Syntheses and Applications of (Thio)Urea‐Containing Chiral Quaternary Ammonium Salt Catalysts

We herein report our efforts to obtain a new class of systematically modified bifunctional (thio)urea‐containing quaternary ammonium salts based on easily obtainable chiral backbones. Among the different classes of catalysts that were successfully synthesized, those based on trans‐1,2‐cyclohexane diamine were found to be the most powerful for the asymmetric α‐fluorination of β‐keto esters. Selectivities up to 93:7 could be obtained by using only 2 mol‐% of the optimized catalyst. The importance of the bifunctional nature of these catalysts was demonstrated by control experiments using simplified monofunctional catalyst analogues, which gave almost racemic product only.