Highly selective and efficient oxidation of sulfides with hydrogen peroxide catalyzed by a chromium substituted Keggin type polyoxometalate

Article abstract of DOI:10.1016/j.tetlet.2014.05.026

The catalytic oxidation of sulfides into the corresponding sulfones by a chromium substituted Keggin type polyoxometalate, (TBA)₄[PW ₁₁CrO₃₉]·3H₂O, was achieved using mild reaction conditions. Excellent yields were obtained using four equivalents of 30% H₂O₂. Under these reaction conditions, the sulfide group was highly reactive and other functional groups such as hydroxyl or a double bond were unaffected. Using a commercially available, eco-friendly, and cheap oxidant, mild reaction conditions, operational simplicity, practicality, short reaction times, high to excellent yields, and excellent chemoselectivity are some of the advantages of this catalytic system.

Full text of DOI:10.1016/j.tetlet.2014.05.026

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly selective and efficient oxidation of sulfides with hydrogen
peroxide catalyzed by a chromium substituted Keggin type
polyoxometalate
Roozbeh Afrasiabi a, Mostafa Riahi Farsani b, Bahram Yadollahi b,
⇑
a Institute for Advanced Studies in Basic Sciences (IASBS), Gavazang, Zanjan 45195-1159, Iran
b Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic oxidation of sulfides into the corresponding sulfones by a chromium substituted Keggin
type polyoxometalate, (TBA)4[PW11CrO39]Á3H2O, was achieved using mild reaction conditions. Excellent
yields were obtained using four equivalents of 30% H2O2. Under these reaction conditions, the sulfide
group was highly reactive and other functional groups such as hydroxyl or a double bond were
unaffected. Using a commercially available, eco-friendly, and cheap oxidant, mild reaction conditions,
operational simplicity, practicality, short reaction times, high to excellent yields, and excellent chemose-
lectivity are some of the advantages of this catalytic system.
Received 23 February 2014
Revised 22 April 2014
Accepted 7 May 2014
Available online xxxx
Keywords:
Substituted polyoxometalate
Catalysis
Ó 2014 Elsevier Ltd. All rights reserved.
Oxidation
Sulfide
Hydrogen peroxide
Organosulfur compounds such as sulfoxides and sulfones are
important synthetic intermediates, for example, in the synthesis
of natural products and biologically significant molecules,1 and
are also utilized for the extraction and separation of some
metals.2,3 The selective oxidation of sulfides into sulfoxides and
sulfones is conventionally performed by using stoichiometric oxi-
dants such as peracids, dioxiranes, NaIO4, MnO2, CrO3, SeO2, and
PhIO, but these stoichiometric systems are not atom-efficient.4,5
In contrast, ‘green oxidants’ such as oxygen and hydrogen peroxide
are very attractive, because these oxidants are readily available,
inexpensive, and environmentally benign.6–8 There are many
reports on the H2O2-based oxidation of sulfides into sulfoxides
and sulfones by homogeneous and heterogeneous organocatalysts,
acid catalysts, enzymes, metal catalysts, and polyoxometalates
(POMs).9
properties has made this class of compounds very attractive
catalysts for the oxidation of a variety of compounds such as
alkenes, alcohols, and sulfides.12–16 In particular, the environmen-
tally important oxidation of sulfides into sulfones, and the develop-
ment of efficient oxidations of various sulfides by H2O2 are still in
demand.17
Herein, a highly efficient and simple route for the oxidation of
sulfides with H2O2 catalyzed by a monosubstituted Keggin-type
POM, [(n-C4H9)4N]4[PW11CrO39]Á3H2O (PWCr)24 is reported.
Notably, in the presence of small amounts of catalyst excellent
conversions of sulfides into the corresponding sulfones using the
appropriate amount of H2O2 was achieved.
The catalytic activity of the PWCr was examined in the oxida-
tion of diphenylsulfide by 30% H2O2 as a model reaction. To a solu-
tion of Ph2S (1 mmol) and PWCr (0.0245 mmol) in different
solvents was added 4 mmol of an oxidant at 25 °C (Scheme 1).25
The results (Table 1) show that acetonitrile was the best solvent
providing the highest yields and selectivity (100%). The use of other
solvents such as methanol, dichloromethane, and chloroform led to
lower catalytic activities being observed.
In POMs, the ability to alter extensively the molecular proper-
ties (potentials, charges, sizes, etc.), coupled with their chemically
robust nature, has led to a wide range of applications. A number of
processes, for example, oxidation and acid-dependent reactions are
catalyzed by Keggin and Wells-Dawson type POMs.10
Transition metal substituted POMs are thermodynamically
stable to oxidation and, furthermore, possess hydrolytic stability
under appropriate pH conditions.11 This unique combination of
The amount of catalyst was varied from 0.0163 to 0.0493 mmol
and the other reaction conditions remained constant. The results
for the oxidation reaction at 25 °C over 15 min are shown in
Table 2. A general trend of increase in the conversion of diphenyl-
sulfide by raising the amount of catalyst was observed. These
results demonstrate that PWCr is a very active catalyst in this
⇑
Corresponding author. Tel.: +98 311 7932742; fax: +98 311 6689732.
E-mail addresses: yadollahi@chem.ui.ac.ir, yadollahi.b@gmail.com (B. Yadollahi).
http://dx.doi.org/10.1016/j.tetlet.2014.05.026
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Afrasiabi, R.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.026

2
R. Afrasiabi et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 2
Optimization of the amounts of H2O2 and the catalyst for the selective oxidation of
diphenylsulfide into diphenylsulfonea
O
O
Entry
H2O2 (mmol)
Catalyst (mmol)
Yieldb (%)
S
S
R1
R2
R1
R2
1
2
3
4
5
6
7
8
0.5
1
1.5
2
3
4
5.4
4
4
4
4
0.0245
0.0245
0.0245
0.0245
0.0245
0.0245
0.0245
0.0163
0.0195
0.0327
0.0493
4
CH3CN, r.t., H2O2
18
35
47
61
94
96
88
62
77
72
R1, R2 = Ar, allyl, linear, cyclic
Scheme 1. Selective oxidation of sulfides by H2O2 in the presence of PWCr as the
catalyst.
9
10
11
Table 1
Oxidation of diphenylsulfide into diphenylsulfone by 30% hydrogen peroxide in
different solventsa
a
Yieldb (%)
Reaction conditions: diphenylsulfide (1 mmol), catalyst, H2O2, CH3CN (3 mL),
Entry
Solvent
room temperature, 15 min.
1
2
3
4
5
6
7
MeOH
CH2Cl2
CHCl3
CH3CN
DMF
Me2CO–AcOH
MeOH–H2O
10
17
21
94
6
b
Yields refer to GC yields.
Using the optimized catalytic conditions, the scope of the
method was extended to the oxidation of different sulfides includ-
ing cyclic, benzylalkyl, substituted arylalkyl, and dialkyl sulfides
bearing different functional groups with 30% aqueous hydrogen
peroxide as the oxidant (Scheme 1). From the results in Table 3,
aromatic and aliphatic sulfones were mostly obtained with more
than 99% selectivity. It was observed that a small amount of the
catalyst (0.0245 mmol) was sufficient for the oxidation of aliphatic
sulfides. In the oxidation of aryl methyl sulfides into the corre-
sponding sulfones, methyl substitution at the para-position of the
phenyl ring had an affect on the reaction time but not on the prod-
uct selectivity (Table 3, entries 3 and 4). This indicated that the
reaction proceeds by an oxygen transfer mechanism. If the reaction
involves electron transfer instead of oxygen transfer, substantial
amounts of benzaldehyde would be formed.22,23 The chemoselec-
tivity was noteworthy under our oxidation conditions: the sulfide
34
11
a
Reaction conditions: diphenylsulfide (1 mmol), catalyst (0.0245 mmol), H2O2
(4 mmol), solvent (3 mL), room temperature, 15 min.
b
Yields refer to GC yields.
reaction system, and even a small amount of the POM catalyst
(0.0245 mmol) leads to significant conversion.
The amount of oxidant (H2O2) in the reaction was optimized by
using 0.0245 mmol of the POM catalyst, acetonitrile (3 mL), and
different amounts of hydrogen peroxide with diphenylsulfide
(1 mmol) at room temperature. Samples were drawn at regular
intervals and analyzed by GC (Table 2). The results indicated that
4 mmol H2O2 is the optimum amount for the oxidation of diphe-
nylsulfide in this catalytic system.
Table 3
Selective oxidation of various sulfides by 30% H2O2 at room temperature in the presence of PWCr as the catalysta,b
Entry
Sulfide
Sulfone
Time (min)
45
S
S
O
S
1
O
O
S
2
3
25
10
O
O
S
S
O
O
S
S
4
60
O
S
S
O
S
5
6
25
8
NO2
O
NO2
O
S
O
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R. Afrasiabi et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 3 (continued)
Entry
Sulfide
Sulfone
Time (min)
10
S
O
OH
S
7
OH
O
S
O
S
Bu
Bu
8
9
25
10
Bu
O
Bu
O
O
S
S
S
O
S
10
10
25
O
S
O
S
11
O
a
Reaction conditions: sulfide (1 mmol), H2O2 (4 mmol), PWCr (0.0245 mmol), 25 °C.
Yields are quantitative on the basis of sulfide conversion and refer to GC yields.
b
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functional group was highly reactive and various other functional
groups were tolerated. Sulfides containing allyl and hydroxyl
groups (Table 3, entries 7, 10, and 11) were selectively oxidized
into their corresponding sulfones, and these reactive functional
groups remained intact. Diallyl sulfide was cleanly converted
into diallylsulfone without epoxidation. It is also interesting to
note that the presence of a strong electron-withdrawing NO2
group on the phenyl ring of the diaryl sulfide, did not affect
considerably the formation of the corresponding sulfone (Table 3,
entry 5).
In summary, we have demonstrated that PWCr can act as a
highly efficient catalyst for the selective oxidation of various sul-
fides into sulfones using H2O2 under mild reaction conditions.
The advantages of this catalytic system are that the reactions occur
at room temperature, are operationally simple, practice, require
short reaction times, give high to excellent yields and chemoselec-
tivity, and use commercially available, eco-friendly, and cheap
H2O2 as the oxidant. These advantages make this POM a promising
catalyst as material for practical and large scale applications.
16. Nisar, A.; Lu, Y.; Zhuang, J.; Wang, X. Angew. Chem., Int. Ed. 2011, 50,
3187–3192.
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Trubitsina, T. A.; Melgunov, M. S.; Burgina, E. B.; Mrowiec-Bialon, J.; Jarzebski,
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Acknowledgement
21. Radkov, E.; Beer, R. H. Polyhedron 1995, 14, 2139–2143.
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24. Preparation of the (TBA)4[PW11CrO39]Á3H2O catalyst: K4[PW11CrO39]ÁnH2O was
synthesized according to the published procedure. The structure was
Support for this research by the University of Isfahan and the
Institute for Advanced Studies in Basic Sciences (IASBS) is
acknowledged.
confirmed by elemental analysis, UV–vis, and infrared spectroscopy.18
A
sample of K4[PW11CrO39]ÁnH2O (0.062 g) was dissolved in 0.5 M NaHSO4 (5 mL,
pH = 1) and dropwise added to an aqueous solution of tetra-n-butylammonium
bromide (TBABr) (0.026 g) in 5 mL H2O. The resulting solid was filtered and
recrystallized from acetonitrile.19–21 IR, UV–vis, and elemental analyses
established that the product had not changed during counter ion exchange
(see Supplementary Data, Figures S1–S4).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014. 05.026.
25. Typical procedure for the catalytic oxidation of sulfides to sulfones: PWCr
catalyst (0.0245 mmol), CH3CN (3 mL), sulfide (1 mmol), and hydrogen
peroxide (4 mmol, 30% aq solution) were added to
a glass tube as the
References and notes
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product was extracted with CH2Cl2 and the combined organic layers were
dried over anhydrous Na2SO4. The solvent was removed under reduced
pressure to give the corresponding pure sulfone. The products were
identified by comparison of their 1H NMR and 13C NMR signals with the
literature data (see Supplementary Data, Figures S5–S18).
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Please cite this article in press as: Afrasiabi, R.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.026