Ion exchange resin catalyzed selective oxidation of sulfides to sulfoxides using hydrogen peroxide

Article abstract of DOI:10.1007/s00706-006-0563-4

An efficient method for the selective oxidation of sulfides to sulfoxides under mild and environmentally safe conditions is achieved using hydrogen peroxide in the presence of Amberlyst 15 and Amberlite IR-400 at room temperature. This procedure can be ap

Full text of DOI:10.1007/s00706-006-0563-4

Monatshefte f€ur Chemie 138, 83–88 (2007)
DOI 10.1007/s00706-006-0563-4
Printed in The Netherlands
Short Communication
Ion Exchange Resin Catalyzed Selective
Oxidation of Sulfides to Sulfoxides Using
Hydrogen Peroxide
ꢀ
Moslem Mansour Lakouraj , Mahmood Tajbakhsh,
and Hamed Tashakkorian
Department of Chemistry, Faculty of Basic Science, University of Mazandaran, Babolsar, Iran
Received July 29, 2006; accepted (revised) September 7, 2006
Published online December 22, 2006 # Springer-Verlag 2006
Summary. An efficient method for the selective oxidation of sulfides to sulfoxides under mild and
environmentally safe conditions is achieved using hydrogen peroxide in the presence of Amberlyst 15
and Amberlite IR-400 at room temperature. This procedure can be applied for dialkyl and diaryl
sulfides with a variety of functional groups. Functional groups such as hydroxyl, methoxy, amino,
aldehyde, and olefinic double bonds remain intact.
Keywords. Amberlyst 15; Amberlite IR-400; Hydrogen peroxide; Sulfide; Sulfoxide.
Introduction
Transformation of sulfides to sulfoxides can be achieved by employing oxygenat-
ing agents as well as halogenating ones [1–5], but halogenating agents give rise to
aromatic halogenation or formation of ꢀ-derivatives, and C–S bond cleavage.
Oxidation of sulfides is the standard method for the preparation of sulfoxides,
although the undesired over-oxidation to sulfones is a quite frequent competing re-
action [6]. To avoid the facile conversion of sulfoxides into sulfones, careful se-
lection of oxidizing agent and reaction conditions are required [7].
Several methods have been introduced for this purpose [8–24]. However, most
of the existing methods use sophisticated reagents, complex catalysts, toxic metal-
lic compounds, or rare oxidizing agents that are difficult to prepare. Many of these
procedures also suffer from poor selectivity. In contrast to other oxidizing agents,
H2O2 is the most attractive oxidant from an environmental viewpoint and shows
safety in operation and is relatively cheap [25, 26]. However, H2O2 alone has to be
ꢀ
Corresponding author. E-mail: Lakouraj@umz.ac.ir

84
M. M. Lakouraj et al.
Scheme 1
used in a controlled manner to reduce the possibility of an over-oxidation reaction
[27]. Therefore, there is a need to improve the utility of H2O2 for the selective
conversion of sulfides to sulfoxides. Ion exchange resins are extensively studied
and used for different purposes in organic synthesis [28]. To the best of our knowl-
edge no report exists for the selective conversion of sulfides to sulfoxides with
commercial H2O2 in the presence of recyclable cation or anion exchange resins as
catalyst.
In continuation of our investigation on the application of the Amberlyst OHꢁ
form and H2O2 for selective conversion of nitriles to amides [29] and epoxidation
of ꢀ,ꢁ-unsaturated carbonyls [30] along with selective oxidation of thioethers to
sulfoxides [31, 32], herein we introduce a simple method for selective oxidation of
thioethers to their corresponding sulfoxides with H2O2 (35%) in the presence of
acidic and=or basic ion exchange resin as a promoter (Scheme 1).
Results and Discussion
In this study, a number of sulfides have been prepared [33] and treated with
Amberlyst 15=H2O2 and=or Amberlite IR-400=H2O2. The results are summarized
in Tables 1 and 2. As can be seen in these Tables, different types of sulfides were
selectively oxidized to their corresponding sulfoxides in excellent yields at room
temperature. Aliphatic sulfides were completely converted to the corresponding
sulfoxides in less than 2 h (Table 1, entries 13–16), whereas aromatic sulfides
required 3–11 h for complete conversion (Table 1, entries 1–11). The substituents
on sulfides showed significant effects on the reaction times. For example, electron
withdrawing groups such as chloro, nitro, and cyano or phenyl groups show some
rate retardation, while methoxy, methyl, and amino groups show little rate
enhancement due to their electron-donating properties. The Amberlyst 15=H2O2
and Amberlite IR-400=H2O2 systems were chemo-selective, tolerating various
functional groups such as methoxy, amino, carbonyl, nitrile, halide, hydroxyl,
and olfinic double bonds. Although the hydroxyl groups are readily oxidized by
common reagents [34], with this reagent the hydroxyl and aldehyde groups resisted
oxidation.
In order to demonstrate the chemoselectivity of this method, we investigated
the competitive reactions of sulfides with sulfoxides, alcohols, aldehydes, and
nitriles. As shown in Scheme 2, sulfides are converted to the corresponding sulf-
oxide in good to high yield with excellent selectivity. However, in the reaction of

Selective Oxidation of Sulfides to Sulfoxides
85
a,b
Table 1. Selective oxidation of sulfides to sulfoxides with Amberlyst 15=H2O2
Entry R1
R2
Time=h Yield=%c Lit. mp (Found mp)=ꢂC Ref.
1
2
3
4
5
6d
7
8
9
Ph
Me
3
98
95
98
85
98
70
98
98
95
95
85
95
98
98
98
98
33–34 (31.5–32.5)
117–121 (116–118)
135–136 (136.5–137.5)
– (128–130)
[35]
[9]
Ph
PhCH2
PhCH2
PhCH2
PhCH2
Ph
6.5
6
4-MeC6H4
4-ClC6H4
4-MeOC6H4
4-O2NC6H4
2-H2NC6H4
PhCH2
[4]
9
–
5.5
– (105–107)
–
15
100–101 (101–103)
oil
[36]
–
Ph
5
PhCH2
CH2CH2OH
CH2COOH
CH2CN
PhCH2
4.5
6.5
7.5
9
133–135 (134.5)
150–152.2 (151–152)
112.5–113 (113–114)
69–73 (70–71)
oil
[35]
[37]
[38]
[32]
[39]
[40]
–
Ph
10
Ph
11
12
13
14
15
16
4-ClC6H4
Cyclohexyl
2.5
0.5
0.1
0.2
2
CH2¼CHCH2 CH2¼CHCH2
40–40.5 (40–41)
oil
Me
CH2CH2CHO
–CH2CH2CH2CH2–
n-Bu
105–107 (104–105)
32.6 (32)
[41]
[42]
n-Bu
a
All reactions were carried out at room temperature using an optimized ratio of 1=0.5=2 of
sulfide=QꢁHþ=H2O2 in methanol; b all products were characterized spectroscopically (1H NMR, IR)
and showed physical and spectral data in accordance with their expected structure and by compari-
son with authentic samples; c yields refer to pure isolated products; d the reaction was carried out in
methanol=dioxone (1:1)
a,b
Table 2. Selective oxidation of sulfides to sulfoxides with Amberlite IR-400=H2O2
Entry R1
R2
Time=h Yield=%c Lit. mp (Found mp)=ꢂC Ref.
1
2
3
4
5
6d
7
8
9
Ph
Me
3.5
7.5
6.5
9
98
92
93
85
95
65
93
92
93
93
92
98
98
98
98
33–34 (31.5–32.5)
117–121 (116–118)
135–136 (136.5)
– (128–130)
[35]
[9]
Ph
PhCH2
PhCH2
PhCH2
PhCH2
Ph
4-MeC6H4
4-ClC6H4
4-MeOC6H4
4-O2NC6H4
2-H2NC6H4
PhCH2
Ph
[4]
–
6
– (105–107)
–
16
100–101 (101–103)
oil
[36]
–
Ph
5.5
5
PhCH2
CH2CH2OH
CH2COOH
PhCH2
133–135 (134.5)
150–152.2 (151–152)
112.5–113 (113–114)
oil
[35]
[37]
[38]
[39]
[40]
–
7
10
Ph
7.5
3
11
12
13
14
15
Cyclohexyl
CH2¼CHCH2 CH2¼CHCH2
0.5
0.1
0.2
2
40–40.5 (40–41)
oil
Me
CH2CH2CHO
–CH2CH2CH2CH2–
n-Bu
105–107 (104–105)
32.6 (32)
[41]
[42]
n-Bu
a
All reactions were carried out at room temperature using an optimized ratio of 1=0.5=3 of
b
sulfide=QþOHꢁ=H2O2; all products were characterized spectroscopically (1H NMR, IR) and
showed physical and spectral data in accordance with their expected structure and by comparison
c
with authentic samples; yields refer to pure isolated products; the reaction was carried out in
d
methanol=dioxone (1:1)

86
M. M. Lakouraj et al.
Scheme 2
sulfide in the presence of nitrile using basic resin, hydrolysis of nitrile to the
corresponding amide was observed.
To test the role of the catalytic activation of the resin, a model run was carried
out with thioanisole in the absence of resin. It was found that using even a six-fold
excess of H2O2, after a long reaction time (12 h) the reaction was not completed.
The effect of solvent on the oxidation efficiency of the reagent was evaluated
by carrying out the oxidation of thioanisole in a series of solvents such as dichloro-
methane, n-hexane, dioxane, and methanol at room temperature. It was found that
polar solvents, such as methanol seem to increase the oxygenation yield due to
good miscibility with reactant and resin. Therefore, reactions were preformed in
methanol as solvent of choice at ambient temperature. The influence of molar
ratios of sulfide=catalyst=H2O2 on the progress of the sulfoxidation reaction of
thioanisole is presented in Table 3. As evident, sulfoxide formation was achieved
Table 3. Optimization of the reaction conditions in methanol at 25ꢂC
PhSMe=H2O2=Catalyst
Yield=% (Time=h)
Amberlite IR-400
Yield=% (Time=h)
Amberlyst 15
1
1
1
1
1
1
1
1
1
1
1
3
3
2
2
3
0
30 (8)
30 (8)
0.5
1
95 (6)
90 (5.5)
95 (5.5)
98 (3)
90 (6)
0.5
0.3
0.5
1
98 (3.5)
85 (4.5)
98 (4.5)
90 (5)
90 (4)
98 (3)
95 (4)
1
98 (4)
98 (3.5)

Selective Oxidation of Sulfides to Sulfoxides
87
using an optimized ratio of 1=0.5=2 of sulfide=QꢁHþ=H2O2 in methanol. In the
case of basic resin, the ratio of sulfide=QþOHꢁ=H2O2 was 1=0.5=3.
In conclusion, the oxidizing agent H2O2 is inexpensive and the reactions are
highly clean affording quantitative yields of sulfoxides. This oxidation system is an
ideal waste-avoiding oxidant since water is the only theoretical by-product. Owing
to the remarkable chemo-selectivity, high yield, and the convenient procedure,
Amberlyst 15 and Amberlite IR-400 are the best catalysts for the oxidative con-
version of sulfide to sulfoxide with H2O2. Furthermore, the procedure is well
suitable for preparative and large scale reactions or industrial usage because of
high efficiency, reusability of the catalysts, and cheapness.
Experimental
Chemicals were purchased from Merck, Fluka, and Aldrich chemical companies. A number of sulfides
were prepared using the standard synthesis method [35]. All sulfoxides were separated and purified by
chromatographic techniques and also identified by comparison of their mp, IR, NMR, and bp with
1
those reported in literature [36]. H NMR spectra were recorded on a JEOL 90MHz NMR spectro-
meter. IR spectra were recorded on a Pye-Unicam SP 1100 spectrophotometer.
General Procedure for Oxidation of Sulfides Using Cation Exchange Resin
To a solution of 1 mmol sulfide in 4 cm3 methanol, 2 mmol H2O2 (0.18 cm3 of a 35% aqueous solution)
and 0.5 mmol Amberlyst 15 were added. The resulting mixture was stirred at room temperature and
monitored by TLC. On completion of the reaction, the cation exchange resin was removed by filtration.
Evaporation of the solvent under reduced pressure gave the desired sulfoxide. Further purification was
performed by recrystallization from ethanol-water.
General Procedure for Oxidation of Sulfides Using Anion Exchange Resin
To a solution of 1 mmol sulfide in 4 cm3 methanol, 3 mmol H2O2 (0.26 cm3 of a 35% aqueous solution)
and 0.5 mmol Amberlite IR-400 were added. The resulting mixture was stirred at room temperature.
The progress of the reaction was monitored by TLC (n-hexane:ethylacetate ¼ 2:1). After completion of
the reaction, the reaction mixture was filtered and the solvent was removed under reduced pressure.
Recrystalization from ethanol-water afforded the pure sulfoxide in excellent yield.
Acknowledgement
We are grateful to the Research Councils of Mazandaran University for their partial financial support.
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