Room-Temperature\nSecondary Ammonium Salt-Catalyzed\nEnantioselective Halogenation via Hydrogen-Bonded Ion Pair

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