Chiral Organic Contact Ion Pairs in Metal-Free Catalytic Asymmetric Allylic Substitutions

Abstract

Chiral contact ion-pair catalysis with particular focus on metal-free processes is gaining in interest. As a result, new perspectives are opened, and highly stereoselective transformations, traditionally performed under metal catalysis, can be realized. Herein, we report the development of an unprecedented asymmetric Brønsted acid-catalyzed allylic alkylation. The concept relies on chiral contact ion-pair catalysis, in which the chiral organic counteranion of an allylic carbocation induces high enantioselectivities and allows access to biologically relevant chromenes in good yields and with excellent enantioselection.

Asymmetric Ion‐Pairing Catalysis

“…[90] Again, N -triflylphosphoramide catalysts were found to be more reactive and enantioselective than phosphoric acid catalysts. The transformation was proposed to proceed via chiral contact ion-pair catalysis: protonolysis of the allylic alcohol by the Brønsted acid catalyst followed by intramolecular attack on the resulting chiral allyl cation•phosphoramidate ion pair.…”

Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

“…[50] Eine asymmetrische allylische Alkylierung mit NTPAs als Brøn-sted-Säurekatalysatoren wurde kürzlich beschrieben. [51] Der Test verschiedener BPs und NTPAs zeigte, dass NTPAs vielversprechende Katalysatoren für die enantioselektive Synthese von 2H-Chromen 84 a ausgehend vom Phenolderivat 83 a sind (Schema 38). Das 3,3'-Phenyl-subsitutierte H 8 -NTPA 18 a ergab hierbei die besten Ausbeuten und Selektivitäten.…”

Chiral Brønsted acid catalyzed enantioselective intermolecular allylic aminations

“…By increasing the catalyst loading to 10 mol%, up to 75% ee was obtained (Table 1, entry 4). Solvents were found to influence this reaction significantly, and CHCl 3 gave the optimal results (Table 1, entries [4][5][6][7][8]. Various amine nucleophiles were screened for this reaction.…”