Mn(III)-catalyzed oxidation of sulfides to sulfoxides with hydrogen peroxide

Article abstract of DOI:10.1016/j.tetlet.2006.05.012

Sulfides were selectively oxidized to the corresponding sulfoxides in good yields with hydrogen peroxide using a manganese(III) Schiff-base complex as a catalyst in glacial acetic acid as solvent under mild conditions.

Full text of DOI:10.1016/j.tetlet.2006.05.012

Tetrahedron Letters 47 (2006) 5195–5197
Mn(III)-catalyzed oxidation of sulfides to sulfoxides
with hydrogen peroxide
Farideh Hosseinpoor and Hamid Golchoubian*
Department of Chemistry, University of Mazandaran, PO Box 453, Babolsar, Iran
Received 27 March 2006; revised 22 April 2006; accepted 4 May 2006
Available online 5 June 2006
Abstract—Sulfides were selectively oxidized to the corresponding sulfoxides in good yields with hydrogen peroxide using a manga-
nese(III) Schiff-base complex as a catalyst in glacial acetic acid as solvent under mild conditions.
Ó 2006 Elsevier Ltd. All rights reserved.
The growing attention and applications of sulfoxides
have encouraged research into new methods for their
synthesis. Organic sulfoxides are valuable synthetic
intermediates for the production of a range of chemi-
cally and biologically active molecules including thera-
peutic agents such as anti-ulcer (proton pump
inhibitors),1 antibacterial, antifungal, anti-atheroscle-
rotic,2 antihypertensive3 and cardiotonic agents4 as well
as psychotropics5 and vasodilators.6
In continuation of our work on the selective oxidation
of hydrocarbons,17,18 we report a very efficient and selec-
tive oxidation of sulfides to the corresponding sulfoxides
using H2O2 catalyzed by Mn(III) Schiff-base complex 1,
under mild conditions. The complex was prepared by a
procedure similar to that used by Karmaker et al.19
but 1,2-diaminoethane was used instead of 1,3-
diaminopropane.
+
The oxidation of sulfides to sulfoxides is a straightfor-
ward synthetic method. There are several reagents and
oxidative procedures available for this transformation.
However, many result in over-oxidation to sulfones.
Therefore, controlling the reaction conditions, that is,
time, temperature and the relative amount of oxidants,
plays an important role to avoid forming oxidative side
products, however, these requirements are often hard to
meet. Thus, there is still considerable interest in the
development of selective oxidants for this transforma-
tion.7–12
N
N
O
III
Mn
-
ClO4
O
Whilst the hydrogen peroxide oxidation of sulfides to
sulfoxides catalyzed by several transition metal com-
plexes has been reported,7,13,14 oxidation catalyzed by
Mn(III) has received less attention.15,16 Hydrogen per-
oxide is considered as an ideal ‘green’ oxidant due to
its strength and lack of toxic by-products.
1
To the best of our knowledge, there is no literature re-
port on the selective oxidation of sulfides to sulfoxides
using a Mn(III) catalyst under these conditions. Oxida-
tion of sulfides was performed at room temperature in
the presence of a catalytic amount of the Mn(III) com-
plex using 30% H2O2 as the oxidant (Scheme 1) and gla-
cial acetic acid as the solvent.20
Keywords: Sulfide; Oxidation; Hydrogen peroxide; Catalyst; Selective
oxidation; Mn(III) complex; Sulfoxide.
*
Methyl phenyl sulfide was selected as a model substrate
for optimization studies.
Corresponding author. Tel./fax: +98 1125242002; e-mail:
h.golchobian@umz.ac.ir
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.012

5196
F. Hosseinpoor, H. Golchoubian / Tetrahedron Letters 47 (2006) 5195–5197
O
oxidation was complete after 25 min at elevated temper-
O
R1-S-R2
O
1 mol% 1
R1-S-R2
R1-S-R2
+
ature, the selectivity decreased significantly (Table 1,
entries 6–8). Further, when the oxidation of methyl phenyl
sulfide was carried out using different concentrations of
hydrogen peroxide, the reaction did not reach comple-
tion when using less than 8 mmol of H2O2. However,
when a larger concentration was employed, neither the
conversion nor selectivity of the reaction was improved
(Table 1, entries 9–11). The oxidation occurred in poor
yield when the reaction was carried out under similar
reaction conditions, but in the absence of complex 1
(Table 1, entry 12) or when using other Mn(III) salts,
such as Mn(III) acetate as the catalyst (Table 1, entry 13).
8 equiv 30% H2O2
r.t, acetic acid
R1, R2 = benzyl, phenyl, alkyl,
aryl, allyl
Scheme 1.
The oxidation took place to afford methyl phenyl sulfox-
ide in 86% yield in the presence of 1 and 8 equiv of 30%
H2O2 in 25 min (Table 1, entry 1). The large excess of
hydrogen peroxide required was a result of its decompo-
sition in the presence of the Mn(III) catalyst. The oxy-
gen released during decomposition played little role in
the oxidation of the sulfides. The oxidation occurred,
albeit in poor yield, by simply bubbling molecular oxygen
through the reaction mixture under similar reaction con-
ditions (Table 1, entry 2). The oxidation of methyl phen-
yl sulfide in various solvents was studied. Among the
solvents examined, glacial acetic acid was the most effec-
tive (Table 1, entries 3–5).
The oxidation of other sulfides such as dibenzyl, diphen-
yl, dialkyl, diallyl, phenyl alkyl, benzyl alkyl, benzyl
phenyl, phenyl allyl and cyclic sulfides were then exam-
ined using the optimized reaction conditions (Table 2).
In all cases, the reactions resulted in 100% conversion
of the sulfides. In the case of benzyl sulfides (Table 2, en-
tries 2 and 6), no oxidation was observed at the benzylic
C–H bonds. Similarly, the carbon–carbon double bonds
in allyl sulfides remained intact during the oxidation
(Table 2, entries 5 and 9). This procedure can also be
applied to the oxidation of a cyclic sulfide (Table 2, entry
Alternatively, the reaction was carried out at different
temperatures under the same conditions. Although the
Table 1. Oxidation of methyl phenyl sulfide
Entry
Complex 1 (mmol)
H2O2 (mmol)
Solvent (2 mL)
Time (min)
Temp. (°C)
Conversion (%)
Sulfoxide selectivity (%)a
1
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
None
Mn(OAc)3
8
—
8
8
8
8
8
8
2
CH3COOH
CH3COOH
CH3CN
CH3NO2
CH3OH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
CH3COOH
25
600
40
30
240
18
7
25
25
25
25
25
25
25
25
25
25
25
40
60
80
25
25
25
25
25
100
7
65
100
90
100
100
100
<10
52
100
45
60
86
100
65
62
79
75
60
50
82
75
74
100
100
2b
3
4
5
6
7
8
9
10
11
12
13b
5
10
8
8
a Isolated yields.
b The reaction was carried out under an atmosphere of O2 instead of in the presence of 30% H2O2.
Table 2. Oxidation of sulfides to sulfoxides with hydrogen peroxide catalyzed by Mn(III) complex 1 at room temperaturea
Entry
Substrate
Time (min)
Conversion (%)
Sulfoxide (%)b,c
1
2
3
4
5
6
7
8
9
Ph–S–Me
(Ph–CH2)2S
Ph–S–Ph
Ph–CH2–S–CH3
(CH2@CH–CH2)2S
Ph–CH2–S–Ph
(CH3–CH2–CH2)2S
(CH3–CH2–CH2–CH2)2S
CH2@CH–CH2S–Ph
25
100
35
9
4
19
4
100
100
100
100
100
100
100
100
100
86
95
91
100
100
84
100
100
92
25
22
10
11
9
7
100
100
92
S
CH3–S–CH2–CH2–CHO
100
a 2 mmol of substrate at room temperature with 0.06 mmol of Mn(III) catalyst 1, and 8 mmol of 30% aqueous H2O2, in glacial acetic acid (2 mL).
b Isolated yields.
c The products were identified by comparison of physical and spectroscopic properties with authentic compounds.

F. Hosseinpoor, H. Golchoubian / Tetrahedron Letters 47 (2006) 5195–5197
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Figure 1. Spectral changes associated with the formation of a possible
oxomagnesium(V) species: (a) absorption spectrum of LMnIII in acetic
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Large scale oxidation of methyl phenyl sulfide (10 mmol)
was also investigated and the results demonstrated that
oxidation took place in good yield (typically 5–10% yield
was lost).
In summary, the results indicate that complex 1 is an effi-
cient catalyst for the oxidation of sulfides to the corre-
sponding sulfoxides with hydrogen peroxide. Although
the exact mechanism of this transformation is still un-
clear, the catalytic system probably involves the forma-
tion of an intermediate oxo-Mn(V) complex. Moreover,
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was stirred at room temperature until TLC indicated the
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as eluent to provide analytically pure product.
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Acknowledgements
We are grateful for the financial support of Mazandaran
University of the Islamic Republic of Iran.
References and notes
1. Lai, S. K. C.; Lam, K.; Chu, K. M.; Wong, B. C.; Hui, W.
M.; Hu, W. H.; Lau, G. K.; Wong, W. M.; Yuen, M. F.;