An efficient and selective oxidation of sulfides and thiols with silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex

Article abstract of DOI:10.1080/10426500902814031

Silica-supported 1,1,3,3-tetramethylguanidine/Br₂ complex is an efficient reagent for the selective oxidation of aliphatic and aromatic sulfides to the corresponding sulfoxides and the oxidative coupling of thiols to disulfides in aqueous solution at room temperature in a short reaction time.

Full text of DOI:10.1080/10426500902814031

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Phosphorus, Sulfur, and Silicon and the
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An Efficient and Selective Oxidation of
Sulfides and Thiols with Silica-Supported
1,1,3,3-Tetramethylguanidine/Br2
Complex
a
a
a
Ahmad Shaabani , Elham Farhangi & Abbas Rahmati
a
Department of Chemistry , Shahid Beheshti University , Tehran,
Iran
Published online: 03 Feb 2010.
To cite this article: Ahmad Shaabani , Elham Farhangi & Abbas Rahmati (2010) An Efficient and
Selective Oxidation of Sulfides and Thiols with Silica-Supported 1,1,3,3-Tetramethylguanidine/Br2
Complex, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:2, 463-468
To link to this article: http://dx.doi.org/10.1080/10426500902814031
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Phosphorus, Sulfur, and Silicon, 185:463–468, 2010
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902814031
AN EFFICIENT AND SELECTIVE OXIDATION OF SULFIDES
AND THIOLS WITH SILICA-SUPPORTED
,1,3,3-TETRAMETHYLGUANIDINE/Br COMPLEX
1
2
Ahmad Shaabani, Elham Farhangi, and Abbas Rahmati
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex is an efficient reagent for the
selective oxidation of aliphatic and aromatic sulfides to the corresponding sulfoxides and
the oxidative coupling of thiols to disulfides in aqueous solution at room temperature in a
short reaction time.
Keywords Oxidation; silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex; sulfide;
thiol
INTRODUCTION
Selective oxidative coupling of thiols to disulfides and oxidation of sulfides to sul-
foxides are of interest from both biological and synthetic chemistry points of view.1
Although a large number of oxidizing agents can affect the conversion of sulfides to sulfox-
–10
1
1–24
ides and the oxidative coupling of thiols to disulfides,
the susceptibility of sulfoxides
and disulfides to further oxidation narrows the choice of reagents for these processes.
Therefore, the development of new oxidants for the selective transformation of sulfides to
sulfoxides and thiols to disulfides is of importance in synthetic organic chemistry.
The use of molecular bromine for the oxidation of sulfides and thiols is restricted
because the formation of HBr (as a reduction product) can affect the selectivity of the
reaction. Under acidic conditions, the main product becomes contaminated by the formation
of side products such as sulfonic acids, sulfinic acids, and bromo-substituted sulfides and
25
sulfoxides. Therefore, if bromine is to be used as an oxidant, it is necessary that these
problems be circumvented by carrying out the reactions under conditions where HBr is not
released as a free acid.
A number of amine-based complexes of bromine have been reported in the literature,26
in which amine acts as a HBr acceptor. However, as indicated in Scheme 1, the amine part
of the complex is only able to neutralize 1 mol of HBr being produced in the oxidation
process. Therefore the reaction media is still acidic from the remaining 1 mol of HBr.
Received 16 December 2008; accepted 11 February 2009.
Financial assistance from the Research Council of Shahid Beheshti University of Iran is gratefully acknowl-
edged.
Address correspondence to Ahmad Shaabani, Department of Chemistry, Shahid Beheshti University, P. O.
Box 19396-4716, Tehran, Iran. E-mail: a-shaabani@cc.sbu.ac.ir
4
63

464
A. SHAABANI ET AL.
H O
2
R S=O+2HBr+amine
2
R S
2
Amine/Br2
R-S-S-R+2HBr+amine
2
RSH
Scheme 1
RESULTS AND DISCUSSION
This article reports the oxidation of sulfides and thiols to the corresponding sulfoxides
and disulfides utilizing 1,1,3,3-tetramethylguanidine/Br2 on hydrated silica gel (Scheme 2).
O
SSTMG/Br2
S
S
R1
R2
R1
R2
CH CN/H O
3
2
10-40 min
90-96%
1
2
R =R = Alkyl or Aryl
1
2
R = Aryl, R = Alkyl
SSTMG/Br2
1
1
1
R SH
R SSR
0-95%
CH CN/H O
3
2
9
3-6 min
Scheme 2
This silica-supported 1,1,3,3-tetramethylguanidine/Br2 (SSTMG/Br2) oxidant is not
only safe to handle owing to full chemisorption of the toxic bromine complex, but both the
1,1,3,3-tetramethylguanidine and silica gel act as a HBr scavenger and prevent the solution
from becoming acidic (Scheme 3).
The silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex was readily prepared
by adding bromine to a magnetically stirred slurried mixture of silica in an n-hexane solution
of 1,1,3,3-tetramethylguanidine. The mixture was filtered, washed with n-hexane, and dried
at room temperature. This yellow-orange, non-hygroscopic, homogenous solid is very stable
at room temperature and is not affected by ordinary exposure to light, air, or water and
has none of the offensive odor of bromine or amine. Easy workup and the stability of the
reagent make it a safe and convenient source of active bromine.
This method offers a simple, general, selective, and highly efficient route for con-
verting sulfides and thiols to their corresponding sulfoxides and disulfides without over-
oxidation. As shown in Table I, the reaction time for oxidation is relatively short, and the
generality of the method was examined using alkyl, aryl, dialkyl, diaryl, and cyclic sulfides
as well as alkyl and aryl thiols. It was discovered that a wide variety of sulfides and thiols
can be selectively oxidized by this inexpensive reagent under mild reaction conditions.

OXIDATION OF SULFIDES AND THIOLS
465
X
O
H
Si OH
O
S
H
N
N
R
R
N
Br
Br
H
H
O
XNH Br
2
Si
O
H
Br
R S
2
H
Br
S
R
H
X
N
R
+
OH2 Br
Br
X=N
R
S
R
Br
H
X N
Br
R
S
R
H O
2
Br
Scheme 3
The possible mechanism for the oxidation of sulfides to the corresponding sulfoxides
using silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex is outlined in Scheme 3.
In conclusion, we report an easy and versatile method for the selective oxidation of sul-
fides and thiols to their corresponding sulfoxides and disulfides without any over-oxidation.
The amine and silica parts of the complex act as HBr acceptors, preventing the solution
from becoming acidic. The formation of 1,1,3,3-tetramethylguanidine/hydrobromide as the
reaction proceeds seems to accelerate the rate of oxidation possibly by increasing the ionic
strength of the solution.
EXPERIMENTAL
Melting points were measured on an Electrothermal 9200 apparatus. IR spectra were
1
recorded on a Shimadzu IR-470 spectrometer. H NMR spectra were obtained on solutions
1
in CDCl3 and DMSO-d6. All reaction products were known and characterized by IR and H
NMR spectra and melting points as compared with those obtained from authentic samples.
All chemical reagents were purchased from Fluka and Merck and were used without
purification.
Preparation of Silica-Supported 1,1,3,3-Tetramethylguanidine/Br2
A solution of bromine (3.84 g, 24 mmol) in n-hexane (20 mL) was added to a
magnetically stirred slurry mixture of silica gel (16.00 g) in n-hexane (100 mL) solution of
1
1
,1,3,3-tetramethylguanidine (2.76 g, 24 mmol). The mixture was stirred for an additional
h, and then the product was collected by vacuum filtration as yellow solid; yield 20.88 g.

466
A. SHAABANI ET AL.
Table I Oxidation of sulfides and thiols with the silica-supported 1,1,3,3-tetramethylguanidine/Br2 complex
◦
Time
(min)
Yield
(%)
M p or Bp ( C)
Entry
Reductant
Product
Found (Reported) [ref.]
a
1
2
(CH3(CH2)3)2S
(CH3(CH2)3)2SO
15
10
94
92
27–29 (29–31.6) [27]
b
232–234 (235–237) [27]
a
3
4
5
6
7
8
9
PhSCH3
PhSOCH3
30
15
20
10
10
25
20
40
96
90
91
95
93
92
94
93
31–33 (33–34) [28]
a
p-BrC6H4SCH3
p-O2NC6H4S CH3
PhSCH2CH3
PhCH2SCH2Ph
PhCH2SPh
p-BrC6H4SOCH3
p-O2NC6H4SOCH3
PhSOCH2CH3
PhCH2SOCH2Ph
PhCH2SOPh
73–75 (74–76) [28]
a
139–141(140–142) [28]
a
141–144 (146) [28]
a
129–132 (133–135) [28]
120–122 (123–124) [28]
68–70 (70.5) [29]
150–153 (53–155) [30]
a
a
PhSPh
PhSOPh
a
1
0
b
11
12
13
14
15
16
17
18
19
CH3(CH2)2SH
CH3(CH2)3SH
CH3(CH2)7SH
C6H11SH
PhCH2SH
PhSH
(CH3(CH2)2S)2
(CH3(CH2)3S)2
(CH3(CH2)7S)2
(C6H11S)2
(PhCH2S)2
(PhS)2
3
4
5
5
3
6
5
5
3
90
92
91
94
95
93
95
92
95
193–194 (193) [31]
b
190–191 (192) [31]
a
72–75 (74–75) [28]
a
120–123 (125–130) [29]
a
67–71 (69–72) [29]
a
57–59 (61–62) [29]
a
p-CH3C6H4SH
o-ClC6H4SH
(p-CH3C6H4S)2
(o-ClC6H4S)2
44–46 (45–46) [32]
a
84–86 (87–88) [30]
a
135–137 (139) [33]
aMelting point.
b
Boiling point.
Oxidation of Sulfides
In a typical reaction, methyl phenyl sulfide (0.124 g, 1 mmol) was added to the
mixture of silica-supported 1,1,3,3-tetramethylguanidine/Br2 (1 g) in aqueous acetonitrile
CH3CN:H2O; 3:1 v/v, 20 mL). The mixture was stirred at room temperature for 30 min,
(
while the progress of the reaction was followed by TLC (n-hexane:ethylacetate 7:1). The
product was extracted into CH2Cl2 (2 × 10 mL) and dried over anhydrous magnesium
sulfate. Evaporation of the solvent gave a product (0.134 g, 96%) of sufficient purity for
◦
28
◦
most purposes; Mp 31–33 C (ref. : 33–34 C).
Oxidation of Thiols
In a typical procedure, p-methyl thiophenol (0.124 g, 1 mmol) was added to a
mixture of silica-supported 1,1,3,3-tetramethylguanidine/Br2 (0.5 g) in aqueous acetonitrile
(CH3CN:H2O; 3:1 v/v, 20 mL). The mixture was stirred at room temperature for 3 min,
while the progress of the reaction was followed by TLC (n-hexane:ethylacetate 7:1). The

OXIDATION OF SULFIDES AND THIOLS
467
product was extracted into CH2Cl2 (2 × 10 mL) and dried over anhydrous magnesium
sulfate. Evaporation of the solvent gave a product (0.118 g, 96%) of sufficient purity for
◦
32
◦
most purposes; Mp 44–46 C (ref. : 45–46 C).
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