Chiral quaternary phosphonium salts as phase-transfer catalysts for environmentally benign asymmetric transformations
Phase-transfer catalysis has been recognized as a powerful and green sustainable method for organic synthesis in both industry and academia. Since the pioneering studies on enantioselective phase-transfer reactions promoted by cinchona alkaloid-derived tetraalkylammonium salt catalysts, this research field has served as an attractive area for the pursuit of green sustainable chemistry. Although various types of chiral quaternary ammonium salts have been developed for highly selective asymmetric phase-transfer reactions in the last three decades, examples of chiral quaternary phosphonium salts as onium salt catalysts are limited, and effective catalysts have been developed in relatively recent years. This review summarizes the development of chiral quaternary phosphonium salts as phase-transfer and related catalysts for environmentally benign asymmetric transformations.