View source for Iminium

{{Short description|1=Polyatomic ion of the form >C=N< and charge +1}}
[[Image:Iminium-Ion.svg|thumb|right|150px|The general structure of an iminium cation]]
In [[organic chemistry]], an '''iminium''' [[cation]] is a [[polyatomic ion]] with the general structure {{chem2|1=[R^{1}R^{2}C\dNR^{3}R^{4}]+}}.<ref>{{GoldBookRef | file = I02958 | title = iminium compounds}}</ref> They are common in synthetic chemistry and biology.
==Structure==
[[File:KIGPUL.png|thumb|Structure of the cation in the salt [Me<sub>2</sub>N=C(H)tolyl]<sup>+</sup>OTf<sup>−</sup> illustrating the near planarity of the iminium functional group.<ref>{{cite journal|title=Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion|author=Guido M. Böttger Roland Fröhlich Ernst-Ulrich Würthwein |journal=European Journal of Organic Chemistry|year=2000|volume=2000|issue=8|pages=1589–1593|doi=10.1002/(SICI)1099-0690(200004)2000:8<1589::AID-EJOC1589>3.0.CO;2-T}}.</ref>]]
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.<ref name= Johnson>{{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 }}</ref><ref>{{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 }}</ref>
==Formation==
Iminium cations are obtained by [[protonation]] and [[alkylation]] of [[imine]]s:
:{{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".<ref>{{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}}</ref>
More exotic routes to iminium cations are known, e.g. from [[ring-opening reaction]]s of [[pyridine]].<ref>{{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}}</ref>
==Occurrence==
[[Image:Visual cycle v2 (cropped).png|460px|thumb|left|Iminium compounds are intermediates in the vision cycle of [[rhodopsin]].]]
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:Eschenmosersalz.png|thumb|right|"[[Eschenmoser's salt]]" is a well known example of an iminium salt.<ref>{{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}}</ref>]]
==Reactions==
Iminium salts hydrolyse to give the corresponding ketone or aldehyde:<ref>{{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}}</ref>
:{{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.<ref name= Johnson/>
===Named reactions involving iminium species===
*[[Aza-Cope rearrangement]]<ref name=grieco>{{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}}</ref>
*[[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 R<sub>2</sub>C=N<sup>+</sup>. They form a subclass of [[nitrenium ion]]s.<ref>{{GoldBookRef | title = iminylium ions | file = I02964}}</ref>
==See also==
*[[Ammonium]]
*[[Imine]]
*[[Quaternary ammonium cation]]
==References==
{{reflist}}
[[Category:Functional groups]]
[[Category:Chemistry suffixes]]