Chiral flavinium catalysts

Planar chiral flavinium salts-prospective catalysts for enantioselective sulfoxidation reactions. Ear. J. Org. 

The hydrogen peroxide-mediated oxidation of p-tolyl methyl sulfide with chiral iminium salt 20 (Figure 19.9), reported in 1993, gives a 32% ee [81]. The use of chiral flavinium salt catalysts afforded a 65% ee for the same transformation [119]. 

It should finally be mentioned that asymmetric and metal-free sulfoxidation can also be achieved by use of flavinium (84) [131] or simpler chiral iminium cations (85) as catalysts [132] (Scheme 10.18). For the axially chiral flavinophane 84 enantiomeric excesses up to 65% were reported by Toda et al. (for methyl p-tolyl sulfide as substrate), at typical catalyst loadings of ca. 10 mol% (relative to the thioether). The same substrate gave the sulfoxide with 32% ee when the iminium cation 85 was used [132],... 

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