A copper(II)–thioamide combination as a robust heterogeneous catalytic system for green synthesis of 1,4-disubstituted-1,2,3-triazoles under click conditions

Zohreh Mirjafary*a, Leila Ahmadib, Masomeh Moradib and Hamid Saeidianb
aDepartment of Chemistry, Tehran Science and Research Branch, Islamic Azad University, Tehran, Iran
bDepartment of Science, Payame Noor University (PNU), PO Box: 19395-4697, Tehran, Iran. E-mail: zmirjafary@srbiau.ac.ir

Received 17th August 2015 , Accepted 8th September 2015

First published on 8th September 2015


Abstract

An efficient and practical synthesis of 1,4-disubstituted-1,2,3-triazoles under click conditions using a copper(II)–thioamide combination as an efficient heterogeneous catalyst is disclosed. Mild reaction conditions and high yields make this method an attractive option for the preparation of triazole derivatives. The key to this procedure was the generation of Cu(I) required for the azide-alkyne cycloaddition, which was achieved by in situ reduction of Cu(II) using thiobenzanilide as reduction agent and ligand.


Introduction

Aromatic heterocyclic compounds with a five-membered ring containing two carbon atoms and three nitrogen atoms are named 1,2,3-triazoles. These compounds constitute a major class of naturally occurring compounds and privileged medicinal scaffolds that exhibit a broad range of biological and pharmaceutical properties, such as anti-HIV, antibacterial, anticancer, antifungal, and antiviral activities.1–6 Fig. 1 shows representative bioactive 1,2,3-triazole-containing drugs.7–11
image file: c5ra16581d-f1.tif
Fig. 1 Representative bioactive 1,2,3-triazoles.

Developing more efficient methods for the construction of compounds containing triazoles is a topic of immense importance. The synthesis of 1,2,3-triazoles strongly relies on copper(I)-catalyzed azide-alkyne [3 + 2] Huisgen cycloaddition reaction (CuAAC reaction) which is the best ‘click’ reaction to date, due to its wide range of applications in chemistry, biochemistry and polymer chemistry.12,13 A major advance in improving of this reaction was accomplished through the use of copper compounds as catalysts. 1,4-Disubstituted 1,2,3-triazoles were synthesized in the presence of copper(I) with high yields under very mild conditions. Various CuAAC reactions involving different sources of copper(I) and solvents have been developed, leading to 1,4-disubstituted-1,2,3-triazoles.14–20 Copper(I) salts are less used because of their general thermodynamic instability. One possible choice to protect the copper(I) center from oxidation or disproportionation and to enhance its catalytic activity in the CuAAC reactions is to use Cu(I) salts supported by nitrogen, sulfur and polydentate or auxiliary ligands.21–23 However, the preparation of such supported copper(I) catalysts are tedious, expensive and not always easy. On the other hand, recycling and reusability of them because of their generally homogeneous nature are difficult.

It should be noted that copper(II) salts did not work well in CuAAC reactions. Sharpless and co-workers established a very robust catalytic system for the CuAAC reactions, which made use of a less expensive copper(II) precatalyst, along with substoichiometric amounts of sodium ascorbate for an in situ reduction.14 Designing of new specific catalytic system for the CuAAC reactions has caused profound effects in optimizing the efficiency of a wide range of 1,4-disubstituted-1,2,3-triazoles. Development of such catalysts has resulted in more economical and environmentally friendly chemistry through replacing expensive, unstable, or toxic catalysts.

In the context of our general interest in the synthesis of heterocycles and following our research on thioamide chemistry,24–29 herein, we propose a facile synthesis of 1,4-disubstituted-1,2,3-triazoles via the CuAAC reaction in the presence of a copper(II)–thioamide combination as an efficient and inexpensive catalytic system (Scheme 1). Thioamides are a class of organosulfur compounds which recently become a very important functional group in coordination chemistry.30–32 The presence of nitrogen atom and the larger and less electronegative sulfur atom as soft donor enables the thioamides-NH to bind to a metal in different ways, giving a variety of complexes.32 Moreover organosulfur compounds generally can reduce copper(II) species.31 With these two promising properties of thioamides in hands, our attention were focused on using thiobenzanilide in the CuAAC reaction as an reduction agent and ligand.


image file: c5ra16581d-s1.tif
Scheme 1 One-pot synthesis of 1,4-disubstituted-1,2,3-triazoles 3 using copper(II)–thioamide combination.

Experimental

General information

All the chemicals required for the synthesis of 1,4-disubstituted-1,2,3-triazoles 3 were purchased from Sigma-Aldrich (St. Louis, MO, USA), Fluka (Neu-Ulm, Germany) and Merck (Darmstadt, Germany) companies and were used as received. The all synthesized compounds 3 gave satisfactory spectroscopic data. A Bruker (DRX-400 Avance) NMR was used to record the 1H NMR and 13C NMR spectra. All NMR spectra were determined in CDCl3 and DMSO at ambient temperature. Gas chromatography-mass spectrometry (GC-MS) (Agilent HP 6890, electron ionization (EI), 70 eV, HP-5 column (30 m × 0.25 mm × 0.2 μm), HP 5793 mass selective detector) was used to record the mass spectra. IR spectra were taken films KBr pellets on a Nicolet spectrometer (Magna 550). Liquid chromatography-mass spectrometry (LC-MS) analysis was performed on an Agilent 1200 LC system (Agilent, Waldbronn, Germany) equipped with Agilent 6410 triple quadrupole tandem mass spectrometer and managed by a Mass Hunter workstation (Agilent Technologies, CA, USA). All the reactions are monitored by thin layer chromatography (TLC) carried out on silica gel with UV light and iodine, as detecting agents.

General procedure for the synthesis of thiobenzanilide

Thiobenzanilide was prepared according to previously reported procedure via Willgerodt–Kindler reaction (Scheme 2).27 benzaldehyde (4 mmol) was added to a mixture of aniline (6 mmol), elemental sulfur (5 mmol) and Na2S (10 mol%) in DMF (5 mL), then heated at 100 °C for 8 h. After completion of the reaction, monitored by TLC (n-hexane/EtOAc: 5/2), the obtained solid was removed by filtration. The unreacted sulfur was removed by adding 5 mL EtOH, heating and then hot filtration. After cooling, thiobenzanilide was crystallized and separated by simple filtration.
image file: c5ra16581d-s2.tif
Scheme 2 Preparation of thiobenzanilide via Willgerodt–Kindler reaction.

General procedure for the synthesis of copper(I)–thioamide catalytic system

CuCl2 (2 mmol) was added into the solution of thiobenzanilide (2 mmol) in ethanol (1 mL) at room temperature. After 40 min a dark red precipitate was observed which was filtrated, washed with ethanol and dried at room temperature (Scheme 3). The obtained precipitate is stable in air and insoluble in organic solvents, except DMSO and DMF.
image file: c5ra16581d-s3.tif
Scheme 3 General procedure for the synthesis of copper(I)–thioamide catalytic system.

General procedure for the synthesis of 1,4-disubstituted-1,2,3-triazoles 3

To a suspension of copper(I)–thioamide catalyst (10 mg) in H2O/EtOH (5 mL, 1[thin space (1/6-em)]:[thin space (1/6-em)]1), was added sodium azide (0.6 mmol) and alkyl halide (0.5 mmol), and the resulting mixture was stirred at room temperature for 1 h. Phenylacetylene (0.5 mmol) was added to the reaction mixture and the mixture was stirred at an ambient temperature for 3 h. The progress of the reaction was monitored by TLC. After completion of the reaction, the catalyst was recovered by centrifugation and filtration. The catalyst was reused in subsequent reaction without losing any significant activity. Water (5 mL) was added to the reaction mixture and extracted with EtOAc (3 × 10 mL) and dried over Na2SO4. The crude was concentrated under vacuum and was purified by preparative TLC (eluent: petroleum ether/ethyl acetate 4[thin space (1/6-em)]:[thin space (1/6-em)]1) to afford the desired products.

Results and discussion

In an initial attempt to synthesis of 1,4-disubstituted-1,2,3-triazoles 3 using a combination of CuCl2 and thiobenzanilide as catalytic system, we focused on systematic evaluation of different conditions for the model reaction (Table 1). The reaction of phenylacetylene, benzyl bromide and sodium azide was checked without catalyst, and we get no desired product 3a (Table 1, entry 1), while good results were obtained in the presence of copper(I)–thioamide catalytic system after 4 h. We then continued to optimize the model reaction by considering the efficiency of solvent. A mixture of H2O/EtOH (1[thin space (1/6-em)]:[thin space (1/6-em)]1) was much better than H2O and no reaction was performed in EtOH (Table 1, entry 7). The effect of temperature was also studied by carrying out the model reaction at room temperature, 50 °C, and 75 °C. It was observed that the yield was not increased as the reaction temperature was raised to 75 °C. On using the optimized amount of copper(I)–thioamide catalytic system, we found that 10 mg could effectively catalyze the reaction of phenylacetylene (0.5 mmol) and benzyl bromide (0.5 mmol) for the synthesis of the desired product 3a. Using more than 10 mg of the catalyst had no significant effect on the yield. It is interesting to note that replacement of CuCl2 with CuCl produced 3a in moderate yield (Table 1, entry 4). Trace yield of 3a was observed in the absence of thiobenzanilide (Table 1, entry 2).
Table 1 Optimization of the copper(I)–thioamide-catalyzed synthesis of 3aa
Entry Catalyst Solvent T (°C) Yield (%)
a Reaction conditions: solvent (5 mL, 1[thin space (1/6-em)]:[thin space (1/6-em)]1), phenylacetylene (0.5 mmol) and benzyl bromide (0.5 mmol), catalyst (10 mg) and reaction time: 4 h.b Catalyst (5 mg).c Catalyst (20 mg).
1 No catalyst H2O/EtOH rt 0
2 CuCl2 H2O/EtOH rt Trace
3 CuCl H2O/EtOH rt 71
4 CuCl/PhCSNHPh H2O/EtOH rt 86
5 CuCl2/PhCSNHPh H2O/EtOH rt 95
6 CuCl2/PhCSNHPh H2O rt 58
7 CuCl2/PhCSNHPh EtOH rt 0
8 CuCl2/PhCSNHPh H2O/EtOH 50 65
9 CuCl2/PhCSNHPh H2O/EtOH 75 69
10 CuCl2/PhCSNHPh H2O/EtOH rt 70b
11 CuCl2/PhCSNHPh H2O/EtOH rt 91c


As mentioned above, organo-thiols and thiones generally can reduce copper(II) and disulfide compounds are formed according to eqn (1):31

 
2RSH + 2Cu2+ → RSSR + 2Cu+ + 2H+ (1)

It is conceivable that thiobenzanilide can exhibit thione–thiol tautomerism in the presence of Cu2+, so formation of the corresponding disulfide is possible. Acceleration of the CuAAC reaction by using CuCl2–thioamide combination supported that copper(II) is reduced to copper(I) (Table 1, entry 5 versus entry 2). During the redox reaction, thiobenzanilide acting as reduction agent is oxidized to the corresponding disulfide according to eqn (2).

 
image file: c5ra16581d-u1.tif(2)

LC-Mass analysis of the alcoholic solution of catalyst system confirmed the formation of the corresponding disulfide of thiobenzanilide (Fig. 2). The disulfide formed relative low intensity [M + H]+ at m/z 425 under positive ion electrospray ionization (ESI) conditions. The most abundant ion in its ESI-MS is observed at m/z 180 as iminium fragment.


image file: c5ra16581d-f2.tif
Fig. 2 LC-MS spectrum of the corresponding disulfide of thiobenzanilide.

Reduction of Cu2+ to Cu+ by thiobenzanilide is also consistent with the result obtained by 1H NMR analysis of copper(I)–thioamide catalytic system in DMSO-d6 (Fig. 3). NMR spectrum of complex bearing Cu2+ due to paramagnetism properties could not be obtained. The 1H NMR appearance of copper–thioamide catalyst revealed that a redox reaction take place in the combination of CuCl2 with thiobenzanilide in ethanol. The univalence of the copper ions in catalytic system was also confirmed by the measured diamagnetism. It can be considered that probably the generated disulfide from thiobenzanilide acts as a ligand. Therefore the disulfide was synthesized through oxidation of thiobenzanilide by using DMSO–HCl system according to Scheme 4.33 The synthesized disulfide reacted with CuCl2 and CuCl to give the corresponding complexes. The latter complex gave 3a in moderate yield (76%), while the former afforded 3a in very low yield.


image file: c5ra16581d-f3.tif
Fig. 3 1H NMR spectra of copper(I)–thioamide catalyst (top) and the corresponding disulfide of thiobenzanilide (bottom).

image file: c5ra16581d-s4.tif
Scheme 4 Synthesis of the corresponding disulfide of thiobenzanilide by using DMSO–HCl system.

Comparison the 1H NMR spectrum of copper–thioamide catalyst with 1H NMR spectrum of the disulfide (Fig. 3) shows that the thioamide acts as ligand not disulfide. The 1H NMR spectrum of copper(I)–thioamide catalyst consisted of a broad line at δ = 11.82 ppm correlating with the NH and multiple lines for the aromatic protons at δ = 7.83–6.90 ppm.

Unfortunately, all attempts to get single crystals of copper(I)–thioamide catalytic system for X-ray crystallography were failed due to its high insolubility. Assignments of selected characteristic IR bands (4000–500 cm−1) for thiobenzanilide and copper(I)–thioamide catalyst are given in ESI (Fig. 1S and 2S). The positions of bands provide significant hints regarding the bonding sites of the thiobenzanilide when complexed with copper(I). Thiobenzanilide can show thione–thiol tautomerism. The ν(S–H) band at 2550 cm−1 is absent in IR spectrum of thiobenzanilide but the ν(N–H) band is observed at 3418 cm−1, indicating that thiobenzanilide exists in the thione rather than the thiol form in the solid state.34 It shows that a adsorption band of the C[double bond, length as m-dash]S bond is a region 1570–1395 cm−1 (1526 cm−1 for thiobenzanilide),35 which is shifted to 1516 cm−1 after being reacted with copper(II), indicating that the sulfur of the C[double bond, length as m-dash]S participates as a coordinating site. This coordination is confirmed by the absence of the strong band at 1361 cm−1 in IR spectrum of copper(I)–thioamide catalyst.36

Energy-dispersive X-ray spectroscopy (EDS) was used for the chemical characterization of copper(I)–thioamide catalyst. EDS analysis exhibited the existence of chlorine atoms in catalytic system (Fig. 4). CHN analysis exhibited 44.48% C, 3.05% H and 3.73% N. These results indicate that copper(I)–thioamide system has a polymeric or cluster structure which explain its stability and high insolubility.31 A possible structure of one unit in the polymeric complex of copper(I)–thioamide catalyst is shown in Fig. 5.


image file: c5ra16581d-f4.tif
Fig. 4 EDS analysis of copper(I)–thioamide catalyst.

image file: c5ra16581d-f5.tif
Fig. 5 A possible structure of one unit in the polymeric complex of copper(I)–thioamide catalyst.

Next we turned our attention to apply copper(I)–thioamide catalyst for the reaction of phenylacetylene with a series of alkyl/aryl halide to obtain the corresponding 1,4-disubstituted-1,2,3-triazoles 3. All the substrates consistently furnished the desired triazoles in good to excellent yields (Table 2). Formation of azides proceeds via a SN2 mechanism (Scheme 5). It is worth noting that direct displacement reactions take place rapidly in benzylic systems. The π systems of the benzylic group provide extended conjugation, which stabilizes the TS in the SN2 mechanism,37 so benzyl halides afforded the corresponding triazoles in excellent yield, as shown in Table 2. Various alkyl halides were subjected to the same reaction conditions to obtain the corresponding 1,2,3-triazoles. The yields were good in general. The structure of the products was confirmed by 1H-NMR, 13C-NMR, GC-MS and comparison with authentic samples prepared by reported methods (see ESI).20,38

Table 2 Copper(I)–thioamide-catalyzed synthesis of 1,4-disubstituted 1,2,3-triazole derivatives 3
Entry Halide Product Yield (%)
1 image file: c5ra16581d-u2.tif image file: c5ra16581d-u3.tif 95
2 image file: c5ra16581d-u4.tif image file: c5ra16581d-u5.tif 84
3 image file: c5ra16581d-u6.tif image file: c5ra16581d-u7.tif 93
4 image file: c5ra16581d-u8.tif image file: c5ra16581d-u9.tif 89
5 image file: c5ra16581d-u10.tif image file: c5ra16581d-u11.tif 66
6 image file: c5ra16581d-u12.tif image file: c5ra16581d-u13.tif 68
7 image file: c5ra16581d-u14.tif image file: c5ra16581d-u15.tif 61
8 image file: c5ra16581d-u16.tif image file: c5ra16581d-u17.tif 59
9 image file: c5ra16581d-u18.tif image file: c5ra16581d-u19.tif 58



image file: c5ra16581d-s5.tif
Scheme 5 Proposed mechanism for copper(I)–thioamide-catalyzed synthesis of 1,4-disubstituted 1,2,3-triazole derivatives 3.

The reusability of the copper(I)–thioamide catalyst was also studied for the model reaction. The catalyst was recovered by simple filtration, washed with ethanol and reused with consistent activity even after eight catalytic cycles (Fig. 6).


image file: c5ra16581d-f6.tif
Fig. 6 Recyclability of copper(I)–thioamide catalyst in one-pot synthesis of 1,2,3-triazoles.

A possible mechanism for this copper(I)–thioamide catalyst multicomponent reaction was proposed in Scheme 5. The reaction was initiated by the metalation of phenylacetylene in the presence of Cu+, resulting copper acetylide followed by coordination of alkyl/benzyl azides to the copper of the acetylide initiates an azide-alkyne 1,3-dipolar cycloaddition to form the desired products 3.

In summary, a very efficient protocol for the synthesis of 1,4-disubstituted 1,2,3-triazole derivatives was reported via a multicomponent reaction in the presence of a cheap and easily recyclable heterogeneous copper(I)–thioamide catalyst. The catalyst was easily prepared with the reaction of CuCl2 and thiobenzanilide. The catalyst was collected easily by filtration and the reusability of the catalyst was successfully tested for eight runs only a very slight loss of catalytic activity. Further studies of applicability of the heterogeneous copper(I)–thioamide catalyst for synthesis of useful organic compounds such as propargylamines are in progress.

Acknowledgements

We are very grateful to Mr Saleh Vahdati Khajeh for his valuable suggestions on this paper to reach reasonable results.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16581d

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