Unifying metal and Brønsted acid catalysis--concepts, mechanisms, and classifications.

Asymmetric catalysis is a key feature of modern synthetic organic chemistry. Traditionally, different combinations of ligands and metals are used to perform highly enantioselective reactions. Since the renaissance of organocatalysis in the early 2000s, tremendous improvement in the field of metal-free catalysis has been achieved. Recently, the combination of transition metals and organocatalysts has allowed the development of new protocols enabling transformations that could not previously be realized. This article aims to present the latest contributions in the field of combined chiral Brønsted acid and metal catalyzed reactions, highlighting the advantages of these catalytic systems as well as describing the uncertainties regarding the molecular structure of the catalytically active species and the reaction mechanisms.