organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, an iminium

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

is a

polyatomic ion A polyatomic ion (also known as a molecular ion) is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that usually has a net charge that is not zero, or in special c ...

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 In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...

and

alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...

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. " 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. 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

Iminylium ions have the general structure R₂C=N⁺. They form a subclass of nitrenium ions.

References

{{reflist Functional groups Chemistry suffixes