Iminium
In organic chemistry, an iminium cation is a polyatomic ion with the general structure. They are common in synthetic chemistry and biology.
Structure
Iminium cations adopt alkene-like geometries: the central C=N unit is nearly coplanar with all four substituents. Unsymmetrical iminium cations can exist as cis and trans isomers. The C=N bonds, which are near 129 picometers in length, are shorter than C-N single bonds. Cis/trans isomers are observed. The C=N distance is slightly shorter in iminium cations than in the parent imine, and computational studies indicate that the C=N bonding is also stronger in iminium vs imine, although the C=N distance contracts only slightly. These results indicate that the barrier for rotation is higher than in the parent imines.Formation
Iminium cations are obtained by protonation and alkylation of imines:They also are generated by the condensation of secondary amines with ketones or aldehydes:
This rapid, reversible reaction is one step in "iminium catalysis".
More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine.
Occurrence
Iminium derivatives are common in biology. Pyridoxal phosphate reacts with amino acids to give iminium derivatives. Many iminium salts are encountered in synthetic organic chemistry.image:Eschenmoser's salt.svg|thumb|right|"Eschenmoser's salt" is a well known example of an iminium salt.
Reactions
Iminium salts hydrolyse to give the corresponding ketone or aldehyde:Iminium cations are reduced to the amines, e.g. by sodium cyanoborohydride. Iminium cations are intermediates in the reductive amination of ketones and aldehydes.
In a related reaction, iminium cations add carbanion equivalents, such as Grignard reagents.
Unsymmetrical iminium cations undergo cis-trans isomerization. The isomerization is catalyzed by nucleophiles, which add to the unsaturated carbon, breaking the C=N double bond.
Named reactions involving iminium species
- Aza-Cope rearrangement
- Beckmann rearrangement
- Duff reaction
- Mannich reaction
- Pictet-Spengler reaction
- Stephen aldehyde synthesis
- Stork enamine alkylation
- Vilsmeier-Haack reaction and Vilsmeier reagent
Iminylium ions