Polymer supported reagents: Oxidative selection between thiols

Article abstract of DOI:10.1081/SCC-120020192

A very simple reaction is described for the efficient oxidative selection between thiols by Dowex l-X8, as a quaternary ammonium resin, on which Cl^- is replaced by dichromate and bisulfate ions (DDB).

Full text of DOI:10.1081/SCC-120020192

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Polymer Supported Reagents: Oxidative Selection
Between Thiols
F. Shirini a , M. M. Lakouraj b , I. Mohammadpour-Baltork c & D. Asadi a
a Department of Chemistry , College of Science , Guilan University , Rasht, Iran
b Department of Chemistry , College of Science , Mazandaran University , Babolsar, Iran
c Department of Chemistry , College of Science , Isfahan University , Isfahan, Iran
Published online: 17 Aug 2006.
To cite this article: F. Shirini , M. M. Lakouraj , I. Mohammadpour-Baltork & D. Asadi (2003) Polymer Supported Reagents:
Oxidative Selection Between Thiols, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 33:11, 1833-1837, DOI: 10.1081/SCC-120020192
To link to this article: http://dx.doi.org/10.1081/SCC-120020192
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 11, pp. 1833–1837, 2003
Polymer Supported Reagents: Oxidative
Selection Between Thiols
2
3
F. Shirini,1, M. M. Lakouraj, I. Mohammadpour-Baltork,
*
and D. Asadi1
1Department of Chemistry, College of Science, Guilan
University, Rasht, Iran
2Department of Chemistry, College of Science,
Mazandaran University, Babolsar, Iran
3Department of Chemistry, College of Science, Isfahan
University, Isfahan, Iran
ABSTRACT
A very simple reaction is described for the efficient oxidative selection
between thiols by Dowex 1-X8, as a quaternary ammonium resin, on
which ClÀ is replaced by dichromate and bisulfate ions (DDB).
Key Words: Dowex 1-X8; Oxidation; Polymer supported reagents;
Selection; Thiols.
*Correspondence: F. Shirini, Department of Chemistry, College of Science,
Guilan University, Rasht, Iran; E-mail: shirini@cd.gu.ac.ir.
1833
DOI: 10.1081/SCC-120020192
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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1834
Shirini et al.
Oxidative coupling of thiols to disulfides under heterogeneous,
neutral, and mild conditions is of importance from biological and
practical points of view.[1,2] Since thiols are easily oxidizable and may
thus be overoxidized, extensive studies have been carried out to discover
methods for their controlled oxidation.[3–15] However, some of the
reported reagents suffer fromdisadvantages such as long reaction time,
difficulty of preparation, difficult work-up, low selectivity, and instability.
Thus, a milder and more selective reagent is still in demand.
In this communication we report that Dowex 1-X8, on which ClÀ is
replaced by Cr2O27À; and HSO4À (DDB), as a previously reported oxidiz-
ing agent,[16] can act as a very efficient reagent for the coupling of thiols in
refluxing acetonitrile, in good to high yields (Table 1). Overoxidation of
the products was not observed by this method.
Table 1. Oxidative coupling of thiols with DDB in refluxing acetonitrile.
Entry
1
Substrate
Time (min)
15
Yield (%)a
90
2
3
4
5
10
5
92
85
90
85
25
40
b
6
7
20
30
—
75
80
8
90
9
90
90
0c
0c
10
aIsolated yield.
bPolymerization was occurred.
cReaction was not occurred and the unchanged starting material was obtained.

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Oxidative Selection Between Thiols
1835
Scheme 1.
Table 2. Comparison of some of the results obtained by our method (1) with
some of those obtained by bis[trinitratocerium(IV)] chromate (2),[6] pyridinium
[15]
chlorochromate (3),[6] and pyraziniumdichromate (4).
Time in hour (yield %)
Entry
Substrate
1
2
3
4
1
2
Thiophenol
2-Mercaptobenzothiazole
0.25 (90)
1.5 (80)
4 (90)
4 (80)
2 (85)
4 (50)
0.7 (91)
3.5 (94)
Benzylic thiols do not undergo oxidation with this reagent (Table 1,
Entries 9 and 10). Therefore this methodology shows selectivity and is
suitable for oxidative selection against thiols. This is exemplified by the
competitive reaction between thiophenol and benzylthiol (Sch. 1).
In order to show the efficiency of this method we have compared some
of the results with relevant ones reported in the literature (Table 2).[6,15]
In conclusion, this method is very simple and efficient for oxidative
selection between thiols. The easy procedure and simple work-up,
simplicity, selectivity, and short reaction time make this method a useful
addition to the present methodologies.
EXPERIMENTAL
All products were characterized through comparison of their spectral
and physical data with those of the known samples.[17] The purity
determination of the products was accomplished by TLC on silica gel
À
polygramSIL G/UV 254 plates. Dowex 1-X8 (Cl form, 20–50 mesh)
and other chemicals were purchased from Merck. Products were separated

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1836
Shirini et al.
and purified by different chromatography techniques, and were also
identified by the comparison of their m.p., IR and NMR spectra, b.p.,
refractive index with those reported for the authentic samples. The
capacity of the reagent was determined by atomic absorption technique
and titration method.
Preparation of DDB
To a solution of CrO3 (0.35 g, 3.5 mmol) in 0.5 M H2SO4 (40 mL),[18]
Dowex 1-X8 (5 g) was added and the mixture stirred for 0.5 h at room
temperature. The resulting dark-orange resin was filtered and washed
with acetone (2 Â 20 mL) and diethylether (2 Â 5 mL) and was finally
dired in vacuo at 50ꢀC for 0.5 h. The infrared spectrumof the dry
reagent showed bands at 930 and 765 cmÀ1, characteristic of dichromate
ion and bands at 820, 847, 877, 1045, 1180, 1220, 1420, and 3250 cmÀ1
characteristic of bisulfate ion.[19]
,
The capacity of the reagent was determined to be 0.25 mmol Cr2O72À
and 1.5 mmol HSOÀ4 per gramof the resin.
General Procedure for the Oxidation of Thiols by DDB
To a solution of thiol (1 mmol) in acetonitrile (5 mL), DDB (2 mmol,
based on capacities) was added and refluxed while stirred for 5–90 min.
Progress of the reaction was monitored by TLC. The reaction mixture
was cooled to the roomtemperature and filtered being followed by
repeated washing with acetonitrile (10 mL). Evaporation of the solvent
followed by column chromatography on silica gel gave the corresponding
disulfide fromgood to high yields.
Regeneration of Dowex 1-X8 in its ClÀ form: To an aqueous solution
of NaOH (2 M, 20 mL) the spent reagent (2 g) was added and the mixture
was stirred for 12 h. The suspension was filtered and washed repeatedly
with a solution of hydrochloric acid (2 M) and finally with water.
The precipitate was dried in vacuo at 80ꢀC for 6 h to give the original
quaternary ammonium resin characterized by its IR spectrum.
ACKNOWLEDGMENT
We are grathful to Guilan University Research Council for financial
support of this work.

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Oxidative Selection Between Thiols
REFERENCES
1837
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Received in the Netherlands August 23, 2002

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