Solvent free oxidation of sulfides to sulfones by H2O2 in the presence of chromium substituted polyoxometalate as catalyst

Article abstract of DOI:10.1016/j.inoche.2014.10.034

Solvent-free oxidation of sulfides into sulfones by 30% hydrogen peroxide has been achieved using chromium substituted Keggin type polyoxometalate under mild reaction conditions in 94-100% yield. This catalytic system showed excellent activity in the oxidation of sulfide groups. The other active functional groups such as hydroxyl and C=C bond have been tolerated.

Full text of DOI:10.1016/j.inoche.2014.10.034

Inorganic Chemistry Communications 50 (2014) 113–116
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Solvent free oxidation of sulfides to sulfones by H2O2 in the presence of
chromium substituted polyoxometalate as catalyst
c
Roozbeh Afrasiabi a, Fariba Jalilian b, Bahram Yadollahi b, , Mostafa Riahi Farsani
⁎
a
Institute for Advanced Studies in Basic Sciences (IASBS), Gavazang, Zanjan 45195-1159, Iran
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
b
c
Department of Chemistry, Islamic Azad University, Shahrekord Branch, Shahrekord, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2014
Received in revised form 19 October 2014
Accepted 30 October 2014
Available online 1 November 2014
Solvent-free oxidation of sulfides into sulfones by 30% hydrogen peroxide has been achieved using chromium
substituted Keggin type polyoxometalate under mild reaction conditions in 94–100% yield. This catalytic system
showed excellent activity in the oxidation of sulfide groups. The other active functional groups such as hydroxyl
and C_C bond have been tolerated.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Substituted polyoxometalate
Catalysis
Oxidation
Sulfide
Hydrogen peroxide
Solvent free
Because of environmental rules limiting the sulfur content in fuel
oils to the few ppm levels, ultra-deep desulfurization has become an im-
portant topic of research. The oxidation of sulfide compounds is of much
current interest in the context of the preparation of synthetically useful
sulfones [1–3] as well as of the desulfurization of fuels [4–6]. Sulfone
compounds are important synthetic intermediates in various chemicals
and are also very useful building blocks for the preparation of elaborated
organic structures [7]. As well, oxidation of organo-sulfur compounds
finds also application in the removal of sulfur-containing pollutants dur-
ing air and wastewater treatment [8]. For the oxidation of sulfides to
sulfones, a range of oxidants have been investigated. For example, stoi-
chiometric oxidation of sulfides by sodium percarbonate, sodium per-
borate, H5IO6, and the other unclean oxidant with different catalysts
was studied [1–3]. Unfortunately, most of these reagents are not satis-
factory for medium- to large-scale synthesis because of the low content
of effective oxygen, toxic oxidants and solvents, the formation of envi-
ronmentally unfavorable co-products, and high cost [9].
oxidants” for its environmentally benign characteristics, only producing
harmless water as by-product, safety in storage and operation, cheap-
ness, readily available and the high effective oxygen content [10]. Also
solid-phase-assisted reaction systems without organic solvents have
been increasingly attracted as environmentally benign organic reaction
systems [11].
In the past years, many catalytic systems such as ionic liquids [12],
H2WO4 [13], Na2WO4 [14], K12WZnMn2(ZnW9O34)2 [15], H3PW12O40
[16], [(C18H37)2(CH3)2N]7PW11O39 [17], polymer-immobilized peroxo-
tungsten [18], and metal catalysts including Se, Mn, and Re [19–21],
have been implied on sulfide oxidation with H2O2.
Polyoxometalates (POMs) and transition metal substituted POMs
are large classes of highly modifiable discrete metal–oxygen anionic
clusters [22] with substantial structural diversity and widely ranging
properties facilitating applications in catalysis [23]. POMs have been
extensively applied as catalysts for the oxidation of a variety of
organic compounds such as alkenes, alcohols, and sulfides [24–26]. For
example, it was reported that [γ-PW10O38V2(μ-OH)2]3− and [TBA]4[γ-
SiW10O34(H2O)2] [27,28] homogeneously catalyze oxidation of various
sulfides. POM-based ionic liquids [29] and H3PW12O40 supported
nanoparticle catalysts were also reported on dibenzothiophene oxida-
tion [30].
Obviously, current demand for environmentally friendly processes
requires the development of green oxidation methods that employ
clean oxidants, such as hydrogen peroxide. During recent years, aque-
ous hydrogen peroxide has been one of the most attractive “green
Environmentally highly important catalytic oxidation of sulfides to
sulfones is still a key challenge [31–33]. Herein, we want to report
a practical and green method for selective oxidation of various
⁎
Corresponding author.
E-mail addresses: yadollahi@chem.ui.ac.ir, yadollahi.b@gmail.com (B. Yadollahi).
http://dx.doi.org/10.1016/j.inoche.2014.10.034
1387-7003/© 2014 Elsevier B.V. All rights reserved.

114
R. Afrasiabi et al. / Inorganic Chemistry Communications 50 (2014) 113–116
Table 1
Optimum amounts of H2O2 and POM catalyst for selective oxidation of diphenyl sulfide to
diphenyl sulfone.a
Entry
H2O2 (equiv)
TBAPWCr (μmol)
Yield (%)b
1
2
3
4
5
6
7
8
3.5
4.5
5.5
6.5
6.5
6.5
6.5
6.5
16.3
16.3
16.3
16.3
19.5
24.5
32.7
49.2
70
90
96
100
85
80
77
74
a
The reactions was performed by 1 mmol diphenyl sulfide at room temperature in
15 min.
b
Yields refer to GC yields.
Fig. 1. FTIR spectrum of TBAPWCr.
The oxidation reaction of diphenyl sulfide, as a model compound,
was carried out by various amounts of catalyst from 16.3 to 49.2 μmol
at 25 °C for 15 min, and other reaction conditions remained constant.
Results showed that the reaction yield and selectivity were affected cru-
cially by the catalyst amount (Table 1). By raising the catalyst amounts,
a general trend of increasing conversion for diphenyl sulfide was ob-
tained. These results demonstrated clearly that TBAPWCr catalyst is
very active in this reaction system, and that even small amount
(16.3 μmol) of the catalyst could lead to significant conversion.
For optimization of hydrogen peroxide amounts, the reactions
were carried out at room temperature by using 16.3 and 49.2 μmol cat-
alyst and different amounts of hydrogen peroxide for a fixed amount of
diphenyl sulfide (1 mmol) (Table 2). In this oxidation system the
selectivity pattern was changed by variety amounts of H2O2 (Fig. 3). Re-
sults have been shown that 6.5 equivalent of H2O2 is an optimum
amount for desired yields and selectivity of sulfone in oxidation of
diphenyl sulfide.
Using the optimized reaction conditions [37], oxidation of various
aromatic and aliphatic sulfides by 6.5 mmol H2O2 was performed. Reac-
tions proceeded smoothly with a substrate/catalyst ratio of 61.5 at
298 K. Various sulfides were selectively oxidized to the corresponding
sulfones with high H2O2 efficiency (Scheme 1).
In this catalytic system not only aryl sulfides (Table 2, entries 1–7)
but also alkyl ones could be oxidized to the corresponding sulfone in ex-
cellent yields (Table 2, entry 8). It is also interesting to mention that
even the presence of strong electron withdrawing NO2 group on the
phenyl ring of aryl sulfides did not noticeably affect the synthesis of sul-
fone (Table 2, entry 5). The oxidation of diallylsulfide, 2-(hydroxyethyl)
phenylsulfide, and phenyl allylsulfide proceeded chemoselectively to
the corresponding sulfones without oxidation of C_C double bonds
and dehydrogenation of the hydroxyl groups (Table 2, entries 11, 7
and 10).
sulfides to corresponding sulfones by aqueous H2O2 in the presence of
chromium substituted Keggin type POM, (TBA)4[PW11CrO39]·3H2O
(TBAPWCr), as a catalyst at solvent free conditions [34,35].
As shown in Fig. 1, in the FTIR spectrum of TBAPWCr the peaks in the
IR spectral range of 600–1100 cm−1 correspond to Keggin structural vi-
brations that could be easily distinguished at 1090, 975, 893, and
808 cm−1. The peaks were attributed to the asymmetry vibrations
P\Oa (internal oxygen connecting P and W), W\Od (terminal oxygen
bonding to W atom), W\Ob (edge-sharing oxygen connecting W),
and W\Oc (corner-sharing oxygen connecting W3O13 units), respec-
tively. Comparison between FT-IR spectrum, in the range of 600–
1100 cm−1, for KPWCr [36] and TBAPWCr indicated that the character-
istic peaks did not shift. These results confirmed that Keggin structure in
PWCr is preserved (see Supplementary data, Fig. S1).
As shown in Fig. 2, the UV–vis spectrum of TBAPWCr illustrated two
broad peaks at 265 nm and 637 nm. The first peak at 265 nm
was assigned to the oxygen to tungsten charge transfer band of
POM. The broad envelope centered at 637 nm is typically for octa-
hedral chromium(III) complexes [36] and could be assigned to spin-
allowed transitions to the excited quarter state. Comparing between
the UV–vis spectra of KPWCr and TBAPWCr was also confirmed that
in the latter Keggin structure was maintained (see Supplementary
data, Fig. S2).
As mention above, the oxidation of diphenyl sulfide by H2O2 in
the presence of TBAPWCr catalyst has been performed in 393:60:1 M
ratios respectively. The stability of POM catalyst in this system was
monitored using multiple sequential oxidation of diphenyl sulfide
with H2O2. By addition of new samples of the sulfide to the reaction
mixture, it has been observed that the catalyst is reusable for at
least three times. Also, the FTIR spectrum of the recovered TBAPWCr
after each run still shows the typical bands of the embedded POMs
(Fig. 4).
The applicability and efficiency of our catalytic system in the
oxidation of sulfides by hydrogen peroxide have been compared with
some of the other reported methods. Result showed that compared to
the most of these methods our catalytic system is superior [1–3,9,
12–30].
In conclusion, the chromium substituted POM, TBAPWCr, showed an
excellent catalytic activity for the oxidation of sulfides with H2O2. The
remarkable feature could be the selective oxidation of sulfides to
Fig. 2. UV–vis spectrum of TBAPWCr.

R. Afrasiabi et al. / Inorganic Chemistry Communications 50 (2014) 113–116
115
Table 2
TBAPWCr catalyzed selective oxidation of sulfides to sulfones using 30% H2O2.a
Entry
1
Sulfide
Sulfone
Time (min)b
20
15
30
2
3
4
5
30
25
6
7
10
30
8
9
25
10
10
20
40
11
a
Reactions conditions: 1 mmol sulfide and 6.5 mmol H2O2 in the presence of TBAPWCr (16.3 μmol) at 25 °C.
Yields are quantitative on the basis of sulfide and refer to GC yields with biphenyl as an internal standard.
b
the sulfones by using an inexpensive reagent under safe and mild con-
ditions without any additional reagents. Additionally, the sulfone prod-
ucts could be easily separated from the reaction mixture.
Acknowledgment
Support for this research by the University of Isfahan and Institute
for Studies in Basic Sciences (IASBS) is acknowledged.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.10.034. This data include MOL file and
InChiKey of the most important compounds described in this article.
O
O
S
S
R1
R2
R1
R2
r.t., H2O2
R1, R2 = Ar, allyl, linear, cyclic
Fig. 3. Screening of hydrogen peroxide amount in the oxidation of diphenyl sulfide, the re-
actions were run for 15 min at 25 °C using 16.3 μmol of TBAPWCr and yields are deter-
mined by GC.
Scheme 1. Selective oxidation of sulfides by hydrogen peroxide in the presence of
TBAPWCr as catalyst.

116
R. Afrasiabi et al. / Inorganic Chemistry Communications 50 (2014) 113–116
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(KBr pellets) were recorded on a FTIR Bruker Vector 22 instrument. UV–vis spectra
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and elemental analysis established that product had not changed during the
counter-ions exchange (see Supplementary data, Figs. S1, S2).
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