Hydrogen-Bonding Catalysis of Alkyl-Onium Salts.

The synthetic utility of alkyl-onium salt compounds is widely recognized in the field of organic chemistry. Among the wide variety of onium salts, quaternary ammonium, phosphonium, and tertiary sulfonium salts have been the most useful compounds in organic syntheses. These compounds have been very useful reagents in the construction of organic building blocks. In addition, onium salts are known as reliable catalysts, which are used to promote important organic transformations by serving as phase-transfer and ion-pair catalysts through the activation of nucleophiles. Although phase-transfer catalysis is a major direction for onium salt catalysis, hydrogen-bonding catalysis of alkyl-onium salts, which is promoted via the activation of electrophiles, has recently become a relevant topic in the field of onium salt chemistry. This Minireview introduces new possibilities and future directions for alkyl-onium salt chemistry based on its use in hydrogen-bonding catalysis and on its overall utility.