CaF<sub>2</sub> Catalyzed Chemoselective Oxidation of Sulfides to Sulfoxides with Hydrogen Peroxide Under Solvent-Free Conditions

Background: Sulfoxides are useful intermediates in organic synthesis. Furthermore, many biologically active compounds and drugs have sulfoxide moieties in their structures. The most popular approach for the preparation of sulfoxides involve selective oxidation of the corresponding sulfides. Although a wide variety of reagent systems have been traditionally used to accomplish this transform. However, many of these procedures utilize environmentally unfavorable reagents, solvents, and catalysts that highlight the matter of eco-efficiency in our environmentally conscious times. The use of 'green oxidants' such as molecular oxygen and hydrogen peroxide is attractive, since they are readily available, inexpensive, and environmentally benign, with formation of water as the only by-product. CaF2 is commercially available, environmentally compatible, cheap, easy to handle, and stable. It was recently shown to be a good substitute for conventional acidic catalytic materials. Methods: The sulfides (1 mmol) were oxidized to the corresponding sulfoxides using 30% H2O2 (3.6 mequiv.) in the presence of catalytic amount of CaF2 (0.25 mmol) under solvent-free conditions at room temperature. The purification was performed using short column chromatography with EtOAc/n-hexane (1/10). All the products are known and were characterized by IR and 1H-NMR and by melting point comparison with those of authentic samples. Results: The structurally diverse sulfides (including aryl, benzylic, heterocyclic, allylic and linear) were subjected to oxidation. The reactions proceeded well to produce the corresponding sulfoxides in good to high yields ranging from 80 to 97 % in short times. thiantrene and diphenyl sulfide, which are relatively unreactive substrates, were also converted to the corresponding sulfoxides to high yields, oxidation of them require higher temperature. Interestingly, the sulfur atom was chemoselectively oxidized in the sulfides compounds that containing oxidation-prone and acid-sensitive functional groups. Conclusion: This procedure offers several major advantages: (1) the use of a commercially available, cheap, and chemically stabile catalyst and oxidant; (2) highly efficient for the selective oxidation of structurally diverse sulfides in good to high yields; (3) excellent chemoselectivity; and (4) the method conforms to several of the guiding principles of green chemistry. We believe the present method to be an improvement with respect to other procedures. Keywords: CaF2, catalyst, hydrogen peroxide, oxidation, selective, sulfide, sulfoxide.