Trimethylphenylammonium tribromide as a new and efficient oxidizing agent for the oxidative coupling of thiols into disulfides and chemoselective oxidation of sulfides into sulfoxides

Article abstract of DOI:10.1080/10426507.2010.527878

Trimethylphenylammonium tribromide has been introduced as a versatile and new oxidizing agent for the preparation of disulfides and sulfoxides from thiols and sulfides, respectively. The reaction progress is simple, and proceeds under mild and homogenous conditions in ambient temperature. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. Copyright Taylor &Francis Group, LLC.

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Trimethylphenylammonium Tribromide
as a New and Efficient Oxidizing Agent
for the Oxidative Coupling of Thiols into
Disulfides and Chemoselective Oxidation
of Sulfides into Sulfoxides
Arash Ghorbani-Choghamarani a , Mohammad Soleiman-Beigi a
Farid Noormohammadi b
&
a Department of Chemistry, Faculty of Science , Ilam University ,
Ilam, Iran
b Islamic Azad University, Science and Research , Branch-Ahvaz,
Ahvaz, Iran
Published online: 11 Aug 2011.
To cite this article: Arash Ghorbani-Choghamarani , Mohammad Soleiman-Beigi & Farid
Noormohammadi (2011) Trimethylphenylammonium Tribromide as a New and Efficient Oxidizing
Agent for the Oxidative Coupling of Thiols into Disulfides and Chemoselective Oxidation of Sulfides
into Sulfoxides, Phosphorus, Sulfur, and Silicon and the Related Elements, 186:8, 1665-1672, DOI:
10.1080/10426507.2010.527878
To link to this article: http://dx.doi.org/10.1080/10426507.2010.527878
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Phosphorus, Sulfur, and Silicon, 186:1665–1672, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.527878
TRIMETHYLPHENYLAMMONIUM TRIBROMIDE AS
A NEW AND EFFICIENT OXIDIZING AGENT FOR THE
OXIDATIVE COUPLING OF THIOLS
INTO DISULFIDES AND CHEMOSELECTIVE OXIDATION
OF SULFIDES INTO SULFOXIDES
Arash Ghorbani-Choghamarani,1 Mohammad Soleiman-Beigi,1
and Farid Noormohammadi2
1Department of Chemistry, Faculty of Science, Ilam University, Ilam, Iran
2Islamic Azad University, Science and Research, Branch-Ahvaz, Ahvaz, Iran
GRAPHICAL ABSTRACT
O
-
Me3PhN+Br3
S
1
S
R1
R2
R1
R2
MeCOOEt/H2O
r.t.
2
Or
-
Me3PhN+Br3
R
SH
RS SR
4
MeCOOEt, r.t.
3
Abstract Trimethylphenylammonium tribromide has been introduced as a versatile and new
oxidizing agent for the preparation of disulfides and sulfoxides from thiols and sulfides, respec-
tively. The reaction progress is simple, and proceeds under mild and homogenous conditions
in ambient temperature.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords Disulfides; sulfides; sulfoxides; thiols; tribromide; trimethylphenylammonium
INTRODUCTION
Synthetic methodology, as the building block of organic synthesis, continually seeks
for new reagents, better reaction conditions, and more efficient and selective methods.1
Recently, a variety of methodologies have been reported for the preparation of organic
Received 19 August 2010; accepted 26 September 2010.
We are grateful to Ilam University, Ilam, Iran for financial support of this research project.
Address correspondence to Arash Ghorbani-Choghamarani, Ilam University, P.O. Box 69315516, Ilam, Iran.
E-mail: arashghch58@yahoo.com; a.ghorbani@mail.ilam.ac.ir
1665

1666
A. GHORBANI-CHOGHAMARANI ET AL.
compounds using molecular bromine.2–4 In order to decrease the toxicity of molecular
bromine, a variety of organic tribromide reagents have been synthesized and used in different
reactions.5–7 In this light, trimethylphenylammonium tribromide attracted our attention, and
we decided to use this reagent for the preparation of disulfides and sulfoxides.
Controlled oxidative coupling of thiols into disulfides is of interest from synthetic,
biological, and industrial points of view.8–10 Oxidation of thiols is the most exploited
method for disulfide synthesis, mainly because several thiols are commercially available
and are easily prepared.11 Thiols and disulfides are important in living cells, since they are
structural features of many biomolecules including proteins. In many biochemical redox
reactions they are interconverted.12
In addition, the chemoselective oxidation of sulfides to the corresponding sulfoxides
is of great importance. The standing interest in the oxidation of sulfides into sulfoxides is due
to its utility in different areas of chemistry, such as violability as synthetic intermediates
for the synthesis of chemically and biologically molecules,13,14 its application in C C
bond-forming,15 and its participation in molecular rearrangements.16
Althoughawidevariety of reagents or reagent systems for thepreparationof disulfides
and sulfoxides has emerged in recent years,17–27 finding an oxidizing agent that is able to
oxidize thiols and sulfides into corresponding disulfides and sulfoxides, respectively, is of
great challenge in organic chemistry.
RESULTS AND DISCUSSION
In continuation of our recent work on the application of new reagents for organic
functional group transformations,28–35 we decided to introduce trimethylphenylammonium
tribromide (there are only a few reports on the application of this reagent in the liter-
ature36–40) as an efficient and new oxidizing agent for the chemoselective oxidation of
sulfides to the sulfoxides, and also oxidative coupling of thiols to the corresponding disul-
fides.
In order to find appropriate solvent for these transformations, we designed separated
reactions for the oxidation of sulfides and oxidative coupling of thiols. Ethylphenyl sulfide
and 2-naphthylthiol were subjected to the oxidation reaction with trimethylphenylammo-
nium tribromide in different solvents, as typical examples. The results of these reactions
(including reaction times and yields) are summarized in Table 1. As can be seen from
O
-
Me3PhN+Br3
S
1
S
R1
R2
R1
R2
MeCOOEt/H2O
r.t.
2
Or
-
Me3PhN+Br3
R
SH
RS SR
4
MeCOOEt, r.t.
3
Scheme 1

TRIMETHYLPHENYLAMMONIUM TRIBROMIDE AS AN OXIZING AGENT
1667
-
Me3PhN+Br3
Me3PhN+Br-
Br
S
Br-
S
Br2
+
Me3PhN+Br3- + RSH
RSBr + HBr + Me3PhN+Br-
Br
S
Br-
Br
OH
H2O
S
+ HBr
RSSR + Br2
RSBr + HBr
RS SR
2RSBr
RSH + Br2
RSH + RSBr
Br
S
O
OH
-HBr
(a)
S
+ HBr
(b)
Scheme 2
Table 1, ethyl acetate is the best solvent for the sulfoxidation as well as oxidative coupling
of thiol in terms of selectivity and reactivity.
With the optimal conditions in hand, a wide variety of sulfides 1 and thiols 3 were
selectively converted into corresponding sulfoxides 2 and disulfides 4 using trimethylpheny-
lammonium tribromide at room temperature (Scheme 1 and Table 2).
Table 1 Oxidation of ethylphenyl sulfide and oxidative coupling of 2-naphthylthiol by trimethylphenylammo-
nium tribromide in different solvents at room temperaturea
Entry
Compound
Solvent
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
Ethylphenyl sulfide
2-Naphthylthiol
2-Naphthylthiol
2-Naphthylthiol
2-Naphthylthiol
2-Naphthylthiol
Acetonitrile/H2O
Acetone/H2O
Chloroform/H2O
Dichloromethane/H2O
n-Hexane/H2O
Ethyl acetate/H2O
Ethyl acetate
Ethanol/H2O
Acetonitrile
Acetone
Chloroform
Dichloromethane
n-Hexane
Ethyl acetate
Ethanol
Immediately
55
5 h
150
27 h
Less than 1 min
6 h
15
20 h
20 h
50
35
20 h
5
92
78
–
c
89
c
–
92
d
–
94
Sluggish
Sluggish
80
98
c
–
2-Naphthylthiol
2-Naphthylthiol
98
Sluggish
5 h
aSubstrate/reagent/solvent/H2O for entries of 1–7 (1 mmol/1.2 mmol/5 mL/2 mL); for entries of 8–14 (1
mmol/0.55 mmol/5 mL/0 mL).
bIsolated yield.
cReaction did not complete.
dTrace conversion.

1668
A. GHORBANI-CHOGHAMARANI ET AL.
Table 2 Conversion of sulfides 1 and thiols 3 to the corresponding sulfoxides 2 and disulfides 4 using
trimethylphenylammonium tribromide in ethyl acetate (in oxidative coupling of thiols) or ethyl acetate/water
(5 mL:2 mL; in sulfoxidation reaction) at room temperaturea
Entry
1
Substrate
Product
Time (min)
Yield (%)b Reference
5
86
35
2
120
98
35
3
4
Less than 1 min
60
96
98
51
35
5
Less than 1 min
92
10
6
7
Less than 1 min
Less than 1 min
76
98
35
30
8
5
51
35
9
5
5
98
76
–
10
35
11
12
10
25
75
82
20
35
13
5
95
54
14
15
16
17
5
98
93
98
95
54
54
54
54
5
5
Less than 1 min
18
60
63
54

TRIMETHYLPHENYLAMMONIUM TRIBROMIDE AS AN OXIZING AGENT
1669
Table 2 Conversion of sulfides 1 and thiols 3 to the corresponding sulfoxides 2 and disulfides 4 using
trimethylphenylammonium tribromide in ethyl acetate (in oxidative coupling of thiols) or ethyl acetate/water
(5 mL:2 mL; in sulfoxidation reaction) at room temperaturea (Continued)
Entry
19
Substrate
Product
Time (min)
270
Yield (%)b
90
Reference
12
20
10
75
41
21
22
HSCH2COOH
HOOCCH2S SCH2COOH
5
98c
95
12
41
Less than 1 min
aSubstrate/reagent for entries of 1–12 (1:1.2 mmol); for entries of 13–22 (1:0.55 mmol).
bYields refer to isolated product purified by column chromatography (CH2Cl2 as eluent).
cYield refers to isolated product purified by column chromatography (acetone as eluent).
Sulfoxidation and oxidative coupling reactions were homogenously performed under
mild conditions. The corresponding products were readily prepared via mixing of substrates
(thiol or sulfide) with trimethylphenylammonium tribromide, then stirring of this homoge-
nous mixture at room temperature for the appropriate times. The pure product could be
obtained easily by passing of reaction mixture through a short column.
A plausible mechanism for these transformations is outlined in Scheme 2 based on
our previously reported work.28,30–32,34–36
In order to investigate the efficiency of this new reagent in comparison with known
reported procedures in the literature, the results for the preparation of 1,2-diphenyldisulfane
(Table 2, entry 22) and 1-(methylsulfinylmethyl)benzene (entry 3), as representative exam-
ples, compared with the best of the well-known data from the literature as outlined in the
Table 3.
In conclusion, we have introduced a convenient and efficient protocol for the green
preparation of sulfoxides and disulfides from sulfides and thiols, respectively, using
trimethylphenylammonium tribromide as a nontoxic oxidizing agent. Furthermore, this
method exhibits mild reaction conditions and experimental convenience.
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies. The
oxidation products were characterized by comparison of their spectral (IR, 1H NMR, and
13C NMR) and physical data with authentic samples.
Oxidation of Benzylphenyl Sulfide to Benzylphenyl Sulfoxide
by Trimethylphenylammonium Tribromide
Trimethylphenylammonium tribromide (0.451 g, 1.2 mmol) was added to a solution
of benzylphenyl sulfide (0.200 g, 1 mmol) in ethyl acetate/water (5 mL:2 mL). The resulting
mixture was stirred at room temperature for 60 min (the reaction progress was monitored
by TLC). After reaction completion, anhydrous Na2SO4 (5 g) was added to the reaction

1670
A. GHORBANI-CHOGHAMARANI ET AL.
Table 3 Comparison of the different methods that were applied for the preparation of 1,2-diphenyldisulfane and
1-(methylsulfinylmethyl)benzene
Entry
Substrate
Thiophenol
Thiophenol
Thiophenol
Thiophenol
Thiophenol
Thiophenol
Thiophenol
Reagent or catalyst
Me3PhN+Br3
I2/CeCl3·7H2O
NaBrO3/NaBr
N-Bromosuccinimide
Rosolic acid
Time (min)
Yield (%)a
Ref.
−
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
<1 min
60
12
5
60
95
99
91
95
95
89
98
85
96
92
93
99
83
96
99
48
This work
41
10
42
43
44
Fe(CF3CO2)3
Mn(III)/O2
15
18
45
46
Thiophenol
Me4N+F−CrO3
120
<1 min
50
180
30
24 h
30
10
−
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Benzylmethyl sulfide
Me3PhN+Br3
This work
47
H2O2/Acetic acid
Fe(salan)/H2O2
Ru(TPP)Cl/O2
Al(salan)/H2O2
TaCl5/H2O2
48
49
50
51
52
53
[n-Bu)4N]4(α-Mo8O26)/H2O2
Oxaziridinium salt
180
aIsolated yield.
mixture. The crude product was purified by short column chromatography, packed by silica
gel (using n-hexane/acetone, 6:4, as eluent). Finally solvents were evaporated by simple
filtration and benzylphenyl sulfoxide obtained in 98% yield (0.212 g) as crystalline white
solid, mp 121–122◦C (Lit. 116–120◦C30).
Oxidative Coupling of 2-Naphthylthiol into
1,2-Di(naphthalen-2-yl)disulfane Using Trimethylphenylammonium
Tri b ro mi d e
To a solution of trimethylphenylammonium tribromide in ethyl acetate (5 mL), 2-
naphthyl thiol was added, and the mixture was stirred at room temperature for 5 min (the
reaction progress was monitored by TLC). After reaction completion, the crude product
was purified by short column chromatography using dichloromethane as eluent. Finally
solvents were evaporated, and 1,2-di(naphthalen-2-yl)disulfane was obtained in 95% yield
(0.302 g) as white solid, mp 141–143◦C (Lit. 142.2–143.5◦C54).
Selected spectral data are presented in the Supplemental Materials (available online).
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