Aryl methyl sulfides asymmetric sulfoxidation with hydrogen

ASYMMETRIC SULFOXIDATION OF ARYL METHYL SULFIDES WITH HYDROGEN PEROXIDE IN WATER... 

Ghiral flavins have been used to obtain an asymmetric sulfoxidation with H2O2 as the oxidant [45a, 52]. Flavin 27, with planar chirality, was used to oxidize different aryl methyl sulfides with 35% H2O2. The hydrogen peroxide was added slowly over 5 days at —20°C to the substrate and the catalyst. The best result was obtained with the p-tolyl derivative, which gave 65% ee (Eq. (8.12)). 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

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