Background: Since the discovery of metal-free catalysts or organocatalysts about twenty
years ago, a number of small molecules with different structures have been used to accelerate organic
transformations. With the development of environmental awareness, to obtain highly efficient
scaffolds, scientists have directed their studies towards synthetic methodologies that minimize
or preferably eliminate the formation of waste, avoid toxic solvents and reagents and use renewable
starting materials as far as possible.
Methods: In this connection, the organocatalytic reactions providing efficiency and selectivity for
most of the transformations have become an endless topic in organic chemistry since several advantages
from both practical and environmental standpoints. Organocatalysts contributing to the transformation
of reactants into products with the least possible waste production, have been serving the
concept of green chemistry.
Results and Conclusion: Organocatalysts have been classified based on their binding capacity to
the substrate with covalent or noncovalent interactions involving hydrogen bonding and electrostatic
interaction. Diverse types of small organic compounds including proline and its derivatives,
phase-transfer catalysts, (thio)urease, phosphoric acids, sulfones, N-oxides, guanidines, cinchona
derivatives, aminoindanol, and amino acids have been utilized as hydrogen bonding organocatalysts
in different chemical transformations.