Iminium

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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.{{cite journal |doi=10.1021/jo010067k |title=Mechanisms of Acid-Catalyzed Z/E Isomerization of Imines |year=2001 |last1=Johnson |first1=James E. |last2=Morales |first2=Nora M. |last3=Gorczyca |first3=Andrea M. |last4=Dolliver |first4=Debra D. |last5=McAllister |first5=Michael A. |journal=The Journal of Organic Chemistry |volume=66 |issue=24 |pages=7979–7985 |pmid=11722194 }}{{cite journal |doi=10.1021/jp9617332 |title=Factors That Influence the CN Stretching Frequency in Imines |year=1997 |last1=Wang |first1=Youliang |last2=Poirier |first2=Raymond A. |journal=The Journal of Physical Chemistry A |volume=101 |issue=5 |pages=907–912 |bibcode=1997JPCA..101..907W }}

Iminium cations are obtained by protonation and alkylation of imines:

:{{chem2|1=R'N\dCR2 + H+ -> [R'NH\dCR2]+}}

:{{chem2|1=R'N\dCR2 + R+ -> [R'RN\dCR2]+}}

They also are generated by the condensation of secondary amines with ketones or aldehydes:

:{{chem2|1=O\dCR2 + R'2NH + H+ <-> [R'2N\dCR2]+ + H2O}}

This rapid, reversible reaction is one step in "iminium catalysis".{{cite journal|title=Iminium Catalysis|first1=Anniinä|last1= Erkkilä|first2=Inkeri|last2=Majander|first3=Petri M.|last3=Pihko|journal=Chem. Rev.|year=2007|volume=107|issue=12|pages=5416–70|doi=10.1021/cr068388p|pmid=18072802}}

More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine.{{cite journal| first1=Klaus|last1=Hafner |first2=Klaus-Peter|last2=Meinhardt | title = Azulene |journal=Organic Syntheses | year = 1984 | volume = 62 | page = 134| doi= 10.15227/orgsyn.062.0134}}

Image:Visual cycle v2 (cropped).png.]]

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" is a well known example of an iminium salt.{{cite encyclopedia|author=E. F. Kleinman|title=Dimethylmethyleneammonium Iodide and Chloride|encyclopedia=Encyclopedia of Reagents for Organic Synthesis|year=2004|publisher=J. Wiley & Sons|location=New York|doi=10.1002/047084289X.rd346}}]]

Iminium salts hydrolyse to give the corresponding ketone or aldehyde:{{cite journal|title=A General Synthesis of Cyclobutanones from Olefins and Tertiary Amides: 3-Hexylcyclobutanone|author=C. Schmit |author2=J. B. Falmagne |author3=J. Escudero |author4=H. Vanlierde |author5=L. Ghosez

|journal=Org. Synth.|year=1990|volume=69|page=199|doi=10.15227/orgsyn.069.0199}}

:{{chem2|1=[R2N\dCR2]+ + H2O -> [R2NH2]+ + O=CR2}}

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.

• Aza-Cope rearrangement{{cite journal |last1=Grieco |first1=P. A. |last2=Larsen |first2=S. D. |year=1990 |title=Iminium Ion-Based Diels–Alder Reactions: N-Benzyl-2-Azanorborene |url=http://www.orgsyn.org/Content/pdfs/procedures/CV8P0031.pdf |journal=Organic Syntheses |volume=68 |page=206 |doi=10.15227/orgsyn.068.0206}}
• Beckmann rearrangement
• Duff reaction
• Mannich reaction
• Pictet-Spengler reaction
• Stephen aldehyde synthesis
• Stork enamine alkylation
• Vilsmeier-Haack reaction and Vilsmeier reagent

Iminylium ions have the general structure R₂C=N⁺. They form a subclass of nitrenium ions.{{GoldBookRef | title = iminylium ions | file = I02964}}

• Ammonium
• Imine
• Quaternary ammonium cation

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Category:Functional groups

Category:Chemistry suffixes