Phase-Transfer Catalysis in Oxidations Based on the Covalent Bonding of Hydrogen Peroxide to Amphiphilic Flavinium Salts

The amphiphilic flavin derivatives 1,10-ethylene-bridged 3-alkylalloxazinium salts (alkyl=butyl, octyl, dodecyl and hexadecyl) were shown to catalyse the chemoselective oxidation of sulfides to sulfoxides with hydrogen peroxide in chloroform/water or dichloromethane/water systems. The efficiency of the catalytic oxidation depended substantially on the pH of the aqueous phase to reach maximum values at pH7. Under such conditions, the complete conversion of aliphatic and aromatic sulfides occurred within a maximum of 24h, whereas the non-catalysed reaction gave only traces of products. This new concept of two-phase oxidation utilises the activation and the increase in the lipophilicity of hydrogen peroxide by its covalent bonding to the alloxazinium catalyst. A practical application in the synthesis of modafinil was demonstrated.