Chiral Brønsted acids in enantioselective carbonyl activations--activation modes and applications.

Chiral phosphoric acids and derivatives have attracted considerable attention as a powerful tool in asymmetric catalysis. Various enantioselective reactions have been developed by using these efficient Brønsted acid organocatalysts. Although initially the activation was restricted to reactive Brønsted basic substrates, recent reports are demonstrating the versatility of phosphoric acid catalysts in the activation of carbonyl compounds in a stereochemically controlled fashion. This tutorial review gives an overview of enantioselective Brønsted acid catalyzed transformations with the main focus on carbonyl activation. Different activation modes, key features of the catalysts and the applied substrates are presented and discussed with the goal to elucidate the origin of stereoselectivity in these Brønsted acid catalyzed transformations.