Abstract
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.
Highlights
The use of chiral ammonium salt phase-transfer catalysts (PTCs) has contributed significantly to the field of asymmetric catalysis.[1]
We report our efforts to obtain a new class of systematically modified bifunctionalurea-containing quaternary ammonium salts based on obtainable chiral backbones
The successful combination of their high potential to activate and control the reactivity of electrophiles combined with the Novacek and Waser unique nucleophile activation potential of quaternary ammonium salts should result in a remarkable bifunctional catalytic system
Summary
The use of chiral ammonium salt phase-transfer catalysts (PTCs) has contributed significantly to the field of asymmetric catalysis.[1]. Investigations concerning syntheses and applications of such catalysts are not common.[7,8,9] To the best of our knowledge, only one example describing the synthesis and application of a thioureacontaining cinchona alkaloid PTC[7] and two reports concerning urea-containing cinchona PTCs[8] have been published (Figure 1). During the preparation of this manuscript, the Zhao group reported an elegant strategy with which to access a diversified library of αamino acid based (thio)urea-containing quaternary ammonium salts that were found to be powerful catalysts for asymmetric aza-Henry reactions and the addition of thiols to imines (Figure 1).[9]
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