Chiral Ureas, Thioureas, and Squaramides in Anion‐Binding Catalysis with Co‐catalytic B rønsted/ L ewis Acids

This chapter describes a relatively new strategy in anion-binding catalysis, involving cooperative action of an achiral Brønsted or Lewis acid with a chiral co-catalyst. The chiral co-catalysts are typically dual H-bond donors such as thiorueas or squaramides capable of anion-binding the conjugate base or counterion of the achiral acid to form chiral ion pair intermediates. Although only a few examples of this strategy have been identified to date, the strategy holds potential for broad application to the synthesis of important chiral compounds via cationic intermediates.