as a Heterogeneous Recyclable Catalyst for the Air Oxidation of Thiols Under Solvent Free Conditions

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.


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].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, [NaP 5 W 30 O 110 ] 14-, Na + is encrypted inside a central cavity formed by five PW 6 O 22 units arranged in a crown [8], as shown schematically in Figure 1.Recently we have explored the application of Preyssler catalyst in various organic reactions [8][9][10][11][12][13][14].This paper reports the results of the oxidation of thiols to disulfides utilizing [NaP 5 W 30 O 110 ] 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.

Results and Discussion
Previously it was shown that in the oxidations of anilines with H 2 O 2 catalyzed by Preyssler and Keggin HPA catalysts, in order to compare the stability of the Preyssler catalyst structure in the presence of H 2 O 2 , 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 H 2 O 2 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 H 2 O 2 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.

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 catalytic capability of this useful polyanion.IR spectra of the resulting solids indicate that the catalyst can be recovered without structural degradation (Figure 2).

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, [NaP 5 W 30 O 110 ] 14-, a heteropolyanion with interesting catalytic properties as a green, reusable and eco-friendly catalyst.

General
Melting points were measured by using the capillary tube method with an Electrothermal 9200 apparatus. 1H-NMR spectra were recorded on a Bruker AC-80 MHZ spectrometer using TMS as an internal standard (CDCl 3 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 H 14 -P 5 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].

Table 2 .
Oxidation of thiols to disulfides.