Planar Chiral Flavinium Salts - Prospective Catalysts for Enantioselective Sulfoxidation Reactions

A novel planar chiral flavinium salt, 3-benzyl-5-ethyl-10-(8-phenylnaphthalen-1-y])isoalloxazinium perchlorate (2b), which bears a phenyl cap that covers one side of the isoalloxazinium skeleton plane, has been prepared as a potential catalyst for the enantioselective H2O2 oxidation of sulfides. The rate of H2O2 oxidation of sulfides in the presence of racemic 2b is comparable to that of the reaction catalysed by 5-ethyl-3,10-dimethylisoalloxazinium perchlorate, which indicates that the bulky shielding substituent does not influence the catalytic activity of the flavinium unit. The turnover frequency for the oxidation of thioanisole with hydrogen peroxide with 2b is 870 h(-1). The enantiomerically pure salts (+)-2b and (-)-2b were prepared from the pure enantiomers (+)-3b and (-)-3b of 3-benzyl-10-(8-phenylnaphthalen-1-yl)isoalloxazine (3b) obtained by HPLC separation of racemic 3b on a chiral stationary phase. The enantiomerically pure salts (+)2b and (-)-2b catalyse the H2O2 oxidation of par-substituted thioanisoles with enantiomeric excesses of 34-44 %. The highest enantioselectivity (54 % cc) was observed in the oxidation of methyl naphthyl sulfide.