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Selective and mild oxidation of suldes to sulfoxides by H2O2 using DBUH-Br3 as catalyst DBUH-Br3 catalyzed selective conversion of suldes to sulfoxides in the presence of H2O2 as oxidizing agent is described. The reaction was performed selectively at room temperature and relatively short reaction times. Sulfoxides are important intermediates in the synthesis of various natural products . Omeprazole and Figpronil pesticides are two typical examples of extensive applications of these important intermediates in pharmaceutical and nechemical industries . Syntheses of sulfoxides are usually achieved by oxidation of their corresponding suldes. During the last years, various procedures for H2O2 oxidation of suldes including the use of various heterogeneous catalyst systems have been reported . The application of this oxidant has been developed due to its effective oxygen content,eco-friendly nature, low cost, and also safety in storage and operation . Recently various heterogenous catalyst-oxidizing agent systems including gold(III)–hydrogen peroxide silica-immobilized vanadyl alkyl phosphonate–sodium bromate, silica sulfuric acid–hydrogen peroxide , and titanium based catalyst-oxidizing agents have been reported for oxidation of suldes. Although, many reagents have been employed for the oxidation of suldes to sulfoxides,the most common problem encountered during the reaction is the over-oxidation of sulfoxides to their corresponding sulfones . Prompted by these reports and due to the potential oxidation of sulfur-containing compounds by H2O2, in the presence of heterogenous catalysts, in this study we wish to report on a new and efcient method for selective oxidation of suldes to sulfoxides by H2O2 as oxidant and in the presence of DBUH-Br3 as catalyst Experimental All chemicals were purchased from Merck and Fluka chemical companies. The oxidation products were characterized by comparison of their spectral and physical data with authentic samples. 1.1. General procedure for the selective oxidation of suldes to sulfoxides To a mixture of various suldes (1 mmol) and DBUH-Br3 (0.1 mmol, 0.039 g) in CH3CN (15 mL), H2O2 30% (v/v)(1.4 mmol) was added gradually. After the completion of the reaction which was monitored by TLC using n-hexane:EtOAc (7:3), the solvent was removed and CH2Cl2 (20 mL) was added and the mixture was washed with water(15 mL), 5% HCl solution (15 mL) and water (15 mL), respectively. The organic phase was then dried over anhydrousNa2SO4 and the solvent was removed under reduced pressure. The resulting products were determined by their 1HNMR spectrums and compared with the authentic samples. The synthesis and X-ray structure of DBUH-Br3 as the catalyst have already been reported elsewhere (Fig. 1)[13].In order to get an insight into the optimum catalytic conditions for the selective oxidation of suldes to sulfoxides,the oxidation reaction of methylphenylsulde by hydrogen peroxide as a model experiment was studied at different temperatures and catalyst/H2O2 ratios. The results are summarized in Table 1. Different solvents were also screened. or the oxidation of methylphenylsulde, and as the result acetonitrile was found to be the best solvent in terms of activity and selectivity. In order to show the catalytic role of DBUH-Br3, the reaction of methylphenylsulde was performed in the absence of DBUH-Br3. The reaction was not completed within 24 h . Consequently, it was performed in different molar ratios of the catalyst in the presence of oxidant in order to obtain the optimum reaction conditions . The chemoselectivity of the system was also investigated. Several suldes containing an unsaturated C–C bond,nitrile or an alcoholic group were subjected to the sulfoxidation reaction, and it was observed that the functional groups remained intact during the reaction . The suggested mechanism of this selective oxidation is shown in Scheme 2. DBUH-Br3 might in situ generate Br2.Subsequently, Br+ can be produced via oxidation with H2O2. The nal step of oxidation involves nucleophilic attack of sulfur of the sulde compound on the Br+ followed by concerted oxygen transfer from water to give the corresponding sulfoxide. In summary, we have described an efcient method for the selective oxidation of suldes to sulfoxides under very mild reaction conditions. Also, chemoselectivity, easy and clean work-up, green oxidant, high yields and the easy preparation of the catalyst were the remarks of this method which could be the priority of the other existing methodologies. |
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