Hydrogen-bond-mediated asymmetric catalysis.

The utilization of hydrogen bonding as an activation force has become a powerful tool in asymmetric organocatalysis. Significant advances have been made in the recent past in this emerging field. Due to space constraints, this Focus Review summarizes only the key aspects with an emphasis on catalysis based on chiral ureas and thioureas, diols, and phosphoric acids. The examples provided neatly demonstrate that chiral ureas and thioureas, diols, and phosphoric acids display effective and unique activation modes of catalysis for a broad spectrum of asymmetric organic transformations, including single-step and multiple-step cascade reactions. These functionalities, which have the ability to afford efficient H-bond activation of electrophiles including C=O, C=N, aziridines, and epoxides, have established their status as "privileged" functional groups in the design of organocatalysts.