Fe<sub>3</sub>O<sub>4</sub> as a magnetically reusable Fenton nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones

Amin Rostami; Sirvan Moradi; Zahra Shokri

doi:10.1016/j.crci.2017.12.003

Fe₃O₄ as a magnetically reusable Fenton nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones

Amin Rostami ^1,² ; Sirvan Moradi ¹ ; Zahra Shokri ¹

¹ Department of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175, Sanandaj, Iran
² Research Center for Nanotechnology, University of Kurdistan, P.O. Box 416, Sanandaj, Iran

Comptes Rendus. Chimie, Volume 21 (2018) no. 2, pp. 80-87.

Résumé

In this study, the oxidation of thiols to sulfonic acids and sulfides to sulfoxides and sulfones was carried out in the presence of Fe₃O₄/H₂O₂ as an efficient heterogeneous Fenton system. The products were obtained in good to excellent yields and short reaction times. Further results showed that the heterogeneous catalyst could be recovered easily using an external magnetic field and reused several times without any loss of its catalytic activity.

Métadonnées

Reçu le : 2017-08-29
Accepté le : 2017-12-11
Publié le : 2018-01-17

DOI : 10.1016/j.crci.2017.12.003

Mots clés : Magnetically reusable, Fenton system, Oxidation, Sulfonic acid, Sulfoxide, Sulfone

Affiliations des auteurs :

Amin Rostami ^{1, 2} ; Sirvan Moradi ¹ ; Zahra Shokri ¹

¹ Department of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175, Sanandaj, Iran
² Research Center for Nanotechnology, University of Kurdistan, P.O. Box 416, Sanandaj, Iran

@article{CRCHIM_2018__21_2_80_0,
     author = {Amin Rostami and Sirvan Moradi and Zahra Shokri},
     title = {Fe\protect\textsubscript{3}O\protect\textsubscript{4} as a magnetically reusable {Fenton} nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones},
     journal = {Comptes Rendus. Chimie},
     pages = {80--87},
     publisher = {Elsevier},
     volume = {21},
     number = {2},
     year = {2018},
     doi = {10.1016/j.crci.2017.12.003},
     language = {en},
}

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TI  - Fe3O4 as a magnetically reusable Fenton nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones
JO  - Comptes Rendus. Chimie
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DO  - 10.1016/j.crci.2017.12.003
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%0 Journal Article
%A Amin Rostami
%A Sirvan Moradi
%A Zahra Shokri
%T Fe3O4 as a magnetically reusable Fenton nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones
%J Comptes Rendus. Chimie
%D 2018
%P 80-87
%V 21
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%I Elsevier
%R 10.1016/j.crci.2017.12.003
%G en
%F CRCHIM_2018__21_2_80_0

Amin Rostami; Sirvan Moradi; Zahra Shokri. Fe₃O₄ as a magnetically reusable Fenton nanocatalyst for the oxidation of thiols to sulfonic acid and sulfides to sulfoxides and sulfones. Comptes Rendus. Chimie, Volume 21 (2018) no. 2, pp. 80-87. doi : 10.1016/j.crci.2017.12.003. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2017.12.003/

Version originale du texte intégral

1 Introduction

Over the recent decades, the catalytic application of nanoparticles (NPs) is of importance because of their high specific surface area leading to increase in the collision between reactants and catalyst, maximize the reaction rates, and reduce consumption of the catalyst [1]. However, the nanocatalysts have several advantages over conventional catalyst systems but the isolation and recovery of these nanocatalysts from the reaction medium is difficult and time-consuming. One of the best solutions to overcome this problem is the use of magnetically recoverable nanocatalysts [2], which can be separated by magnetic decantation after the reaction, therefore avoiding traditional filtration processes. Besides, most of these catalysts are recyclable [3–7]. Fe₃O₄ magnetic nanoparticles (MNPs) are one of the well-known magnetic materials, which have attracted more attention because they are easy to prepare, environmentally benign, economic, rather biocompatible, and easily separated from the reaction media using an external magnetic field [8]. Fe₃O₄ NPs have been applied in various organic transformations such as regioselective synthesis of propargylic amines [9], synthesis of α-aminonitriles [10], synthesis of quinoxalines [11], Sonogashira–Hagihara reaction [12], and N-protection of amines [13]. In addition, Fe₃O₄ MNPs have been used as a heterogeneous catalyst in the Fenton and Fenton-like processes [14,15].

Fenton process is a method that results in the formation of HO by a reaction between H₂O₂ as an oxidant and Fe²⁺/Fe³⁺ ions as a catalyst (Scheme 1) [16].

The produced free radicals, HO, are strong oxidant agents for organic compounds [16,17].

Keeping the above facts in mind and based on our previous work about application of magnetically reusable nanocatalysts in organic synthesis [18–20], we have synthesized Fe₃O₄ MNPs and used it as an effective heterogeneous catalyst for the oxidation of thiols to sulfonic acids and sulfides to sulfoxides and sulfones using hydrogen peroxide as an inexpensive and environmental oxidant.

Sulfonic acids and their derivatives have many applications such as dyes, surfactants, pesticides, animal feeds, pharmaceuticals, and catalysts. Many aliphatic sulfonic acid derivatives show pharmacological activity, as well as several aromatic sulfonic acids have many medicinal uses and are suitable for treatment of a range of infections [21]. On the other hand, sulfoxides and sulfones are versatile intermediates and precursors in the preparation of fine chemicals and pharmaceuticals such as antifungal and antihypertensive agents [22]. Sulfoxides are also important in CC bond construction and molecular rearrangements [23].

2 Experimental section

2.1 General

All substances were purchased from the Merck and Aldrich Chemical Companies and used without purification. Products were characterized by assessment of their physical and spectral data with those of authentic samples. Fourier transform infrared (FTIR) and NMR spectra were recorded using Thermo Nicolet Nexus 670 FTIR and Bruker NMR spectrometers (¹H NMR 250.1 MHz, ¹³C NMR 69.1 MHz), respectively. The X-ray powder diffraction data were collected using an X'Pert MPD Philips diffractometer with Cu Kα radiation source (λ = 1.54050 Å) at 40 kV voltage and 40 mA current. The particle morphology was determined by scanning electron microscopy (SEM) using FESEM-TESCAN MIRA3. Magnetic properties of the samples were obtained using a vibrating sample magnetometer (VSM, model MDKFT) under magnetic fields up to 20 kOe.

2.2 General procedure for the preparation of Fe₃O₄ MNP_S

In a typical synthesis, FeCl₃·6H₂O (5.838 g, 0.0216 mol) and FeCl₂·4H₂O (2.147 g, 0.0108 mol) were sonicated in deionized water (100 mL) for 30 min. Afterward, NH₄OH (25%, 10 mL) was added to the reaction mixture under N₂ atmosphere at 85 °C. The reaction continued for another 25 min. After cooling to room temperature, the NPs were then separated from the reaction mixture using an external magnetic field, washed with doubly distilled water and NaCl solution (0.02 M), and dried at room temperature [24].

2.3 General procedure for oxidation of thiols to sulfonic acids

In a round-bottomed flask (25 mL) equipped with a magnetic stirrer, a mixture of thiol (1 mmol) and Fe₃O₄ MNPs (0.025 g) in CH₃CN or EtOH was prepared. Next, H₂O₂ (10 mmol, 2 g) was added to the mixture and stirred under reflux conditions. After completion of the reaction as judged by thin-layer chromatography (eluent, n-hexane/EtOAc, 1/1), the catalyst was separated using an external magnetic field. Then, the mixture of the reaction was extracted with EtOAc (3 × 8 mL) and the organic layer dried over anhydrous Na₂SO₄. Evaporation of the solvent affords the pure sulfonic acid.

2.4 General procedure for the oxidation of sulfides to sulfoxides or sulfones

Fe₃O₄ MNPs (0.01 or 0.025 g) were added to a mixture of sulfide (1 mmol) and 30% H₂O₂ (10 or 20 mmol) in EtOH (2.5 mL). Then, the mixture was stirred at room temperature for an appropriate time. Thin-layer chromatography monitored the progress of the reaction. After completion of the reaction, the solvent was removed under reduced pressure to give the corresponding pure sulfoxide or sulfones in most cases.

3 Results and discussion

3.1 Catalyst preparation

Fe₃O₄ NPs were prepared via chemical coprecipitation of FeCl₃·6H₂O and FeCl₂·4H₂O in aqueous ammonia as reported in the literature (Scheme 2) [24].

The nanocatalyst was characterized by X-ray diffraction (XRD), VSM, FTIR spectroscopy, and SEM.

3.2 Catalyst characterizations

The size and morphology of Fe₃O₄ NPs were measured by SEM analysis (Fig. 1).

XRD pattern was applied to show that Fe₃O₄ MNPs were properly synthesized. XRD patterns of these NPs are shown in Fig. 2, reveal peaks at 2θ = 30.248, 35.628, 43.338, 53.638, 57.298, and 62.808, respectively, which are corresponding to the peaks of pure Fe₃O₄ (JCPDS 01-1111).

The FTIR spectrum of Fe₃O₄ MNPs as shown in Fig. 3a, indicates a band at 3420 cm⁻¹, which associated to stretching vibrations of the OH bonds on the surface of iron atoms. Also, the band at 571 cm⁻¹ relates to vibrations of FeO bonds of iron oxide [25]. The magnetic property of the catalyst was determined by the VSM at 298 K (Fig. 3b). The magnetization curves revealed that the saturation magnetization (Ms) was 66.976 emu g⁻¹ at an applied field of 10,000 Oe and showed superparamagnetic property of the catalyst at room temperature.

Fig. 3
a) FTIR of Fe₃O₄ MNPs. (b) Magnetization curve of Fe₃O₄ MNPs.

3.3 Catalytic activity

To optimize reaction conditions for the oxidation of thiols to sulfonic acids, we surveyed the influence of different parameters, for example, solvent, temperature, and different amounts of Fe₃O₄ MNPs on the oxidation of thiophenol to benzenesulfonic acid as a model reaction (Table 1). As shown in Table 1, the best conditions were obtained in the presence of Fe₃O₄ MNPs (0.025 mg) using H₂O₂ (30%, 10 mmol, 2 g) in CH₃CN under reflux conditions (Table 1, entry 9).

Entry	Fe₃O₄ (g)	Solvent (mL)	Temperature (°C)	Time	Yield (%)
1	0.01	CH₃CN	Reflux	150 min	40
2	0.015	CH₃CN	Reflux	150 min	55
3	0.02	CH₃CN	Reflux	150 min	65
4	0.025	CH₃CN	Reflux	150 min	83
5	0.025	CH₃CN	rt	22 h	70
6	0.025	CH₃CN	45	14 h	70
7	0.025	CH₃CN	55	9 h	75
8	0.025	CH₃CN	65	5 h	83
9	0.025	CH₃CN	Reflux	150 min	83
10	0.025	n-Hexane	Reflux	150 min	–
11	0.025	Et₂O	Reflux	150 min	–
12	0.025	CH₂Cl₂	Reflux	150 min	–
13	0.025	CHCl₃	Reflux	150 min	–
14	0.025	DMF	Reflux	150 min	30
15	0.025	EtOH	Reflux	150 min	45
16	0.025	H₂O/EtOH	Reflux	300 min	83

^a Reaction conditions: thiophenol (1 mmol), H₂O₂ (30%, 10 mmol, 2 g), solvent (2.5 mL).

We also investigated the effect of solvent on the oxidation of thiosalicylic acid as another thiolic compound with different polarity. The results of these experiments are presented in Table 2.

Entry	Fe₃O₄ (g)	Solvent (mL)	Temperature (°C)	Time (min)	Yield (%)
1	0.025	n-Hexane	Reflux	150	–
2	0.025	Et₂O	Reflux	150	–
3	0.025	CH₂Cl₂	Reflux	150	–
4	0.025	CHCl₃	Reflux	150	–
5	0.025	DMF	Reflux	150	60
6	0.025	EtOH	Reflux	150	90
7	0.025	H₂O/EtOH	Reflux	300	90
8	0.025	CH₃CN	Reflux	150	90

^a Reaction conditions: thiosalicylic acid (1 mmol), H₂O₂ (30%, 10 mmol, 2 g), solvent (2.5 mL).

The results presented in Tables 1 and 2 show that the solvent plays an important role in the type and yield of the product. In nonpolar solvents (immiscible in water), the reaction proceeds to coupling of thiols or one of the intermediates (Scheme 3) [26,27] because oxidants and thiols are located in two different phases and the possibility of the collision is low. However, in the presence of the solvents that are miscible in water and are capable of dissolving the thiols and other intermediates, the reaction has gone to the considered product because oxidant, thiols, and its intermediates are in the same phase.

Scheme 3
The oxidation steps of thiols to sulfonic acids.

The best results for the oxidation of thiophenol in terms of reaction time and the product yield were attained in CH₃CN, whereas for oxidation of thiosalicylic, ethanol was the best solvent of choice.

With optimal conditions in hand, to extend the scope of this method, oxidation reactions for different types of thiols were studied in both CH₃CN and EtOH (Table 3).

Entry	Substrate	Product^a	Time	Yield
CH₃CN	EtOH	CH₃CN	EtOH
1			150	150	83	40
2			120	120	85	50
3			180	180	80	33
4			60	45	90	90
5			120	120	83	52
6			90	60	90	90
7			60	45	90	93
8			150	180	83	78
9			45	60	83	81
10			50	30	95	95
11			30	15	93	97
12			120	90	80	85

^a All products are known and were characterized by IR and NMR spectroscopy as compared with those reported [28].

The results in Table 3 show that the thiols with more polarity were oxidized faster in ethanol (Table 3, entries 4, 6, 7, 11, and 12); however, the thiols with the lower polarity reacted faster in acetonitrile (Table 3, entries 1, 2, 3, 5, 8, and 9). Also, the reaction rate was increased with electron-donating substituents, but aromatic thiols with electron-withdrawing substituents reacted more slowly.

The chemoselectivity of this synthetic method was also studied with the oxidation 2-mercaptoethanol as a model for thiols with a hydroxyl functional group. The results showed that both of the functional groups were oxidized with the same reactivity and did not show any selectivity (Table 3, entry 10).

The catalytic activity of Fe₃O₄ MNPs in the presence of H₂O₂ for the oxidation of a series of structurally diverse sulfides was also investigated (Scheme 4).

Scheme 4
The oxidation of sulfides to sulfoxides and sulfones.

As shown in Table 4, Fe₃O₄ MNPs (0.01 g) and H₂O₂ (30%, 10 mmol) were suitable for oxidation of sulfides to sulfoxides, and the use of excess H₂O₂ (20 mmol) in the presence of Fe₃O₄ MNPs (0.025 g) produced the corresponding sulfones in a clean reaction.

Entry	Substrate	Product^a^,^b 1a	Time (min)	Yield (%)	Product^b^,^c 2a	Time (min)	Yield (%)
1			30	93		180	95
2			5	97		75	97
3			8	95		210	93
4			5	95		180	90
5			60	90		210	87
6			150	87		390	85
7			120	50		24	20
8			30	97		150	97

^a Reaction conditions: sulfide (1 mmol), H₂O₂ (30%, 10 mmol), catalyst (0.01 g), EtOH (2.5 mL), rt.

^b All products are known and were characterized by IR and NMR spectroscopy as compared with those reported [23,29–32].

^c Reaction conditions: sulfide (1 mmol), H₂O₂ (30%, 20 mmol), catalyst (0.025 g), EtOH (2.5 mL), reflux.

A possible mechanism for oxidation of thiols and sulfides was proposed on the basis of the Fenton reaction (Scheme 5) [33].

3.4 Catalyst recovery

To investigate the recyclability and reusability of the catalyst for the oxidation of thiols to sulfonic acids, we examined the oxidation of 2-mercaptoacetic acid as a model compound. After the completion of the reaction, the catalyst was separated magnetically and washed with Et₂O to remove the residual product. The separated catalyst was reused five times without a considerable decrease in activity as shown in Fig. 4.

Fig. 4
Recoverability of Fe₃O₄ MNPs in the oxidation of 2-mercaptoacetic acid.

3.5 Selected spectral data

3.5.1 2-Sulfoacetic acid

FTIR (KBr): υ (cm⁻¹) = 1051, 1217, 1640, 1723, 3447; ¹H NMR (D₂O, 250.1 MHz): δ (ppm) = 4.17 (s, 2H, CH₂), 4.69 (s, 1H, DOH); ¹³C NMR (D₂O, 62.9 MHz): δ (ppm) = 56.08, 169.23 (Table 3, entry 11).

3.5.2 Phenylmethanesulfonic acid

FTIR (KBr): υ (cm⁻¹) = 1058, 1196, 1413, 1449, 1580, 2998, 3093, 3430; ¹H NMR (D₂O, 250.1 MHz): δ (ppm) = 4.44 (s, 2H, CH₂), 7.66 (s, 5H); ¹³C NMR (D₂O, 62.9 MHz): δ (ppm) = 57.25, 128.66, 129.24, 130.88, 132.09 (Table 3, entry 8).

3.5.3 2-Sulfoacetic acid

FTIR (KBr): υ (cm⁻¹) = 1056, 1216, 1642, 1724, 3458; ¹H NMR (DMSO, 250.1 MHz): δ (ppm) = 2.34 (s, 1H, SO₃H), 2.48 (s, 3H, CH₃), 3.42 (s, 2H, CH₂), 4.22 (s, 1H, DOH); ¹³C NMR (DMSO, 62.9 MHz): δ (ppm) = 57.19, 168.25 (Table 3, entry 10).

3.5.4 Methylsulfinylbenzene

IR (KBr): υ (cm⁻¹) = 696, 1029, 1128, 1296, 1451, 1488, 3030; ¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 7.5–7.7 (m, 5H, ArH), 2.72 (s, 3H, CH₃) (Table 4, entry 1) [31].

3.5.5 Methylsulfonylbenzene

IR (KBr): υ (cm⁻¹) = 744, 1082, 1145, 1291, 1324, 1444, 1572, 2925, 3090; ¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 7.3–7.53 (m, 5H, ArH), 2.72 (s, 3H, CH₃) (Table 4, entry 1) [31].

3.5.6 Sulfinyldibenzene

IR (KBr): υ (cm⁻¹) = 688, 734, 1021, 1077, 1436, 1471, 1576, 3060; ¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 7.56 (d, 4H, ³J_HH = 6.7, ArH), 7.29–743 (m, 6H, ArH) (Table 4, entry 7).

3.5.7 Sulfonyldibenzene

IR (KBr): υ (cm⁻¹) = 678, 734, 1156, 1329, 1370, 1441, 1475, 1574, 3077; ¹H NMR (CDCl₃, 250 MHz) δ (ppm) = 7.94 (d, 4H, ³J_HH = 7.7, ArH), 7.45–7.58 (m, 6H, ArH) (Table 4, entry 7) [27].

3.5.8 2-(Phenylsulfinyl)ethanol

IR (KBr): υ (cm⁻¹) = 691, 737, 1041, 1085, 1435, 1474, 1579, 2925, 3060, 3405; ¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 7.33 (d, 2H, ³J_HH = 7.25 Hz, ArH), 7.2–7.3 (m, 3H, ArH), 3.70–3.75 (t, 2H, OCH₂), 3.05–3.10 (t, 2H, CH₂SO), 3.00 (m, 1H, OH) (Table 4, entry 3) [28].

3.5.9 2-(Phenylsulfonyl)ethanol

¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 7.95 (d, 2H, ³J_HH = 7.75 Hz, ArH), 7.58–7.70 (m, 3H, ArH), 3.99–4.03 (m, 2H, OCH₂), 3.44–3.49 (m, 1H, OH), 3.34–3.38 (m, 2H, CH₂SO₂) (Table 4, entry 3).

3.5.10 2-(Methylsulfinyl)ethanol

IR (KBr): υ (cm⁻¹) = 933, 1031, 1058, 1424, 1456, 2915, 3395; ¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 3.53–3.58 (m, 2H, OCH₂), 3.49 (m, 1H, OH), 2.48–2.53 (m, 2H, CH₂SO), 1.94 (s, 3H, CH₃) (Table 4, entry 4).

3.5.11 2-(Methylsulfonyl)ethanol

¹H NMR (CDCl₃, 250 MHz): δ (ppm) = 4.13–4.17 (m, 2H, OCH₂), 3.6–3.8 (m, 1H, OH), 3.2–3.3 (m, 2H, CH₂SO₂), 3.03 (s, 3H, CH₃) (Table 4, entry 4).

4 Conclusion

We have demonstrated that robust and magnetically reusable Fe₃O₄ MNPs can catalyze the oxidation of a wide variety of thiols and sulfides using hydrogen peroxide as an inexpensive and environmental oxidant. This simple reported procedure offers several significant advantages such as the use of a commercially available, eco-friendly oxidant, the mild reaction conditions, short reaction times and high yields, simple workup procedure, and recyclability of the catalyst.

Acknowledgments

We are grateful to the Research Council of Kurdistan University for the financial support of this work.

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