The selective oxidation of sulfides to sulfoxides is an important transformation,1 since sulfoxides are useful building blocks especially as chiral auxiliaries.2,3 Furthermore sulfoxides also play an important role for the acquisition of many biologically and chemically important compounds. A number of oxidizing agents such as hydrogen peroxide (H2O2), sodium periodate (NaIO4), m-CPBA, ozone, oxygen and r-BuOOH have been employed. Although, H2O2 is environmentally benign and NaIO4 may be stored and used safely, they oxidize sulfides slowly; for example, NaIO4 requires 12 hours or more, while H9O2 in the absence of catalyst requires 18 hours or more. Moreover H2O2 should be used in a controlled manner to avoid over-oxidation. Therefore most of the oxidizing agents are either hazardous peroxy acids or involve toxic heavy metals or rare oxidants that are difficult to prepare thus detracting their practical application in many cases. Hydrogen peroxide in the presence of complexes or salts of metals such as vanadium, molybdenum, tungsten as catalysts selectively furnishes sulfoxides. A variety of oxidants in combination with metal Schiff base complexes as catalysts have also been used for this purpose; however, this method has low operational stability due to facile oxidative degradation of porphyrin rings, phthalocyanines and salens. Thus, many of the reagents and catalysts used suffer from drawbacks such as over-oxidation, long reaction times, low selectivity and low yields. Oxidizing agents based on quaternary ammonium salts have been exploited in many organic reactions due to their solubility in several solvents which provide advantages in terms of mildness, operational simplicity, selectivity, high reaction rates, low reaction temperatures and absence of side-reactions. However, some of them require catalysts for activation. For example, tetrabutylammonium periodate (TBAPI) alone is not able to convert sulfides to sulfoxides. In the presence of a Lewis acid (AICl3 or BF3·Et2O) as catalyst and at reflux temperature, TBAPI oxidizes sulfides to sulfoxides in CH3CN. Cetyltrimethylammonium bromide (CTAB) is also reported to catalyze the oxidation of sulfides to sulfoxides using hypervalent iodine reagents.
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