Self-catalyzed oxidation of sulfides with hydrogen peroxide: A green and practical process for the synthesis of sulfoxides

Article abstract of DOI:10.1002/adsc.200700206

A self-catalyzed selective oxidation of sulfides to sulfoxides has been developed. The scope of the protocol is demonstrated in the selective oxidation of 17 different substrates. High yields and chemoselectivity (in general > 90 % ) are achieved in most cases.

Full text of DOI:10.1002/adsc.200700206

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DOI: 10.1002/adsc.200700206
Self-Catalyzed Oxidation of Sulfides with Hydrogen Peroxide:
A Green and Practical Process for the Synthesis of Sulfoxides
a
a,b
a,b
a,b,
Feng Shi, Man Kin Tse, Hanns Martin Kaiser, and Matthias Beller *
a
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
University of Rostock, Center for Life Science Automation (CELISCA), Friedrich-Barnewitz-Str. 8, 18119
b
Rostock-Warnemünde, Germany
Phone: (+49)-381-1281-0; fax: (+49)-381-1281-5000; e-mail: matthias.beller@catalysis.de
Received: April 24, 2007; Revised: July 13, 2007
[
10]
[11]
[12]
[13]
Abstract: A self-catalyzed selective oxidation of
sulfides to sulfoxides has been developed. The
scope of the protocol is demonstrated in the selec-
tive oxidation of 17 different substrates. High yields
and chemoselectivity (in general >90%) are achiev-
ed in most cases.
sten,
manganese,
copper,
titanium,
plati-
[
14]
[15]
num, and magnesium based systems were used to
obtain good yields.
During our continuous investigation for selective
oxidation reactions using aqueous hydrogen peroxide
as the terminal oxidant,
[
16]
we found serendipitously
that the sulfoxides themselves, produced during the
reaction, can be very effective catalysts for the selec-
tive oxidation of sulfide to sulfoxide (Scheme 1). This
Keywords: green chemistry; selective oxidation;
self-catalyzed; solvent free; sulfoxide
For the sustainable development of our economy, the
further use of green chemistry is an important issue.
In this respect preventing waste, but not treating or
cleaning it up, is a basic requirement. In a given
chemical process, reducing the use of toxic chemicals is one of the relatively few cases of self-catalyzed re-
Scheme 1. Selective oxidation of sulfides to sulfoxides.
[
1]
[
17]
and waste remitting with high atom-efficiency are key actions. This process needs no additional catalysts,
points. One of the major challenges of organic synthe- solvents and proceeds highly selectively.
sis is undoubtedly to develop more environmentally
friendly oxidation processes. As an example, herein anisole in the presence of different transition metal
we report an efficient oxidation of sulfides to sulfox- catalysts under solvent-free conditions, all reactions
In our initial exploratory experiments applying thio-
[
2]
ides.
gave good yields of methylsulfinylbenzene after 12 h
The selective oxidation to give sulfoxides has at- at room temperature. To our surprise, the control ex-
tracted much attention over the years since sulfoxides periment without catalyst also showed high yield of
constitute useful building blocks in organic synthesis the corresponding sulfoxide (>98%, GC yield). At
for the preparation of biologically active compounds 708C, the oxidation of thioanisole to methylsulfinyl-
[
3]
and activation of enzymes. Traditionally, the synthe- benzene with 1 equiv. 30 wt% H O gave even full
2
2
sis is performed with a stoichiometric amount of or- conversion within 1 h! When the reaction was carried
ganic or inorganic oxidant and a large amount of out at room temperature, the yield to methylsulfinyl-
[
4]
toxic waste is produced. In order to run this trans- benzene was only 63% (GC yield) after 6 h. As
formation in a “greener” manner, there are three fac- shown in Figure 1 the reaction started at a very low
tors that can be improved: use of (i) environmentally rate. However, when a small amount of methylsulfi-
benign oxidants, (ii) less or even no organic solvent nylbenzene is produced, the reaction is accelerated
and (iii) non-toxic catalysts. Hence, aqueous hydrogen and became faster and faster. After the formation of
peroxide has been used as the terminal oxidant since 4.7% sulfoxide in the mixture (5 min), ~90% of thio-
[
5]
the beginning of the last century, because it is anisole could be further converted into methylsulfi-
cheap, environmentally benign, readily available, of nylbenzene in the next 15 min. It is clear that the con-
high atom-efficiency (47%), and theoretically only centration of methylsulfinylbenzene, but not the con-
[
6]
generates water as the by-product. Different kinds centration of thioanisole or hydrogen peroxide deter-
[
7]
[8]
[9]
of catalysts including acids, iron, vanadium, tung- mined the initial reaction rate. In order to clarify this
Adv. Synth. Catal. 2007, 349, 2425 – 2430
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Feng Shi et al.
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Figure 1. Typical reaction conditions: 5.0 mmol (620 mg) thioanisole and 5.0 mmol (0.50 mL) 30 wt% H O and the appro-
2
2
priate catalyst [no sulfoxide or sulfone was added for (a)–(c), 5 mol% methylsulfinylbenzene, 20 mol% methylsulfinylben-
zene, 20 mol% methylsulfonylbenzene, or 20 mol% dimethyl sulfoxide were initially present in (d), (e), (f) and (g) respec-
tively] were heated at 68–708C. (a) thioanisole (%) vs. reaction time; (b) methylsulfinylbenzene (%) vs. reaction time; (c)
[18]
methylsulfonylbenzene (%) vs. reaction time; (d), (e), (f) and (g) methylsulfinylbenzene (%) vs. reaction time.
effect, methylsulfinylbenzene was further added to than that with sulfoxide. The addition of dimethyl
the reaction mixture. 16% or 76% of thioanisole are sulfoxide exhibited comparable activity to methylsul-
converted in 5 min with 5 or 20 mol% of added meth- fonylbenzene but it was much lower than with meth-
ylsulfinylbenzene. The reaction rates became similar ylsulfinylbenzene, Figure 1 (h). Therefore, the struc-
after 20 min, because much more sulfoxide than the ture of sulfoxide but not only the functional group
pre-added amount was produced. This is a direct evi- itself has a great effect on its activity.
dence for the acceleration of the sulfide oxidation by
In order to explore the generality for this protocol,
the self-generating sulfoxide. Interestingly, both phos- we further tested this selective oxidation with various
phine oxides and nitroxyl radicals decrease the rate of sulfides (Table 1). Typical sulfides, such as thioanisole,
the sulfide oxidation. Hence, the addition of 20 mol% methyl p-tolyl sulfide and ethyl phenyl sulfide gave
of trioctylphosphine oxide and 20 mol% TEMPO the corresponding sulfoxides in excellent yields, 92–
gave a significantly lower yields (50–60% after 97% (Table 1, entries 1–3). It is noteworthy that only
30 min) compared with the blank experiment (90% 1% sulfones formed in the oxidation of thioanisole
after 30 min). Also the corresponding sulfone is not and methyl p-tolyl sulfide. This indicates that the
the catalyst in the reaction system because up to present protocol is very selective and easily controlla-
6
0 min, only ~1% of methylsulfonylbenzene is ob- ble. Functional groups are also tolerated in this proce-
served by GC-FID and GC-MS. In fact, the reaction dure. Methyl phenyl sulfide derivatives with ÀOMe,
in the presence of 20 mol% sulfone is much slower ÀCl, ÀCN, ÀCH Br and ÀCH CH Cl substituents re-
2
2
2
[a]
Table 1. Selective oxidation of sulfides to sulfoxides with hydrogen peroxide.
[b]
[c]
Entry
Substrate
Product
Sulfoxide:Sulfone
99:1
Yield [%]
1
97
94
92
2
3
99:1
94:6
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Self-Catalyzed Oxidation of Sulfides with Hydrogen Peroxide
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Table 1. (Continued)
[b]
[c]
Entry
Substrate
Product
Sulfoxide:Sulfone
99:1
Yield [%]
4
98
91
5
6
97:3
98:2
88
7
8
91:9
94:6
52
90
9
91:1
90
1
1
0
1
99:1
99:1
90
87
1
1
2
3
88:12
99:1
69
93
1
1
4
5
96:4
96:4
87
81
[
[
d]
1
1
6
7
83:17
86:14
80
84
e]
[
[
[
[
[
a]
2
.0 mmol sulfide, 2.0 mmol H O (30 wt% in water, 1 equiv., VWR), 708C (oil bath temperature), 1 h.
2 2
b]
Determined by GC-MS.
Isolated yield for sulfoxide.
c]
d]
e]
2
1
4.0 mmol H O , 12 h.
6.0 mmol H O , 2 h.
2 2
2
2
Adv. Synth. Catal. 2007, 349, 2425 – 2430
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Feng Shi et al.
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sulted in excellent yields (Table 1, entries 4–8). With Experimental Section
2-(methoxyphenyl)methylsulfane, the isolated yield
reached 98% (Table 1, entry 4). However, a lower General Procedure for the Self-Catalyzed Selective
yield (52%) is obtained with 4-(methylthio)benzyl Oxidation of Sulfide to Sulfoxide
bromide. Here, GC-MS analysis showed that hydroly-
sis of the bromomethyl group to the corresponding 4-
methylthiobenzyl alcohol and oxidation of the benzyl
bromide group to 4-methylthiobenzylaldehyde are the
major side reactions.
All reactions were carried out in an oil bath (69–708C, oil
bath temperature). To a glass reactor (40 mL), 2.0 mmol
(
0.292 g) di-n-butyl sulfide and 2.0 mmol H O (30 wt% in
2 2
water, from VWR, 0.20 mL) were added, respectively. The
reaction was vigorously stirred (500–750 rpm) at 708C for
The selective oxidation of compounds with two 1 h. The mixture was then cooled to room temperature and
functional groups within one molecule is always an in- extracted by CH
Cl (20 mL”3). After drying with anhy-
drous Na SO4, the organic mixture was removed in vacuum
2
2
teresting topic in oxidation chemistry. Hence, sulfides
2
and a colorless liquid (0.301 g) was obtained. The sample
was analyzed by GC-MS, which indicated that the ratio be-
tween sulfoxide and sulfone was 96:4. 1H NMR analysis
showed that the ratio between sulfoxide and sulfone was
containing oxidizable ÀNH , ÀCH OH, ÀCHO and
2
2
ÀC(O)À groups were tested (Table 1, entries 9–13).
To our delight, for compounds containing amine and
alcohol groups, the oxidation occurred selectively on
the sulfur atom. Even with 4-methylthiobenzaldehyde,
a good isolated yield of 69%, was obtained.
9
5:5.
Analytically pure dibutyl sulfoxide was obtained by
column chromatography (silica gel 60, 70–230 mesh) using
Moreover, for the selective oxidation of aliphatic CH Cl to ethyl acetate as the gradient eluent. After remov-
2
2
sulfides, good yields are also achieved (Table 1, en- al of the solvent and drying under high vacuum for 2 h, a
tries 14 and 15). Methyl (methylthio)acetate was white solid was obtained; yield: 282 mg (87%).
easily oxidized into methyl methylacetate sulfoxide, a
[
19]
synthetically useful reagent, in 81% yield. Notably, Example of a Scaling-Up for the Selective Oxidation
this protocol can be easily scaled up. As a demonstra- of Dibutyl Sulfide
tionof a 200-mmol scale reaction, 29.2 g of di-n-butyl
sulfide were oxidized to di-n-butyl sulfoxide. Merely
To a round-bottom flask (250 mL), 200 mmol (29.2 g) of di-
n-butyl sulfide and 200 mmol H O (30 wt% in water, from
2
2
by extracting the reaction mixture was a quantitative VWR, 20 mL) were added respectively. The reaction mix-
yield of product obtained with>93% purity.
ture was vigorously stirred (500–750 rpm) at 708C for 1 h.
The formation of sulfoxides from diphenyl sulfide During the reaction, the system should be open to air and
[
21]
well cooled with a condenser.
cooled to room temperature and extracted by ethyl acetate
200 mL”4). After drying with anhydrous Na SO , the or-
The mixture was then
and benzyl phenyl sulfide are more difficult to ach-
ieve under the known standard oxidation procedures.
Even in the presence of highly active catalysts, much
more hydrogen peroxide and longer reaction times
were necessary in order to accomplish satisfactory re-
(
2
4
ganic mixture was removed in vacuum and a colorless liquid
34.6 g) was obtained. The sample was analyzed by GC-MS,
which indicated that the ratio between sulfoxide and sulfone
(
[10b]
sults.
In the presence of an excess of hydrogen per-
1
was 93:7. H NMR analysis showed that the ratio between
oxide, the yield for diphenyl sulfoxide reached 80%
(
sulfoxide and sulfone was 93:7. 1-(Butylsulfinyl)butane: R =
f
1
Table 1, entry 16) and the yield of benzyl phenyl sulf- 0.84 (ethyl acetate); colorless semi-solid; H NMR
oxide reached 84% (Table 1, entry 17).
(300.1 MHz, CDCl ): d=0.93 (6H, t, J=7.3 Hz), 1.35–1.54
3
13
In conclusion, a self-catalyzed selective oxidation of (4H, m), 1.66–1.76 (4H, m), 2.54–2.70 (4H, m); C NMR
(
7
75.5 MHz, CDCl ): d=13.7, 22.1, 24.6, 52.1; GC-MS (E.I.,
3
+
sulfides to sulfoxides has been developed for the first
time. The scope of the protocol is demonstrated in
the selective oxidation of 17 different substrates. High
yield and chemoselectivity (in general >90%) are
achieved in most cases. Compared with the previous
protocols, this system has the advantage that no or-
ganic solvents or any additional catalyst is necessary.
This makes the reaction environmentally benign,
more practical and easily manageable. The study of
0 eV): m/z (rel. int.)=162 (M , 1), 106 (12), 89 (32), 63
(
16), 57 (35), 56 (12), 55 (22), 41 (100), 39 (36).
Acknowledgements
This work has been supported by the State of Mecklenburg-
Western Pommerania, the Deutsche Forschungsgemeinschaft
the reaction mechanism and the development of a ste- (SPP 1118 and Leibniz prize) and the Federal Ministry of
reoselective version of this reaction are now undergo- Education and Research (BMBF). Dr. Feng Shi thanks the
[
20]
ing in our laboratory.
Alexander-von-Humboldt-Stiftung for an AvH-Fellowship.
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4
1
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2
2
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8
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9
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[
21] Even on this scale, we did not observe any explosion
during the reaction. For safety reason, the system
should be open to air and well cooled with a condens-
er.
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Adv. Synth. Catal. 2007, 349, 2425 – 2430