Selective sulfoxidation

The synthesis of sulfoxides from sulfides has been widely explored, and numerous oxidants have been investigated to achieve an efficient and selective sulfoxidation [38]. However, most of the reagents require carefully controlled conditions, including the quantity of oxidants, to avoid the formation of sulfone side products. Control to avoid formation of sulfones is particularly difficult since the first oxidation to sulfoxides requires relatively high energy [38]. m-Chloroperbenzoic acid (mCPBA) has been intensively used in the synthesis of prazole derivatives [39]. 

Sulfoxides are easily obtained by oxidation of sulfides. They can be further oxidized to sulfones. Many reagents have been used, and the possibility to synthesize selectively sulfoxides and sulfones, often in the presence of other sensitive functionalities, is an important feature for synthetic applications. The reaction is well documented [75], and only a few notable aspects are mentioned here. 

Another reaction of heteroatom oxidation is that of S-oxidation, which leads to the synthesis of sulfoxides, a reaction not very common in the plant cell biochemical factory. Enantiomerically pure sulfoxides are important chiral synthons in asymmetric synthesis, in particular in enantio-selective carbon-carbon bond formation [77]. The sulfoxide functional group is involved in different biological activities, and optically pure sulfoxides are of great pharmaceutical interest [82]. However, plant peroxidases, such as horseradish peroxidase, catalyze the enantio-selective sulfoxidation of alkyl aryl sulfides ... 

Table 2 Physical properties of selected sulfoxides and sulfones...

Selective sulfoxidation of thioethers on Ti-containing zeolites under mild conditions... 

Catalyst Substrate Conversion /%/ Selectivity Sulfoxide (%f Sulfone % ... 

The material 6 showed a remarkable catalytic activity in the oxidation of thioethers 13 to sulfoxides 14 by urea hydroperoxide (UHP) or H O (Scheme 2). Although the conversion and selectivity (for 14 over 15, >90%) was reasonable with UHP for the substrates with smaller substituents, 13a and 13b, the ones with bulkier substrates 13c and 13d failed to produce any measurable conversion. The conversion increases to 100% by changing UHP with H O. The catalytic activity of 6 for selective sulfoxidation remains similar even after 30 cycles. Despite the fact that no asymmetric induction was found in the catalytic sulfoxidations, enantioen-riched sulfoxides were obtained by enantioselective sorption of the resulting racemic mixture by the chiral pores of 6, which occured simultaneously with the catalytic process. Thus, after catalytic oxidation of 13a, (5)-14a was preferentially absorbed by the pore of 6 leaving exactly equal amount of the excess ) -enantiomer in the solution phase (-20% ee). The combination of high catalytic activity and enantioselective sorption property of 6 provides a unique opportunity to device a one-step process to produce enantioenriched products. 

The oxidative decontamination process for mustard, a CWA threat of major concern, targeted by us and others, is selective sulfoxidation, eq 1, because the sulfoxide has been known for decades to be far less toxic than mustard itself (2, 3). 

Heterogeneous catalytic oxidation of sulfides in ionic liquids by anhydrous H2O2 or urea hydroperoxide with MCM-41 and related mesoporous catalysts containing Ti or Ti and Ge was studied by Hardacre and coworkers [51]. The Ti-based catalyst gave a quite selective sulfoxidation. Addition of Ge to Ti increased the rate of the oxidation but reduced the selectivity toward sulfoxide [51a]. 

The screening of a metagenomic library derived from loam soil afforded a novel two-component SMO, which has been heterologously expressed in E. coli [52], This biocatalytic system was able to perform selective sulfoxidations of alkyl aryl sulfides, providing the recovered (R)-sulfoxides with good to high selectivities and moderate conversions after 16h. Ethyl phenyl sulfide was the best substrate for this enzyme, as the final sulfoxide was obtained with 92% ee. It therefore represents an interesting SMO alternative. 

Scheme 8.8 Selective sulfoxidation of thioanisole under sunlight with Rose Bengal (Reprinted with permission [97]. Copyright 2013 Royal Society of Chemistry)...

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