organocatalyst

An organocatalyst is a small organic molecule that increases the rate of a chemical reaction by providing an alternative, lower-energy pathway for the transformation. Unlike many metal-based catalysts, organocatalysts do not rely on transition metals and are built from elements such as carbon, hydrogen, nitrogen, oxygen, sulfur, or phosphorus. They are used to promote a range of enantioselective and non-enantioselective reactions under relatively mild conditions, and they are valued for potential advantages in green chemistry and pharmaceutical synthesis.

Organocatalysis operates through covalent or noncovalent interactions. In covalent organocatalysis, the catalyst temporarily attaches to a substrate to form reactive intermediates, as seen in enamine catalysis and iminium ion catalysis. In enamine catalysis, secondary amines form enamine intermediates with carbonyl compounds; in iminium catalysis, amines activate aldehydes or ketones toward nucleophilic attack. Noncovalent organocatalysis uses hydrogen bonding, ion pairing, or other weak interactions to stabilize transition states. Common classes include proline- and proline-derived catalysts for enamine activation; imidazolidinone and related catalysts for iminium activation (MacMillan-type); and hydrogen-bond donor catalysts such as cinchona alkaloid derivatives, ureas, thioureas, and squaramides.

The field gained prominence in the late 20th and early 21st centuries, with key contributions from researchers such as MacMillan and List. In 2021, the development of asymmetric organocatalysis was recognized with the Nobel Prize in Chemistry awarded to Benjamin List and David W. C. MacMillan. Organocatalysis is used in a variety of bond-forming reactions, including aldol, Michael, and Mannich reactions, as well as cycloadditions, providing routes to enantioenriched products without metal catalysts. Limitations include varying substrate scope and, in some cases, sensitivity to reaction conditions or synthesis of catalysts.