Recent Advances in Asymmetric Phase-Transfer Catalysis

The use of chiral nonracemic onium salts and crown ethers as effective phase‐transfer catalysts have been studied intensively primarily for enantioselective carbon–carbon or carbon–heteroatom bond‐forming reactions under mild biphasic conditions. An essential issue for optimal asymmetric catalysis is the rational design of catalysts for targeted reaction, which allows generation of a well‐defined chiral ion pair that reacts with electrophiles in a highly efficient and stereoselective manner. This concept, together with the synthetic versatility of phase‐transfer catalysis, provides a reliable and general strategy for the practical asymmetric synthesis of highly valuable organic compounds. Chiral ion pair lends a hand: Asymmetric phase‐transfer catalysis, as represented by chiral quaternary onium salts, has become an increasingly important method for the synthesis of optically active organic molecules. The multiple ion‐exchange process catalytically generates a chiral ion pair, whose chiral onium cation creates an effective asymmetric environment around the nucleophilic anion and enables stereoselective bond formation (see scheme).