Chiral Ammonium Betaines as Ionic Nucleophilic Catalysts

摘要

Asymmetric nucleophilic catalysis has been extensively studied over the last several decades and plays an increasingly important role in modern asymmetric synthesis. The general definitive feature of this catalysis is that a Lewis basic catalyst reacts with a substrate to give a reactive ionic intermediate through the formation of a new covalent bond, which will be eventually cleaved by the elimination of the catalyst. In this respect, an anionic molecule could function as a potentially more nucleophilic catalyst for initiating the reaction, compared to the commonly utilized electronically neutral molecules, but it generates a rather stable intermediate bearing no charge (see below). Hence, research toward exploiting the reactivity of anionic molecules for the development of a new type of nucleophilic catalysis, that is, enantioselective ionic nucleophilic catalysis, has met with limited success.