalysts in a variety of chemical reactions, because of their low cost, high selectivity and numerous industrial applications.17-19 Moreover, the use of Cu(II) complexes as ILs have rapidly increased for organic transformations in recent years.20-24


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A notable copper-catalyzed reaction is Huisgen 1,3-dipolar cycloaddition in which organic azides and terminal alkynes are united to afford 1,4-disubstituted 1,2,3-triazoles in a region selective manner.25-27 The copper-catalyzed azide–alkyne cycloaddition (CuAAC), broadly known as the azide/alkyne “click” reaction.28 "Click chemistry" concept refers to a group of reactions that are fast, simple to use, high yielding, versatile, wide in scope, modular, stereospecific, generated only safe by-products that can be removed without chromatography. Besides, the reaction had to proceed under green conditions, with readily available and inexpensive starting materials.29 1,2,3-Triazoles family is an important category of organic compounds regarding their various biological and pharmacological activities such as anti-allergic, anti-bacterial, and anti-HIV.30-36 So far, alternative copper-catalyzed processes have been developed to synthesize regioselective 1,2,3-triazoles.37-43 At the outset, simple Cu(I) species were directly used in this transformation. In the following, because of the instability of Cu(I) salts, the active species were generated in situ from reduction of Cu(II) salts such as CuBr2, CuSO4 and Cu(OTf)2 with ascorbate salts.44,45 These systems can be extremely efficient, but, they suffer from serious drawbacks including the side effect of heavy metal impurities on molecular biological activity and copper contamination of isolated desired product.46 Hence, copper species were immobilized on different solid supports such alumina,47 zeolite

48

, magnetic nanoparticles

49

and silica

50

to solve these problems.

However, long reaction times, the use of large amounts of copper or/and easy catalyst leaching are the limitations and drawbacks of many of the previously reported methods. To overcome the above mentioned problems and as a consequence of our efforts on the development of efficient nano catalytic systems for useful synthetic organic transformations,51-53 herein, we wish to report an efficient three-component synthesis of mono- and multi-fold 1,4-


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disubstituted 1,2,3-triazoles in the presence of immobilized copper(II)-containing ionic liquid on silica nanoparticles as a reusable catalyst (Scheme 1). Scheme 1. Click Reaction Catalyzed by SNIL-Cu(II)

EXPERIMENTAL SECTION General Remarks The chemicals were purchased from Fluka and Merck chemical companies. FT-IR spectra were recorded on a JASCO 6300D spectrophotometer. 1H and

13

C NMR (400 and 100 MHz)

spectra were recorded on a Bruker Avance 400 MHz spectrometer using CDCl3 as solvent. Elemental analysis was performed on a LECO CHNS-932 analyzer. Thermogravimetric analysis (TGA) was carried out on a Mettler TG50 instrument under air flow at a uniform heating rate of 10 °Cmin-1 in the range 30-650 °C. The scanning electron micrographs were taken on a Hitachi S-4700 scanning electron microscope (SEM). The transmission electron microscopy (TEM) was carried out on a Philips CM10 transmission electron microscope operating at 100 kV. The copper content of the catalyst was determined by a Jarrell-Ash 1100 ICP analysis. Preparation of Silica Nanoparticles (SN)


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A mixture of Triton X-100 (35.4 g), hexanol (32 mL), methyl cyclohexane (150 mL), water (8 mL) and aqueous ammonia (2 mL) was stirred at room temperature for 30 minutes. Then tetraethyl orthosilicate (TEOS) was added to solution and stirred for 24 h. After that, 3chloropropyltrimethoxysilane (CPTMS) was added and stirred at room temperature for 24 h. The reaction mixture was centrifuged at 4000 rpm for 30 minutes; the solid material was washed two times with ethanol and one times with water. Synthesis of 1,2-Bis(4-pyridylthio)ethane A mixture of 4-bromopyridinium chloride (5.1 mmol, 1 g), 1,2-ethanedithiol (0.3 mL, 3.5 mmol) and sodium hydroxide (7.7 mmol, 0.3 g), in DMF (10 mL) was stirred at 80 ˚C for 24 h. Reaction progress monitored by TLC (eluted with ether/ methanol, 2:1). The mixture with water and EtOAc and the catalyst was separated by filtration. The aqueous layer was extracted, and evaporated under reduced pressure. Then, residue was purified by recrystallization in methanol to afford the pure product. Immobilization of Ionic Liquid on Silica Nanoparticles (SNIL) In a round-bottomed flask equipped with a condenser and a magnetic stirrer, a mixture of silica nanoparticles (1.0 g) and 1,2-bis(4-pyridylthio)ethane, (0.4 g, 1 mmol) in DMF (10 mL) was stirred at 80 °C for 6 h. Next, the reaction mixture was centrifuged at 4000 rpm for 20 minutes and the resulting solid washed with methanol (2 × 20 mL) and dried in a vacuum oven at 40 °C. Then, to a mixture of obtained nanoparticles (1.0 g) in dry toluene (10 mL) was added methyl iodide (3 mmol) and stirred at room temperature. Finally, the solution was centrifuged at 4000 rpm for 20 minutes and the precipitate washed with ether (2 × 20 mL) and dried in a vacuum oven at 50 °C. Immobilization of Cu(II) Triflate on NSIL (SNIL-Cu(II))


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A mixture of Cu(OTf)2 (1 mmol, 3.6 mg) in ethanol (10 mL), was added to SNIL (1 g). The reaction mixture was refluxed for 24 h. The reaction mixture was centrifuged; the solid material was washed with methanol (2 × 20 mL) and then dried in a vacuum oven at 40 °C to afford the SNIL-Cu(II) catalyst. General Procedure of Synthesis 1,4-Disubstituted 1,2,3-Triazoles from Organic Halides A mixture of SNIL-Cu(II) (0.05 mol%) and sodium ascorbate (2 mol %) in PEG-400/H2O (1:1, 2 mL) was allowed to stirrer for 2 min and then aryl halide (1 mmol), sodium azide (1.2 mmol) and alkyl/phenyl acetylene (1 mmol) were added to mixture. The reaction mixture was stirred at room temperature and monitored by TLC (eluting with ether/ethyl acetate, 2:1). After completion of the reaction, water and ethyl acetate were added and the catalyst was separated by centrifugation and washed with acetone and water, and dried under vacuum. Organic layer was separated and dried over Na2SO4. The products were purified by recrystallization from nhexane/EtOAc. General Procedure Synthesis of Multi-Fold 1,4-Disubstituted 1,2,3-Triazoles A mixture of SNIL-Cu(II) (0.1 mol %) and sodium ascorbate (4 mol%) in PEG-400/H2O (1:1, 4 mL) was allowed to stirrer for 2 min, and then, 1,4/1,2- bis(bromomethyl) (1 mmol), sodium azide (2.4 mmol) and alkyl/phenyl acetylene (2 mmol) were added to the mixture. The reaction mixture was stirred at room temperature. The reaction progress was monitored by TLC until total conversion of the precursor. The work-up was performed as described. RESULTS AND DISCUSSION Synthesis and Characterization of SNIL-Cu(II) Synthesis and Modification of Silica Nanoparticles


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First, silica nanoparticles were synthesized by modification of previously reported water-in-oil (W/O) microemulsion process (Scheme 2).54 The particle size and dispersity of nanoparticles can be easily controlled by tuning the microemulsion properties according to various scientific studies.54-56 The organic moiety of nanoparticles was determined about 12% by thermogravimetric analysis (TGA) (Figure 2a). Preparation of Supported Ionic Liquid on Silica Nanoparticles (SNIL) The preparation route for supported ionic liquid on silica nanoparticles (SNIL) is shown in Scheme 2. The ligand, 1,2-bis(4-pyridylthio)ethane, was prepared by the reaction of 4bromopyridinium chloride with 1,2-ethandithiol. The reaction of this ligand with modified nanoparticles, afforded the supported ligand. Then, nanoparticles were reacted with methyl iodide in order to prepare the supported ionic liquid. These processes were monitored by elemental analysis, FT-IR spectroscopy and TGA. The FT-IR was applied to characterize the samples prepared in each step (Figure 1). The bands correspond to the asymmetric and symmetric stretching vibration of Si-O-Si bond can be clearly observed at 1105 and 811 cm-1 in the FT-IR spectra of functionalized silica nanoparticles (SN) and silica nanoparticles supported ionic liquid (SNIL). Furthermore, the bands at 2863 and 2975 cm-1, were assigned to C-H stretching vibrations. After preparation of SNIL, a new characteristic vibrational bands correspond to the stretching vibration of pyridinium ring can clearly be observed at 1635 and 1595 cm-1 (Figure 1b). Also, the C-S band of ligand at 980 cm-1 was masked in the FT-IR spectrum, due to the presence of strong Si-O band in the range of 10001200 cm-1.


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Scheme 2. Schematic Preparation of Immobilized Cu(II) Containing Ionic Liquid on Silica Nanoparticles (SNIL-Cu(II))

O O

Si

Cl

O

CPTMS + TEOS

O

Si

Cl

Si

ClN

IS

S

Si

(OTf)2

Cl

1) N

O

S

S

N

DMF, 80 oC, 24 h

Cu(OTf)2 EtOH, 78 oC 24 h

2) CH3I, Toluene r.t, 6 h W/O Microemulsion

CH3

Cu(II)

O O

N

Surface modifide silica nanoparticles

SNIL

SNIL-Cu(II)

Figure 1. The FT-IR spectra of (a) Functionalized silica nanoparticles (SN) and (b) Supported ionic liquid on silica nanoparticles (SNIL).

The nitrogen content of the SNIL, measured by CHNS analysis, showed a value of about 2.9%. Based on this value, the amount of supported IL on the silica nanoparticles is about 1 mmolg-1.


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The weight loss of the nanoparticles between 30-650 oC as a function of temperature was determined using TGA, which is an irreversible process because of the thermal decomposition (Figure 2). The first step of weight loss (between 30-200 °C) in the case of silica nanoparticles corresponds to the removal of physically adsorbed solvents, whereas, in the other cases, the main weight loss in the second step (between 200-500 °C) is due to the removal of organic moieties on the surface. The observed total weight loss for SNIL is about 32%, which demonstrates that the IL grafting on nanoparticles had been successfully achieved.

Figure 2. Thermogram of: (a) Surface modified silica nanoparticles (SN) and (b) Supported ionic liquid on silica nanoparticles (SNIL).

After preparation and characterization of SNIL, copper was immobilized on the nanoparticles via complexation of copper(II) triflate with ligand to produce the silica nanoparticles supported ionic liquid copper complex, SNIL-Cu(II), (Scheme 2). The copper content of this catalyst, measured by ICP analysis, was obtained to be 0.21 mmolg-1 of SNIL-Cu(II).


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The morphology of the surface of SN and SNIL-Cu(II) was studied by scanning electron microscopy (SEM) (Figures 3a and 3b). As can be seen, the particles are spherical and have sizes within the range of nanometers. Furthermore, the energy dispersive X-ray (EDX) results, obtained from SEM analysis for the SN and SNIL-Cu(II) are shown in Figures 3c and 3d, which clearly shows the presence of copper in the SNIL-Cu(II) catalyst.

Figure 3. SEM image of: (a) SN and (b)SNIL-Cu(II). SEM-EDX spectrum of: (c) SN and (d) SNIL.


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The results for element distribution mapping of SNIL-Cu(II) are shown in Figure 4. These images show that the copper complex is homogeneously dispersed on silica nanoparticles and the catalytic active sites are isolated which is responsible for high catalytic activity of this catalyst under mild conditions.

Figure 4. Energy dispersive X-ray (EDX) mapping analysis of SNIL-Cu(II).

Further characterization of SNIL-Cu(II) was performed by transmission electron microscopy (TEM) (Figure 5). The dark regions or black spots in the photograph demonstrate the copper species while the colorless parts belong to silica nanoparticles, this is due to the higher electron density of copper compared to silica nanoparticles.

Figure 5. TEM Image of SNIL-Cu(II).


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After, synthesis and characterization of the catalyst, the catalytic activity of this heterogeneous catalyst was investigated in the click reaction. Initially, the effect of different copper(II) catalysts such as CuCl2, Cu(OTf)2, Cu(OTf)2-IL and SNIL-Cu(II) was investigated in the three-component reaction between 4-bromobenzyl bromide (1 mmol), phenylacetylene (1 mmol), and sodium azide (1.1 mmol) in the presence of Na-ascorbate was chosen as model reaction. The results, which are summarized in Table 1, demonstrate that Cu(OTf)2 immobilized on SNIL is the best catalyst and shows higher catalytic compared to CuCl2, Cu(OTf)2 and Cu(OTf)2-IL as homogeneous catalysts. The high catalytic activity compared to homogeneous counterpart can be attributed to dispersion of catalytic active site on high surface area silica nanoparticles. Such behavior has been reported previously for supported catalysts.57 The model reaction was also carried out in the presence of 0.1 mol% of conventional copper (I) species such as CuI and CuCl. Under these conditions, the desired product was obtained 32% and 45%, respectively (entries 5 and 6). The catalytic activity of Cu(OTf)2 is less than CuI and CuCl because the first step in the case of copper(II) species is its reduction to copper (I) in the presence of Na-ascorbate which increases the reaction time, while in the case of CuI and CuCl the reaction times are shorter. Table 1. Investigation of catalytic activity of Different Copper Salts in the Click Reaction.a

Entry 1 2 3 4 5 6

Copper Salt CuCl2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 CuCl CuI

Catalyst Type SNIL-Cu(II) SNIL-Cu(II) Cu(OTf)2-ILc -

[Cu] mol% 0.05 0.05 0.1 0.1 0.1 0.1

Time 15 min 15 min 4h 1h 1h 1h

Yield (%)b 14 75 32 58 45 32

a

Reaction conditions: 4-bromobenzyl bromide (1 mmol), phenylacetylene (1 mmol), sodium azide (1.2 mmol), and Na-ascorbate (2 mol%) in PEG-400 (2 mL) at r.t. b All yields are isolated yields. c 4,4'-(ethane-1,2-diylbis(sulfanediyl))bis(1-


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methylpyridin-1-ium) iodide was used as ionic liquid.

Then, the reaction parameters such as the catalyst amount, solvent type and temperature were examined to obtain the most appropriate conditions for click reaction. The results are summarized in Table 2. The model reaction was carried out in different solvents such as PEG, H2O, EtOH, DMSO, CH3CN, toluene, hexane, aqueous PEG and DMF (Table 2, entries 1-10). It was found that, the solubility of sodium azide is an efficient parameter on the yield of the desired product. In aprotic polar and nonpolar solvents, the reaction gives very low yield probably because of poor solubility of sodium azide; while, in polar protic solvents, higher yield of the corresponding triazole was obtained. Among the solvents examined, PEG-400/H2O (1:1) was found to be the best reaction medium (Table 2, entry 6). Also, the effects of the amount of catalyst was explored. As can be seen, the yield decreased obviously with decreasing the catalyst amount (Table 2, entry 11) by its increasing did not affect the yield of the desire product (Table 2, entry 12). Furthermore, increasing the reaction temperature to 75 °C did not decrese the reaction time (Table 2, entries 13 and 14). Obviously, in the absence of Na-ascorbate as reducing agent for conversion of Cu(II) to Cu(I), the corresponding 1,2,3-triazole obtained in low yield after 4 h (Table 2, entry 15). Therefore, we concluded that 0.05 mol% of the catalyst and 2 mol% of Na-ascorbate in PEG/H2O (1:1) at room temperature are the most appropriate reaction conditions for this transformation.

Table 2. Optimization of reaction conditions for three component click reaction catalyzed by SNIL-Cu(II).a

Entry 1 2

Cayalyst (mol%) 0.05 0.05

Solventa PEG H 2O

T RT RT

Time (h) 0.25 1

Yield (%)b 75 58


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3 0.05 EtOH RT 2 60 4 0.05 DMSO RT 2 12 5 0.05 DMF RT 2 25 6 0.05 PEG/H2O (1:1) RT 0.25 99 7 0.05 DMF/H2O (1:1) RT 2 50 8 0.05 CH3CN RT 2 52 9 0.05 Hexane RT 4 12 10 0.05 Toluene RT 4 28 11 0.03 RT 2 51 PEG/H2O (1:1) 12 0.08 RT 0.25 99 PEG/H2O (1:1) 13 0.05 50 °C 0.25 99 PEG/H2O (1:1) 75 °C 0.25 98 14 0.05 PEG/H2O (1:1) 15c 0.05 RT 4 Trace PEG/H2O (1:1) a Reaction was performed using 2 mL of solvent. b Isolated yield. c Without sodium ascorbate.

To survey the scope and versatility of the SNIL-Cu(II) in the click reaction, a variety of benzyl bromide, alkynes and sodium azide were reacted under the optimal reaction conditions. As shown in Scheme 3, both aromatic and aliphatic terminal acetylenes gave the corresponding triazoles in excellent yields, high purity, and excellent regioselectivity, in which only 1,4regioisomeric products were formed (3a-i). More importantly, the benzyl bromides containing electron-donating and electron-withdrawing substituents reacted efficiently to afford the desired products in high yields. The turnover frequency (TOF) of the SNIL-Cu(II) reached to 7920 h−1. To the best of our knowledge, this is the highest TOF obtained for copper catalyzed Huisgen cycloaddition. In order to further widen the applicability of this catalytic system and due to the various applications of the multi-fold 1,4-substituted triazole isomers in the field of supramolecular chemistry, chemical biology, and materials chemistry, their preparation were also studied.58-64 As shown in Scheme 4, the "bis-click" reaction of 1,2- or 1,4-bis(bromomethyl)benzene with terminal alkynes and sodium azide was performed efficiently in the presence of SNIL-Cu(II) catalyst at room temperature and the corresponding bis(triazole) 5a-c were obtained in 80-90% yields.


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Scheme 3. SNIL-Cu(II) Catalyzed Three-Component Synthesis of 1,4-Disubstituted 1,2,3Triazoles from Benzyl Halidesa

a

Reaction conditions: benzyl bromide (1 mmol), alkyne (1 mmol), sodium azide (1.2 mmol),

SNIL-Cu(II) (0.05 mol%), Na-ascorbate (2 mol%) in PEG-400/H2O (1:1) (2ml) at r.t. All yields are isolated yields.

Scheme 4. SNIL-Cu(II) Catalyzed Synthesis of Multi-fold 1,4-Disubstituted 1,2,3-Triazoles from Benzyl Halidesa


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a

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Isolated yield.

Furthermore, three-component multicyclization of 1,3,5-tris(bromomethyl)benzene as core unit with alkynes and sodium azide led to the formation of a tris(triazole) as flower-shaped molecules in 72-94% catalytic system has much activity yield (Scheme 4, 7a-c). Notably, these results show that the present catalytic system has much higher activity and TOF for preparation of multi-fold 1,4-disubstituted 1,2,3-triazoles compared to Cu(II)[email protected] This may be due to the ionic liquid nature of this catalyst which acts as a pseudo-homogeneous catalyst.65


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Compared to most of the previously reported methods, the yield with the Cu(II)-PsIL catalyst is higher, the reaction time is shorter and the turn over frequency (TOF) is higher, indicating the efficiency of this method (Table 3). Compared to polystyrene supported copper complex, the silica supported copper catalyst is more efficient. This can be attributed to the nano nature of silica particles in which the catalytic active species dispersed on a material with higher surface area.57

Table 3. Comparison of conventional protocols used for 1,4-disubstituted 1,2,3-triazoles.

a

Time [min]

Yield [%]a

TOF [-1h]

Ref.

PEG/H2O (1:1), 65 ˚C

15

99

7920

This work

Cu(II)-TD@nSiO2b

Ethanol/H2O (2:1), r.t

17

99

1166

53

Cu(II)-PsIL

PEG-400, 65 ˚C

15

99

1980

57

CuNP

H2O, 70 ˚C

180

95

7.3

66

[MNPs@FGly][Cl]

H2O/t-BuOH (1:3), 55 ˚C

120

97

97

67

SiO2-CuI

H2O, r.t

16

90

69

68

CuFe2O4

H2O, 70 ˚C

180

93

6.2

69

Nano copper (I)

Ethanol, reflux

60

92

92

70

P4VPy-CuI c

H2O, reflux

20

89

215

71

Catalyst

Conditions

SNIL-Cu(II)

Isolated yield.

b

c

Copper immobilized on nanosilicatriazinedendrimer.

Copper iodide nanoparticles on poly(4-vinyl pyridine).


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The recycling and reusability of a catalyst are important from economic and environmental points of view. Therefore, the reusability of SNIL-Cu(II) was examined in the reaction of 4bromobenzyl bromide with phenyl acetylene and in the presence of sodium azide under the optimized conditions. After completion of reaction, the reaction mixture was diluted with water and ethyl acetate and the catalyst was separated by centrifugation and reused. The results showed that the catalyst could be reused six consecutive times without significant loss of its catalytic activity (Figure 6). Comparison of the FT-IR spectrum of the recovered catalyst after the 6th run with fresh catalyst showed no obvious changes in the structure of the catalyst which showed the stability of the catalyst during the catalytic cycles (See SI, Figure S35).

Figure 6. Reusability of the SNIL-Cu(II) catalyst in the reaction of benzyl bromide, phenyl acetylene and sodium azide.

The amount of copper leached was determined by ICP-OES. The result of ICP analysis for the reaction showed no copper species. To establish the heterogeneity of three component click reaction, a hot filtering test was performed for this reaction at 75 °C.72,73 The reaction mixture


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was filtered (centrifuged) at 75 °C after 7 min, at which the SNIL-Cu(II) and the desired product were readily removed. The filtrate was stirred but no further progress was observed (Figure 7).

Figure 7. Hot filtration test in the click reaction catalyzed by SNIL-Cu(II).

A possible three component click reaction pathway in the presence of SNIL-Cu(II) catalyst is illustrated in Scheme 5. First, Cu(II) is reduced to Cu(I) by the reaction of Na-ascorbate to produce the compound 1 which in turns react with an alkyne to give a copper acetylide 2.74,75 This copper acetylide complex can be converted to a dinuclear copper complex 3 (the use of a weakly coordinating trifluoromethanesulfonate (OTf) ligand is important for formation of dinuclear complex). Both compounds 2 and 3 can catalyzed the Huisgen 1,3-dipolar cycloaddition.76 As mentioned previously,76 the pathway involved the dinuclear complex 3 is the kinetically favored pathway. Since in the present catalytic system, high catalytic activity is observed, the reaction may continue via formation of compound 3. Then, the compound 3 reacts with NaN3 and benzyl bromide to give the compound 4 which final converts to corresponding 1,2,3-triazole and releases the catalyst for the next catalytic cycle. The formation of Cu acetylide was monitored by FT-IR technique (Figure 8). Gratifyingly, the characteristic absorption peak


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corresponding to stretching vibration of Cu-C≡C (2) in the heterogeneous nano catalyst was observed at 1941 cm−1.77 In contrast, in the absence of Na-ascorbate, no peak corresponds to CuC≡C was observed. These results indicate that the heterogeneous Cu(II) catalyst was reduced by sodium ascorbate to give the Cu(I) species which in turns reacted with an alkyne to give the Cu acetylide. To check the potential ability of this catalyst in the Glaser reaction, first phenylacetylene and SIL-Cu(II) was stirred in presence of Na-ascorbate at room temperature for 45 min. Then, the sodium azide and benzyl bromide were added to the reaction mixture. As expected, the click reaction was performed and 1,2,3-triazole product was isolated with high yield and purity in short reaction time. While, if the Glaser coupling reaction could be occurred, the 1,2,3-triazole wasn’t produced. Scheme 5. Proposed Mechanism


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Figure 8. FT-IR spectrum of: (a) mixture of SNIL-Cu(II), phenylacethylene and Na-ascorbate and (b) phenyl acethyelene.

Conclusion In conclusion, we have demonstrated a novel copper-containing ionic liquid immobilized on silica nanoparticles (SNIL-Cu(II)) for synthesis of mono- and multi-fold 1,4-disubstituted 1,2,3triazoles via three-component reaction. The noteworthy features of this catalytic system are high catalytic activity, ease of separation and reusability of the catalyst, and environmentally-benign reaction conditions. The SNIL-Cu(II) provides the highest TOF so far obtained for a copperimmobilized nano catalyst-promoted Huisgen 1,3-dipolar cycloaddition under green conditions. ASSOCIATED CONTENT There is no Funding Source for this manuscript.


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Corresponding Authors Majid Moghadam

Shahram Tangestaninejad

[email protected]

[email protected].

Supporting Information Copies of 1H and

13

C NMR spectra of the products. This material is available free of charge via the

Internet at http://pubs.acs.org.

ABBREVIATIONS SN, functionalized silica nanoparticles; SNIL, silica nanoparticles supported ionic liquid; SNILCu(II), silica nanoparticles supported copper containing ionic liquid; TEOS,

tetraethyl

orthosilicate; CPTMS, 3-chloropropyltrimethoxysilane. ACKNOWLEDGEMENT The authors thank the Research Council of the University of Isfahan.

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For Table of Contents Use Only: Copper Dithiol Complex Supported on Silica Nanoparticles: A Sustainable, Efficient and Eco-friendly Catalyst for Multi-component Click Reaction Mahnaz Tavassoli, Amir Landarani-Isfahani, Majid Moghadam,* Shahram Tangestaninejad,* Valiollah Mirkhani, Iraj Mohammadpoor-Baltork

SNIL-Cu(II) provided a highly stable, active, reusable, and solid-phase catalyst for preparation of a series of mono- and multi-fold 1,4-disubstituted 1,2,3-triazoles.


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Copper Dithiol Complex Supported on Silica ... - ACS Publications

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