Alkylimidazolium/alkylpyridinium octamolybdates catalyzed oxidation of sulfides to sulfoxides/sulfones with hydrogen peroxide

Article abstract of DOI:10.1016/j.catcom.2016.03.020

β-Mo₈O₂₆ based alkyl imidazolium and pyridinium salts of general formula [Bmim]₄Mo₈O₂₆ (Bmim = 1-butyl-3-methylimidazolium), [Hmim]₄Mo₈O₂₆ (Hmim = 1-hexyl-3-methylimidazolium), [Dhmim]₄Mo₈O₂₆ (Dhmim = 1.2-dimethyl-3-hexylimidazolium) and [Hpy]₄Mo₈O₂₆ (Hpy = 1-hexylpyridinium) have been used as catalysts for the oxidation of sulfides using 30% hydrogen peroxide as oxidant. The examined β-Mo₈O₂₆ salts prove to be highly active and are self-separating. A high selectivity towards either sulfoxides or sulfones can be nicely controlled by variation of the reaction conditions. In both cases, the catalysts can be recycled and reused for several times without significant loss of activity, representing a good stability of the catalysts.

Full text of DOI:10.1016/j.catcom.2016.03.020

Alkylimidazolium/alkylpyridinium octamolybdates catalyzed oxidation of
sulfides to sulfoxides/sulfones with hydrogen peroxide
Jin-Xin Ye, Jing-Yun Wang, Xin Wang, Ming-Dong Zhou
PII:
DOI:
Reference:
S1566-7367(16)30112-1
doi: 10.1016/j.catcom.2016.03.020
CATCOM 4622
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
5 January 2016
7 March 2016
30 March 2016
Please cite this article as: Jin-Xin Ye, Jing-Yun Wang, Xin Wang, Ming-Dong Zhou,
Alkylimidazolium/alkylpyridinium octamolybdates catalyzed oxidation of sulfides to
sulfoxides/sulfones with hydrogen peroxide, Catalysis Communications (2016), doi:
10.1016/j.catcom.2016.03.020
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ACCEPTED MANUSCRIPT
Alkylimidazolium / Alkylpyridinium octamolybdates catalyzed oxidation of
sulfides to sulfoxides /sulfones with hydrogen peroxide
Jin-Xin Ye, a Jing-Yun Wang,a Xin Wang,a and Ming-Dong Zhoua*
a School of Chemistry and Materials Science, Liaoning Shihua University, Dandong
Road 1, Fushun113001, China. Tel: +862456863837, Fax: +862456863837. E-mail:
mingdong.zhou@lnpu.edu.cn
Abstract
β-Mo8O26 based alkyl imidazolium and pyridinium salts of general formula
[Bmim]4Mo8O26 (Bmim = 1-butyl-3-methylimidazolium), [Hmim]4Mo8O26 (Hmim =
1-hexyl-3-methylimidazolium), [Dhmim]4Mo8O26 (Dhmim
=
1,2-dimethyl-3-
hexylimidazolium) and [Hpy]4Mo8O26 (Hpy = 1-hexylpyridinium) have been used as
catalysts for the oxidation of sufides using 30 % hydrogen peroxide as oxidant. The
examined β-Mo8O26 salts prove to be highly active and are self-separating. A high
selectivity towards either sulfoxides or sulfones can be nicely controlled by variation
of the reaction conditions. In both cases, the catalysts can be recycled and reused for
several times without significant loss of activity, representing a good stability of the
catalysts.
Keywords: Sulfide oxidation, Polyoxomolybdate, Homogeneous catalysis, Hydrogen
peroxide
1. Introduction
There is a growing need for environmentally friendly chemical producing processes in
industry. A major goal is to develop green catalysts or catalytic methods with “atom
economic” use of raw materials, low energy consumption, and without or minimize
the production of wastes. Sulfide oxidation is one of important industrial processes,
which is the most straightforward method for producing sulfoxides or sulfones [1,2].
Many transition metal based catalysts are highly efficient to catalyze the oxidation of
sulfides to sulfoxides / sulfones using H2O2 as environmentally friendly oxidant [4-8].
Among various examined catalysts, polyoxometalates (POMs) have been received
much attention both in academia and in industry in the past decades [9,10]. Some
3-
3-
Keggin type POM catalysts such as PW12O40 or PMo12O40 based organic salts are
found to be able to activate H2O2 molecule and are very efficient phase transfer
catalysts for the oxidation of olefins, alcohols, aromatic compounds, as well as
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sulfides [11,12]. The simple organic β-octamolybdate salts bearing quaternary
ammonium cation have also been reported as highly active catalysts for oxidation
reactions [13-17]. While comparing to the well-established Keggin type POM catalyst,
β-octamolybdate is advantageous since it is cheaper, more stable and easy to prepare.
Nevertheless, to the best of our knowledge, their catalytic application has not yet been
well developed so far.
Our laboratory has developed several imidazolium or pyridinium based β-
octamolybdates catalysts [13], and they have been applied as excellent self-separating
catalyst for olefin epoxidations. Recently, these catalysts were also successfully
applied in sulfide oxidations. Sulfide can be oxidized to either sulfoxide or sulfone,
and the selectivity control in these transformations is very important. However, when
reviewing the literature work, we find that most established catalytic methods only
described the oxidation of sulfide to one of the oxidation product (either sulfoxide or
sulfone). The highly selective oxidation of sulfide to both sulfoxide and sulfone using
β-octamolybdate based catalytic system has never been described. We have recently
found that, by controlling the reaction condition, the imidazolium or pyridinium based
β-octamolybdates can catalyze the selective oxidation of sulfide to both sulfoxide and
sulfone in high selectivity. Moreover, the catalysts are also self-separating, and can be
reused for several times without significant loss of activity. Therefore, we wish to
report one primarily results of this work herein.
2. Results and Discussion
Four alkylimidazolium or alkylpyridinium octamolybdates including [Bmim]4Mo8O26
(Bmim = 1-butyl-3-methylimidazolium) 1, [Hmim]4Mo8O26 (Hmim = 1-hexyl-3-
methylimidazolium)
2,
[Dhmim]4Mo8O26
(Dhmim
=
1,2-dimethyl-3-
hexylimidazolium) 3 and [Hpy]4Mo8O26 (Hpy = 1-hexylpyridinium) 4 have been
synthesized and characterized as described previously [13] (See preparation details
and characterization data in the Supplementary Material). They have been examined
as catalysts for sulfide oxidations using 30% hydrogen peroxide as oxidant. In general,
the sulfides could be selectively oxidized to sulfoxide at room temperature in high
yield. On the other hand, if sulfone is the targeted product, it is favorable to perform
the reaction at higher temperatures (> 60 °C) with excess amount of oxidant. The
selective sulfide oxidations towards both sulfoxides and sulfones have been examined
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by using compounds 1-4 as catalysts and 30% hydrogen peroxide as oxidant. In both
cases, the reaction optimizations were carried out by using compound 1 as a catalyst,
thioanisole as a standard sulfide substrate, and 30% hydrogen peroxide as the oxidant
(Scheme 1).
Scheme 1 Oxidation of thioanisole
2.1 Selective oxidation of sulfides to sulfoxides
Sulfoxides can be used as synthetic intermediates for the preparation of a variety of
chemically or biologically value-added compounds such as drugs, flavors
andgermicides [13-15]. To our delight, the oxidation of thioanisole under the optimal
condition - [Bmim]4Mo8O26 (0.5 mol%), H2O2 (1.5 equiv.), CH3CN (1 mL), rt (see
details in Table S1) - affords the desire products in good yield and selectivity. A time-
dependent yield of sulfoxide and sulfone was examined by applying 0.1 mol % of
catalysts 1-4 in order to give a more detailed determination of the catalyst
performance (Figure 1). In general, all the catalysts show similar time-dependent
curves and oxidation yields towards both products. The turn over frequencies (TOF)
of 1-4 are 132, 110, 135 and 97 min-1 (after 5 min), respectively. In general, the alkyl
pyridinium cation based β-Mo8O26 compound 4 show comparatively lower activity
than the alkyl imidazolium based compounds. Catalysts 1-4 result in 86, 82, 82, 81 %
of 2a respectively after 1 hour of the reaction, whereas 1 proved to be the most
efficient catalyst. It can be observed that, ca. 10-15 % of sulfone are formed even at 5
min of the reaction, and the yields are kept almost unchanged with the increase of
reaction time.
After adding H2O2, the catalyst solids were completely dissolved and finally resulted
in a yellow solution, indicating the formation of active species. It has been reported
that the dissociated oxodiperoxo compounds might be the real catalytic active species
for the oxidation reactions [27]. While after the consumption of H2O2, the white
catalyst solids can be precipitated in CH3CN. Therefore, the catalysts can be recycled
simply by filtration. The recycled catalysts were purified by washing with water, dried
under vacuum and then used directly for the next catalytic run. The catalytic results
for the recycled catalyst 1 are shown in Figure 2. The overall recycling yields are
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above 95 % after each run. The recycled catalyst [Bmim]4Mo8O26 can be used at least
8 times without significant loss of activity, indicating a good stability of the catalyst.
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Time (min)
Figure 1 Time-dependent yield using catalysts 1-4 for the oxidation of thioanisole to
2a (●1, *2, ■3, ▲4) or 2b (○1, ×2, □3, ∆4).
90
80
70
60
50
40
30
20
10
0
1
2
SO
SO2
3
4
5
6
7
8
Runs
Figure 2 Reuse of catalyst 1 in the oxidation of thioanisole.
The scope of reaction was then extended to other substrates. It is found that the
electronic effects of different functional groups on the phenyl ring of aromatic
sulfides significantly affect the activity and selectivity (Table 1, entries 1-5). The
aromatic sulfides bearing electron-donating groups display higher activity than the
ones bearing electron-withdrawing groups. The reaction conversions are in the order
p-methoxy thioanisole>p-methyl thioanisole>thioanisole>p-chlorine thioanisole>p-
nitro thioanisole, which is in accordance with the order of the electron donating
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abilities of different functional groups. However, the order of selectivity towards
sulfoxide is opposite. Such a result is also reasonable since high activity might
accelerate the oxidation of sulfides to sulfones. 100 % selectivity towards sulfoxide
can be achieved for strong electron-withdrawing nitro derived 4-nitro thioanisole.
While in the case of [(n-C4H9)4N]4(α-Mo8O26)/H2O2 catalytic system, the catalyst
displays higher activity as compared to our system and all the functional groups
derived thioanisoles result in almost 100 % conversions [13]. Accordingly, the subtle
functional group effects on the activities are not easily observed. Furthermore, most
aliphatic sulfides exhibit higher activity than aromatic sulfides, in which almost 100%
of sulfoxide conversion and selectivity could be obtained for aliphatic sulfides even
with 1 equiv. of H2O2 applied (Table 1, entries 6-9). However, due to the strong steric
effect of tert-butyl groups, the reaction displays comparatively low activity for tert-
butyl sulfide, resulting in 90 % of oxidation yield after 3 h of the reaction (Table 1,
entry 10).
Table 1 Oxidation of different sulfides to sulfoxides with H2O2 using [Bmim]4Mo8O26
as catalysta
Yield(%)
SO
74
Sel. (%)
SO
76
Conv.
(%)
Entry R1
R2
SO2
24
19
11
1
4-OCH3C6H4
CH3
98
2
4-CH3C6H4
C6H5
CH3
95
76
80
3
CH3
95
84
88
4
4-Cl-C6H4
4-NO2-C6H4
C2H5
CH3
90
79
11
88
5
CH3
82
82
0
100
>99
>99
97
6b
7b
8b
9b
C2H5
100
100
100
100
>99
>99
97
trace
trace
2
CH3
CH2=CHCH2
CH2=CHCH2
(CH2)3CH3
CH2=CHCH2
(CH2)3CH3
95
5
95
5

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10c
C(CH3)3
C(CH3)3
90
86
4
96
aThe reaction was carried out using substrate (0.5 mmol), [Bmim]4Mo8O26 (0.5mol%),
b
H2O2 (1.5 equiv.) in CH3CN (1 mL) at rt for 40 min; 1 equiv. of H2O2 was applied;
cthe reaction was performed for 3 h.
2.2 Selective oxidation of sulfides to sulfones
Based on the above studies, the capability of the catalytic system towards oxidation of
sulfides to sulfones was further investigated. The optimal condition obtained was
o
[Bmim]4Mo8O26 (0.2 mol%), H2O2 (3.0 equiv.), CH3CN (1 mL) at 60 C for 1h (see
details in Table S2). Under optimal condition, the oxidation of sulfides to sulfones
reaction was examined for different substrates, and the results are shown in Table 2. It
is satisfactory to see that most reactions result in almost 100% conversion of
substrates with 100 % of selectivity towards sulfones. While in the case of p-chlorine
thioanisole and p-nitro thioanisole bearing electron-withdrawing groups on the phenyl
ring, only a poor to moderate selectivity towards sulfones can be obtained. The
catalysts also show similar time-dependent curves of yields for catalysts 1-4. In this
system, the white catalyst powder was also easily precipitated from the solution after
the reaction. Thus, the catalyst was recycled by filtration, washed with water, dried
under vacuum and used directly for the next run (Fig. S1-S2).
Table 2 Oxidation of different sulfides to sulfones with H2O2 using [Bmim]4Mo8O26
as catalysta
Conv.
(%)
Yield
(%)
Entry
R1
R2
1
2
3
4
5
6
7
4-OCH3C6H4
4-CH3C6H4
C6H5
CH3
100
100
100
100
100
100
100
100
CH3
>99
CH3
>99
4-Cl-C6H4
4-NO2-C6H4
C2H5
CH3
54 (46c)
19 (80c)
100
CH3
C2H5
CH3
CH2=CHCH2
>99
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8
CH2=CHCH2
(CH2)3CH3
C(CH3)3
CH2=CHCH2
(CH2)3CH3
C(CH3)3
100
100
100
>99
>99
>99
9
10
aThe reaction was carried out using substrate (0.5 mmol), [Bmim]4Mo8O26 (0.2
b
mol%), H2O2 (3 equiv.) in CH3CN (1 mL) at 60 °C for 1 h. Selectivity towards
sulfone. cYield of sulfoxide.
Conclusions
The selective oxidation of sulfides to either sulfoxides or sulfones is studied using
imidazolium or pyridinium based octamolybdate salts as efficient catalysts in the
presence of hydrogen peroxide as green oxidants. A high selectivity towards either
sufoxides or sulfones can be nicely controlled by variation of reaction temperature,
amounts of catalysts and the amounts of oxidant applied. All the catalysts display
good self-separating property and stability. Recycling experiments indicate that the
catalysts can be recycled and reused for several times without significant loss of
activity. The catalytic system provides a green and easy handling strategy for the
synthesis of sulfoxides or sulfones under mild conditions.
Acknowledgements
M.D.Z thanks the National Science Foundation of China (21101085), Natural Science
Foundation of Liaoning Province (2015020196), Fushun Science & Technology
Program (FSKJHT 201423), and Liaoning Excellent Talents Program in University
(LJQ2012031) for the financial supports.
References
[1] K. Jeyakumar, D.K. Chand, Tetrahedron Lett.47 (2006) 4573-4576.
[2] M. Jereb, Green Chem.14 (2012) 3047-3052.
[3] Y.S. Chi, C.P. Li, Q.Z Jiao, Q.S. Liu, P.F. Yan, X.M. Liu, W.B. Urs, Green
Chem.13 (2011) 1224-1229.
[4] M. Kirihara, J. Yamamoto, T. Noguchi, A. Itou, S. Naito, Y. Hirai, Tetrahedron65
(2009) 10477-10484.
[5] B. Zhang, S. Li, S. Yue, M. Cokoja, M.D. Zhou, S.L. Zang, F.E. Kühn, J.
Organomet. Chem.744 (2013) 108-112.
[6] Ch. V. Reddy, J. G. Verkade, J. Mol. Catal. A: Chem.272(2007) 223-240.
[7] W. Al-Maksoud, S. Daniele, A.B. Sorokin, Green Chem.10 (2008) 447-451.
7

ACCEPTED MANUSCRIPT
[8] C. Yang, Q. Jin, H. Zhang, J. Liao, J. Zhu, B. Yu, J. Deng, Green Chem.11(2009)
1401-1405.
[9] N. Mizuno, K. Yamaguchi, Chem. Rev.6 (2006) 12-22.
[10] N. Mizuno, K. Yamaguchi, K. Kamata, Coord. Chem. Rev.249 (2005) 1944-
1956.
[11] K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi, N. Mizuno,
Science300 (2003) 964-966.
[12] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal.41 (1996) 113-252.
[13] M.D. Zhou, M.J. Liu, L.L. Huang, J. Zhang, J.Y. Wang, X.B. Li, F.E. Kühn, S.L.
Zang, Green Chem.17 (2015) 1186-1193.
[14] M.L. Guo, H.Z. Li, Green Chem.9 (2007) 421-423.
[15] S.L. Linguito, X. Zhang, M. Padmanabhan, A. V. Biradar, T. Xu, T.J. Emge, T.
Asefa, J. Li, New J. Chem.37 (2013) 2894-2901.
[16] J. Du, J. Yu, J. Tang, J. Wang, W. Zhang, W.R. Thiel, M. Jia, Eur. J. Inorg.
Chem. 2011, 2361-2365.
[17] C.J. Carrasco, F. Montilla, E. Álvarez, M. Herbert, A. Galindo, Polyhedron54
(2013) 123-130.
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Graphical Abstract
β-Mo8O26 based alkylimidazolium and alkylpyridinium salts are used as green and
efficient self-separating catalysts for highly selective oxidation of sulfides to
sulfoxides or sulfone with hydrogen peroxide.
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Highlights

β-Mo8O26 based alkyl imidazolium and pyridinium salts were used as catalysts
for the oxidation of sufides

A high selectivity towards either sulfoxides or sulfones can be nicely
controlled

The catalysts can be recycled and reused for several runs without significant
loss of activity
10