Iminium
An iminium cation in organic chemistry is a functional group with the general structure [R1R2C=NR3R4]+.[1] They are common in synthetic chemistry and biology.
Structure
Imininium cations adopt alkene-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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/17/KIGPUL.png/220px-KIGPUL.png)
Formation
Iminium cations are obtained by protonation and alkylation of imines:
- RN=CR'2 + H+ → [RNH=CR'2]+
- RN=CR'2 + R"+ → [RR"N=CR'2]+
They also are generated by the condensation of secondary amines with ketones or aldehydes:
- O=CR'2 + R2NH + H+ [R2N=CR'2]+ + H2O
This rapid, reversible reaction is one step in "iminium catalysis".[3]
More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine.[4]
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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Eschenmosersalz.png/220px-Eschenmosersalz.png)
Reactions
Iminium salts hydrolyse to give the corresponding ketone or aldehyde:[6]
- [RR"N=CR'2]+ + H2O → [RR"NH2]+ + O=CR'2
Iminium cations are readily reduced to the amines, e.g. by sodium cyanoborohydride. They are intermediates in the reductive amination of ketones and aldehydes.
Named reactions involving iminium species
- Aza-Cope rearrangement[7]
- Beckmann rearrangement
- Duff reaction
- Mannich reaction
- Pictet-Spengler reaction
- Stephen reaction
- 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.[8]
See also
References
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "iminium compounds". doi:10.1351/goldbook.I02958
- ^ "Electrophilic Reactivity of a 2‐Azaallenium and of a 2‐Azaallylium Ion". European Journal of Organic Chemistry. 2000: 1589–1593. doi:10.1002/(SICI)1099-0690(200004)2000:8<1589::AID-EJOC1589>3.0.CO;2-T.
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(help). - ^ Erkkilä, Anniinä; Majander, Inkeri; Pihko, Petri M. (2007). "Iminium Catalysis". Chem. Rev. 107: 5416–5470. doi:10.1021/cr068388p.
- ^ Hafner, = Klaus; Meinhardt, Klaus-Peter (1984). "Azulene". 62: 134. doi:10.15227/orgsyn.062.0134.
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(help)CS1 maint: extra punctuation (link) - ^ E. F. Kleinman (2004). "Dimethylmethyleneammonium Iodide and Chloride". Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rd346.
- ^ "A General Synthesis of Cyclobutanones from Olefins and Tertiary Amides: 3-Hexylcyclobutanone". Org. Synth. 69: 199. 1990. doi:10.15227/orgsyn.069.0199.
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(help) - ^ Grieco, P. A.; Larsen, S. D. (1990). "Iminium Ion-Based Diels–Alder Reactions: N-Benzyl-2-Azanorborene" (PDF). Organic Syntheses. 68: 206. doi:10.15227/orgsyn.068.0206.
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "iminylium ions". doi:10.1351/goldbook.I02964