Room-Temperature Secondary Ammonium Salt-Catalyzed Enantioselective Halogenation via Hydrogen-Bonded Ion Pair

Abstract

Quaternary ammonium salts play an important role in asymmetric catalysis. However, the asymmetric induction in this class of catalytic cycle is often inefficient because the orientation of the counteranion is not well-defined due to the tetrahedral geometry of the quaternary ammonium cation. Herein, we report a proof-of-concept study on formulating hydrogen-bonded ion-pair catalysis for asymmetric halogenation. Secondary ammonium salt catalyzed the asymmetric bromination-induced semipinacol rearrangement and halocyclization reactions under mild conditions. The phosphate anion interacts with the ammonium cation tightly via a strong bidentate noncovalent interaction that results in asymmetric induction with high directionality. Mechanistic studies suggest that the hydrogen-bonded ion pair was responsible for the high enantioselectivity.