Rapid and convenient method for the synthesis of symmetrical disulfides

Article abstract of DOI:10.1080/10426507.2012.717145

We present here the results on the use of 1,3,5-triazo-2,4,6- triphosphorine-2,2,4,4,6,6-hexachloride as an efficient promoter in the conversion of thiols to the corresponding symmetrical disulfides under solvent-free conditions. Aromatic thiols bearing electron donating and electron withdrawing groups, heteroaromatic, and alkyl thiols reacted efficiently to afford excellent yields of disulfides in short reaction times after easy work-up. Different functional groups including carboxyl, methoxy, methylthio, and halogen are tolerated. Supplementary materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental files: Additional figures and tables.

Full text of DOI:10.1080/10426507.2012.717145

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Rapid and Convenient Method for the
Synthesis of Symmetrical Disulfides
a
a b
a
Kiumars Bahrami
, Mohammad M. Khodaei , Vida Shakibaian &
a
Homa Targhan
a
Department of Organic Chemistry, Faculty of Chemistry , Razi
University , Kermanshah , Iran
b
Nanoscience and Nanotechnology Research Center (NNRC) , Razi
University , Kermanshah , Iran
Accepted author version posted online: 09 Aug 2012.Published
online: 22 Jul 2013.
To cite this article: Kiumars Bahrami , Mohammad M. Khodaei , Vida Shakibaian & Homa Targhan
(2013) Rapid and Convenient Method for the Synthesis of Symmetrical Disulfides, Phosphorus, Sulfur,
and Silicon and the Related Elements, 188:8, 981-988, DOI: 10.1080/10426507.2012.717145
To link to this article: http://dx.doi.org/10.1080/10426507.2012.717145
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Phosphorus, Sulfur, and Silicon, 188:981–988, 2013
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Taylor & Francis Group, LLC
C
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2012.717145
RAPID AND CONVENIENT METHOD FOR THE
SYNTHESIS OF SYMMETRICAL DISULFIDES
1
,2
1
Kiumars Bahrami, Mohammad M. Khodaei,
1
1
Vida Shakibaian, and Homa Targhan
Department of Organic Chemistry, Faculty of Chemistry, Razi University,
Kermanshah, Iran
1
2
Nanoscience and Nanotechnology Research Center (NNRC), Razi University,
Kermanshah, Iran
GRAPHICAL ABSTRACT
Abstract We present here the results on the use of 1,3,5-triazo-2,4,6-triphosphorine-
2,2,4,4,6,6-hexachloride as an efficient promoter in the conversion of thiols to the
corresponding symmetrical disulfides under solvent-free conditions. Aromatic thiols bearing
electron donating and electron withdrawing groups, heteroaromatic, and alkyl thiols reacted
efficiently to afford excellent yields of disulfides in short reaction times after easy work-up.
Different functional groups including carboxyl, methoxy, methylthio, and halogen are
tolerated.
Supplementary materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental
files: Additional figures and tables.
Keywords Thiols; disulfides; solvent-free conditions; oxidative conversion; 1,3,5-triazo-2,4,6-
triphosphorine-2,2,4,4,6,6-hexachloride (TAPC)
INTRODUCTION
One of the main aims of green chemistry is the reduction of the use of organic
solvents because of the economical and environmental concerns associated with them, and
1
therefore, the development of solvent-free synthetic methods is of the utmost importance.
Received 18 June 2012; accepted 27 July 2012.
Address correspondence to Kiumars Bahrami, Department of Organic Chemistry, Faculty of Chemistry,
Razi University, Kermanshah 67149-67346, Iran. E-mail: Kbahrami2@hotmail.com
9
81

9
82
K. BAHRAMI ET AL.
Compounds containing a disulfide linkage have been used for the preparation of
2
3
4
5
dynamic combinatorial libraries, catenanes, macrocycles, rotaxanes, andmicelles. These
examples demonstrate the wide applications of disulfides and show that the synthesis of
6
disulfide bonds is a pivotal transformation in modern research.
Oxidative conversion of thiols is the most typical method of preparation disulfides
because there are a large number of commercially available thiols and the interconversion
7
between thiols and disulfides is easy. On the other hand, disulfides are relatively more
stable toward organic reactions, such as oxidation, alkylation, and acylation, compared to
the corresponding free thiols; the thiol group can conveniently be protected as a disul-
6b
fide. These aspects are responsible for the continuous interest in the development of
new, selective, and efficient protocols for the preparation of disulfides. Reagents such as
8
a
8b
8c
8d–g
8h
silica-PCl5/NaNO2, metal ions, molecular oxygen, halogens,
sodium perborate,
8i
8j
8k
H2O2, tripropylammonium fluorochromate, 1,3-dibromo-5,5-dimethylhydantoin, and
8l
9–12
DMSO among others have been utilized for the preparation of symmetrical disulfides.
However, some of these protocols suffers from drawbacks such as long reaction
times,11d toxicity, high cost of the required reagent(s), unpleasant work-up, heavy
8j
11e
11g
8
b
12a
metal contamination, in certain cases, moderate to low yields, the production of foul-
8l
smelling dimethyl sulfide as a waste product, and organic solvents are always used. Owing
to the wide range of chemical and biological activity of disulfides, the development of an
alternative synthetic methodology is of paramount importance.
Due to our interest in new applications for 1,3,5-Triazo-2,4,6-triphosphorine-
2
,2,4,4,6,6-hexachloride (TAPC),13 we decide to study the use of it as the reagent in the
conversion of thiols to symmetrical disulfides (Scheme 1).
TAPC
R
SH
RS SR
Solvent-free, 25 °C
1
2
Scheme 1
Chlorocyclophosphazenes, as a type of phosphazene compound, have many special
characteristics. They have various applications ranging from elastomers to anticancer drug
14
delivery systems. TAPC, a member of the class of cyclophosphazenes, has been widely
used in organic reactions.15 According to the reference method, TAPC has been prepared
by reacting ammonium chloride with phosphorus pentachloride. These starting materials
are cheaper and more readily available.
16
To the best of our knowledge, there have been no reports employing TAPC as a
promoter for the conversion of thiols to disulfides. Initially, we chose 4-chlorothiophenol as
a substrate to establish the best condition for the preparation of the corresponding disulfide
◦
in the presence of different amount of TAPC at 25 C under solvent-free conditions.
After careful examinations, the use of 50 mol% of TAPC under solvent-free conditions
◦
for 2 min at 25 C was found to be the optimal condition for the disulfide synthesis (Table 1,
entry 5). In the absence of TAPC, the reaction did not occur at all. An increase in the amount
of TAPC did not improve the yields significantly. Subsequently, the scope of the reaction
substrate was extended. Under the optimized conditions a variety of aromatic, heterocyclic,
and aliphatic thiols were converted smoothly to produce disulfides. Excellent yields and high
purity in most cases indicated by both the NMR and TLC analyses. Aromatic thiols having

RAPID METHOD FOR SYMMETRICAL DISULFIDES
983
Table 1 Effect of increasing amount of TAPC on the preparation of symmetrical disulfidea
S
Cl
Cl
SH
Cl
S
Entry
TAPC (mmol)
Yield%b
1
2
3
4
5
0
0
53
74
85
93
0.1
0.2
0.4
0.5
a
◦
Reaction conditions: The reactions were performed with 4-chlorothiophenol (1 mmol) for 2 min, at 25 C.
Isolated yield.
b
either electron donating or withdrawing substituents reacted efficiently to afford excellent
yields of disulfides despite electronic effects. The utility of this promoter in the conversion of
heteroaromatic thiols was also tested. Thus, 2-mercaptobenzothiazole (Table 2, entry 8) was
quantitatively transformed to the desired disulfide. Moreover, the scope of this methodology
was found not to be limited to aromatic thiols, as aliphatic thiols underwent conversion
to the corresponding disulfides in excellent yields (Table 2, entries 11–12). Furthermore,
2-mercaptoacetic acid (Table 2, entry 10) was utilized to obtain the corresponding disulfide
in 90% yield without interference from the carboxyl functional group.
As can be seen from the results in Table 2, the selectivity of the present method is fairly
wide, as carboxyl, methoxy, methylthio, and halogen functionalities do not interfere during
the formation of the product disulfides. Unfortunately, furan-2-methanethiol (Table 2, entry
9) failed to produce acceptable yield of the disulfide product. This thiol produced a mixture
of products, which were difficult to separate; after laborious separation, we obtained an
unacceptable yield of the desired disulfide.
In order to compare the results with those obtained in solution, we tried to study the
coupling reaction in CH2Cl2. The results showed no appreciable difference between the
results obtained in solution and under solvent-free conditions. But reaction in CH2Cl2 is
likely to raise environmental problems, and hence the challenge for a sustainable environ-
ment calls for the use of alternative procedures avoiding the use of harmful solvents. Thus,
solvent-free conditions were selected for the synthesis of disulfide compounds.
To illustrate the efficiency of this method, Table 3 compares our results with reported
data on the relevant methods. As can be seen, TAPC is superior to these reagents in terms
of yields and reaction times under these conditions.
To demonstrate the validity and regioselectivity of this reagent system, we studied
this transformation in the presence of alkene, nitrile, aldehyde hydroxyl, and sulfide, and the
results are summarized in Scheme 2. These studies obviously show that this reagent system
can be applied for the chemoselective the conversion of thiols to symmetrical disulfides in
the presence of the above-mentioned functional groups in multifunctional compounds.
The actual mechanism of the present protocol and the precise role of TAPC are not
clear at this stage. However, it is plausible that at the first stage TAPC reacts with two
19
molecules of thiol (1) to produce a reactive species (3) and by-product hydrochloric acid,
and then the former is promoted by the latter to undergo phosphorus–sulfur bond cleavage.

9
84
K. BAHRAMI ET AL.
Table 2 TAPC-promoted conversion of thiols to symmetrical disulfidesa
Yield
b
◦
(
%) /time Mp/ C (Lit
Entry
1
Disulfide 2
(min)
92 (2)
Mp)
Reference
11d
58 (60)
2
3
4
5
6
90 (2) 38–40 (41–43)
10i
11e
11d
10i
94 (2)
93 (2)
95 (2)
80 (80–82)
70 (71–72)
176–177
(
175–177)
90 (2) 141–143 (140)
17a
7
8
90 (2)
92 (2)
66 (69–72)
178–179
11f
11f
(
179–180)
9
Trace
90 (2)
—
—
1
0
Oil (Oil)
11f
1
1
1
2
93 (2)
95 (2)
Oil (Oil)
Oil (Oil)
11e
17b
a
1
The purified products were characterized by mp and H NMR and compared with the original spectra.
b
Yields refer to pure isolated products.

RAPID METHOD FOR SYMMETRICAL DISULFIDES
985
S
Cl
Cl
SH
Cl
S
TAPC
9
0%
H C CH CN
2
Solvent-free, 25 °C
H C
CH CN
2
Unchanged
S
S
OMe
MeO
SH
MeO
TAPC
9
1%
Solvent-free, 25 °C
CHO
OH
CHO
OH
Unchanged
S
S
NO2
O N
SH
O2N
2
TAPC
92%
Solvent-free, 25 °C
S
S
Unchanged
Scheme 2
The resulting sulfenyl chloride (5) acts as a sulfur electrophile to couple with the thiolate
anion to yield the corresponding disulfide (2) and chloride anion (see Scheme 3). It is
significant that the by-product (4), is soluble in water and as a result, does not contaminate
the product. Simple extraction afforded pure product. Future work will include additional
studies of the scope and mechanism of this transformation, and the utilization of this
transformation to a variety of organosulfur chemistry processes.
In conclusion, we have successfully demonstrated a novel application of TAPC for the
efficient synthesis of disulfide derivatives. This simple procedure can be applied to a wide
variety of aromatic and aliphatic thiols. The ambient reaction conditions, easy availability of
starting materials, shorter reaction times, simple manipulation, and excellent product yields
make this catalytic system an alternative method for the synthesis of disulfide derivatives.
Moreover, because the reaction can be performed without solvent, any tedious work-up is
avoided. Further extensions of the reaction substrates and the investigation of the reaction
mechanism are in progress in our laboratory.

9
86
K. BAHRAMI ET AL.
Cl
P
SR
P
N
2
RSH + Cl
1
RS
N
−
2 HCl
(
X)2
(X)2
TAPC
3
SR
−
Cl
+
RS
P
N
P
N + RS Cl
−
−RS
5
(
X)2
(X)2
4
3
soluble in water
−
RS
+
RS Cl
RS SR
−
5
−Cl
2
Cl
P
X = Cl
N
Scheme 3
EXPERIMENTAL
Materials and Methods
The thiols, phosphorus pentachloride, and ammonium chloride were commercially
available. Melting points were determined in a capillary tube and are not corrected. The
1
13
H NMR and C NMR spectra were recorded on a Bruker-200 NMR spectrometer using
TMS as an internal standard.
Table 3 Comparison of some of the results obtained with TAPC and some reported methods
Entry
1
Substrate
ConditionsRef.
Yield%
Time
2d
This work
KO2/toluene/N2/25 C
PCC/solvent-free
93
72
92
97
90
47
99
93
89
86
90
95
87
87
86
2 min
12 h
1.2 h
1 h
2 min
1.7 h
2.6 h
2 min
1 h
◦
11d
12a
◦
11i
PhCH2Ph3PHSO5/CH3CN/80 C
2
2j
This work
12a
PCC/solvent-free
◦
11h
Al(NO3)3·9H2O/SiO2-SO3H/CH2Cl2/25 C
This work
◦
K3PO4/CH3CN/25 C
PCC/solvent-free
10a
3
4
2k
2l
12a
3 h
◦
11i
PhCH2Ph3PHSO5/CH3CN/80 C
2.5 h
2 min
24 h
4 h
This work
◦
18
MoO2Cl2/DMSO/70 C
PCC/solvent-free
12a
◦
12h
[
Rh(cod)2]BF4/8 PPh3/CH2Cl2/4 C
1 h

RAPID METHOD FOR SYMMETRICAL DISULFIDES
987
General Procedure for the Preparation of Disulfides
A mixture of thiol (1 mmol) and TAPC (0.5 mmol, 0.174 g) was prepared. The
mixture was stirred at room temperature for 2 min (Table 2). The progress of the reaction
was monitored by TLC. After the completion of the reaction, H2O (10 mL) was added to
the reaction mixture and filtered to afford pure product. In the case of liquid disulfides, the
residue was then extracted with EtOAc (4 × 5 mL), and the combined extracts were dried
over MgSO4. The filtrate was evaporated, and the corresponding disulfide was obtained as
the only product.
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