Selective oxidation of bulky sulfides to sulfoxides over titanosilicates having an MWW structure in the presence of H2O2 under organic solvent-free conditions

Article abstract of DOI:10.1016/j.tetlet.2013.07.006

A selective hydrogen peroxide oxidation of sulfides to sulfoxides using Ti-MWW and Ti-IEZ-MWW catalysts was developed. This reaction was carried out under organic solvent-free conditions, and the only side-product was water. Improvement of catalytic activity toward bulky sulfides will be achieved by optimization of the topology in the Ti zeolite.

Full text of DOI:10.1016/j.tetlet.2013.07.006

Tetrahedron Letters 54 (2013) 4918–4921
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Tetrahedron Letters
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Selective oxidation of bulky sulfides to sulfoxides over titanosilicates
having an MWW structure in the presence of H O under organic
2
2
solvent-free conditions
Yoshihiro Kon a, , Toshiyuki Yokoi , Masato Yoshioka , Yumiko Uesaka , Harumi Kujira , Kazuhiko Sato
⇑
,
Takashi Tatsumi
a
b
b
a
a
a,
b,
⇑
National Institute of Advanced Industrial Science and Technology (AIST), Central 5 Higashi 1-1-1, Tsukuba 305-8565, Japan
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A selective hydrogen peroxide oxidation of sulfides to sulfoxides using Ti-MWW and Ti-IEZ-MWW cata-
lysts was developed. This reaction was carried out under organic solvent-free conditions, and the only
side-product was water. Improvement of catalytic activity toward bulky sulfides will be achieved by opti-
mization of the topology in the Ti zeolite.
Received 3 April 2013
Revised 26 June 2013
Accepted 1 July 2013
Available online 6 July 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Oxidation
Green chemistry
Hydrogen peroxide
Ti zeolite
Sulfoxide
Selective oxidation of sulfides to sulfoxides is a key reaction in
synthetic organic chemistry. Sulfoxides are well known as valuable
synthetic intermediates for the construction of various useful com-
2 2
There have been many reports of sulfoxidation using H O ,
6
including application of a micro-reactor, the tuning of various or-
7
ganic solvents to facilitate the reactivity, a self-catalyzed reac-
8
9,10
pounds, including antiulcer, antibacterial, antifungal, and antihy-
tion, and homogeneous catalytic systems.
These reactions,
1
pertensive agents. There are
a
large number of synthetic
however, require large amounts of hazardous organic solvent
and/or high temperature. It is also difficult to separate the catalysts
from the products after the reaction. It would therefore be better to
use solid catalysts for sulfoxidation reactions so that they could
more easily be removed and reused after the reactions.1
methods for highly selective syntheses of sulfoxides. Oxidants such
as NaIO , dioxirane, trichloroisocyanuric acid, BnPh P with HSO
KO /Me SiCl, MnO , sodium perborate, hypervalent iodine com-
4
3
5
,
2
3
2
1–14
pounds, and peroxy acid have been used for the oxidation of sul-
2
fides to give sulfoxides in good yields. However, equimolar
Ti zeolites (TS-1, TS-2, and Ti-beta) are well-known solid cata-
1
4,15
amounts of deoxygenated compounds, including halogens, heavy
metals, and organic compounds, are also produced as waste when
lysts used to catalyze H
2
O
2
sulfoxidation reactions.
TS-1 and
TS-2 have an MFI structure with three-dimensional 10-mem-
bered-ring (3D 10-MR) pores, and Ti-beta has a BEA structure with
3D 12-MR pores. However, these reactions require large amounts
of organic solvents such as haloalkanes, alcohols, and MeCN, and
the use of organic solvents should be decreased to create environ-
mentally friendly chemical processes. In the oxidation of sulfides,
the performance of Ti zeolite catalysts depends strongly on the sul-
3
using these oxidants.
From both an environmental and economic perspective, molec-
ular oxygen and hydrogen peroxide (H
2 2
O ) are ideal oxidants be-
cause they have a high content of active oxygen and their co-
4
2 2
product is water. H O oxidation is particularly useful for organic
syntheses with high selectivity because liquid substrates can be
1
2b,14
effectively mixed and oxidized by contact with the liquid phase
fide structure and the nature of the solvent.
Therefore, a high-
aqueous solution.5
–10
yield synthesis of sulfoxide will be achieved by optimization of the
zeolite structure to overcome the disadvantage of organic solvent-
free conditions. We report herein the high-yield synthesis of vari-
ous sulfoxides using well-designed Ti zeolite catalysts under or-
ganic solvent-free conditions.
2
of the H O
2
⇑
Corresponding authors. Tel./fax: +81 29 861 4511 (K.S.); tel.: +81 45 924 5238;
fax: +81 45 924 5282 (T.T.).
E-mail addresses: y-kon@aist.go.jp (Y. Kon), k.sato@aist.go.jp (K. Sato), ttatsu-
mi@cat.res.titech.ac.jp (T. Tatsumi).
To achieve high levels of activity with the Ti zeolite, we adopted
a new type of Ti zeolite having an MWW structure, ‘Ti-MWW,’
Tel: +81 29 861 4425; fax: +81 29 861 4511.
0
040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.006

Y. Kon et al. / Tetrahedron Letters 54 (2013) 4918–4921
4919
which combines large cavities formed by 12-MR with a system of
independent 10-MR channels. Ti-MWW was obtained by the calci-
nation of Ti-MWW(P) which was hydrothermally synthesized. Ti-
IEZ-MWW was prepared by a silylation of Ti-MWW(P) with dieth-
oxydimethylsilane (DEDMS) under acidic conditions followed by
provide methyl phenyl sulfone in only 6% and 10% yields for Ti-
MWW and Ti-IEZ-MWW catalysts, respectively (Table 1, entries 3
and 4). Ti-MWW having a 10-MR interlayer pore has proved to
be much more active than TS-1 in the epoxidation of linear alkenes
1
8
with H
2
O .
2
In the sulfoxidation of thioanisole, Ti zeolite having
1
6
the calcination.
In addition to TS-1, Ti-MWW, and Ti-IEZ-
MWW structure also showed higher activity than that of TS-1.
These Ti-MWW zeolite catalysts showed the same catalytic activi-
ties as that of Ti-MCM-41 mesoporous silica with a larger pore size
MWW, a typical mesoporous titanosilicate, Ti-MCM-41, was pre-
1
7
pared, and its catalytic performance was evaluated as a control.
The Ti environment in the zeolite was evaluated by the UV–vis
spectroscopy technique. The UV–vis spectra indicated that all tita-
nosilicates mainly have tetrahedrally coordinated Ti species, which
than zeolite (76% yield with 85% selectivity, Table 1, entry 4).19
A
comparison of the Ti-MWW and Ti-IEZ-MWW catalytic activities
per Ti atom in Ti zeolites was also carried out. The TOF per Ti atom
in Ti-MWW was found to be 230, and that for Ti-IEZ-MWW was
280. Ti-IEZ-MWW showed higher catalytic activity for sulfoxida-
tion of thioanisole than that of Ti-MWW.
are the active species for oxidation with H
2 2
O . The atomic ratio of
Si/Ti in titanosilicate was estimated by ICP analysis; the ratios were
5
4
7, 85, 131, and 72 for TS-1, Ti-MWW, Ti-IEZ-MWW, and Ti-MCM-
1, respectively.
Differences in the catalytic activities of TS-1, Ti-MWW, and Ti-
IEZ-MWW were determined based on sulfoxidation of the various
sulfides (Table 2). Investigation of mechanisms of sulfoxidation
reactions catalyzed by Ti-zeolite has been reported as described
We selected thioanisole as a screening substrate. The results are
shown in Table 1. The reaction was conducted with 30% H O
2 2
(
1.0 equiv to thioanisole) and titanosilicate (10 mg) in open air at
1
4,15
2
5 °C for 2 h with vigorous stirring. It is well known that thioani-
below.
2
In the presence of water, the coordination of an H O
sole is easily oxidized to give methyl phenyl sulfoxide without cat-
molecule to the Ti–OOH active site forms a cyclic structure as
shown in Figure 1. The nucleophilic attack of the sulfur atom to
an oxygen atom of the Ti–OOH active site leads to the sulfoxide
formation. The electronic character of the Ti–OOH active site in
Ti-MWW and Ti-IEZ-MWW was investigated by using XSO values.20
The XSO values of Ti-MWW and Ti-IEZ-MWW were 0.09 and 0.11,
respectively. These low XSO values were in a similar range to those
of Ti-beta (0.07 and 0.19) and indicated the electrophilic charac-
ter of the Ti–OOH active site. The reactivity can be linked to the
topology of Ti-zeolites, due to the relative easiness of the accessi-
alyst at higher temperatures.8
,10
A lower temperature (25 °C)
without organic solvent reaction conditions resulted in lower lev-
els of sulfoxidation (18%, Table 1, entry 1). TS-1 catalyzed the sul-
foxidation of thioanisole to give the corresponding sulfoxide in 53%
yield (Table 1, entry 2). The sulfoxidations of thioanisole using Ti-
MWW and/or Ti-IEZ-MWW showed high conversions, yields, and
selectivities (conversions; 100%, and 79%, yields; 89%, and 70%,
and selectivities; 89%, and 89%, respectively, Table 1, entries 3
and 4). Overoxidation of methyl phenyl sulfoxide was found to
2
1
Table 1
a
2 2
The oxidation of thioanisole with TS-1, Ti-MWW, Ti-IEZ-MWW, and Ti-MCM-41 catalysts using 30% aqueous H O
O
S
3
0%H O .aq. (1.0 eq.)
2 2
O
O
S
titanosilicate (10 mg)
S
+
2
5 °C, 2 h
Entry
Zeolite catalyst
Si/Ti ratiob
BET surface areac
Conversiond (%)
Yieldd (%)
Selectivitye (%)
TOF per Ti atom
sulfoxide
sulfone
1
2
3
4
5
None
TS-1
Ti-MWW
Ti-IEZ-MWW
Ti-MCM-41
—
—
19
65
100
79
18
53
89
70
76
0
8
10
6
95
82
89
89
85
—
72
57
86
85
471
499
521
1128
190
230
280
310
89
11
a
b
c
2 2
Reaction conditions: thioanisole (1.0 mmol), 30% H O (1.0 mmol), titanosilicate catalyst (10 mg), 25 °C, 1000 rpm, 2 h.
The atomic ratio of Si/Ti in titanosilicate was estimated by ICP analysis.
The BET surface area in titanosilicate was calculated from the amount of adsorbed N on the surface.
2
Yield and conversion on the basis of thioanisole, determined by GC analysis with biphenyl as an internal standard.
d
e
Yield/conversion  100.
Table 2
a
2 2
The oxidation of various sulfides with TS-1, Ti-MWW, and Ti-IEZ-MWW catalysts using 30% aqueous H O
3
0%H O .aq. (1.0 eq.)
2 2
O
S
O
O
Ti zeolite (10 mg)
S
S
+
R1
R2
R
1
R2
R
1
R2
2
5 °C, 2 h
Entry
1
Substrate
Product
Conversionb (%)
19d
Yieldb (%)
Selectivityc (%)
95d
S
O
S
18d
e
e
6
1
7
5e
53
82
89
89
f
f
h
f
00
89 (85)
g
g
h
g
9
70 (70)
(
continued on next page)

4
920
Y. Kon et al. / Tetrahedron Letters 54 (2013) 4918–4921
Table 2 (continued)
Entry
Substrate
Product
Conversionb (%)
Yieldb (%)
8d
Selectivityc (%)
80d
S
O
S
2
10d
e
e
e
2
9
5
7
5
1
21
88
46
78
93
90
f
f
f
g
g
g
S
S
O
S
3
1d
1d
>99d
Cl
Cl
Br
e
f
e
e
1
9
3
6
8
7
14
93
34
88
95
92
f
f
g
g
g
O
S
4
1d
1d
>99d
Br
e
f
e
e
1
8
2
8
4
3
14
76
18
78
90
78
f
f
g
g
g
S
O
S
5i
11d
7d
64d
e
e
e
4
6
6
4
4
9
25
40
48
57
63
70
f
f
f
g
g
g
O
O
S
O
6j
S
S
9d
7d
78d
O
O
e
f
e
e
4
5
4
5
6
1
41
54
37
91
96
90
f
f
g
g
g
O
S
7k
11d
5d
8e
29
50
45d
e
e
1
3
6
6
6
4
50
f
f
f
81
g
g
g
78
S
O
S
8l
26d
25d
96d
e
e
e
3
9
7
2
6
1
27
80
84
83
f
f
f
g
g
g
62
87
S
O
S
9l
5
5
6d
8e
6d
7e
12
29
>99d
88e
5
5
2f
f
>99
f
1
3
g
g
g
0
97
a
2 2
Reaction conditions: sulfide (1.0 mmol), 30% H O (1.0 mmol), Ti zeolite catalyst (10 mg), 25 °C, 1000 rpm, 2 h, unless otherwise stated.
Yield and conversion on the basis of sulfide, determined by GC analysis with biphenyl as an internal standard.
b
c
d
e
f
Yield/conversion  100.
No catalyst.
Using TS-1 catalyst.
Using Ti-MWW catalyst.
Using Ti-IEZ-MWW catalyst.
Isolated yield.
g
h
i
4
2
4
2
0 °C, 1000 rpm, 6 h.
5 °C, 1000 rpm, 6 h.
0 °C, 1000 rpm, 18 h.
5 °C, 1000 rpm, 4 h.
j
k
l
H
bility of nucleophilic S atom on the sulfide to the O atom on the Ti–
OOH active site.
In the case of p-substituted thioanisoles, TS-1 gave methyl p-to-
lyl sulfoxide in 21% yield, and p-chlorophenyl- and p-bromophenyl
methyl sulfoxide in 14% yields (Table 2, entries 2–4). Ti-MWW
showed higher activity than TS-1 to give the corresponding phenyl
O
SiO
H
SiO Ti
SiO
O
H
O
Figure 1. Proposed Ti–OOH active site in the sulfoxidation catalyzed by Ti-zeolite.

Y. Kon et al. / Tetrahedron Letters 54 (2013) 4918–4921
4921
methyl sulfoxides in 76–93% yields with good (90–95%) selectivi-
ties (Table 2, entries 2–4). Ti-IEZ-MWW gave the products in
moderate (18–46%) yields (Table 2, entries 2–4). Although Ti-
MWW maintained high activities in the case of p-chloro- and p-
bromo thioanisoles, those of TS-1 went down (Table 2, entries 3
and 4).
The high-yield synthesis of bulkier sulfoxide will be achieved by
optimization of the topology of Ti zeolite to overcome the disad-
vantage of organic solvent-free conditions.
Acknowledgements
These results showed that Ti-MWW with a 10-MR interlayer
pore is much more active than TS-1 in the sulfoxidation of thioani-
soles under organic solvent-free conditions. The thioanisoles are
small enough to easily come into contact with the Ti–OOH active
site located in the titanosilicate.14 The reactions of phenyl vinyl
sulfide and ethyl (phenylthio) acetate gave the corresponding sul-
fides in moderate yields using TS-1, Ti-MWW, and Ti-IEZ-MWW
catalysts (25% and 41% yields for TS-1, 40% and 54% yields for Ti-
MWW, and 48% and 37% yields for Ti-IEZ-MWW, respectively, Ta-
ble 2, entries 5 and 6). The epoxidation of phenyl vinyl sulfide did
not proceed. In this case, no tremendous differences in the sulfox-
idation activities were observed due to the reduced accessibility
resulting from employing slightly bulkier aryl sulfides.
This work was partially supported by the New Energy and
Industrial Technology Development Organization (NEDO), Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.07.006.
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yield) > TS-1 (7% yield). Elongation of the interlayer would also
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1
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In summary, the oxidation of sulfides to sulfoxides using 30%
20. The XSO values reflect the extent of oxidation at the SO site in thianthrene 5-
oxide. SO = (nucleophilic oxidation)/(total oxidation) = (SSO + SOSO )/
+ SOSO + 2SOSO ): (a) Adam, W.; Golsch, D.; Görth, F. C. Chem. Eur. J.
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X
2
2
H
2
O
2
aqueous solution with titanosilicate catalysts was carried
out. This reaction can be considered to be a green sustainable pro-
cess using titanosilicate catalyst in the presence of H under or-
(
1
SSO
2
2
O
2 2
Mizuno, N. Chem. Eur. J. 2007, 13, 639–648.
2
2
1. Adam, W.; Mitchell, C. M.; Saha-Möller, C. R.; Selvam, T.; Weichold, O. J. Mol.
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ganic solvent-free conditions. Thioanisoles were effectively
oxidized by Ti-MWW to generate the corresponding sulfoxides in
good yields with good selectivities. Ti-IEZ-MWW was applicable
to the sulfoxidation of bulkier sulfides such as diphenyl sulfide.
2. Ti-IEZ-MWW catalysts also showed a much higher conversion and specific
activity than Ti-MWW in cyclohexene oxidation with H O : Wang, L.; Wang,
2
2
Y.; Liu, Y.; Wu, H.; Li, X.; He, M.; Wu, P. J. Mater. Chem. 2009, 19, 8594–8602.