Acta Chim. Slov. 2008, 55, 257260
257
Scientific paper
Carbon-based Solid Acid Catalyzed Highly Efficient
Selective Oxidations of Sulfides
to Sulfoxides or Sulfones
with Hydrogen Peroxide
Abbas Zali,*1 Arash Shokrolahi,1 Mohammad Hossein Keshavarz1
and Mohammad Ali Zarei2
1 Chemistry Department, Malek-ashtar University of Technology, Shahin-shahr, P.O.Box 83145-115, I. R. Iran
2 Chemistry Department Faculty of Materials, Malek-ashtar University of Technology, Tehran
* Corresponding author: E-mail: abaszali@mut-es.ac.ir; abbaszali@gmail.com
Received: 15-05-2007
Abstract
Carbon-based solid acid catalyst was found to be highly efficient, eco-friendly; recyclable heterogeneous catalyst for selective oxidation of sulfides to sulfoxides or sulfones in excellent yields under mild reaction conditions using 30% hydrogen peroxide as an oxidant.
Keyword: Carbon-based solid acid, oxidation, hydrogen peroxide, heterogeneous catalyst, sulfoxides, sul-fones
1. Introduction
Replacement of conventional toxic and pollutant Brnsted and Lewis acid catalysts with environmentally benign and reusable solid heterogeneous catalysts is an active area of current research. New catalysts may include acidic zeolites, sulfated zirconia and acidic groups containing resins. Using solid acid catalyst have some advantages such as lower of equipments, ease of products separation, recycling of the catalyst and environmental acceptability as compared to liquid acid catalyst.1
Moreover, ideal solid acid catalysts should have high stability and numerous strong protonic acid sites. Carbon-based solid acid catalyst has many advantages. It is insoluble in common organic solvents, causes low corrosion, and shows environmental acceptability. Also the products could be easily separated from the reaction mixture and the catalyst is recoverable without decreasing its activity. Therefore, it can be successfully used instead of sulfuric acid as catalyst.25
Organosulfur compounds, such as sulfoxides and sulfones, are useful compounds in organic chemistry. In
particular, sulfoxides are valuable synthetic intermediates for the production of different varieties of chemically and biologically active molecules.616 The main synthetic route for preparation of these valuable materials is oxidation of corresponding sulfides.17
Various reagents including high valent metal and nonmetal oxides, mineral salts, e.g. potassium permanganate, potassium bromate, organic peroxides and hydrogen peroxide, have been used for the oxidation of functional groups.18,19 There is also some limitations for using of mineral oxidants because they require rigorous control of the reaction conditions. Moreover, the lack of selectivity in multistage organic synthesis, tedious work-up and toxicity are their main disadvantages.20 Aqueous H2O2 is an ideal oxidant for its high content of active oxygen (47%) and cleanness of the method.2123
This work represent the continuation of our studies using carbon-based solid acid in organic transforma-tions.24 We introduce an efficient, rapid, and clean procedure for the oxidation of sulfides through using carbon-based solid acid and hydrogen peroxide in a suitable solvent. There is also a good selectivity in the oxidation of
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sulfides to sulfoxides or sulfones, which depend on the reaction conditions (see Scheme 1).
2. Experimental
Chemicals were purchased from Fluka, Merck and Aldrich chemical companies. The products were characterized by comparison of their spectral (IR, 1H NMR), TLC and physical data with the authentic samples. Carbon-based solid acid was prepared according to the reported pro-cedure.2
2. 1. General Experimental Procedure for the Preparation of Sulfoxides
A mixture of carbon-based solid acid (0.10 g), sulfide substrate (1 mmol), hydrogen peroxide (aq. 30%, 1 mmol) and 1,2-dichloroethane (5 ml) was refluxed for the specified reaction time (see Table 1). The progress of reaction was monitored by TLC and GC method. At the end of reaction, catalyst was removed by filtration and the resulting solution was concentrated under reduced pressure. Then, residue was purified by column chromatography on silica gel. After removing the solvent, the corresponding sulfoxide was obtained in good to excellent yield (Table 1).
Table 1. Oxidation of sulfides (1 mmol) to sulfoxides with hydrogen peroxide (1 mmol)/carbon-base solid acid (0.10g).
Table 2. Oxidation of sulfides (1 mmol) to sulfones with hydrogen peroxide (5mmol)/carbon-base solid acid (0.20g).
Entry RSR Reaction time Yield
(min) (%)a
1 R=R=C6H5 10 98
2 R=C6H5; R=CH3 10 94
3 R=C6H5; R=CH2C6H5 10 95
4 R=C6H5; R=n-C4H9 10 88
5 R=R CH2C6H5 8 95
6 R=R=n-C4H9 7 95
7 R=R=n-C3H7 7 95
8 R= CH2C6H5; R=n-C4H9 10 90
9 Dibenzothiophene 30 70
a Isolated yields.
2. 2. General Experimental Procedure for the Preparation of Sulfones
A mixture of carbon-based solid acid (0.20 g), sulfide substrate (1 mmol), hydrogen peroxide (aq. 30%, 5 mmol) and 1,2-dichloroethane (5 ml) was refluxed for the specified reaction time (see Table 2). After completion of the reaction, carbon-based solid acid catalyst was removed by filtration and the resulting solution was concentrated under reduced pressure. Then, the residue was subjected to column chromatography using silica gel to obtain pure compound in good yield (Table 2).
Entry R-S-R Reaction Yield
time (min) (%)a
1 R=R=C6H5 10 95
2 R=C6H5; R=CH3 10 95
3 R=C6H5; R=CH2C6H5 10 98
4 R=C6H5; R=n-C4H9 10 93
5 R=R =CH2C6H5 10 95
6 R=R=n-C4H9 7 95
7 R=R=n-C3H7 7 97
8 R= CH2C6H5; R=n-C4H9 10 98
9 Dibenzothiophene 45 75
a Isolated yields.
3. Results and Discussion
Carbon-based solid acid was prepared according to the procedure reported by Hara et al.2 The density of SO3H group was measured using NaOH (0.01 mol/L) as titrant by acidbase potentiometric titration method. The amount of SO3H groups attached to polycyclic aromatic carbon was obtained to be 4.75 mmol/g.
We have used the carbon-based solid acid as a heterogeneous catalyst in the presence of hydrogen peroxide as an oxidizing agent for the oxidation of both aliphatic and aromatic sulfides to the corresponding sulfoxides or sulfones. The optimum ratio of sulfide to H2O2 (1:1 equivalent) in the presence of carbon-based solid acid (0.1 g) is found to be ideal for complete conversion of sulfides to sulfoxides (with a few amount of sulfone), which the reaction remains incomplete in lesser amounts.
The use of excess reagent (the higher molar ratio of H2O2 relative to the sulfide (5:1) in the presence of carbon-based solid acid (0.2 g)) affords the corresponding sulfone in a clean reaction. The yield and selectivity of oxidation of diphenyl sulfide to the corresponding sulfoxide or sulfone as a function of time is indicated in Table 3.
As shown in Tables 1 and 2, the generality of this approach has been demonstrated by a facile and effortless oxidation of a wide variety of alkyl, aryl and aromatic sulfides. To check the reusability and recyclability of the catalyst, we have carried out the oxidation of dibutyl sulfide to dibutyl sulfone under similar reaction conditions. After completion of the reaction, the catalyst was separated by filtration and the same reaction course was repeated with fresh substrate and oxidant (aqueous 30% H2O2). It was found that the activity of catalyst has been remained unchanged after four catalytic cycles.
To compare the carbon-based solid acid with other catalysts including zeolite HZSM-5, USY-BEA, am-berlyst-15 resins, Nafion NR-50, and silica sulfuric acid,
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Table 3. Oxidation of diphenyl sulphide with hydrogen peroxide/carbon-base solid acid under different reaction conditions.
Entry Solvent Carbon-base solid acid H2O2 Reation time Yield (%)
(g/mmol of substrate) (mmol) (min) sulfoxides sulfones
1 Acetonitrile 0.20 5 20 - 90
2 Dichloromethane 0.20 5 20 - 85
3 1,2-Dichloroethane 0.20 5 10 - 95
4 1,2-Dichloroethane 0.05 1 15 86 -
5 1,2-Dichloroethane 0.10 1 10 90 Trace
6 1,2-Dichloroethane 0.20 1 10 90 5
7 1,2-Dichloroethane 0.00 5 60 - Trace
8 1,2-Dichloroethane 0.05 5 20 5 80
9 1,2-Dichloroethane 0.10 5 15 - 89
10 1,2-Dichloroethane 0.15 5 15 - 92
11 1,2-Dichloroethane 0.20 5 10 - 95
12 1,2-Dichloroethane 0.30 5 8-10 - 95
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the oxidation of diphenyl sulfide as a model compound with H2O2 was studied (see Table 4). Carbon-based solid acid affects the reaction course so that the reaction was completed in a shorter time (10-min) with a high yield.
As shown in Scheme 1, the reaction probably involves the in situ formation of persulfonic acid by the reaction of sulfonic acid groups of the carbon-based solid acid with hydrogen peroxide, followed by the oxygen transfer to the organic substrate.
4. Conclusion
In summary, we have reported a new, versatile, and efficient methodology for the selective oxidation of sulfides to sulfoxides or sulfones with aqueous hydrogen peroxide as an oxidant, in the presence of catalytic amounts of carbon-based solid acid. The usefulness of this methodology lies in the availability of the reagents, easy and clean work-up, and possibility of large scale synthesis. Moreover, the reagents are very cheap and the reaction proceeds
Scheme 1
Table 4. Comparison of the efficiency of carbon-based solid acid and the other catalysts for the oxidation of diphenyl sulfide to diphenyl sulfone with hydrogen peroxide.
Entry Catalyst Solvent Catalyst load (g) Time (min) Yield, (%) Ref.
1 Nafion NR-50 1,2-Dichloroethane 0.25 35 98 3
2 HZSM-5 1,2-Dichloroethane 0.25 60 40 3
3 USY-BEA 1,2-Dichloroethane 0.25 50 55 3
4 Amberlyst-15 1,2-Dichloroethane 0.25 45 75 3
5 Silica sulfuric acid Acetonitrile 0.20 45 96 25
6 Carbon-based solid acid Dichloromethane 0.20 20 85 This work
7 Carbon-based solid acid 1,2-Dichloroethane 0.20 10 95 This work
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with relatively good yields. We believe that the present methodology would be an important improvement with respect to the other methodologies.
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Povzetek
V prispevku je opisana uporaba heterogenega, trdnega kislega katalizatorja na ogljikovi osnovi za seletivno oksidacijo sulfidov v sulfokside in sulfone. Reakcija poteka pod milimi reakcijskimi pogoji s 30 % vodikovim peroksidom kot oksidantom in z visokimi izkoristki. Uporabljeni heterogeni katalizator se je izkazal kot zelo u~inkovit, okolju prijazen in enostaven za recikliranje.
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