Design Strategies for Enhanced Hydrogen‐Bond Donor Catalysts

Dual hydrogen‐bond donor (HBD) scaffolds can be strategically designed to enable their optimal performance as catalysts. The deliberate installation of activating groups on HBDs containing conventional functionalities, such as ureas and thioureas, has emerged as one fruitful direction in the identification of catalysts with enhanced activity and improved stereocontrol. Alternatively, new families of HBDs based on structurally unique, inherently more acidic functional groups, such as squaramide and aminopyridinium systems, have proven advantageous over conventional HBD catalysts in select processes. The appropriate incorporation of activating design elements into HBD catalyst frameworks is currently determined empirically and dictated by the specific process under development; each individual reaction benefits from tuning of the HBD catalyst acidity, steric environment, and/or electronic environment. This review presents recent design elements integrated into dual HBD catalyst scaffolds and their influence on HBD catalyst performance.