Hydrogen Bonding and Internal or External Lewis or Bronsted Acid Assisted (Thio)urea Catalysts

The obtainment of more active organocatalysts has promoted the search for new modes of activation and new organocatalytic systems. Inspired on the mode of activation of enzymes, organocatalysis has recently focused on the challenge to create an enzyme-like active site giving access to hydrogen-bond donors (HBDs) with enhanced activity. Therefore, the assembly of catalytic species, connected through multiple weak inter- or intramolecular interactions, is a young and emerging topic of research which could become in the future a powerful tool for asymmetric catalysis. In the area of (thio)ureas, different alternatives such as internal hydrogen bonds or the use of an internal or an external Lewis or Bronsted acid assisted catalysts have been developed. The pivotal publications recently reported covering this family of organocatalysts are proof of this importance. This review treats to illustrate these important and scarce examples and to show a plausible mode of activation for each one.