Efficient and convenient oxidation of sulfides to sulfones using H₂O₂ catalyzed by V₂O₅ in ionic liquid [C₁₂mim][HSO₄]

Abstract

A simple, efficient, and eco-friendly procedure for the oxidation of sulfides to sulfones using H₂O₂ catalyzed by V₂O₅ in ionic liquid [C₁₂mim][HSO₄] has been developed. This atom-economical protocol affords the target products in good to high yields. The products can be separated by a simple extraction with organic solvent, and the catalytic system can be recycled and reused without loss of catalytic activity.

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Sulfones are an important class of chemicals that are used extensively for the preparation of a variety of fine or special chemicals such as drugs, natural products, and catalysts.^1,2 In particular, sulfones are important building blocks for the synthesis of various biologically active molecules.² Therefore, there has been tremendous interest in developing efficient methods for the synthesis of these molecules, and a well known method constitutes the oxidation of sulfides. Traditional methods for performing such a transformation generally involve the use of stoichiometric amounts of the strongest oxidizing reagents (i.e., HNO₃, KMnO₄, MnO₂, etc.)³ and suffer from considerable drawbacks such as high cost, low yield, harsh or delicate reaction conditions, and a large amount of waste byproducts. Oxidation using potassium hydrogen persulfate,⁴ NaClO₄,⁵ H₅IO₆,⁶ NaClO,⁷ UHP,⁸m-chloroperbenzoic acid,⁹ NaIO₄,¹⁰ dimethyldioxirane,¹¹ oxone¹² and other reagents¹³ has also been developed for this conversion. However, these protocols are generally associated with one or more disadvantages, such as long reaction times, high temperatures, low yields, complex handling procedures, problematic side reactions, the use of expensive, toxic and moisture sensitive reagents, and environmental concerns associated with the use of poorly manageable catalysts. Hydrogen peroxide (H₂O₂) is an economical, environmentally friendly and atom-efficient oxidant which leads to only water after the reaction.¹⁴ This reagent has, however, high activation energy, making catalysis necessary,¹⁵ and a number of catalytic oxidation processes based on the combination of transition metals or organocatalysts and H₂O₂ have been developed for such a conversion.¹⁶ However, most of these methods still suffered from the use of expensive reagents, lower yields, long reaction times, environmental hazards, poor recovery of expensive metal catalysts and laborious workup procedures. Consequently, there is a great need to develop new and environment-benign procedures that address these drawbacks for such a conversion.

Ionic liquids (ILs) are a class of organic salts with unusually low melting temperatures that have attracted much attention from the scientific community in recent years. As a class of potential greener solvents, ILs exhibit many unique physicochemical properties, such as negligible volatility and nonflammability under ambient conditions, large liquid range, high thermal stability, wide electrochemical window, and strong ability to dissolve many chemicals.^17,18 Therefore, ILs have found wide applications in chemical synthesis,¹⁹ biocatalytic transformations,²⁰ electrochemistry,²¹ and analytical and separation science.²² In continuation of our interest in exploring green oxidation reactions in ionic liquids, we report herein a new, efficient and environmentally friendly protocol for the oxidation of sulfides to sulfones using H₂O₂ catalyzed by vanadium pentoxide (V₂O₅) in the ionic liquid 1-dodecyl-3-methylimidazolium hydrogensulfate ([C₁₂mim][HSO₄]) under mild conditions (Scheme 1). Furthermore, we demonstrate that the catalytic system can be recycled and reused without any significant loss of catalytic activity.

Scheme 1 Catalytic oxidation of sulfides to sulfones.

The initial study was carried out using methyl(phenyl)sulfane as substrate to optimize the reaction conditions, and the results are summarized in Table 1. Initial reaction screening led to disappointing results. In the absence of catalyst and ionic liquid, the reaction proceeded very slowly, and the yield was only 20% after 24 h (Table 1, entry 1). The results mean that the oxidant H₂O₂ alone does not work effectively in the reaction. The effects of different ionic liquids, such as [C₆mim][HSO₄], [C₁₀mim][HSO₄], [C₁₂mim][HSO₄], [C₁₂mim][PF₆], [C₁₂mim][OTf], [C₁₂mim][BF₄], and [C₁₂mim]Br, were tested with V₂O₅ as catalyst in the reaction (Table 1, entries 2–8), and it was observed that [C₁₂mim][HSO₄] demonstrated the best performance (Table 1, entry 4). The different effects of ILs on the reaction may be attributed to their different abilities to stabilize and dissolve the oxidant H₂O₂ and the catalyst. Under the same conditions, the IL that stabilizes and dissolves them more easily will lead to a larger increase in the effective reactant concentration, which increases the encounter probabilities between the substrate and the reactive species; thus, a higher rate or yield of the reaction is observed. For a blank test (Table 1, entry 11), a lower yield of the product was obtained when the same reaction conditions were used in the absence of ionic liquid. In addition, the catalyst is crucial for this reaction, and a lack of it leads to a much lower yield (Table 1, entry 12).

Table 1 Optimization of the conditions for the oxidation of methyl(phenyl)sulfane using H₂O₂^a

Entry	Ionic liquid	Catalyst	Time/h	Yield^b/(%)
a The reactions were carried out using methyl(phenyl)sulfane (10 mmol), catalyst (2 mmol), IL (10 mL), and H2O2 (30%, 25 mmol) at room temperature (25 °C). b Isolated yield. c The first run. d The second run.
1	—	—	24	20
2	[C6mim][HSO4]	V2O5	6	75
3	[C10mim][HSO4]	V2O5	4	87
4	[C12mim][HSO4]	V2O5	4	90
5	[C12mim][PF6]	V2O5	4	82
6	[C12mim][OTf]	V2O5	4	79
7	[C12mim][BF4]	V2O5	4	74
8	[C12mim]Br	V2O5	6	70
9	[C12mim][HSO4]	V2O5	4	90^c
10	[C12mim][HSO4]	V2O5	4	89^d
11	—	V2O5	8	59
12	[C12mim][HSO4]	—	8	68
13	[C12mim][HSO4]	MnO2	4	76
14	[C12mim][HSO4]	ZnO	4	65
15	[C12mim][HSO4]	Fe2O3	4	84
16	[C12mim][HSO4]	CuO	4	72

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Besides V₂O₅, we also tried to use other types of catalysts in this model reaction (Table 1, entries 13–16), and the results showed that V₂O₅ demonstrated the best performance in terms of yield and reaction rate. Furthermore, the catalytic system could be typically recovered and reused with no appreciable decrease in yield and reaction rate (Table 1, entries 9 and 10). Therefore, the combination of H₂O₂, V₂O₅ and [C₁₂mim][HSO₄] was chosen as the optimal condition for further exploration.

With these results in hand, we subjected other sulfides to the oxidation reactions, and the results are listed in Table 2. It is clear that various types of aryl and alkyl sulfides can be successfully oxidized to the corresponding sulfones in good to high yields (Table 2). Various types of aryl-alkyl and alkyl sulfides can be successfully oxidized to the corresponding sulfones (Table 2, entries 1–6 and 10); because the aryl sulfides were less reactive, more drastic reaction conditions were needed to achieve good yields (Table 2, entries 7–9). It was also observed that the electronic nature of the substituents on the aromatic ring has some impact on the reaction rate. Aryl-alkyl sulfides, especially those with electron-donating substituents (Table 2, entries 3 and 4), were more reactive than those with electron-withdrawing substituents (Table 2, entries 5 and 6), providing excellent yields.

Table 2 Catalytic oxidation of sulfides to sulfones^a

Entry	Substrate	Product	Time/h	Yield/(%)^b
a Unless otherwise noted, the reactions were carried out using sulfide (10 mmol), V2O5 (2 mmol), [C12mim][HSO4] (10 mL), and H2O2 (30%, 25 mmol) at room temperature. b Isolated yield. c The reaction was carried out at 45 °C.
1			4	90
2			4	94
3			4	93
4			4	95
5			6	83
6			6	85
7			8	84^c
8			8	87^c
9			8	82^c
10			4	93

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According to the literature²³ and the observations in our reactions, taking the oxidation of methyl(phenyl)sulfane with H₂O₂ as an example, a possible mechanism is proposed (Scheme 2). In the reaction, the catalyst V₂O₅ provides a source of VO₃⁻ and VO₂⁺ (1), and 1 reacts with the oxidant H₂O₂ to form the red oxoperoxo VO(O₂)⁺ (2) and the VO(O₂)OOH species (3). Then, 3 reacts with the substrate to form the transition state 4. Then, 4 eliminates the oxoperoxo (2) and water to yield the intermediate substrate 5, which reacts with 3 to form the transition state 6. 6 then very rapidly affords 2 and water to yield the desired product. 2 is then re-oxidized to 3 by H₂O₂ to complete the catalytic cycle. It appears that the formations of 4 and 6 are the rate-determining steps.

Scheme 2 Possible mechanism for the oxidation of methyl(phenyl)sulfane using H₂O₂.

In conclusion, we have demonstrated an efficient and convenient oxidation of sulfides to sulfones using the H₂O₂–V₂O₅ catalytic system in the ionic liquid [C₁₂mim][HSO₄]. Compared to the synthetic methods reported in the literature,^3–13,16 not only were the yields of products enhanced or greatly improved but the operating units were also easily worked up. Mild reaction conditions, simplicity of operation, high yields, stability, easy isolation of products, and excellent recyclability of the catalytic system are the attractive features of this methodology. The scope, definition of the mechanism and synthetic application of this reaction are currently under study in our laboratory.

Experimental

All the chemicals were obtained from commercial sources without any pretreatment. All reagents were of analytical grade. The ionic liquids were synthesized according to the procedure in the literature.¹⁸¹H NMR spectra were recorded using a Bruker 500 MHz spectrometer with CDCl₃ as solvent and tetramethylsilane (TMS) as internal standard. High performance liquid chromatography (HPLC) experiments were performed using a liquid chromatograph (Dionex Softron GmbH, America) consisting of a pump (P680) and an ultraviolet–visible light detector (UVD) system (170U). The experiments were performed on a Discovery C18 column (ø 4.6 × 250 mm). Elemental analysis was performed using a Vario EL III instrument (Elementar Analysensysteme GmbH, Germany).

General procedure for oxidation reactions. For a typical experiment, to 10 mL of [C₁₂mim][HSO₄] were sequentially added sulfide (10 mmol), V₂O₅ (2 mmol), and an aqueous solution of H₂O₂ (30%, 25 mmol). The obtained mixture was stirred vigorously at room temperature for an appropriate time (Table 2), and the reaction progress was monitored by HPLC. Upon completion of the reaction, ethyl acetate was used to extract the organic compounds (3 × 20 mL). The ethyl acetate solution was washed with water (3 × 20 mL) and then dried over anhydrous Na₂SO₄. The solvent was removed, and the residue was distilled under vacuum to give the desired pure product. The rest of the ionic liquid and the catalyst were recovered by decantation of water produced in the reaction and concentration under vacuum. Fresh substrates were then recharged to the recovered catalytic system and then recycled under identical reaction conditions. The target substrates were characterized by ¹H NMR and elemental analysis or compared with their authentic samples. Spectroscopic data for selected products are as follows.

1-Methyl-4-(methylsulfonyl)benzene (Table 2, entry 3). ¹H NMR (500 MHz, CDCl₃): δ (ppm) = 2.46 (s, 3H, CH₃), 3.57 (s, 3H, CH₃), 7.37–7.42 (m, 2H, Ar–H), 7.86–7.91 (m, 2H, Ar–H); elemental analysis % calc. (% found): C 56.42 (56.44), H 5.90 (5.92), O 18.76 (18.80), S 18.81 (18.84).

1-Methoxy-4-(methylsulfonyl)benzene (Table 2, entry 4). ¹H NMR (500 MHz, CDCl₃): δ (ppm) = 3.61 (s, 3H, CH₃), 3.87 (s, 3H, CH₃), 7.21–7.25 (m, 2H, Ar–H), 7.83–7.87 (m, 2H, Ar–H); elemental analysis % calc. (% found): C 51.55 (51.60), H 5.39 (5.41), O 25.74 (25.77), S 17.18 (17.22).

2,3-Dihydrobenzo[b]thiophene 1,1-dioxide (Table 2, entry 8). ¹H NMR (500 MHz, CDCl₃): δ (ppm) = 3.25 (t, 2H, CH₂), 3.83 (t, 2H, CH₂), 7.45–7.78 (m, 4H, Ar–H); elemental analysis % calc. (% found): C 57.09 (57.12), H 4.77 (4.79), O 18.98 (19.02), S 19.04 (19.06).

Dibenzo[b,d]thiophene 5,5-dioxide (Table 2, entry 9). ¹H NMR (500 MHz, CDCl₃): δ (ppm) = 7.56–7.63 (m, 4H, Ar–H), 7.92–8.05 (m, 4H, Ar–H); elemental analysis % calc. (% found): C 66.61 (66.65), H 3.68 (3.73), O 14.79 (14.80), S 14.77 (14.83).

Acknowledgements

We thank the Natural Science Foundation of Jiangsu Province and the Science and Technology Program of Jiangxi Provincial Education Bureau (no. GJJ13535) for supporting this research.

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