Functionalized multi-walled carbon nanotubes as an efficient reusable heterogeneous catalyst for green synthesis of N-substituted pyrroles in water

Abstract

In this protocol, the functionalization of multi-walled carbon nanotubes (MWCNTs) was carried out and the resulting sulfonated MWCNTs were characterized by FT-IR, XRD, SEM, BET, EDX, XPS and Raman spectroscopy that are each discussed separately in the text. Then, the MWCNT–SO₃H composite was applied as an efficient, recyclable heterogeneous catalyst for the synthesis of N-substituted pyrroles via the reaction of 2,5-dimethoxy tetrahydrofuran with primary amines under clean and mild conditions. In this reaction, the N-substituted pyrroles were obtained as beneficial and significant products in short reaction times (30–65 min) and good to excellent yields (40–92%) with high purity. The products were obtained through a simple work up procedure and characterized by FT-IR, ¹H NMR and ¹³C NMR. After the end of the reaction, the nanocatalyst was recovered and reused several times without efficient loss of its activity for the preparation of N-substituted pyrroles.

---

Introduction

Since carbon nanotubes were detected by Iijima in 1991, they have received remarkable attention. These elongated tubular macromolecules, consisting of regular carbon hexagons in a concentric procedure with both ends normally capped by fullerene-like structures, can be seen as a graphite sheet rolled into a nanoscale tubular form as single-walled carbon nanotubes (SWCNTs), or with additional graphene tubes around the core of a SWCNT as multi-walled carbon nanotubes (MWCNTs).¹

Functionalized carbon nanotubes possess vast potential as components of sensors and nanoscale electronics. The panorama of applications has led to the successful functionalization of single-walled carbon nanotubes (SWCNTs) and multi-waledl carbon nanotubes (MWCNTs).^2–10 The chemical modification of CNTs is a method employed to increase their number of applications. Different methods such as changing the chemical behaviour using highly concentrated acids,^11,12 wrapping, sheathing or grafting the CNTs with polymer chains^13–16 and functionalizing in plasma conditions^17–19 have been used to better the functionalities of the CNTs surfaces. The chemical oxidation of nanotubes is generally achieved using gaseous or liquid oxidants. A more general procedure used for the oxidation of CNTs is the use of aqueous solutions of oxidizing agents. The oxidation of MWCNTs using acid-sonication has been studied by Xing et al., who demonstrated that the dispersion stability and solubility of the treated MWCNTs in water improved significantly.²⁰ Ozone in the presence of an oxidative ozonide cleavage reagent (H₂O₂) has been described as a good oxidizing agent.²¹ Some methods for sulfonating nanotubes have been reported to be hydrothermally and microwave assisted under various conditions.^22–26

The substituted pyrroles are a major class of heteroaromatic molecules that are components in a diverse range of biologically active natural products and industrially useful compounds.

Pyrroles are present in different bioactive drug molecules such as immunosuppressant, anti-inflammatory and antitumor agents.²⁷ In view of their high importance, many methodologies have been developed for the synthesis of pyrrole structures.²⁸

Among them, the Paal–Knorr²⁹ synthesis reminds the most useful preliminary method for the produced pyrroles. Some of the catalysts that have been utilized in these methods are; glacial acetic acid,³⁰ P₂O₅,³¹ FeCl₃·7H₂O,³² TfOH³³ and β-cyclodextrin.³⁴

In continuation of our previous research on catalytic reactions for the synthesis of heterocyclic compounds,^35,36 herein, we hope to report the efficient sulfonation of MWCNTs and the application of the resulting MWCNTs–SO₃H composites as a heterogeneous catalyst for the synthesis of N-aryl pyrroles. These heterocycles were produced through the treatment of 2,5-dimethoxy tetrahydrofuran with various primary aromatic amines in water as a green media.

Experimental

General information

All commercially available reagents were used without further purification and purchased from the Merck Chemical Company in high purity. The used solvents were purified by standard procedures. IR spectra were obtained as KBr pellets on a Perkin-Elmer 781 spectrophotometer and on an Impact 400 Nicolet FTIR spectrophotometer. ¹H NMR and ¹³C NMR were recorded in CDCl₃ and d₆-DMSO solvents on a Bruker DRX-400 spectrometer with tetramethylsilane as an internal reference. UV-vis spectra were recorded with a Perkin-Elmer 550 spectrophotometer in a CH₂Cl₂ solvent. The nanostructures were characterized using a Holland Philips Xpert X-ray powder diffraction (XRD) diffractometer (CuK, radiation, k = 0.154056 nm), at a scanning speed of 2° min⁻¹ from 10° to 100° (2Ø). The morphological study of the nanocomposites was investigated by scanning electron microscopy (SEM, Leo 1455VP). The Electron Dispersive X-ray (EDX) analysis of the catalyst was performed on a Philips Holland model XL30. The surface area analysis (BET) was performed using an automated gas adsorption analyzer (NANO SORD). The Raman spectra were recorded with an Almega Thermo Nicolet Dispersive Raman spectrometer excited at 532 nm. X-ray photoelectron spectroscopy (XPS) spectra were measured on an ESCA-3000 electron spectrometer with non-monochromatized Mg Kα X-rays as the excitation source. Melting points were obtained with a Yanagimoto micro melting point apparatus and are uncorrected. The purity determination of the substrates and reaction monitoring were accomplished by TLC on silica-gel polygram SILG/UV 254 plates (from Merck Company).

General procedure for the synthesis of MWCNTs–SO₃H. Preparation of the MWCNTs–SO₃H catalyst was performed in two steps, as described in Scheme 1. MWCNTs (1 g) and deionized water (100 ml) were added to a beaker and sonicated for 30 min. After sonication, the solvent was removed under vacuum and the obtained MWCNTs were transferred to another flask containing 25 ml HNO₃ (63%) and 25 ml HCl (37%), and stirred at 80 °C for 4 h under a nitrogen atmosphere. Then, the solution was filtered under vacuum, washed throughout with deionized water and dried at 100 °C for 10 h. In this step, the MWCNTs–COOH material was obtained. The MWCNTs–COOH (1 g) and deionized water (50 ml) were sonicated for 15 min. Then, the water was filtered and the MWCNTs–COOH material was dried. Then, 40 ml H₂SO₄ (98%) was added to the set-up at 250–270 °C for 20 h under a nitrogen atmosphere. After cooling the solution, the liquid was filtered and washed throughout with deionized water several times. The obtained solid was dried at 100 °C for 12 h. After this step, the MWCNTs–SO₃H composite was obtained and used as a catalyst in the synthesis of N-aryl pyrroles.

Scheme 1 Preparation of the MWCNTs–SO₃H catalyst.

General procedure for the synthesis of N-aryl pyrroles. MWCNTs–SO₃H (0.04 g) was added as a catalyst to a mixture of 2,5-dimethoxy tetrahydrofuran (1.2 mmol) and different primary aromatic amines (1 mmol) in water (5 ml) as solvent. Then, the reaction mixture was heated (80 °C) and stirred at different times in water. The progress of the reactions was monitored by TLC (ethyl acetate : n-hexane 6 : 4). After the completion of the reaction, the catalyst along with the solid product was collected by filtration from the reaction mixture. The filtrated solid was dissolved in chloroform, the catalyst was separated from the reaction mixture and washed with chloroform. The solvent was removed in vacuum to give the crude product. Then, the crude product was recrystallized from ethanol, the N-aryl pyrroles were obtained as pure products. The recovered catalyst (0.019 g) was reused in the reaction for 4 runs. All of the pure products were characterized by comparison of their physical and spectral data with authentic samples.^37–47

Results and discussion

Preparation and characterization of the catalyst

In the first step, the MWCNTs were converted to MWCNTs–COOH with HCl (37%) and HNO₃ (63%). This step was carried out in order to maintain the purity of the MWCNTs and to remove fullerene, amorphous carbon and catalyst particles such as Fe, Co, Ni and increase the reactivity. This step also introduced oxygen comprising groups, mainly the carboxyl groups on the MWCNTs.^48,49 After this step, sulfuric acid (98%) was added to MWCNTs–COOH to obtain MWCNTs–SO₃H.

The prepared catalyst was characterized by SEM, EDX, XRD, FT-IR, BET, XPS and Raman spectroscopy. The spectroscopic data of FT-IR confirmed that the functionalization of the MWCNTs with sulfonic acid groups had occurred (Fig. 1a and b).

Fig. 1 (a) FT-IR spectrum of the MWCNTs, (b) FT-IR spectrum of MWCNTs–SO₃H.

This spectrum shows peaks at about 1400 and 1100 cm⁻¹ which correspond to the SO₂ asymmetric and symmetric stretching modes, respectively. In addition, in the low frequency part of the spectrum, the line at 670 cm⁻¹ is assigned to the S–O stretching mode and the peak shown at 582.96 cm⁻¹ is assigned to the C–S stretching mode, suggesting the existence of covalent sulfonic acid groups. The broad band centred at about 3400 cm⁻¹ is the contribution of the acidic OH group. The XRD images of the MWCNTs before and after the H₂SO₄ treatment are shown in Fig. 2a and b. As can be seen in Fig. 2, the XRD pattern for the MWCNTs (Fig. 2a) has a slight difference in peak width compared to the peak related to MWCNTs–SO₃H (Fig. 2b) suggesting that the structure of the MWCNTs was not changed after the functionalization to MWCNTs–SO₃H.

Fig. 2 (a) XRD spectrum of the pristine MWCNTs, (b) XRD spectrum of MWCNTs–SO₃H.

The SEM images of the MWCNTs before and after the H₂SO₄ treatment are shown in Fig. 3. Compared with the primitive MWCNTs, the functionalized MWCNTs were covered by a layer and they are severely entangled and form a big agglomeration that results in thickened MWCNTs. Also, the EDX image of MWCNTs–SO₃H is shown in Fig. 4. For MWCNTs–SO₃H, the results obtained by EDX were as follows (%): C, 95.80; O, 1.73; S, 2.47. This image confirmed that oxygen and sulphur were contained in the functionalized MWCNTs. Furthermore, the EDX image infers that functional groups such as sulfonic acid were attached to the MWCNTs.

Fig. 3 (a) SEM image of the MWCNTs, (b) SEM image of MWCNTs–SO₃H.

Fig. 4 The EDX pattern of the MWCNTs–SO₃H catalyst.

Another technique used to characterize MWCNTs is Raman spectroscopy. In the 1300–1600 cm⁻¹ region of the spectrum, two bands are observed showing the characteristics of MWCNTs. These bands point to the graphite band (G-band) and the disorder and defects of the structure, named the D-band. A higher ratio between the intensities of the D-band and the G-band, noted as the I_D/I_G value, corresponds to a higher proportion of sp³ carbon atoms. Fig. 5 shows the Raman spectra of the MWCNTs and MWCNTs–SO₃H. As shown in Fig. 5a, the spectrum exhibits two peaks at about 1345 and 1583 cm⁻¹. The feature at 1583 cm⁻¹ is the G-band and the origin of the line at 1345 cm⁻¹ is the D-band with regards to the MWCNTs. Also, in Fig. 5b, the two peaks at 1349 and 1583 cm⁻¹ are the D-band and the G-band of MWCNTs–SO₃H, respectively. It was calculated that the I_D/I_G values of the MWCNTs and MWCNTs–SO₃H are 0.85 and 1.70, respectively. This increase in the I_D/I_G value indicates that some of the sp² carbon atoms were converted to sp³ carbon atoms on the side walls of MWCNTs. This matter can be related to the functionalization of the MWCNTs by sulfonic acid groups.

Fig. 5 Raman spectra of (a) the MWCNTs, and (b) MWCNTs–SO₃H.

XPS spectra were measured to investigate the chemical valences of the functional groups attached to the surface of the MWCNTs. As shown in Fig. 6, the spectra of the C1s region, O1s region and S2p region were assigned. The main peak appears at 284.9 eV with a non-symmetrical edge. This peak generally refers to the sp² carbon–carbon double bond or the sp³ carbon–carbon single bond.⁵⁰ Also, this peak is related to the sp²-hybridized graphite carbon. The peak at 288.7 eV can be determined to be due to the carbon atoms bound to oxygen or sulphur in functionalized groups e.g. carboxylic acid or sulfonic acid groups.⁵⁰ The O1s region shows that all the peaks occur in the range of 530.9 to 534.2 eV, with a peak maxima at ∼533.1 eV.²² In the S2p region, the main peak of sulphur in the sulfonic acid groups was observed at 169.2 eV. The peak at 169.2 eV is related to a higher oxidation state of sulphur in the SO₃H groups.²²

Fig. 6 The XPS spectra of MWCNTs–SO₃H in the regions of C1s, O1s, and S2p.

The surface area of the initial MWCNTs and MWCNTs–SO₃H as a target solid catalyst was determined by a BET technique. The corresponding results were indicated in Table 1. The decrease in the total and unit surface of the MWCNTs after sulfonation proved the presence of –SO₃H as acidic groups on the surface of the solid catalyst (Table 1). It can be concluded that the sulfonation of carbon nanotubes had taken place, because the physical adsorption of nitrogen gas after functionalization using sulphuric acid was decreased to nearly 50% lower than that for the raw MWCNTs.

Table 1 Surface area of the MWCNTs and MWCNTs–SO₃H

Name	S.A (BET) (m^2 g^−1)	S.A (total) (m^2)
MWCNTs	62.775	0.816
MWCNTs–SO3H	30.925	0.619

---

In order to investigate the catalytic activity of MWCNTs–SO₃H, it was compared compared with other catalysts. The reaction of 2,5-dimethoxy tetrahydrofuran with 4-methoxy aniline in a 1.2 : 1 molar ratio was done in the presence of various catalysts under thermal conditions in water as a solvent. It was found that for the reactions in the presence of CuFe₂O₄, Fe₃O₄, TiO₂, ZnS nano particles, MWCNTs, MWCNTs–COOH, MWCNTs/H₂SO₄ and Ph-SO₃H, the products were not obtained in suitable yields (Table 2), while the reaction in the presence of MWCNTs–SO₃H as a heterogeneous catalyst in the same conditions achieved appropriate results on the basis of yield and reaction time (Table 2, entry 5). Then, the reaction was carried out in the presence of MWCNTs–SO₃H (Scheme 2).

Table 2 Investigation of the different catalysts at 80 °C in water

Entry	Catalyst	Time (h)	Yield (%)
1	CuFe2O4	12	40
2	Fe3O4	5	30
3	TiO2	12	—
4	ZnS	8	—
5	MWCNTs–SO3H	1	30
6	MWCNTs	7	—
7	MWCNTs–COOH	5	10
8	MWCNTs/H2SO4 (a drop)	5	—
9	Ph-SO3H	3	17

---

Scheme 2 Reaction of 2,5-dimethoxy tetrahydrofuran and primary aromatic amines.

Then, the optimization of the catalyst amount in the reaction was investigated. The reaction of 2,5-dimethoxy tetrahydrofuran with 4-methoxy aniline in a 1.2 : 1 molar ratio was carried out in different concentrations of catalyst under thermal conditions in water. The corresponding results are presented in Table 3. By consideration of the results in Table 3, the optimum amount of MWCNTs–SO₃H used as a catalyst in this reaction was obtained as 0.04 g per one mol of aniline derivative (Table 3, entry 4).

Table 3 Optimization of the catalyst amount at 80 °C in water

Entry	Amount of catalyst (g)	Time (min)	Yield (%)
1	0.01	120	30
2	0.02	120	45
3	0.03	90	60
4	0.04	45	92
5	0.05	45	92

---

After the optimization of catalyst amount, the reaction was carried out in various solvents (Table 4). The best results were obtained using water as solvent (Table 4, entry 4). Then, the reaction of 2,5-dimethoxy tetrahydrofuran with 4-methoxy aniline (1.2 : 1 molar ratio) in the presence of MWCNTs–SO₃H catalyst (0.04 g) in aqueous solution was carried out at various temperatures (Table 5). The best results were obtained at 80 °C (Table 5, entry 3).

Table 4 Optimization of different solvents in the presence of MWCNTs–SO₃H (0.04 g) at 80 °C

Entry	Solvent	Time (min)	Yield (%)
1	EtOH	30	5
2	CH3CN	30	40
3	DMF	30	80
4	H2O	30	90

---

Table 5 Optimization of temperature in the presence of MWCNTs–SO₃H (0.04 g) in water

Entry	Temp. (°C)	Time (min)	Yield (%)
1	60	45	70
2	70	45	83
3	80	45	92
4	90	45	90
5	100	45	87

---

To ascertain the limitation of this protocol, the reactions of 2,5-dimethoxy tetrahydrofuran with several primary aromatic amines were carried out according to the general experimental procedure. The corresponding products and their results are summarized in Table 6. In this study, N-aryl pyrroles as products in an efficient method were prepared through the reaction of 2,5-dimethoxy tetrahydrofuran and various primary aromatic amines in the presence of a heterogeneous catalytic amount of MWCNTs–SO₃H (0.04 g) under thermal conditions in water. A comparison of the present method with previous works using other catalysts was done and the related results were indicated in Table 6, entries 13–16.^37,51

Table 6 Synthesis of various N-aryl pyrroles using MWCNTs–SO₃H (0.04 g) as catalyst under thermal conditions (80 °C) in water

Entry	Amine	Product	Time (min) : yield^a (%)
a Isolated yields.b Reaction conditions: 10% MgI2(OEt2)n, CH3CN, 80 °C, Lit.^51c Reaction conditions: AcOH, MW, 170 °C, Lit.^37
1			45 : 88
2			50 : 89
3			45 : 92
4			50 : 87
5			30 : 92
6			30 : 90
7			55 : 88
8			50 : 88
9			55 : 78
10			65 : 40
11			60 : 72
12			60 : 70
13^b			720 : 62
14^b			360 : 66
15^c			10 : 70
16^c			10 : 77

---

Recycling of the catalyst

For the practical application of heterogeneous catalysts, the life-time of the MWCNTs–SO₃H and its level of reusability are very important features. At the end of each reaction, the catalyst was isolated by filtration, washed exhaustively with chloroform and ethanol, and dried at 100 °C for 24 h before being used with fresh materials. The catalyst was reused for 4 runs; the yields ranged from 95% to 90% (Fig. 7).

Fig. 7 Reusability of the catalyst in the reaction.

The structures of N-aryl pyrroles as products were confirmed by physical and spectroscopic data such as their melting point, IR, ¹H NMR and ¹³C NMR spectroscopy. The ¹H NMR spectrum of 1-(3-nitrophenyl)-1H-pyrrole shows signals around δ = 6.33–7.34 ppm that are assigned to the protons of the aromatic rings (Fig. 8). In the ¹³C NMR spectra of the above mentioned compound, signals are shown at around δ = 110.0–150.0 ppm that are assigned to the carbon atoms of the aromatic rings (Fig. 9).

Fig. 8 ¹H NMR spectrum of 1-(3-nitro phenyl)-1H-pyrrole.

Fig. 9 ¹³C NMR spectrum of 1-(3-nitro phenyl)-1H-pyrrole.

A plausible mechanism⁵² for the formation of N-aryl pyrroles is presented in Scheme 3. As can be seen, in the first step, the acetal group of 2,5-dimethoxy tetrahydrofuran was protonated to form compound a. Then, the nucleophilic attack of hydroxyl group of the substrate to the carbon atom of the acetal occurred to produce the intermediate b. Next, b was protonated by MWCNTs–SO₃H and dehydrated to convert to the dicarbonyl compound (d). Then, d was protonated by MWCNTs–SO₃H, then nucleophilic attack of the amino group to the carbonyl group of e took place to produce f. Finally, f was protonated, cyclized and dehydrated to give N-aryl pyrrole as a desired product.

Scheme 3 Plausible reaction mechanism for the preparation of N-aryl pyrroles.

Conclusion

In this research, the synthesis of N-aryl pyrroles using 2,5-dimethoxy tetrahydrofuran with different primary aromatic amines was described. This reaction was performed in the presence of a catalytic amount of MWCNTs–SO₃H (0.04 g) under thermal conditions. The corresponding products were obtained in excellent yields, high purity and short reaction times. The products were confirmed by physical and spectroscopic data such as their melting point, FTIR, ¹H NMR, ¹³C NMR spectroscopy.

Acknowledgements

The authors are grateful to the University of Kashan for supporting this work by grant number 159148/52.

References

1. Y. Wang, Z. Iqbal and S. V. Malhotra, Chem. Phys. Lett., 2005, 402, 96–101.
2. M. J. O'Connell, P. Boul, L. M. Ericson, C. Huffman, Y. Wang, E. Haroz, C. Kuper, J. Tour, K. D. Ausman and R. E. Smalley, Chem. Phys. Lett., 2001, 342, 265–271.
3. M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S. Mclean, S. R. Lustig, R. E. Richardson and N. G. Tassi, Nat. Mater., 2003, 2, 338.
4. E. T. Michelson, I. W. Chiang, J. L. Zimmerman, P. J. Boul, J. Lozano, J. Liu, R. E. Smalley, R. H. Hauge and J. L. Margrave, J. Phys. Chem., 1999, 103, 4318.
5. J. Chen, A. M. Rao, S. Lyuksyutov, M. E. Itkis, M. A. Hamon, H. Hu, R. W. Cohn, P. C. Eklund, D. T. Colbert, R. E. Smalley and R. C. Haddon, J. Phys. Chem., 2001, 105, 2525.
6. V. Georgakilas, K. Kordatos, M. Prato, D. M. Guldi, M. Holzinger and A. Hirsch, J. Am. Chem. Soc., 2002, 124, 760.
7. J. L. Bahr, J. Yang, D. V. Kosynkin, M. J. Bronikowski, R. E. Smalley and J. M. Tour, J. Am. Chem. Soc., 2001, 123, 6536.
8. F. Pompeo and D. E. Resasco, Nano Lett., 2002, 2, 369.
9. H. Peng, L. B. Alemany, J. L. Margrave and V. N. Khabashesku, J. Am. Chem. Soc., 2003, 125, 15174.
10. K. A. Williams, P. T. M. Veenhuizen, B. G. de la Torre, R. Eritja and C. Dekker, Nature, 2002, 420, 761.
11. I. D. Rosca, F. Watari, M. Uo and T. Akasaka, Carbon, 2005, 43, 3124.
12. V. Datsyuk, M. Kalayva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis and C. Galiotis, Carbon, 2008, 46, 833.
13. A. Eitan, K. Jiang, R. Andrews and L. S. Schadler, Chem. Mater., 2003, 15, 3198.
14. D. E. Hill, Y. Lin, A. M. Rao, L. F. Allard and Y. P. Sun, Macromolecules, 2002, 35, 35.
15. B. Z. Tang and H. Y. Xu, Macromolecules, 1999, 32, 2569.
16. Y. Lin, A. M. Rao, B. Sadanadan, E. A. Kenik and Y. P. Sun, J. Phys. Chem., 2002, 106, 1294.
17. G. T. Lewis, G. R. Nowling, R. F. Hicks and Y. Cohen, Langmuir, 2007, 23, 10756.
18. D. Shao, Z. Jiang, X. Wang, J. Li and Y. Meng, J. Phys. Chem., 2009, 113, 861.
19. Z. Hou, B. Cai, H. Liu and D. Xu, Carbon, 2008, 46, 405.
20. Y. C. Xing, L. Li, C. C. Chusuei and R. V. Hull, Langmuir, 2005, 21, 9.
21. H. Naeimi, A. Mohajeri, L. Moradi and A. M. Rashidi, Appl. Surf. Sci., 2009, 256, 631–635.
22. G. S. Duesberg, R. Graupner, P. Downes, A. Minett, L. Ley, S. Roth and N. Nicoloso, Synth. Met., 2004, 142, 263–266.
23. Y. Wang, Z. Iqbal and S. Mitra, J. Am. Chem. Soc., 2006, 128, 95.
24. H. Yu, Y. Jin, Z. Li, F. Peng and H. Wang, J. Solid State Chem., 2008, 181, 432.
25. R. F. Alamdari, M. Golestanzadeh, F. Agend and N. Zekri, Can. J. Chem., 2013, 91, 982–991.
26. M. M. Doroodmand, S. Sobhani and A. Ashoori, Can. J. Chem., 2012, 90, 701–707.
27. J. M. Muchowski, Adv. Med. Chem., 1992, 1, 109.
28. O. A. Tarasova, N. A. Nedolya, V. Y. Vvedensky, L. Brandsma and B. A. Trofimov, Tetrahedron Lett., 1997, 38, 7241.
29. B. M. Trost and G. A. Doherty, J. Am. Chem. Soc., 2000, 122, 3801.
30. M. Artico, R. Silvestri, S. Massa, A. G. Loi, S. Corrias, G. Piras and P. L. Colla, J. Med. Chem., 1996, 39, 522.
31. Y. Fang, D. Leysen and H. C. Ottenheijm, Synth. Commun., 1995, 25, 1857.
32. N. Azizi, A. Khajeh-Amiri, H. Ghafuri, M. Bolourtchian and M. R. Saidi, Synlett, 2009, 14, 2245.
33. M. Abid, L. Teixeira and B. Torok, Tetrahedron Lett., 2007, 48, 4047.
34. R. Sridhar, B. Srinivas, V. P. Kumar, V. P. Reddy, A. V. Kumar and K. R. Rao, Adv. Synth. Catal., 2008, 350, 1489.
35. H. Naeimi and S. Mohamadabadi, Dalton Trans., 2014, 43, 12967–12973.
36. R. Ghahremanzadeh, Z. Rashid, A.-H. Zarnani and H. Naeimi, Dalton Trans., 2014, 43, 15791–15797.
37. K. C. Miles, S. M. Mays, B. K. Southerland, T. J. Auvil and D. M. Ketcha, ARKIVOC, 2009, xiv, 181–190.
38. H. J. Jung, L. Chang Kiu and Y. Ji Sook, J. Heterocycl. Chem., 2000, 37, 15–24.
39. Y. Haitao, X. Chao, M. Zhiwei and C. Ruyu, Eur. J. Org. Chem., 2011, 18, 3353–3360.
40. F. Ferenc, F. Katalin, T. Angelika and T. Laszlo, Tetrahedron, 1997, 53, 4883–4888.
41. Y. Chao-Wu, H. Chen-Wei, C. Ji-Wang and C. Grace, Org. Lett., 2012, 14, 3688–3691.
42. R. Katla, M. S. Narayana and N. Y. V. Durga, Synth. Commun., 2012, 42, 2471–2477.
43. Z. Rui, X. Lixin, W. Xinyan, C. Chuanjie, S. Deyong and H. Yuefei, Adv. Synth. Catal., 2008, 350, 1253–1257.
44. C. Weiqiang and W. Jianhui, Organometallics, 2013, 32, 1958–1963.
45. K. Hui-Jun, L. Bao-Le, L. Pei-He, L. Ning, L. Ya-Nan, M. Fei-Ping, M. Li-Ping and Z. Zhan-Hui, Appl. Catal., 2013, 457, 34–41.
46. R. V. Prakash, K. A. Vijay and R. K. Rama, Tetrahedron Lett., 2011, 52, 777–780.
47. F. Corelli, S. Massa, G. Stefancich, M. Artico, S. Panico and N. Simonetti, Farmaco, Ed. Sci., 1984, 39, 95–109.
48. A. G. Osorio, I. C. L. Bueno and C. P. Bergmann, Appl. Surf. Sci., 2008, 225, 2485.
49. H. Peng, L. B. Alemany, J. L. Margrave and V. N. Khabishesku, J. Am. Chem. Soc., 2003, 125, 15174.
50. H. Ago, T. Kugler, F. Cacialli, W. R. Salaneck, M. S. P. Shaffer, A. H. Windle and R. H. Friend, J. Phys. Chem. B, 1999, 103, 8116–8121.
51. X. Zhang and J. Shi, Tetrahedron, 2011, 67, 898–903.
52. S. Rivera, D. Bandyopadhyay and B. K. Banik, Tetrahedron Lett., 2009, 50, 5445–5448.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12185j

---