An Improved Protocol for the Direct Asymmetric Aldol Reaction in Ionic Liquids, Catalysed by Onium Ion-Tagged Prolines

Two onium ion-tagged prolines, imidazolium bis(trifluoromethylsulfonyl)imide-substituted proline 6 and butyldimethylammonium bis(trifluoro-methylsulfonyl)imide-substituted proline 7, were synthesised and their catalytic activity in the direct asymmetric aldol condensation was studied in ionic liquids. For the reaction of acetone with various aldehydes, using 5 % of the catalyst, the yields of the aldols varied between 50-85 % while the ee values were in the 80-85% range. Other ketones were studied too, the yields obtained in those cases being in the 35-78 % range while the enantioselectivities varied between 75-94 %.