Selective oxidation of sulfides to sulfoxides/sulfones by 30% hydrogen peroxide

Article abstract of DOI:10.1080/10426507.2011.649501

A selective and efficient procedure for the oxidation of various sulfides with sodium tungstate dihydrate with 30% hydrogen peroxide in the presence of trioctylmethylammonium dihydrogen phosphate, respectively, to the corresponding sulfoxides and sulfones is reported. The oxidation reaction is carried out at -5 to 0 °C in the presence of hydroxypropyl -β cyclodextrins for sulfoxides or at 50-60 °C for sulfones. The mild reaction conditions, easy workup, and good yields of the products are the major advantages of this method. Copyright Taylor and Francis Group, LLC 2012.

Full text of DOI:10.1080/10426507.2011.649501

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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Selective Oxidation of Sulfides to
Sulfoxides/Sulfones by 30% Hydrogen
Peroxide
Ying He a , Xiaoyun Ma a , Hai Feng Ji a , Xin Bing Zha a , Hongliang
Jiang a & Ming Lu a
a School of Chemical Engineering , Nanjing University of Science and
Technology , Nanjing , P.R. China
Accepted author version posted online: 04 Jan 2012.Published
online: 30 May 2012.
To cite this article: Ying He , Xiaoyun Ma , Hai Feng Ji , Xin Bing Zha , Hongliang Jiang & Ming Lu
(2012) Selective Oxidation of Sulfides to Sulfoxides/Sulfones by 30% Hydrogen Peroxide, Phosphorus,
Sulfur, and Silicon and the Related Elements, 187:7, 822-830, DOI: 10.1080/10426507.2011.649501
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Phosphorus, Sulfur, and Silicon, 187:822–830, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.649501
SELECTIVE OXIDATION OF SULFIDES TO
SULFOXIDES/SULFONES BY 30% HYDROGEN PEROXIDE
Ying He, Xiaoyun Ma, Hai Feng Ji, Xin Bing Zha,
Hongliang Jiang, and Ming Lu
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, P.R. China
GRAPHICAL ABSTRACT
O
O
S
- -
HP
D
C
H
H
30% 2O2
30% 2O2
β
R1
R2
S
R1
R2
R1
R2
S
a
N
O4 2O/
W 2H PT
C
a
N
O4 2O/
W 2H PT
C
2
2
O
Abstract A selective and efficient procedure for the oxidation of various sulfides with sodium
tungstate dihydrate with 30% hydrogen peroxide in the presence of trioctylmethylammonium
dihydrogen phosphate, respectively, to the corresponding sulfoxides and sulfones is reported.
The oxidation reaction is carried out at –5 to 0 ◦C in the presence of hydroxypropyl-β-
cyclodextrins for sulfoxides or at 50–60 ◦C for sulfones. The mild reaction conditions, easy
workup, and good yields of the products are the major advantages of this method.
Keywords Oxidation; hydrogen peroxide; trioctylmethylammonium dihydrogen phosphate;
sodium tungstate dihydrate; sulfoxides; sulfones
INTRODUCTION
Organosulfur compounds such as sulfoxides and sulfones are important as inter-
mediates in the synthesis of natural products and biologically significant molecules, as
ligands in asymmetric catalysis, and as oxo-transfer reagents.1–3 Oxidation of sulfides is
the simplest, straight, and widely accepted for their synthesis.4 In view of the importance
of sulfoxides and sulfones, various reagents and oxidative procedures are now available
for their preparation, such as peracids, dioxiranes, NaIO4, MnO2, CrO3, SeO2, and PhIO.5
Although different approaches have been reported, there are various limitations such as the
use of strong acidic or basic conditions, higher temperatures, long reaction times, hazardous
organic solvents and reagents, high cost, and formation of many toxic wastes. Therefore,
there is a need for a simple, less expensive, general, and safer method for the conversion of
sulfides to sulfoxides and sulfones selectively.
Received 9 October 2011; accepted 8 December 2011.
We gratefully acknowledge financial support from the National Basic Research Program of China.
Address correspondence to Ming Lu, School of Chemical Engineering, Nanjing University of Science and
Technology, Nanjing 210094, P.R. China. E-mail: orienthawk@163.com
822

OXIDATION OF SULFIDES TO SULFOXIDES/SULFONES
823
On the contrary, hydrogen peroxide is sustainable and economical. It is a cheap and
clean oxidant with high oxygen content, and is transformed into harmless water.6 Thus,
the catalytic oxidation of sulfides with H2O2 is attractive. There are some references to
sulfide oxidation using H2O2 with a catalytic amount of CH3COOH7 or transition metals
as catalysts such as Sc(OTf)3,8 TBHP with p-TSA as catalyst,9 TiCl3,10 Venturello-type
peroxo complex Q3{(PO4)[W(O)(O2)2]4},11 polyoxymetalates WZnMn2(ZnW9O34)2,2–12
Copper(II) complexes,13 LDH-WO4,2–14 ZrCl4,15 immobilized cerium alkyl phosphonate,16
and Na2WO4/trioctylmethylammonium hydrogen sulfate.17 However, the development of
an efficient oxidation of various kinds of sulfides with hydrogen peroxide is still demanded.
Herein, we reported a very efficient and practical oxidation of sulfides with H2O2
in the presence of trioctylmethylammonium dihydrogen phosphate and hydroxypropyl-β-
cyclodextrins (HP-β-CD), developed under mild conditions and affording products with
high yields. It is considerably safe, energy saving, and environment friendly.
RESULTS AND DISCUSSION
Oxidation to Sulfoxides
The methods of oxidation of sulfides to sulfoxides and sulfones were occasionally
found to be unsatisfactory. One of the difficulties is the relative ease of further oxidation of
the sulfoxide to the sulfone. In connection with our further investigation on the synthetic
utility of quaternary ammonium dihydrogen phosphate, we have found that the sulfoxides
and sulfones could be synthesized by the oxidation of various sulfides with sodium tungstate
dihydrate with 30% hydrogen peroxide in the presence of trioctylmethylammonium dihy-
drogen phosphate as PTC (Scheme 1).
O
S
O
S
- -
HP
D
C
H
H
30% 2O2
30
2O2
%
β
R1
R2
R1
R2
R1
R2
S
a
N W 2H PT
O4 2O/
C
a
N W 2H PT
O4 2O/
C
2
2
O
R1, R2 = benzyl, phenyl, alkyl, aryl
Scheme 1
Effect of the Amount of PTC
In order to optimize the reaction conditions for the oxidation of sulfides to sulfoxides,
methyl phenyl sulfide was selected as a model substrate. The reaction was carried out with
a molar ratio methyl phenyl sulfide/Na2WO4·2H2O/30% H2O2 = 1:0.002:1.05 at –5 to
0 ◦C. Initially, we investigated the oxidation in the absence of PTC. It was found that the
reaction was slow (15 h) and gave the sulfoxide in low yield (5%). Therefore, we tried to
add 0.002 molar ratio of [CH3(n-C8H17)3N]H2PO4 acting as a phase transfer catalyst to
some degree to facilitate the solubility of the substrate in water and found that the reaction
time was shortened and the hydrogen peroxide was consumed completely within 6 h (86%).
However, further increasing the amount of PTC under the same conditions did not enhance
the formation of the sulfoxide significantly. The results show that quaternary ammonium
salts work effectively to enhance the reaction rate by increasing the reagent solubility. The
selectivity of the formation of sulfoxide was relatively low due to the formation of sulfone. In

824
Y. HE ET AL.
order to diminish the formation of sulfone by overoxidation, dilute hydrogen peroxide was
used at room temperature. An obvious increasing yield of sulfoxide to 96% was observed
by using 5% H2O2 due to the reduced oxidation ability of aqueous hydrogen peroxide.
Effect of Temperature
In order to show the role of the temperature in the selectivity of these reactions, we
conducted the oxidation of methyl phenyl sulfide at different temperatures and observed
that the temperature can essentially affect the reaction rate and the yield and ratio of
the oxidation products. Higher reaction temperatures lead to a dramatic decrease in the
selectivity to sulfoxide. Moreover, elevated temperature promotes the decomposition of
the hydrogen peroxide. The outcome of the reaction was followed and the results are
summarized in Figure 1.
The reaction rate considerably increases as the temperature rises. However, at high
conversion of sulfide, the oxidation does not stop at the sulfoxide level and proceeds
yielding overoxidized products, such as the corresponding sulfone. The selectivity analysis
shows that the sulfoxide is a primary and unstable product, while the corresponding sulfone
appears as a secondary and stable product at high conversion. The selectivity depends on the
reaction conditions (see Figure 1). At a relatively low temperature, the process stops at the
stage of formation of methyl phenyl sulfoxide as the main product when 1.05 equivalents
molar ratio of the oxidant agent are used. A temperature of –5 to 0 ◦C may be regarded as
optimal condition for formation of methyl phenyl sulfoxide.
Effect of the Amount of Hydroxypropyl-β-Cyclodextrin
Although the selective reaction did occur at room temperature with high yield when
5% H2O2 was used, this catalyst system is operationally the most convenient system with
HP-β-CD and 30% H2O2 can also be used with equal success (Table 1, entry 3). Reactions
performed with HP-β-CD proceeded well. The yield of sulfoxide increased to 93% in 6 h
when only 0.001 equivalent of HP-β-CD was applied (Table 1, entry 2). When 0.004
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Temperature(°C)
Conversion of the sulfide Yield of the sulfoxide Yield of the sulfone
Figure 1 Effect of the temperature on the oxidation of methyl phenyl sulfide with H2O2. Reaction conditions:
methyl phenyl sulfide: 1 mol, Na2WO4·2H2O: 0.2 mmol, 30% H2O2: 0.105 mol, dichloroethane: 10 mL, 5 h.

OXIDATION OF SULFIDES TO SULFOXIDES/SULFONES
825
a
Table 1 Effects of the amount of HP-β-CD on the oxidation of methyl phenyl sulfide by H2O2
HP-β-CD
(equiv. molar)
Conversion
(%)
Product yield (%)b
Sulfoxide
Entry
Time (h)
1
2
3
4
5
None
0.001
0.004
0.008
0.01
5
6
7
9
12
96
98
100
100
100
89
93
98
98
98
aReaction conditions: methyl phenyl sulfide: 0.1 mol, Na2WO4·H2O: 0.2 mmol, 30% H2O2: 0.105 mol,
dichloroethane: 10 mL, reaction temperature: –5 to 0 ◦C.
bIsolated yields.
equivalents of HP-β-CD were used, the desired reaction was most efficient and the yield
of sulfoxide increased to remarkable 98% in 7 h (Table 1, entry 3). Thus, we can clearly
observe that the yield increased with the increase in the amount of HP-β-CD. However, the
addition of further amounts of HP-β-CD under the same conditions did not significantly
enhance the yield of sulfoxide (Table 1, entry 5). We even noticed that excessive amounts
of HP-β-CD were disadvantageous for the oxidation reaction because it greatly prolonged
the reaction time (Table 1, entries 4, 5).
The reaction was carried out by the in situ formation of the HP-β-CD complex of
the substrate in water and stirring below 0 ◦C to give the corresponding sulfoxides in high
yields. Only minor overoxidation to the sulfone was observed. The reaction afforded the
sulfoxide in 98% yield after 7 h when dichloroethane was used as solvent (Table 1, entry 3).
On the basis of the results in Table 1, 0.004 equivalents molar HP-β-CD are effective for
the transformation of sulfides into sulfoxides in the oxidizing system.
HP-β-CD are cyclic oligosaccharides, which catalyze reactions by supramolecular
catalysis involving reversible formation of host–guest complexes by noncovalent bonding
as seen in enzymes (see Scheme 2). They exhibit a cavity whose inside diameter is ∪7Å
a
a
W
2
N W
O4
H
2O2
N
H
O O O
( 2)2( )
2
2
O
O
O
-
H
2O
O
Q+
W
O2
S
-
O
S
Q+
W
O3
H
2O2
O
O
Scheme 2 Proposed catalytic process, Q+ = trioctylmethylammonium ion.

826
Y. HE ET AL.
which is large enough to accommodate an aromatic ring.18 The interior of the HP-β-CD is
not hydrophobic, but considerably less hydrophilic than the aqueous environment and, thus,
able to host other hydrophobic molecules. In contrast, the exterior is sufficiently hydrophilic
to impart HP-β-CD water solubility. The oxidation of a substrate could be preceded by its
inclusion into the cavity of HP-β-CD. It may also assist in controlling the chemoselectivity,
thereby decreasing the nucleophilicity of the hydrogen peroxide and preventing further
2–
oxidation of the sulfoxide to the sulfone. The catalytic cycle with the WO4 catalysts is
thought to involve tungsten peroxy complexes. The tungsten peroxy complexes react fast
with the sulfide through interface reaction with transfer of an oxygen atom to give the
sulfoxide with hydrogen peroxide in the presence of PTC. Further research on the catalytic
system is underway.
A wide variety of sulfides (aliphatic, aromatic, benzylic, and heterocyclic) were
subjected to oxidation, and the results are presented in Table 2. All reactions occur with
higher selectivity for sulfoxide formation.
Oxidation to Sulfones
In order to demonstrate the efficiency and applicability of the system further, the
chemoselective oxidation of sulfides to sulfones was also investigated. In order to accelerate
Table 2 Hydrogen peroxide oxidation of sulfides to sulfoxidesa
Entry
1
Substrate
Product
Time (h)
7
Yield (%)b
98
S
O
S
2
3
4
5
6
7
97
97
96
O
S
S
S
S
S
O
O
S
5
6
5
96
95
O
S
S
12
O
S
S
aUnless otherwise stated, the reactions were run using sulfide: 100 mmol, Na2WO4·2·H2O: 0.2 mmol, 30%
H2O2: 105 mmol, HP-β-CD: 40 mmol, [CH3(n-C8H17)3N]H2PO4: 0.2 mmol. If necessary, ethyl acetate was used
as solvent. Reaction temperature: –5 to 0 ◦C.
bIsolated yields.

OXIDATION OF SULFIDES TO SULFOXIDES/SULFONES
827
the reaction rate, a 0.01 molar ratio (catalysts/substrate) was used to facilitate the biphasic
reaction. Meanwhile, excessive hydrogen peroxide was needed due to the decomposition of
hydrogen peroxide at elevated temperature. A 2.5 molar ratio (hydrogen peroxide/substrate)
was recommended. It was found that a wide variety◦of sulfides were oxidized to their
corresponding sulfones in excellent yields at 50–60 C. The results are summarized in
Table 3.
Table 3 Hydrogen peroxide oxidation of sulfides to sulfonesa
Entry
1
Substrate
Product
Time (h)
2
Yield (%)b
96
S
O
S
O
2
1.5
96
O
S
S
O
3
4
2
2
97
96
S
S
S
O
O
O
S
O
5
6
2
4
96
O
S
S
O
98c
O
N
C
S
N
C
S
O
O
7
3
96
S
S
O
aUnless otherwise stated, the reactions were run using sulfide: 100 mmol, Na2WO4·2H2O: 1 mmol, 30%
H2O2: 250 mmol, [CH3(n-C8H17)3N]H2PO4: 1 mmol. If necessary, ethyl acetate was used as solvent. Reaction
temperature: 50–60 ◦C.
bIsolated yields.
cThe optimum conditions for the oxidation were as follows: A 100 mL round-bottomed flask equipped with a
magnetic stirring bar was charged with 0.066 g (0.2 mmol) of Na2WO4·2H2O, 0.093 g (0.2 mmol) of [CH3(n-
C8H17)3N]H2PO4 and 10.1 g (0.1 mol) of ethylthioacetonitrile. Successively, 17.7 g (25 mmol) of 5% H2O2 were
added drop-wise at room temperature. The reaction mixture was stirred till no oxidative property was detected
with KI-starch. Then, 20.4 g of 30% H2O2 was dropped in. After the addition was complete, the mixture was
stirred at 50 ◦C for 4 h. It was then carefully neutralized with ice-cold saturated NaHCO3 solution and separated
directly or, if necessary, extracted with acetic ester. The organic layers were washed with brine and concentrated
under reduced pressure to give ethylsulfonylacetonitrile (13 g, 98%).

828
Y. HE ET AL.
CONCLUSIONS
In conclusion, we report an easy method for the selective oxidation of sulfides to
the corresponding sulfoxides and sulfones with the following advantages: (a) Our reagent
is cheap and can be easily handled without losing its activity for several months. (b) The
oxidation procedure is simple and controllable under mild conditions. (c) The yields of
sulfoxide and sulfone are high.
EXPERIMENTAL
All substrates and HP-β-CD were commercially available and used without further
purification. Eighty-five percent H3PO4, sodium tungstate dihydrate, 30% hydrogen per-
oxide, trimethyl phosphate, and tri-n-octylamine were obtained from sinopharm chemical
1
reagent company. H NMR spectra were recorded with a Bruker DRX-500 spectrometer
(solvent: CDCl3, 1H: 500 MHz, 13C: 100 MHz), and chemical shifts are given as δ (ppm)
relative to tetramethylsilane (TMS) as an internal standard. Melting points were determined
on a Thomas Hoover capillary apparatus and were uncorrected.
Trioctylmethylammonium dihydrogen phosphate was prepared according to the
literature procedure.19 A 250 mL round-bottomed flask equipped with a magnetic stirring
bar was charged with 17.7 g (50.0 mmol) of tri-n-octylamine and 20 mL toluene. Under
stirring, 7.21 g (51.5 mmol) of trimethyl phosphate were added in portions at room tem-
◦
perature, and the mixture was heated at 140 C for 24 h. Then, the mixture was cooled
to room temperature, 20 mL of 10% H3PO4 was added, and the biphasic mixture was
◦
vigorously stirred for 24 h at 90 C. The organic phase was separated and then washed
with 100 mL purified water. Finally, removal of volatile material in vacuo gave 22.6 g
1
of trioctylmethylammonium dihydrogen phosphate (97%). H NMR: δ 0.86 (t, 9H, J =
7.0 Hz), 1.25–1.36 (m, 30H), 1.65 (br, 6H), 3.25–3.58 (t, 9H ), 9.58 (br, 2H).
General Procedure for the Oxidation to Sulfoxides
A typical experimental procedure is as follows: A 100 mL round-bottomed flask
equipped with a magnetic stirring bar was charged with 66 mg (0.2 mmol) of sodium
tungstate dihydrate, 93 mg (0.2 mmol) of trioctylmethylammonium dihydrogen phosphate,
941 mg (0.4 mmol) of HP-β-CD, 100 mmol of sulfide, and 1 g of purified water was added
successively. If necessary, ethyl acetate or dichloroethane were used as solvent. Also, 12.5 g
(105 mmol) of 30% H2O2 was added drop-wise below 0 ◦C. The reaction mixture was stirred
until no H2O2 could be detected with KI-starch paper. After completion of the reaction, it
was quenched with aqueous NaHSO3 solution, separated, and concentrated under reduced
pressure. The corresponding sulfoxides were isolated by silica-gel column chromatography.
General Procedure for the Oxidation to Sulfones
A four-necked, round-bottomed flask equipped with a mechanical stirrer, a reflux
condenser, a thermometer, and a 25 mL dropping funnel was charged with 164 mg
(0.5 mmol) of Na2WO4·2H2O, 232 mg (0.5 mmol) of [CH3(n-C8H17)3N]H2PO4, and 50
mmol of sulfide substrate. The mixture was vigorously stirred at room temperature for 10
min and 14.2 g (125 mmol) of aqueous 30% H2O2 was added drop-wise via the dropping
funnel over a period of 0.5 h while maintaining the temperature of the reaction mixture at

OXIDATION OF SULFIDES TO SULFOXIDES/SULFONES
829
room tempera◦ture. After the completion of the addition, the mixture was stirred for another
2 h at 50–60 C and then cooled to room temperature. Separation, filtration of the solid,
or distillation of the liquid through a short column under vacuum led to the corresponding
sulfones.
Selected Spectroscopic Data
Methyl phenyl sulfoxide.20 (Table 4, entry 1). 1H NMR: δ 2.73 (s, 3H), 7.50–7.56 (m, 3H),
7.63–7.68 (m, 2H). 13C NMR: δ 44.30, 123.83, 129.73, 131.42, 145.90.
Ethyl phenyl sulfoxide.21 (Table 4, entry 4). 1H NMR: δ 1.20 (t, 3H), 2.75–2.91 (m, 2H),
7.31–7.40 (m, 3H), 7.42–7.58 (m, 2H). 13C NMR: δ 5.53, 49.80, 123.80, 128.80,
130.60, 142.96.
Dibenzyl Sulfoxide.22 M. p.: 133–135 ◦C (Table 4, entry 5). 1H NMR: δ 3.85 (d, 2H, J =
12.8 Hz), 3.92 (d, 2H, J = 12.8 Hz), 7.25–7.38 (m, 10H).13C NMR: δ 57.12, 128.25,
128.84, 130.06, 130.03.
Diphenyl sulfoxide.21 M. p.: 69–71 ◦C (Table 4, entry 6). 1H NMR: δ 7.2–7.35 (m, 6H),
7.45–7.60 (m, 4H). 13C NMR: δ 124.65, 129.25, 131.00, 145.14.
◦
1
Methyl phenyl sulfone.23 M. p.: 90–91 C (Table 5, entry 1). H NMR: δ 3.06 (s, 3H),
7.55–7.60 (m, 2H), 7.65–7.68 (m, 1H), 7.94–7.97 (m, 2H). 13C NMR: δ 44.64, 127.29,
129.31, 133.62, 140.54.
Ethylsulfonylacetonitrile.24 (Table 5, entry 6). 1H NMR: δ 3.994–4.070 (t, 2H),
3.296–3.311 (m, 2H), 1.47–1.50 (t, 3H).
Diphenyl sulfone.22 M. p.: 128–130 ◦C (Table 5, entry 7). 1H NMR: δ 7.47–7.60 (m, 6H),
7.93–7.95 (m, 4H). 13C NMR: δ 127.65, 129.26, 133.15, 141.61.
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