Tin(II) polyoxometalate as an efficient catalyst for the selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1515/znb-2006-0516

The applicability of the tin(II) polyoxometalate catalyst, [(n-C ₄H₉)₄N]₅PSnMo₂W ₉O₃₉ · 9H₂O, for sulfoxidation of diaryl, dibenzyl, aryl benzyl, dialkyl, cyclic, and

Full text of DOI:10.1515/znb-2006-0516

Tin(II) Polyoxometalate as an Efficient Catalyst for the Selective Oxidation
of Sulfides to Sulfoxides
Ezzat Rafieea, Iraj M. Baltorkb, Shahram Tangestaninejadb, Alireza Azada, and Sepideh
Moineea
a Department of Chemistry, Faculty of Science, Razi University, Kermanshah, 67149, Iran
b Department of Chemistry, University of Isfahan, Isfahan, 81746-73441, Iran
Reprint requests to Dr. E. Rafiee. Fax: +98-831-427-4559. E-mail: Ezzat rafiee@yahoo.com,
e.rafiei@razi.ac.ir
Z. Naturforsch. 61b, 601 – 606 (2006); received December 6, 2005
The applicability of the tin(II) polyoxometalate catalyst, [(n-C4H9)4N]5PSnMo2W9O39 · 9H2O,
for sulfoxidation of diaryl, dibenzyl, aryl benzyl, dialkyl, cyclic, and heterocyclic sulfides with 30%
hydrogen peroxide was examined under organic halogen-free condition. It is noteworthy that dif-
ferent functional groups including carbon-carbon double bonds, ketones, oximes, aldehydes, ethers,
alcohols, and acetals were tolerated under this reaction condition.
Key words: Tin(II) Polyoxometalate, Sulfoxidation, Sulfoxide, Sulfone, Lewis Acid
Introduction
Oxidation of sulfides is the most straightforward
method for the synthesis of sulfoxides and sulfones
[1 – 3], both of which are important as commodity
chemicals and, in some cases, as pharmaceuticals [4].
Although there are a large number of procedures for
oxidation of sulfides to sulfoxides [5 – 29], few of these
are highly chemoselective and can be stopped at the
sulfoxide level [30, 31]. Some of these methods show
limitations such as long reaction times, low yields
of the products, expensive reagents, poor selectivity,
strongly acidic or basic media, acidic work-up, and
hazardous or toxic oxidizing agents. Therefore intro-
duction of a clean, mild and efficient method for con-
version of sulfides to sulfoxides is still needed.
Scheme 1.
ble against oxidative degradation, unlike metallopor-
phyrins and other metal complexes with organic lig-
ands. These compounds combine the selectivity ad-
vantages of homogeneous catalysts with the stability
advantages of heterogeneous catalysts. There are more
than 50 papers addressing the oxygenation or oxidation
of a wide variety of substrates in the presence of poly-
oxometalate derivatives, and this literature has been re-
cently reviewed [28, 34, 35].
Results and Discussion
Aqueous hydrogen peroxide (H2O2) is an ideal ox-
In continuation of our interest in the catalytic ac-
idant due to its effective oxygen content, low cost, tivities of polyoxometalates [38], here, our studies
safety in storage and operation, and environmentally in the oxidation of aromatic, aliphatic, cyclic and
friendly character, although a catalyst is necessary to heterocyclic sulfides using H2O2 (30%) as oxidant
activate this oxidant [23, 32]. As part of our ongoing in the presence of catalytic amounts of tetrabutyl-
search for application of polyoxometalates in the oxi- ammonium salts of Sn(II)-substituted polyoxomet-
dation of organic substrates [26, 33], the use of tin(II)- alates, [PSnMo2W9O39]5−, are reported. This cata-
substituted polyoxometalates as catalysts for the ox- lyst is readily available and easily prepared from α-
idation of organic sulfides to sulfoxides (Scheme 1) K7PMo2W9O39 · 19H2O by a slight modification of
was demonstrated in the preceding paper. Metal- the previously reported method [33]. The formation of
substituted polyoxometalates, so-called inorganic met- a Keggin structure and the composition of the com-
alloporphyrins, have attracted much attention as oxida- pounds were confirmed by FTIR, 31P NMR, UV-vis,
tion catalysts because they are thermodynamically sta- and elemental analysis. Thermal gravimetric analysis
c
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E. Rafiee et al. · Oxidation of Sulfides by Tin(II) Polyoxometalate
Table 1. Oxidation of sul-
fides to sulfoxides with
30% H2O2 catalyzed by
[(n-C4H9)4N]5PSnMo2-
W9O39 ·9H2O.
Entry
Sulfide
Sulfoxide
Time
Yield of
(min) sulfoxide (%)a
1
1a
2a
15
95
a
Isolated yields, all products
were identified by comparison
of their spectral data with those
of authentic samples.
2
3
1b
1c
2b 25
85
80
2c
60
4
5
1d
1e
2d 15
95
92
2e
30
6
7
8
1f
2f
35
50
90
80
95
1g
1h
2g
2h 40
9
1i
2i
2j
35
40
94
10
11
1j
95
81
1k
2k 100
2l 110
12
1l
82
13
14
1m
1n
2m 20
2n 10
91
95
15
16
1o
1p
2o
10
88
86
2p 10
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E. Rafiee et al. · Oxidation of Sulfides by Tin(II) Polyoxometalate
603
Table 2. Competitive ox-
idation of sulfides in the
Entry
Substrates
Products
Time Yield (%)a
(min)
presence
of
substrates
with different function-
al groups using [(n-
1
15
30
15
95
0
C4H9)4N]5PSnMo2W9O39
9H2O.
·
—
a
Isolated Yields, reactions were
done at r. t. and molar ratio of
substrates: H2O2: Catalyst was
1 : 1 : 1.2 : 0.04.
2
3
92
—
—
0
94
0
4
10
95
0
—
was performed on the catalyst. The result indicated that with selectivities of 90– 95%, and only trace amounts
the hydration number was 9. of sulfone were observed for some of the sulfides (be-
The oxidation reaction was preliminarily investi- low 5% for Table 1, entries 1, 4, 7, 8, 10, 14). The
gated for methyl phenyl sulfide. Different solvents results presented in Table 1 also show that the oxida-
such as dichloromethane, chloroform, acetonitrile, ace- tion of dialkyl sulfides to the corresponding sulfoxides
is easier than that of alkyl aryl sulfides (Table 1, entries
tone, and benzene were examined. It was observed that
acetonitrile is the solvent of choice for this purpose. 12 – 14). We noted that as expected, the susceptibility
The reaction rates were very slow for the other sol- of sulfides to H2O2 is highly dependent on the sub-
stituents. However, with unsaturated sulfides the sul-
vents. The molar ratio of sulfide to catalyst 1 : 0.04 was
found to be ideal for complete conversion of sulfides foxides are formed without any concomitant epoxi-
to sulfoxides, while with lesser amounts (for exam- dation of double bonds. The mechanism of thioether
ple 1 : 0.03) the reaction remains incomplete. A con- oxidation with H2O2 in the presence of heteropoly-
tungstates was studied by Kholdeeva et al. [29].
trol experiment confirmed that, at the time cited in Ta-
ble 1, little oxidation occurred in the absence of cata-
lyst (below 7%). The optimum molar ratio of sulfide
to H2O2 30% was found to be 1.0: 1.2 and when a
higher amount of oxidant was employed, a significant
increase in the formation of sulfones was observed.
A wide variety of diaryl, dialkyl, dibenzyl, aryl ben-
zyl, alkyl aryl, cyclic, and heterocyclic sulfides were
then reacted using this remarkably simple procedure,
and the results are presented in Table 1. It is evident
from the results, as summarized in Table 1, that cat-
Competitive oxidation of sulfides in the presence of
oxime, ether, aldehyde, alcohol and acetal functions
was monitored (Table 2). These observations clearly
suggest that this method can be applied for the chemos-
elective oxidation of sulfides in the presence of the
above-mentioned groups in multifunctional molecules.
Conclusion
The results presented in this paper demonstrate that
alytic reactions yield sulfoxides as the major products this catalytic system is an efficient, rapid, clean, sim-
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E. Rafiee et al. · Oxidation of Sulfides by Tin(II) Polyoxometalate
ple, mild, and inexpensive protocol for selective oxida- Spectroscopic data of products:
tion of a variety of structurally diverse sulfides to their
Methyl phenyl sulfoxide (2a). M. p. 30 – 31 ◦C. –
corresponding Sulfoxides, with very easy workup.
Moreover, the excellent chemoselectivity of this cat-
alyst towards the sulfur group makes it a good choice
for use in the oxidation of functional sulfides.
IR (KBr): υ = 1025 cm−1. – 1H NMR (200 MHz): δ = 2.73
(s, 3H), 7.48 – 7.56 (m, 3H), 7.64 – 7.67 (m, 2H).
Benzyl phenyl sulfoxide (2b). M. p. 122 – 123 ◦C. –
IR (KBr): υ = 1034 cm−1. – 1H NMR (500 MHz, CDCl3):
δ = 4.04 (d, J = 12.6 Hz, 1H), 4.14 (d, J = 12.6, 1H), 7.02 –
7.03 (m, 2H), 7.28 – 7.36 (m, 3H), 7.41 – 7.52 (m, 5H).
Experimental Section
Diphenyl sulfoxide (2c). M. p. 72 – 73 ◦C. – IR (KBr): υ =
1038 cm−1. – 1H NMR (200 MHz, CDCl3): δ = 7.41 – 7.50
(m, 6H), 7.61 – 7.66 (m, 4H).
The sulfides 1c, 1k, 1l, 1n, 1o, 1p were purchased from
the Merck chemical company. The other sulfides were pre-
pared according to the described procedure [36]. The yields
refer to isolated pure products. Melting points were deter-
mined using a digital Gallenkamp apparatus and are uncor-
rected. IR spectra were run on a Philips PU 9716 spectropho-
tometer and FTIR spectra were performed using a Bomem
MB 104 spectrophotometer. Thermogravimetric analyses
(TGA) were performed under nitrogen flow using a Perkin-
Elmer TGA 7 instrument. The tungsten content in the catalyst
was measured by inductively coupled plasma (ICP atomic
emission spectroscopy) on a Spectro Ciros CCd spectrome-
Dibenzyl sulfoxide (2d). M. p. 132 – 133 ◦C. – IR (KBr):
1
υ = 1026 cm−1. – H NMR (500 MHz, CDCl3): δ = 3.91
(d, J = 13.0 Hz, 2H), 3.98 (d, J = 13.0 Hz, 2H), 7.31 – 7.41
(m, 10H).
4-Bromobenzyl 4-methylphenyl sulfoxide (2e). M. p. 159 –
161 ◦C. – IR (KBr): υ = 1030 cm−1. – 1H NMR (500 MHz,
CDCl3): δ = 2.46 (s, 3H), 3.99 (s, 2H), 6.88 (d, J = 8.2 Hz,
2H), 7.27 – 7.31 (m, 4H), 7.42 (d, J = 8.2, 2H).
(Benzimidazol-2-yl) benzyl sulfoxide (2f). M. p. 154 –
1
ter. H NMR spectra were recorded on Bruker Avance 200
◦
1
156 C. – IR (KBr): υ = 1023 cm−1. H NMR (200 MHz,
CDCl3): δ = 4.72 (s, 2H), 7.02 – 7.31 (m, 5H), 7.42 – 8.03
(m, 4H).
or 500 MHz NMR spectrometers with CDCl3 as the solvent
and TMS as the internal standard. 31P NMR spectra were
recorded on a Bruker Avance 400 DSX 400 NMR spectrom-
eter using 85% H3PO4 as a reference.
(Benzimidazol-2-yl) 4-nitrobenzyl sulfoxide (2g). M. p.
182 – 183 ◦C. – IR (KBr): υ = 1025 cm−1. – 1H NMR
(200 MHz, CDCl3): δ = 4.83 (s, 2H), 7.08 – 7.37 (m, 4H),
7.47 – 8.12 (m, 4H).
Preparation of the catalyst
The synthesis of [(n-C4H9)4N]5PSnMo2W9O39 · 9H2O
starts with the preparation of α-K7PMo2W9O39 · 19H2O
from β-Na8HPW9O34 · 24H2O and sodium molybdate
[33, 37]. To a solution of α-K7PMo2W9O39 · 19H2O
(3.5 g, 1.2 mmol) in 25 ml of water, SnCl2 (0.3 g)
was added in excess. After 2 h of stirring, tetrabut-
ylammonium bromide was added until a violet pow-
der of [(n-C4H9)4N]5PSnMo2W9O39 · 9H2O precipitated.
The powder was filtered and dried in a vacuum desic-
cator. FTIR (KBr): υ = 803, 890, 971, 1072 cm−1. –
31P NMR: δ = −12.12, −13.83, −14.08. – UV/vis:
λ = 266 nm. – C80H198Mo2N5O48PSnW9 (3993.0): calcd.
C 23.94, Mo 4.78, N 1.75, P 0.77, Sn 2.96, W 41.26; found
C 24.56, Mo 4.90, N 1.90, P 0.82, Sn 3.15, W 40.57. Thermal
gravimetric analysis (TGA) indicated 9 molecules of H2O.
n-Butyl phenyl sulfoxide (2h). Oil. – IR (KBr): υ =
1018 cm−1 1H NMR (200 MHz, CDCl3): δ = 0.91 (t,
.
J = 7.0 Hz, 3H), 1.15 – 1.53 (m, 4H), 2.88 (t, J = 7.0 Hz,
2H), 7.36 – 7.49 (m, 3H), 7.89 – 7.92 (m, 2H).
3-Bromobenzyl phenyl sulfoxide (2i). M. p. 178 –
179 ◦C. – IR (KBr): υ = 1028 cm−1. – 1H NMR (200 MHz,
CDCl3): δ = 3.98 (d, J = 12.70 Hz, 1H), 4.04 (d,
J = 12.70 Hz, 1H), 6.85 (m, 4H), 7.44 (m, 5H).
2-Chlorobenzyl 4-chlorophenyl sulfoxide (2j). M. p. 73 –
74 ◦C. – IR (KBr): υ = 1040 cm−1. – 1H NMR (500 MHz,
CDCl3): δ = 4.20 (d, J = 12.56 Hz, 1H), 4.30 (d, J = 12.56,
1H), 7.16 (d, J = 7.05 Hz, 2H), 7.25 (t, J = 7.04, 2H), 7.41
(d, J = 8.5, 2H), 7.47 (d, J = 8.5, 2H).
Dibenzothiophene S-oxide (2k). M. p. 182 – 183 ◦C. –
IR (KBr): υ = 1038 cm−1. – 1H NMR (500 MHz, CDCl3):
δ = 7.53 – 7.70 (m, 4H), 7.83 – 8.04 (m, 4H).
General procedure for the oxidation of sulfides
◦
Thioxanthen-9-one 10-oxide (2l). M. p. 201 – 203 C. –
To a stirred solution of the sulfide (1 mmol) in acetonitrile
IR (KBr): υ = 1058, 1670 cm−1
.
1H NMR (500 MHz,
(3 ml) in the presence of [(n-C4H9)4N]5PSnMo2W9O39
9H2O (0.04 mmol), hydrogen peroxide 30% (1.2 mmol) was
added at r. t. The progress of the reaction was monitored by
·
CDCl3): δ = 7.71 – 7.83 (m, 4H), 8.13 – 8.38 (m, 4H).
Allyl phenyl sulfoxide (2m). Oil. – IR (KBr): υ = 1043,
TLC (eluent: textitn-hexane/ethyl acetate). After the time re- 1660 cm−1. – 1H NMR (500 MHz, CDCl3): δ = 3.56 (d,
ported in Table 1, the sulfoxide was purified by flash chro- J = 6.5 Hz, 2H), 4.89 (d, J = 6.5, 1H), 5.03 (d, J = 10.0 Hz,
matography over silica gel and fully characterized.
1H), 5.55 – 5.64 (m, 1H), 7.38 – 7.59 (m, 5H).
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E. Rafiee et al. · Oxidation of Sulfides by Tin(II) Polyoxometalate
Diallyl sulfoxide (2n). Oil. – IR (KBr): υ = 1035,
605
Dipropyl sulfoxide (2p). Oil.
–
IR (KBr): υ =
1620 cm−1. – 1H NMR (500 MHz, CDCl3): δ = 3.57 1020 cm−1. – 1H NMR (200 MHz, CDCl3): δ = 1.10 (t,
(d, J = 7.1 Hz, 4H), 5.24 – 5.38 (m, 4H), 5.72 – 5.96 J = 7.2, 6H), 1.81 – 1.98 (m, 4H), 2.50 – 2.74 (m, 4H).
(m, 2H).
Acknowledgements
Dibutyl sulfoxide (2o). M. p. 34 – 35 ◦C. – IR (KBr): υ =
1022 cm−1. – 1H NMR (200 MHz, CDCl3): δ = 0.92 (t,
The authors thank the University of Razi Research Coun-
cil and Kermanshah Oil Refining Company for support of
this work.
J = 7.2, 6H), 1.41 – 1.56 (m, 4H), 1.70 – 1.80 (m, 4H), 2.58 –
2.73 (m, 4H).
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