Modulating the acidity: Highly acidic Brønsted acids in asymmetric catalysis

Recently, chiral highly acidic Brønsted acids have emerged as powerful catalysts for enantioselective C-C and C-X bond-forming reactions. Their strong acidity renders them valuable tools for the activation of imines, carbonyl compounds, and other weakly basic substrates. As a result, new perspectives are opened and highly stereoselective transformations based on the concept of chiral contact-ion-pair catalysis can be realized. This Minireview gives an overview of the design and application of these new organocatalysts and presents recent results in this rapidly growing field. Chiral highly acidic Brønsted acids B*-H (see scheme, bottom) are efficient organocatalysts for numerous enantioselective reactions. Owing to their high acidity, these catalysts are suitable for the activation of imines, carbonyl compounds, and other weakly basic substrates. The formation of chiral contact-ion pairs (see scheme, top) implicates completely new modes of activation for asymmetric synthesis.