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In organic chemistry, an iminium cation is a polyatomic ion 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. 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.^[3]^[4]

Formation

Iminium cations are obtained by protonation and alkylation of imines:

R'N=CR₂ + H⁺ → [R'NH=CR₂]⁺

R'N=CR₂ + R⁺ → [R'RN=CR₂]⁺

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

O=CR₂ + R'₂NH + H⁺ ⇌ [R'₂N=CR₂]⁺ + H₂O

This rapid, reversible reaction is one step in "iminium catalysis".^[5]

More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine.^[6]

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.^[7]

Reactions

Iminium salts hydrolyse to give the corresponding ketone or aldehyde:^[8]

[R₂N=CR₂]⁺ + 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.^[3]

Named reactions involving iminium species

Iminylium ions

Iminylium ions have the general structure R₂C=N⁺. They form a subclass of nitrenium ions.^[10]

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..
^ ^a ^b Johnson, James E.; Morales, Nora M.; Gorczyca, Andrea M.; Dolliver, Debra D.; McAllister, Michael A. (2001). "Mechanisms of Acid-Catalyzed Z/E Isomerization of Imines". The Journal of Organic Chemistry. 66 (24): 7979–7985. doi:10.1021/jo010067k. PMID 11722194.
^ Wang, Youliang; Poirier, Raymond A. (1997). "Factors That Influence the CN Stretching Frequency in Imines". The Journal of Physical Chemistry A. 101 (5): 907–912. Bibcode:1997JPCA..101..907W. doi:10.1021/jp9617332.
^ 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