Copper catalyzed oxidation of sulfides to sulfoxides with aqueous hydrogen peroxide

Article abstract of DOI:10.1016/j.tetlet.2005.03.194

Copper(II) complex 1 catalyzes the oxidation of sulfides to sulfoxides with 30% H₂O₂ in high yields. Addition of a catalytic amount of TEMPO to the reaction mixture enhances the conversion and selectivity. Complex 1 can be recycled without loss of activity.

Full text of DOI:10.1016/j.tetlet.2005.03.194

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3819–3822
Copper catalyzed oxidation of sulfides to sulfoxides with
aqueous hydrogen peroxide
Subbarayan Velusamy, Akkilagunta V. Kumar, Rakesh Saini and T. Punniyamurthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
Received 22 February 2005; revised 19 March 2005; accepted 29 March 2005
Available online 12 April 2005
Abstract—Copper(II) complex 1 catalyzes the oxidation of sulfides to sulfoxides with 30% H2O2 in high yields. Addition of a
catalytic amount of TEMPO to the reaction mixture enhances the conversion and selectivity. Complex 1 can be recycled without
loss of activity.
Ó 2005 Elsevier Ltd. All rights reserved.
Synthetic building blocks containing sulfoxide func-
N
H
O
N
H
O
tional groups are particularly useful for the construction
of various important compounds.1 These derivatives are
usually prepared by oxidation of sulfides and several
oxidative procedures are applicable for this transform-
ation.2–4 Generally, it is important to stop the oxidation
at the sulfoxide stage by controlling the electrophilic
character of the oxidant, but this requirement is often
hard to meet and failure results in over oxidation to sulf-
ones. Thus, there is still considerable interest in the
development of selective oxidants for this important
transformation.5 We have recently reported that cop-
per(II) complex 1 catalyzes the oxidation of alkanes
and alcohols to the corresponding carbonyl com-
pounds in the presence of 30% H2O2.6 Complex 1 is
water soluble and catalyzes oxidations without addi-
tives generating water as the only by-product, and
does not require halogenated solvents.7 In continua-
tion of our studies on the oxidation of organic
compounds,8 we report herein the oxidation of sul-
fides to sulfoxides using complex 1 in the presence of
30% H2O2 in high yields (Scheme 1). The addition of
TEMPO to the reaction mixture increases the selectiv-
ity and conversion. To the best of our knowledge, this
is the first report on the oxidation of sulfides with a
copper catalyst which is recyclable without loss of
activity.
Cu
1
1 mol% 1
5 mol% TEMPO
RSR'
RR'SO
2 equiv 30% H2O2
4.5-24 h, 20 oC
40->99%
CH3CN
R, R' = alkyl, aryl, allyl
Scheme 1.
The oxidation of methyl phenyl sulfide, as a standard
substrate, was first studied in the presence of copper(II)
complex 1 and 30% aqueous H2O2 at ambient temper-
ature (Table 1, entries 1, 3, and 4). We were pleased to
find that the oxidation occurred to afford a 8:1 mixture
of sulfoxide and sulfone in 70% yield in the presence of
1 mol % of the complex 1 and 2 equiv of 30% H2O2 in
acetonitrile. Addition of 3 mol % TEMPO to the reac-
tion mixture led the oxidation to a 17:1 mixture of sulf-
oxide and sulfone in 90% yield. The selectivity and
conversion of the sulfoxide were further increased to
>99% when the amount of TEMPO was increased to
5 mol %. A control experiment without complex 1 pro-
vided the sulfoxides in only 5% yields and no sulfone
was observed (entry 2). These studies suggest that
complex 1 together with TEMPO catalyzed the oxid-
ation of sulfide to the sulfoxide in the presence of 30%
H2O2.
Keywords: Oxidation; Sulfide; Hydrogen peroxide; CuSalenH4;
TEMPO.
*
Corresponding author. Tel.: +91 361 2691083; fax: +91 361 2690762;
e-mail: tpunni@iitg.ernet.in
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.194

3820
S. Velusamy et al. / Tetrahedron Letters 46 (2005) 3819–3822
Table 1. Oxidation of methyl phenyl sulfidea
O
S
O
S
Me
S
O
1/TEMPO
Me
Me
+
2 equiv 30% H2O2
20 oC, 4.5 h
CH3CN
Entry
Complex 1
TEMPO
Conversion (%)b
Sulfoxide (%)b
Sulfone (%)b
1
1 mol %
None
None
70
5
89
>99
94
11
—
6
2
5 mol %
3 mol %
5 mol %
5 mol %
3
1 mol %
1 mol %
Recovered
90
4
5c
>99
>99
>99
>99
—
—
a Substrate (5 mmol), complex 1 (1 mol %), TEMPO (5 mol %) and 30% H2O2 (10 mmol) were stirred for 4.5 h at 20 °C in CH3CN (2 mL).
b Determined from 400 MHz 1H NMR of the crude product.
c Recovered copper(II) complex was used.
a
Table 2. Copper(II) complex 1 and TEMPO catalyzed selective oxidation of sulfides to sulfoxides with 30% H2O2
Entry
Substrate
Time (h)
Sulfoxide
Yield (%)b,c
83
O
S
S
C6H13
1
15
C6H13
O
S
S
S
2
20
52
O
S
3
4
11
16
92
89
O
S
S
C6H13
C6H13
Me
Me
O
S
S
S
5
6
7
21
14
15
57
96
87
Me
Me
Me
O
S
Me
O
S
S
S
C6H13
C6H13
MeO
MeO
MeO
O
S
8
11
23
22
94
51
71
MeO
O2N
O2N
O
S
S
C6H13
C6H13
9
O2N
O2N
O
S
S
10
O
S
S
C6H13
11
23
51
C6H13
Br
Br

S. Velusamy et al. / Tetrahedron Letters 46 (2005) 3819–3822
3821
Table 2 (continued)
Entry
Substrate
Time (h)
22
Sulfoxide
Yield (%)b,c
Br
12
13
48
S
S
O
Br
S
O
S
23
69
Br
Br
O
S
S
C11H23
C6H13
14
15
21
24
54
40
C11H23
O
S
S
C6H13
O
S
16
17
23
24
62
S
C11H23
H23C11
H11C5
S
O
48d
C11H23
H11C5
S
C11H23
a Substrate (5 mmol), complex 1 (1 mol %), TEMPO (5 mol %) and 30% H2O2 (10 mmol) were stirred in acetonitrile (2 mL) at 20 °C. The identity of
the products was ascertained by 1H and 13C NMR, mass, IR and elemental analysis.
b Isolated yield.
c No sulfone was observed.
d Accompanied by sulfone <2%.
To study the scope of this procedure, the oxidation of
other sulfides was studied (Table 2).9 A series of sub-
strates, aryl alkyl, aryl allyl and dialkyl sulfides, could
be oxidized to the corresponding sulfoxides. The reactiv-
ity and conversion were dependent on the nature of the
substituents. In the case of allyl sulfides, no oxidation
was observed at the carbon–carbon double bond. Simi-
larly, benzylic sulfides could be oxidized to the corre-
sponding sulfoxides without affecting the benzylic C–H
bond. Dialkyl sulfides were moderately reactive provid-
ing the corresponding sulfoxide. These oxidations could
usually be stopped at the sulfoxide stage without over
oxidation to the sulfone.
the presence of 30% aqueous H2O2 at ambient temper-
ature. The addition of TEMPO enhances the sulfoxide
selectivity and yield. From an environmental and eco-
nomic standpoint, this procedure provides a simple
method for the formation of sulfoxides from sulfides.
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