H14[NaP5W30O110] as a heterogeneous recyclable catalyst for the air oxidation of thiols under solvent free conditions

Article abstract of DOI:10.3390/12092223

The catalytic oxidation of thiols to the corresponding disulfides using Preyssler's catalyst H₁₄[NaP₅W₃₀O ₁₁₀] has been studied. These highly selective oxidations gave good yields of the target disulfides.

Full text of DOI:10.3390/12092223

Molecules 2007, 12, 2223-2228
molecules
ISSN 1420-3049
© 2007 by MDPI
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Communication
H14[NaP5W30O110] as a Heterogeneous Recyclable Catalyst for
the Air Oxidation of Thiols Under Solvent Free Conditions
Rahim Hekmatshoar 1,*, Sodeh Sajadi 1, Majid M. Heravi 1 and Fatemeh F. Bamoharram 2
1 Department of Chemistry, School of Science, Alzahra University, Vanak, Tehran, Iran;
E-mails: sodehsadjadi@yahoo.com; mmh1331@yahoo.com
2 Department of Chemistry, School of Sciences, Azad University, Khorasan Branch, Mashad, Iran;
E-mail: fbamoharram@mshdiau.ac.ir
* Author to whom correspondence should be addressed; E-mail: rhekmatus@yahoo.com; Tel.:(+98)-
21-88044040; Fax: (+98)-21-88035187
Received: 12 June 2007; in revised form: 15 September 2007 / Accepted: 16 September 2007 /
Published: 25 September 2007
Abstract: The catalytic oxidation of thiols to the corresponding disulfides using
Preyssler’s catalyst H14[NaP5W30O110] has been studied. These highly selective oxidations
gave good yields of the target disulfides.
Keywords: Disulfides; green catalyst; heteropolyacid; Preyssler; reusable catalyst.
Introduction
The conversion of thiols to disulfides is important from both the biological and synthetic chemistry
points of view. In biological systems, thiols are oxidized by flavins, cytochromes and dehydroascorbic
acid to control the cellular redox potential and prevent oxidative damage. On the other hand, disulfides
are important reagents in organic synthesis and can be used to prepare sulfinyl and sulfenyl
compounds [1]. Several methods based on oxidative S-S coupling have already been reported. These
include, among others, cerium (IV) salts, transition metal oxides [2], air in combination with transition
metal catalysts [3], ferric chloride, sodium chlorite, nitric oxide, hydrogen peroxide, halogens [4] and
heterogeneous permanganate [5].

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Some of these methods suffer from obvious disadvantages such as long reaction times, limited
availability of the oxidant, toxicity of reagents and difficult isolation of products. For example,
halogens are inexpensive, but they are nonselective, the hydrogen halide by-products complicate the
oxidation reaction and they aren’t particularly safe [4]. Consequently, the introduction of readily
available, safe and stable reagents for the oxidation of thiols to disulfides is still a necessity.
The catalytic function of heteropolyacids (HPAs) and related polyoxometalate compounds have
attracted much attention, particularly over the last two decades [6]. Polyoxametalates (POMs) are a
class of molecularly define organic metal-oxide clusters, possessing intriguing structures and diverse
properties [7]. These compounds exhibit high activity in acid-base type catalytic reactions, hence they
are used in many catalytic areas as homogeneous and heterogeneous catalysts. The application of the
Preyssler catalyst is mostly limited and only a few demonstrations of its catalytic activity have been
reported. This heteropolyacid nevertheless has important advantages such as: strong Bronsted acidity
with 14 acidic protons, high thermal stability, high hydrolytic stability (pH 0–12), reusability, safety,
low quantity of waste, separability, low corrosiveness, high oxidation potential and general
“greenness”, along with an exclusive structure, which have attracted much attention on this catalyst. In
the so called Preyssler’s anion, [NaP5W30O110]14-, Na+ is encrypted inside a central cavity formed by
five PW6O22 units arranged in a crown [8], as shown schematically in Figure 1.
Figure 1. Preyssler structure.
Recently we have explored the application of Preyssler catalyst in various organic reactions [8-14].
This paper reports the results of the oxidation of thiols to disulfides utilizing [NaP5W30O110]14- as
catalyst. The greatest advantages of this procedure (Scheme 1) arise from its “greenness”, reusable
catalyst, high yields, the fact that in most cases the reaction was solvent free and side reaction products
resulting from acid catalysis were not found.
Scheme 1.
H14 [NaP5W30O110
]
R-S-S-R
2 RSH
heat, air

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Results and Discussion
Previously it was shown that in the oxidations of anilines with H2O2 catalyzed by Preyssler and
Keggin HPA catalysts, in order to compare the stability of the Preyssler catalyst structure in the
presence of H2O2, with respect to that of Keggin HPAs, separation of the Preyssler catalyst was
perfoemd after the catalytic reaction. Interestingly, in contrast to Keggin HPAs, Preyssler’s anion
catalyzes oxidations of primary aromatic amines by H2O2 without any degradation of its structure, a
very important feature in catalytic processes, specially, in industry. It is well known that Keggin HPAs
at 60–70 °C in the presence of H2O2 are degraded to peroxopolyoxometalates and the reaction actually
takes place via the oxygen atom transfer from the peroxopolyoxometalate to the substrate.
Consequently we were interested in developing of applications of the Preyssler catalyst in other
oxidation reactions because of its exclusive structure and properties.
In our studies it has been found that Preyssler’s anion catalyzes the air oxidation of thiols to
disulfides in excellent yields in a solvent free system. The results of attempted oxidations of these
thiols are presented in Table 1. Separation of the Preyssler catalyst was made after the catalytic
reaction. Both alkyl and aryl thiols gave similar results. Sterically hindered thiols took slightly longer
to react. We also found that the presence of an electron-donating or an electron-withdrawing group on
the aromatic ring did not affect upon the reaction times and yields.
Table 2. Oxidation of thiols to disulfides.
Entry
Thiol
p-Tolylthiol
Disulfide
Di-p-ditolyldisulfide
Diphenyldisulfide
Yield/%
98
1
2
Thiophenol
97
3
Naphtylthiol
Di-naphtyldisulfide
Dicyclohexyldisulfide
Bis[p-chlorophenyl]disulfide
Diethyldisulfide
91
4
Cyclohexylthiol
p-Chlorophenylthiol
Ethanethiol
95
5
97
6
92
7
Propylthiol
Dipropyldisulfide
95
8
Octylthiol
Dioctyldisulfide
98
9
Phenyl-methanethiol
sec-Butylthiol
Dibenzyldisulfide
94
10
Di-sec-butyldisulfide
97
Reusability of the Catalyst
In order to know whether the catalyst would succumb to poisoning and lose its catalytic activity
during the reaction, we investigated the reusability of the catalyst. For this purpose we first carried out
the reaction in the presence of the catalyst. After completion of the reaction, the catalyst was removed
and washed with diethyl ether and subjected to a second run of the reaction process with the same
substrate. The first experiment and subsequent experiments gave almost identical yields. We have thus
found that Preyssler catalyst can be reused several times, without any appreciable loss of activity and
after recovery the catalytic activity was decreased only 2–4%, pointing to the stability and retention of

Molecules 2007, 12
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catalytic capability of this useful polyanion. IR spectra of the resulting solids indicate that the catalyst
can be recovered without structural degradation (Figure 2).
Figure 2. IR spectrum of Preyssler’s anion before (a) and after (b) use.
Conclusions
In conclusion, a new and efficient method for oxidation of thiols developed. The significant
features of the system are as follows: less toxic and hazardous chemicals can be used; high yields are
obtained without side products; using of Preyssler’s anion, [NaP5W30O110]14-, a heteropolyanion with
interesting catalytic properties as a green, reusable and eco- friendly catalyst.
Experimental
General
Melting points were measured by using the capillary tube method with an Electrothermal 9200
1
apparatus. H-NMR spectra were recorded on a Bruker AC-80 MHZ spectrometer using TMS as an
internal standard (CDCl3 solution). IR spectra were recorded from KBr disk on the FT-IR Bruker
Tensor 27. GC spectra were carried out on a Network GC System-Agilent 5973 spectrometer. All
products gave satisfactory spectral data in accord with the assigned structures [4].
Catalyst Preparation
H14-P5 was prepared by passage of a solution of the potassium salt in water through a column (50
cm×1 cm) of Dowex50W×8 in the H+ form and evaporation of the eluate to dryness under vacuum
[15].

Molecules 2007, 12
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Oxidation of Thiols: General Procedure
In a sample run, a dry 25 mL round bottomed flask was charged with thiol (1 mmole) and 0.01
mmole of heteropolyacid. The reaction mixture was heated to 90 °C in an oil bath. For solid thiols
chloroform was used as solvent. Time of reaction was between 1-2 hours, the progress of reaction was
monitored by TLC (chloroform was used as solvent for TLC). After completion of the reaction, the
mixture was diluted with chloroform and catalyst was removed by simple filtration. After evaporation
of solvent, products were obtained in good yields.
Acknowledgements
The authors are thankful from Azzahra Research Council for the partial financial support.
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