In organic chemistry, an iminium cation is a polyatomic ion with the general structure [R1R2C=NR3R4]+.[1] They are common in synthetic chemistry and biology.
In organic chemistry, an iminium cation is a polyatomic ion with the general structure [R1R2C=NR3R4]+.[1] They are common in synthetic chemistry and biology.
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.[3][4]
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".[5]
More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine.[6]
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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.
Iminium salts hydrolyse to give the corresponding ketone or aldehyde:[8]
Iminium cations are reduced to the amines, e.g. by sodium cyanoborohydride. Iminium cations are intermediates in the reductive amination of ketones and aldehydes.
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.[3]
Iminylium ions have the general structure R2C=N+. They form a subclass of nitrenium ions.[10]