Dual role of ionic liquids as phase transfer catalyst and solvent for glycosidation reactions

Abstract

This report describes the dual role of ionic liquids as phase transfer catalysts and reaction media in heterogeneous glycosidation reactions. Thorough study using a diverse set of ionic liquids provided insight into the relationship between ionic liquid structure and catalytic activity in these reactions. For example, glycosidation was efficient in ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate (HxMIm.PF₆), and the O- and S-glycosides were produced exclusively in moderate to good yields. As an outcome of the preliminary screen, a tailored novel ionic liquid, 1-hydroxyhexyl-3-methylimidazolium hexafluorophosphate (HOHxMIm.PF₆) was rationally designed to be immiscible with water and traditional organic solvents. This provided an advantage in ionic liquid recycling and product recovery via convenient triphasic extraction. The versatility of this methodology was demonstrated through glycosidation reaction on a wide variety of substrates including phenols (17–79%), thiophenols (24–97%), chalcone (44%), and flavone (50–67%). Furthermore, this study shows that the ionic liquid could be employed for at least three runs without apparent loss in activity.