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

, an iminium cation 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 has a net charge that is not zero. The term molecule may or may no ...

with the general structure . They are common in synthetic chemistry and biology.

Structure

Iminium cations adopt

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

-like geometries. The central C=N unit is nearly coplanar with all four substituents. The C=N distances, which are near 129 picometers in length, are shorter than C-N single bonds. Cis/trans isomers are observed. KIGPUL

Formation

Iminium cations are obtained by

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

and

alkylation Alkylation is the transfer of an alkyl group from one molecule to another. 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 effecti ...

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

s: :RN=CR'_2 + H+ -> NH=CR'_2 :RN=CR'_2 + R''+ -> R''N=CR'_2 They also are generated by the condensation of secondary amines with ketones or aldehydes: :O=CR'_2 + R2NH + H+ <=> 2N=CR'_2 + H2O 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 Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...

Occurrence

Iminium derivatives are common in biology.

Pyridoxal phosphate Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The International Union of Biochemistry and Molecular Biology has catalogued more than 140 PLP-dependent a ...

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: : R''N=CR'_2 + H2O -> R''NH2 + O=CR'_2 Iminium cations are readily reduced to the amines, e.g. by

sodium cyanoborohydride Sodium cyanoborohydride is the chemical compound with the formula Na B H3 CN. It is a colourless salt, but commercial samples can appear tan. It is widely used in organic synthesis for the reduction of imines. The salt tolerates aqueous conditions ...

. They are intermediates in the

reductive amination Reductive amination (also known as reductive alkylation) is a form of amination that involves the conversion of a carbonyl group to an amine via an intermediate imine. The carbonyl group is most commonly a ketone or an aldehyde. It is considered ...

of ketones and aldehydes.

Named reactions involving iminium species

Aza-Cope rearrangement Rearrangements, especially those that can participate in cascade reactions, such as the aza-Cope rearrangements, are of high practical as well as conceptual importance in organic chemistry, due to their ability to quickly build structural complexi ...

Beckmann rearrangement The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is a rearrangement of an oxime functional group to substituted amides. The rearrangement has also been successfully performed on haloimines and nitrone ...

Duff reaction The Duff reaction or hexamine aromatic formylation is a formylation reaction used in organic chemistry for the synthesis of benzaldehydes with hexamine as the formyl carbon source. It is named after James Cooper Duff, who was a chemist at the Col ...

Mannich reaction In organic chemistry, the Mannich reaction is a three-component organic reaction that involves the amino alkylation of an acidic proton next to a carbonyl () functional group by formaldehyde () and a primary or secondary amine () or ammonia (). ...

* Pictet-Spengler reaction * Stephen aldehyde synthesis *

Stork enamine alkylation The Stork enamine alkylation involves the addition of an enamine to a Michael acceptor (e.g, an enone, α,β -unsaturated carbonyl compound) or another electrophilic alkylation reagent to give an alkylated iminium product, which is hydrolyzed by d ...

* Vilsmeier-Haack reaction and

Vilsmeier reagent The Vilsmeier reagent is an organic compound with the formula CH3)2NCHCll. It is a salt consisting of the N,N-dimethyliminium cation ( CH3)2N=CHClsup>+) and chloride anion. Depending on the particular reaction, the anion can vary. In typical POC ...

Iminylium ions

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

nitrenium ion A nitrenium ion (also called: aminylium ion or imidonium ion (obsolete)) in organic chemistry is a reactive intermediate based on nitrogen with both an electron lone pair and a positive charge and with two substituents (). Nitrenium ions are isoel ...

References

{{reflist Functional groups Chemistry suffixes