Oxidative coupling of thiols by pyridinium chlorochromate in solution and solvent free conditions

Article abstract of DOI:10.1081/SCC-100105325

Oxidative coupling of aromatic and aliphatic thiols is achieved efficiently by pyridinium chlorochromate in solution and solvent free conditions. Omitting the solvent does not change the reaction time and product yield significantly, while the need of using the solvent is suppressed and workup procedure becomes easier.

Full text of DOI:10.1081/SCC-100105325

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OXIDATIVE COUPLING OF THIOLS BY PYRIDINIUM
CHLOROCHROMATE IN SOLUTION AND SOLVENT FREE
CONDITIONS
a b
b
c
Peyman Salehi
, Afsaneh Farrokhi & Mostafa Gholizadeh
a
Department of Phytochemistry , Medicinal Plants Research Institute, Shahid Beheshti
University , Tehran, Iran
b
Department of Chemistry , Razi University , Kermanshah, Iran
c
Department of Chemistry , Teacher Training University , Sabzevar, Iran
Published online: 15 Aug 2006.
To cite this article: Peyman Salehi , Afsaneh Farrokhi & Mostafa Gholizadeh (2001) OXIDATIVE COUPLING OF THIOLS BY
PYRIDINIUM CHLOROCHROMATE IN SOLUTION AND SOLVENT FREE CONDITIONS, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 31:18, 2777-2781, DOI: 10.1081/SCC-100105325
To link to this article: http://dx.doi.org/10.1081/SCC-100105325
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SYNTHETIC COMMUNICATIONS, 31(18), 2777–2781 (2001)
OXIDATIVE COUPLING OF THIOLS
BY PYRIDINIUM CHLOROCHROMATE
IN SOLUTION AND SOLVENT
FREE CONDITIONS
1
,
1
Peyman Salehi, * Afsaneh Farrokhi, and
2
Mostafa Gholizadeh
1
Department of Chemistry, Razi University,
Kermanshah, Iran
2
Department of Chemistry, Teacher Training
University, Sabzevar, Iran
ABSTRACT
Oxidative coupling of aromatic and aliphatic thiols is achie-
ved efficiently by pyridinium chlorochromate in solution and
solvent free conditions. Omitting the solvent does not change
the reaction time and product yield significantly, while the
need of using the solvent is suppressed and workup procedure
becomes easier.
Many years have passed from the introduction of pyridinium chloro-
chromate (PCC) as a reagent for oxidation of primary and secondary alco-
1
hols. Due to its mildness and high selectivity, this oxidant has found wide
2
–4
applications in organic synthesis.
*Corresponding author. Department of Phytochemistry, Medicinal Plants Research
Institute, Shahid Beheshti University, Tehran, Iran.
2
777
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

ORDER
REPRINTS
2
778
SALEHI, FARROKHI, AND GHOLIZADEH
Oxidative coupling of thiols to disulfides under mild conditions is of
5
,6
importance from biological and practical points of view.
among functional groups which can over oxidized and therefore extensive
studies have been carried out for their controlled oxidation.
Thiols are
7
–19
In this paper we wish to report an efficient method for the conversion
of thiols to disulfides by the well known reagent, pyridinium chlorochro-
mate in solution and solvent free conditions.
Oxidative coupling of different types of mercaptans was investigated in
the absence of solvent by pyridinium chlorochromate (Scheme, Table). In an
easy procedure the reactants mixed together in a mortar and stood together
for the appropriate period (Table) without any further agitation. Both
aromatic and aliphatic thiols reacted efficiently and the corresponding dis-
ulfides isolated in good to excellent yields.
Scheme.
In order to compare the obtained results with those performing in
solution we tried to study the coupling reactions in dichloromethane. As
shown in Table there are not appreciable differences between the results
obtained in solution and neat conditions.
In conclusion omitting the solvent in oxidative coupling of thiols by
PCC does not change the reaction time and product yield significantly while
the need of using the solvent is suppressed and workup procedure becomes
easier. The method could be used for the synthesis of disulfides from corre-
sponding thiols in a preparative scale.
EXPERIMENTAL
All of the products are known compounds and were characterized by
1
comparison of their spectral data ( H NMR, IR) and physical properties
with those of authentic samples. The purity determination of substrates and
reaction monitoring were accomplished by TLC using silica gel polygram
SIL G/UV 254 plates. All yields refer to isolated products.

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OXIDATIVE COUPLING OF THIOLS
2779
Table. Oxidative Coupling of Thiols by Pyridinium Chlorochromate in Dichloro-
methane or Solvent Free Conditions
Solvent Free Oxidation Oxidation in CH
2
Cl
2
Entry
1
R-SH
Time (h)
Yield (%) Time (h) Yield (%)
1.9
94
91
92
1.5
1.6
1.4
97
2
3
1.9
1.2
89
90
4
2.1
93
2
90
5
6
7
1
93
91
86
0.9
1.9
1.9
97
95
94
1.8
3
8
9
0
1
CH
CH
3
(CH
(CH
2
)
)
3
SH
SH
4.2
4
1.3
1.7
88
87
55
47
4.4
4.2
1.7
2.2
89
91
62
50
3
2
7
1
1
HOCH CH SH
2
2
HOOCCH SH
2

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780
SALEHI, FARROKHI, AND GHOLIZADEH
General Procedure for Oxidative Coupling of Thiols by
Pyridinium Chlorochromate Under Solvent Free Conditions
Pyridinium chlorochromate (1 mmol) was added to thiol (1 mmol) in a
mortar. Starting materials were mixed and stood together for the appro-
priate period (Table) at room temperature. Progress of the reaction was
followed by dissolving a sample in acetone and using thin layer chromato-
graphy on silica gel (MeOH/HOAc: 5/1). After completion of the reaction
hydrochloric acid (20%) was added and the mixture extracted with ether
(
and dried (MgSO ). Final evaporation of solvent gave the products in
2 Â 20 mL) and chloroform (2 Â 20 mL). The organic layer was separated
4
47–94% yields. [Isolation of water soluble disulfides (Table, entries 10, 11)
was achieved by washing the sticky mixture with ether and chloroform
followed by chromatography on glass plates (Silica gel, CCl /Ether/
4
MeOH: 3/3/1)].
General Procedure for the Conversion of Mercaptans to
Disulfides by Pyridinium Chlorochromate in Dichloromethane
Pyridinium chlorochromate (1 mmol) was added to a solution of mer-
captan (1 mmol) in dichloromethane (10 mL). The mixture was stirred
magnetically at room temperature until the complete consumption of the
substrate. Solvent was evaporated on a rotary evaporator. Water was added
(
40 mL) and the mixture was extracted with ether (2 Â 20 mL) and chloro-
form (2 Â 20 mL). The organic layer was dried (MgSO ) and concentrated.
4
The corresponding disulfides were obtained in 50 to 97% yields. [Water
soluble products (Table, entries 10, 11) were isolated by evaporation of
the solvent followed by thin layer chromatography on glass plates (Silica
gel, CCl /Ether/MeOH: 3/3/1)].
4
ACKNOWLEDGMENT
We are grateful to Razi University Research Council for financial
support of this work.
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OXIDATIVE COUPLING OF THIOLS
2781
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Received in the Netherlands February 15, 2000

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