Amino alcohol-modified β-cyclodextrin inducing biomimetic asymmetric oxidation of thioanisole in water

Abstract

Inspired by β-CD, a macrocyclic oligomers of D-(+)-glucopyranose and a renewable material, which could be obtained from starch, that can promote a lot of organic reactions in water, a green solvent, several amino alcohol-modified β-CDs CD-1 to CD-7 were synthesized in the yields of 36-61%. Their conformations in vacuum and in aqueous solution were optimized by quantum calculation. Their complexes with sodium molybdate prepared in situ were characterized by (1)H NMR and were applied in the asymmetric oxidation of thioanisole. Their performance in inducing enantioselectivity was investigated in detail. For the optimal one, CD-1, moderate enantioselectivity (56% ee) was achieved in aqueous CH(3)COONa-HCl buffer solution (pH 7.0). The abilities of CD-1 to CD-7 to induce asymmetry are highly dependent on the pH value of the reaction medium and the structure of the modifying group. The origin of the moderate enantioselectivity and the reaction mechanism were investigated with the aid of (1)H ROESY NMR studies and quantum calculation. The moderate enantioselectivity was attributed to the two different binding models between CD-1 and thioanisole, which could be defined as intramolecular catalysis and intermolecular catalysis, in which intramolecular catalysis gave (S)-methyl phenyl sulfoxide and intermolecular catalysis gave (R,S)-methyl phenyl sulfoxide.