Iminium

The general structure of an iminium cation

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

Iminium 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.

Structure of the cation in the salt [Me₂N=C(H)tolyl]⁺OTf⁻ illustrating the near planarity of the iminium functional group.^[2]

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⁺

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[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.

"Eschenmoser's salt" is a well known example of an iminium salt.^[5]

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 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.^[8]

References

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "iminium compounds". doi:10.1351/goldbook.I02958
^ Guido M. Böttger Roland Fröhlich Ernst‐Ulrich Würthwein (2000). "Electrophilic Reactivity of a 2‐Azaallenium and of a 2‐Azaallylium Ion". European Journal of Organic Chemistry. 2000 (8): 1589–1593. doi:10.1002/(SICI)1099-0690(200004)2000:8<1589::AID-EJOC1589>3.0.CO;2-T..
^ Erkkilä, Anniinä; Majander, Inkeri; Pihko, Petri M. (2007). "Iminium Catalysis". Chem. Rev. 107 (12): 5416–70. doi:10.1021/cr068388p. PMID 18072802.
^ Hafner, = Klaus; Meinhardt, Klaus-Peter (1984). "Azulene". Organic Syntheses. 62: 134. doi:10.15227/orgsyn.062.0134.
^ E. F. Kleinman (2004). "Dimethylmethyleneammonium Iodide and Chloride". Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rd346.
^ C. Schmit, J. B. Falmagne, J. Escudero, H. Vanlierde, L. Ghosez (1990). "A General Synthesis of Cyclobutanones from Olefins and Tertiary Amides: 3-Hexylcyclobutanone". Org. Synth. 69: 199. doi:10.15227/orgsyn.069.0199.
^ 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

[1] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "iminium compounds". doi:10.1351/goldbook.I02958

[2] Guido M. Böttger Roland Fröhlich Ernst‐Ulrich Würthwein (2000). "Electrophilic Reactivity of a 2‐Azaallenium and of a 2‐Azaallylium Ion". European Journal of Organic Chemistry. 2000 (8): 1589–1593. doi:10.1002/(SICI)1099-0690(200004)2000:8<1589::AID-EJOC1589>3.0.CO;2-T..

[3] Erkkilä, Anniinä; Majander, Inkeri; Pihko, Petri M. (2007). "Iminium Catalysis". Chem. Rev. 107 (12): 5416–70. doi:10.1021/cr068388p. PMID 18072802.

[4] Hafner, = Klaus; Meinhardt, Klaus-Peter (1984). "Azulene". Organic Syntheses. 62: 134. doi:10.15227/orgsyn.062.0134.

[5] E. F. Kleinman (2004). "Dimethylmethyleneammonium Iodide and Chloride". Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rd346.

[6] C. Schmit, J. B. Falmagne, J. Escudero, H. Vanlierde, L. Ghosez (1990). "A General Synthesis of Cyclobutanones from Olefins and Tertiary Amides: 3-Hexylcyclobutanone". Org. Synth. 69: 199. doi:10.15227/orgsyn.069.0199.

[grieco-7] 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.

[8] IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "iminylium ions". doi:10.1351/goldbook.I02964

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