Practical and versatile oxidation of sulfides into sulfoxides and oxidative coupling of thiols using polyvinylpolypyrrolidonium tribromide

Article abstract of DOI:10.1080/10426507.2013.844140

A simple and highly selective oxidation of sulfides to sulfoxides and an oxidative coupling of thiols with polyvinylpolypyrrolidoniume tribromide has been developed. A series of sulfides and thiols was oxidized selectively at room temperature in good to excellent yields.

Full text of DOI:10.1080/10426507.2013.844140

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Phosphorus, Sulfur, and Silicon and the
Related Elements
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Practical And Versatile Oxidation
Of Sulfides Into Sulfoxides And
Oxidative Coupling Of Thiols Using
Polyvinylpolypyrrolidonium Tribromide
Arash Ghorbani-Choghamarani a , Gouhar Azadi a , Bahman Tahmasbi
a , Mojtaba Hadizadeh-Hafshejani a & Zainab Abdi a
a Department of Chemistry , Faculty of Science, Ilam University, P.O.
Box 69315516 , Ilam , Iran
Accepted author version posted online: 25 Sep 2013.Published
online: 12 Mar 2014.
To cite this article: Arash Ghorbani-Choghamarani , Gouhar Azadi , Bahman Tahmasbi , Mojtaba
Hadizadeh-Hafshejani & Zainab Abdi (2014) Practical And Versatile Oxidation Of Sulfides Into
Sulfoxides And Oxidative Coupling Of Thiols Using Polyvinylpolypyrrolidonium Tribromide, Phosphorus,
Sulfur, and Silicon and the Related Elements, 189:4, 433-439, DOI: 10.1080/10426507.2013.844140
To link to this article: http://dx.doi.org/10.1080/10426507.2013.844140
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Phosphorus, Sulfur, and Silicon, 189:433–439, 2014
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.844140
PRACTICAL AND VERSATILE OXIDATION OF SULFIDES
INTO SULFOXIDES AND OXIDATIVE COUPLING OF
THIOLS USING POLYVINYLPOLYPYRROLIDONIUM
TRIBROMIDE
Arash Ghorbani-Choghamarani, Gouhar Azadi,
Bahman Tahmasbi, Mojtaba Hadizadeh-Hafshejani,
and Zainab Abdi
Department of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516,
Ilam, Iran
GRAPHICAL ABSTRACT
O
PVP-Br3, EtOAc
r.t.
S
S
1
2
R1
R2
R
R
PVP-Br3, EtOH
r.t.
S
SH
R
R
R
H
H
C2 CH
C2
Br3
N
PVP-Br3:
HO
n
Abstract A simple and highly selective oxidation of sulfides to sulfoxides and an oxidative
coupling of thiols with polyvinylpolypyrrolidoniume tribromide has been developed. A series
of sulfides and thiols was oxidized selectively at room temperature in good to excellent yields.
Keywords Sulfide; sulfoxide; thiol; disulfide; polyvinylpolypyrrolidoniume tribromide
INTRODUCTION
The modern ecological aspect of oxidation research focuses on thereplacement
of current toxic and dangerous compounds with ecofriendly oxidants. In this line,
Received 21 May 2013; accepted 8 September 2013.
This work was supported by the research facilities of Ilam University, Ilam, Iran.
Address correspondence to Arash Ghorbani-Choghamarani, Department of Chemistry, Faculty of Science,
Ilam University, P.O. Box 69315516, Ilam, Iran. E-mail: a.ghorbani@mail.ilam.ac.ir
433

434
A. GHORBANI-CHOGHAMARANI ET AL.
Scheme 1 Structure of polyvinylpolypyrrolidoniume tribromide (PVP- Br3). (Color figure available online).
polymer-supported reagents have had much attention in recent years due to their selectivity,
stability, and easy handling.1–3 They could serve as heterogeneous, clean, and regenerable
reagents in the oxidation reactions.
The oxidation of sulfides to sulfoxides and coupling of thiols to disulfides has been
the aim of considerable research due to the importance of sulfoxides and disulfides as
useful intermediates in various chemically and biologically significant molecules.4–7 The
chemoselective oxidation of sulfides is also another point of interest.
Various oxidizing reagents used for this purpose include among others: nitric acid,
dinitrogen tetroxide, chromic acid, manganese dioxide, ozone, peracids, selenium diox-
ide, sodium periodate, hypervalent iodine reagents, sodium perborate, halogens, tetra-
butylammonium peroxydisulfate,8 and cerium(IV) salts,9 transition metal oxides,10 N-
phenyltriazolinedione,11 urea–hydrogen peroxide,12 halogens,13 atmospheric oxygen,14
H2O2,15 and solvent-free permanganate.16
However, the reported methods rarely offer the ideal combination of simplicity of
method, rapid and selective reactions, and high yields of products, and often suffer from
a lack of generality and economic applicability. As a result, the introduction of readily
available, safe, and stable reagents for the oxidation research is still a necessity.
RESULTS AND DISCUSSION
As part of our program in environmentally beneficial catalysis and applications of
polymeric reagents,17–22 we are interested in designing a “green” process to effectively
carry out the oxidation reaction. We report herein an inexpensive, safe and environmentally
friendly method to oxidize a variety of functional groups under extremely mild conditions.
Polyvinylpolypyrrolidoniume tribromide (Scheme 1) is stable and efficient oxidant
that can be prepared via the combination of polyvinylpolypyrrolidone with hydrobromic
acid followed by the reaction of resulting salt with bromine.23
Simple work-up and the stability of these reagents make it a safe and convenient
source of active bromine in comparison to molecular bromine which is a highly toxic
oxidizing agent.
Initially, in order to find appropriate solvent for the oxidation of sulfides and coupling
of thiols, we designed separated reactions for these transformations in different organic
solvents. Dibenzyl sulfide and 4-bromothiophenol were selected as model substrates to
optimize reaction conditions (Scheme 2). The results of the screening process have been
summarized in Table 1.

PRACTICAL AND VERSATILE OXIDATION OF SULFIDES INTO SULFOXIDES AND OXIDATIVE 435
Table 1 Oxidation of dibenzyl sulfide and 4-Bromothiophenol by PVP-Br3 at room temperature in different
solventsa
Entry
Substrate
Solvent
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
4-Br-Thiophenol
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
Dibenzyl sulfide
CH3CN
EtOH
CH3CH2OAc
H2O
Acetone
n-Hexane
CH2Cl2
CHCl3
CH3CN
n-Hexane
CH3CH2OAc
EtOH
8
8
98
99
93
27
48h
10
19
—
5
c
—
—
d
97
e
—
99
99
93
99
4
25
18
—
41h
24h
—
5
f
—
Acetone
H2O
CH2Cl2
CHCl3
Trace
g
—
h
—
95
aSubstrate/PVP-Br3: 1 mmol/1.2 gr (for oxidation of 4-bromothiophenol) and 1.9 gr (for oxidation of dibenzyl
sulfide).
bIsolated yield.
c,gReaction did not complete.
d,e,f,hReaction did not complete and impurity was observed on TLC.
Scheme 2 Oxidation of dibenzyl sulfide and 4-bromothiophenol by PVP-Br3 in different solvents. (Color figure
available online).
However, ethanol and ethyl acetate proved to be the best solvent in terms of con-
version and selectivity. Under these conditions, dibenzyl sulfide and 4-bromothiophenol
were converted to dibenzyl sulfoxide and 1,2-bis(4-bromophenyl) disulfane in 99% isolated
yields after 18 and 8 min, respectively (Table 1, entries 2 and 11).

436
A. GHORBANI-CHOGHAMARANI ET AL.
Scheme 3 Oxidation of sulfides and thiols into sulfoxides and disulfides using PVP-Br3 at room temperature.
(Color figure available online).
After optimizing the conditions, all reactions with other sulfides and thiols were
carried out in ethyl acetate and ethanol using polyvinylpolypyrrolidoniume tribromide as
oxidizing agent at room temperature (Scheme 3); the substrates and the yields of products
are listed in Table 2.
All oxidation reactions were performed under completely heterogeneous conditions
at room temperature with good to excellent yields. In some cases, lower yields were
obtained because of the loss of the volatile products during their isolation. There are several
advantages of these reactions over other methods in the literature. The reactions were very
clean, gave excellent reproducible yields, and excellent selectivity was obtained from these
reactions. Thus, this method can be considered as a relatively green technology having
more advantages and wider applicability compared to the conventional oxidation reagents.
A possible mechanism for these transformations is depicted in Scheme 4, based on
previously reported works.28–32
CONCLUSION
In conclusion, an environmentally benign and polymer-supported reagents has been
reported for safe preparation of disulfides and sulfoxides. Rapid reaction, operational
simplicity, good yield, and high purity of products are attractive features of this protocol.
EXPERIMENTAL
The chemicals and solvents were purchased from Fluka, Merck, and Aldrich chemical
companies without further purifications. All products are known and were characterized by
comparison of their spectral and physical data with authentic samples.
Oxidation of sulfides (general procedure): In a 25 mL round-bottom flask, to a so-
lution of sulfide (1 mmol) in ethyl acetate (5 mL), 1.9 gr of polyvinylpolypyrrolidoniume
tribromide was added successively and the mixture was stirred magnetically at room tem-
perature for the indicated time in Table 2 (the progress of the reaction monitored by TLC).
After completion of the reaction, the pure product was isolated by passing of reaction

PRACTICAL AND VERSATILE OXIDATION OF SULFIDES INTO SULFOXIDES AND OXIDATIVE 437
Table 2 Oxidation of sulfides and thiols into sulfoxides and disulfides respectively, using PVP-Br3 at room
temperaturea
Time Yieldb
(min) (%)
Solvent
EtOH
M.p found M.p Reported
Entry
1
Substrate
Product
(◦C)
(◦C)
8
7
99
97
91–93
89–92 [22]
2
EtOH
45
39–42 [22]
3
4
15
6
99
95
EtOH
EtOH
143–144 141–144 [22]
56–57 55–58 [24]
5
8
94
EtOH
160–162 164–166 [22]
6
7
8
40
9
90
99
99
EtOH
EtOH
EtOH
Oil
Oil
Oil [24]
Oil [24]
90
190–192 273–285 [22]
9
18
15
99
95
CH3CH2OAc 138–139 130–134 [25]
10
C-H3(CH2)11SH
C-H3(CH2)11S-S-
(CH2)11CH3
CH3CH2OAc
88–89
31
[26]
11
12
105
40
99c CH3CH2OAc
31 [25]
99
CH3CH2OAc Liquid
Liquid [27]
13
25
94
CH3CH2OAc Liquid
Oil [28]
14
15
15
12
96
78
CH3CH2OAc Liquid
CH3CH2OAc Liquid
Liquid [27]
Liquid [25]
16
17
18
12
35
70
77
CH3CH2OAc
Oil
Oil [25]
99d CH3CH2OAc Liquid
Liquid [27]
99e
CH3CN
126–127 116–120 [25]
aSubstrate/PVP-Br3: 1 mmol/1.2 (for oxidation of thiols)or 1.9 gr (for oxidation of sulfides).
bIsolated yield.
c,d,eReaction proceeded in the presence of 2.9, 2, and 2.6 gr of PVP-Br3, respectively.

438
A. GHORBANI-CHOGHAMARANI ET AL.
Scheme 4 General mechanism for the oxidation of sulfides and thiols. (Color figure available online).
mixture through a short column using dichlromethane and acetone (95:5) as eluent. The
filtrate was evaporated and pure sulfoxides were obtained in good to excellent yields.
Oxidative coupling of thiols (general procedure): A 25 mL round-bottom flask was
charged with thiol (1 mmol), polyvinylpolypyrrolidoniume tribromide (1.2 gr), and EtOH
(5 mL) as solvent. The reaction mixture was stirred at room temperature, and the progress
of the reaction was monitored by TLC. After completion of the reaction, corresponding
disulfides easily obtained by passing of reaction mixture through a short column using
dichlromethane and acetone (95:5) as eluent.
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