Ugi Reaction
The Ugi reaction is a multi-component reaction (MCR) involving a ketone or aldehyde, an amine, an isocyanide, and a carboxylic acid to form a bis-amide (also called an α-acylaminoamide). The Ugi reaction was named after Ivar Karl Ugi, who first reported this reaction in 1959.
Reaction Mechanism
The mechanism proceeds through several distinct equilibria leading to an irreversible final step (Mumm rearrangement):
- Imine Formation: The amine and the carbonyl compound condense to form an imine (or iminium ion if a secondary amine or acid catalyst is present), releasing a molecule of water.
- Nucleophilic Addition: The electrophilic carbon of the imine is attacked by the nucleophilic carbon of the isocyanide. At the same time, the acidic proton from the carboxylic acid protonates the imine nitrogen. This generates an unstable nitrilium ion intermediate.
- Carboxylate Attack: The carboxylate counter-ion attacks the highly electrophilic nitrilium ion, forming an acyl-imino ester intermediate.
- Mumm Rearrangement: The final, irreversible step is an intramolecular acyl transfer (Mumm rearrangement). The amino group attacks the nearby carbonyl group of the ester, shifting the acyl group and yielding the stable bis-amide product.
