Efficient and convenient oxidation of sulfides to sulfones using H 2O2 catalyzed by V2O5 in ionic liquid [C12mim][HSO4]

Article abstract of DOI:10.1039/c3cy00719g

A simple, efficient, and eco-friendly procedure for the oxidation of sulfides to sulfones using H₂O₂ catalyzed by V ₂O₅ in ionic liquid [C₁₂mim][HSO₄] has been developed. This atom-economical protocol affords the target products in good to high yields. The products can be separated by a simple extraction with organic solvent, and the catalytic system can be recycled and reused without loss of catalytic activity.

Full text of DOI:10.1039/c3cy00719g

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Efficient and convenient oxidation of sulfides to
sulfones using H2O2 catalyzed by V2O5 in ionic
liquid [C12mim][HSO4]
Yu-Lin Hu, a Xiao-Bing Liub and Dong Fanga
*
Cite this: DOI: 10.1039/c3cy00719g
Received 22nd September 2013,
Accepted 26th October 2013
DOI: 10.1039/c3cy00719g
www.rsc.org/catalysis
A simple, efficient, and eco-friendly procedure for the oxidation of
sulfides to sulfones using H2O2 catalyzed by V2O5 in ionic liquid
[C12mim][HSO4] has been developed. This atom-economical protocol
affords the target products in good to high yields. The products
can be separated by a simple extraction with organic solvent, and
the catalytic system can be recycled and reused without loss of
catalytic activity.
catalysts. Hydrogen peroxide (H2O2) is an economical, envi-
ronmentally friendly and atom-efficient oxidant which leads
to only water after the reaction.14 This reagent has, however,
high activation energy, making catalysis necessary,15 and a
number of catalytic oxidation processes based on the combi-
nation of transition metals or organocatalysts and H2O2 have
been developed for such a conversion.16 However, most of
these methods still suffered from the use of expensive reagents,
lower yields, long reaction times, environmental hazards, poor
recovery of expensive metal catalysts and laborious workup
procedures. Consequently, there is a great need to develop
new and environment-benign procedures that address these
drawbacks for such a conversion.
Ionic liquids (ILs) are a class of organic salts with unusu-
ally low melting temperatures that have attracted much atten-
tion from the scientific community in recent years. As a class
of potential greener solvents, ILs exhibit many unique physico-
chemical properties, such as negligible volatility and nonflam-
mability under ambient conditions, large liquid range, high
thermal stability, wide electrochemical window, and strong
ability to dissolve many chemicals.17,18 Therefore, ILs have
found wide applications in chemical synthesis,19 biocatalytic
transformations,20 electrochemistry,21 and analytical and sep-
aration science.22 In continuation of our interest in exploring
green oxidation reactions in ionic liquids, we report herein a
new, efficient and environmentally friendly protocol for the
oxidation of sulfides to sulfones using H2O2 catalyzed by
vanadium pentoxide (V2O5) in the ionic liquid 1-dodecyl-3-
methylimidazolium hydrogensulfate ([C12mim][HSO4]) under
mild conditions (Scheme 1). Furthermore, we demonstrate
that the catalytic system can be recycled and reused without
any significant loss of catalytic activity.
Sulfones are an important class of chemicals that are used
extensively for the preparation of a variety of fine or special
chemicals such as drugs, natural products, and catalysts.1,2
In particular, sulfones are important building blocks for the
synthesis of various biologically active molecules.2 Therefore,
there has been tremendous interest in developing efficient
methods for the synthesis of these molecules, and a well
known method constitutes the oxidation of sulfides. Traditional
methods for performing such a transformation generally involve
the use of stoichiometric amounts of the strongest oxidizing
reagents (i.e., HNO3, KMnO4, MnO2, etc.)3 and suffer from
considerable drawbacks such as high cost, low yield, harsh
or delicate reaction conditions, and a large amount of waste
byproducts. Oxidation using potassium hydrogen persulfate,4
NaClO4,5 H5IO6,6 NaClO,7 UHP,8 m-chloroperbenzoic acid,9
NaIO4,10 dimethyldioxirane,11 oxone12 and other reagents13 has
also been developed for this conversion. However, these protocols
are generally associated with one or more disadvantages, such
as long reaction times, high temperatures, low yields, com-
plex handling procedures, problematic side reactions, the use
of expensive, toxic and moisture sensitive reagents, and environ-
mental concerns associated with the use of poorly manageable
The initial study was carried out using methyl(phenyl)sulfane
as substrate to optimize the reaction conditions, and the results
are summarized in Table 1. Initial reaction screening led to dis-
appointing results. In the absence of catalyst and ionic liquid,
the reaction proceeded very slowly, and the yield was only 20%
a College of Chemistry and Chemical Engineering, Yancheng Teachers University,
Yancheng 224002, PR China. E-mail: huyulin1982@163.com;
Fax: +86 0515 88233183; Tel: +86 0515 88233183
b College of Chemistry and Chemical Engineering, Jinggangshan University,
Ji'an 343009, PR China
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Besides V2O5, we also tried to use other types of catalysts in
this model reaction (Table 1, entries 13–16), and the results
showed that V2O5 demonstrated the best performance in terms
of yield and reaction rate. Furthermore, the catalytic system
could be typically recovered and reused with no appreciable
decrease in yield and reaction rate (Table 1, entries 9 and 10).
Therefore, the combination of H2O2, V2O5 and [C12mim][HSO4]
was chosen as the optimal condition for further exploration.
With these results in hand, we subjected other sulfides to
the oxidation reactions, and the results are listed in Table 2.
It is clear that various types of aryl and alkyl sulfides can be
successfully oxidized to the corresponding sulfones in good
to high yields (Table 2). Various types of aryl-alkyl and alkyl
sulfides can be successfully oxidized to the corresponding
sulfones (Table 2, entries 1–6 and 10); because the aryl
sulfides were less reactive, more drastic reaction conditions
were needed to achieve good yields (Table 2, entries 7–9). It
was also observed that the electronic nature of the substituents
on the aromatic ring has some impact on the reaction rate.
Aryl-alkyl sulfides, especially those with electron-donating
substituents (Table 2, entries 3 and 4), were more reactive
than those with electron-withdrawing substituents (Table 2,
entries 5 and 6), providing excellent yields.
Scheme 1 Catalytic oxidation of sulfides to sulfones.
Table 1 Optimization of the conditions for the oxidation of methyl(phenyl)sulfane
a
using H2O2
Entry
Ionic liquid
Catalyst
Time/h
Yieldb /(%)
1
2
3
4
5
6
7
8
—
—
24
6
4
4
4
4
4
6
4
4
8
8
4
4
4
4
20
75
87
90
82
79
74
70
90c
89d
59
68
76
65
84
72
[C6mim][HSO4]
[C10mim][HSO4]
[C12mim][HSO4]
[C12mim][PF6]
[C12mim][OTf]
[C12mim][BF4]
[C12mim]Br
[C12mim][HSO4]
[C12mim][HSO4]
—
[C12mim][HSO4]
[C12mim][HSO4]
[C12mim][HSO4]
[C12mim][HSO4]
[C12mim][HSO4]
V2O5
V2O5
V2O5
V2O5
V2O5
V2O5
V2O5
V2O5
V2O5
V2O5
—
According to the literature23 and the observations in our
reactions, taking the oxidation of methyl(phenyl)sulfane with
H2O2 as an example, a possible mechanism is proposed
(Scheme 2). In the reaction, the catalyst V2O5 provides a source
9
10
11
12
13
14
15
16
MnO2
ZnO
Fe2O3
CuO
−
+
of VO3 and VO2 (1), and 1 reacts with the oxidant H2O2
to form the red oxoperoxo VO(O2)+ (2) and the VO(O2)OOH
species (3). Then, 3 reacts with the substrate to form the
transition state 4. Then, 4 eliminates the oxoperoxo (2) and
water to yield the intermediate substrate 5, which reacts with
3 to form the transition state 6. 6 then very rapidly affords 2
and water to yield the desired product. 2 is then re-oxidized to
3 by H2O2 to complete the catalytic cycle. It appears that the
formations of 4 and 6 are the rate-determining steps.
a
The reactions were carried out using methyl(phenyl)sulfane
(10 mmol), catalyst (2 mmol), IL (10 mL), and H2O2 (30%, 25 mmol)
b
c
d
at room temperature (25 °C). Isolated yield. The first run. The
second run.
after 24 h (Table 1, entry 1). The results mean that the oxi-
dant H2O2 alone does not work effectively in the reaction.
The effects of different ionic liquids, such as [C6mim][HSO4],
[C10mim][HSO4], [C12mim][HSO4], [C12mim][PF6], [C12mim][OTf],
[C12mim][BF4], and [C12mim]Br, were tested with V2O5 as cata-
lyst in the reaction (Table 1, entries 2–8), and it was observed
that [C12mim][HSO4] demonstrated the best performance
(Table 1, entry 4). The different effects of ILs on the reaction
may be attributed to their different abilities to stabilize and
dissolve the oxidant H2O2 and the catalyst. Under the same
conditions, the IL that stabilizes and dissolves them more
easily will lead to a larger increase in the effective reactant
concentration, which increases the encounter probabilities
between the substrate and the reactive species; thus, a higher
rate or yield of the reaction is observed. For a blank test
(Table 1, entry 11), a lower yield of the product was obtained
when the same reaction conditions were used in the absence
of ionic liquid. In addition, the catalyst is crucial for this
reaction, and a lack of it leads to a much lower yield (Table 1,
entry 12).
In conclusion, we have demonstrated an efficient and con-
venient oxidation of sulfides to sulfones using the H2O2–V2O5
catalytic system in the ionic liquid [C12mim][HSO4]. Compared
to the synthetic methods reported in the literature,3–13,16 not
only were the yields of products enhanced or greatly improved
but the operating units were also easily worked up. Mild
reaction conditions, simplicity of operation, high yields, sta-
bility, easy isolation of products, and excellent recyclability
of the catalytic system are the attractive features of this
methodology. The scope, definition of the mechanism and
synthetic application of this reaction are currently under study
in our laboratory.
Experimental
All the chemicals were obtained from commercial sources
without any pretreatment. All reagents were of analytical grade.
The ionic liquids were synthesized according to the procedure
in the literature.18 1H NMR spectra were recorded using
a Bruker 500 MHz spectrometer with CDCl3 as solvent and
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a
Table 2 Catalytic oxidation of sulfides to sulfones
Entry
1
Substrate
Product
Time/h
4
Yield/(%)b
90
2
3
4
4
94
93
4
5
4
6
95
83
6
6
85
7
8
8
8
84c
87c
9
8
4
82c
10
93
a
Unless otherwise noted, the reactions were carried out using sulfide (10 mmol), V2O5 (2 mmol), [C12mim][HSO4] (10 mL), and H2O2 (30%, 25 mmol)
at room temperature. b Isolated yield. c The reaction was carried out at 45 °C.
tetramethylsilane (TMS) as internal standard. High performance
liquid chromatography (HPLC) experiments were performed
using a liquid chromatograph (Dionex Softron GmbH, America)
consisting of a pump (P680) and an ultraviolet–visible light
detector (UVD) system (170U). The experiments were performed
on a Discovery C18 column (ø 4.6 × 250 mm). Elemental analysis
was performed using a Vario EL III instrument (Elementar
Analysensysteme GmbH, Germany).
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Scheme 2 Possible mechanism for the oxidation of methyl(phenyl)sulfane using H2O2.
General procedure for oxidation reactions. For a typical
experiment, to 10 mL of [C12mim][HSO4] were sequentially
added sulfide (10 mmol), V2O5 (2 mmol), and an aqueous
solution of H2O2 (30%, 25 mmol). The obtained mixture was
stirred vigorously at room temperature for an appropriate
time (Table 2), and the reaction progress was monitored by
HPLC. Upon completion of the reaction, ethyl acetate was
used to extract the organic compounds (3 × 20 mL). The ethyl
acetate solution was washed with water (3 × 20 mL) and then
dried over anhydrous Na2SO4. The solvent was removed, and
the residue was distilled under vacuum to give the desired
pure product. The rest of the ionic liquid and the catalyst
were recovered by decantation of water produced in the
reaction and concentration under vacuum. Fresh substrates
were then recharged to the recovered catalytic system and
then recycled under identical reaction conditions. The target
substrates were characterized by 1H NMR and elemental analysis
or compared with their authentic samples. Spectroscopic data
for selected products are as follows.
Acknowledgements
We thank the Natural Science Foundation of Jiangsu Province
and the Science and Technology Program of Jiangxi Provincial
Education Bureau (no. GJJ13535) for supporting this research.
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