Alloxazine–cyclodextrin conjugates for organocatalytic enantioselective sulfoxidations

Abstract

Four structurally different alloxazine-cyclodextrin conjugates were prepared and tested as catalysts for the enantioselective oxidation of prochiral sulfides to sulfoxides by hydrogen peroxide in aqueous solutions. The alloxazinium unit was appended to the primary face of α- and β-cyclodextrins via a linker with variable length. A series of sulfides was used as substrates: n-alkyl methyl sulfides (n-alkyl = hexyl, octyl, decyl, dodecyl), cyclohexyl methyl sulfide, tert-butyl methyl sulfide, benzyl methyl sulfide and thioanisol. α-Cyclodextrin conjugate having alloxazinium unit attached via a short linker proved to be a suitable catalyst for oxidations of n-alkyl methyl sulfides, displaying conversions up to 98% and enantioselectivities up to 77% ee. β-Cyclodextrin conjugates were optimal catalysts for the oxidation of sulfides carrying bulkier substituents; e.g. tert-butyl methyl sulfide was oxidized with quantitative conversion and 91% ee. Low loadings (0.3-5 mol%) of the catalysts were used. No overoxidation to sulfones was observed in this study.