The general structure of an iminium cation

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

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

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 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.

"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]

See also

References

  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. 1 2 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.
  4. ↑ 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.
  5. ↑ Erkkilä, Anniinä; Majander, Inkeri; Pihko, Petri M. (2007). "Iminium Catalysis". Chem. Rev. 107 (12): 5416–70. doi:10.1021/cr068388p. PMID 18072802.
  6. ↑ Hafner, Klaus; Meinhardt, Klaus-Peter (1984). "Azulene". Organic Syntheses. 62: 134. doi:10.15227/orgsyn.062.0134.
  7. ↑ E. F. Kleinman (2004). "Dimethylmethyleneammonium Iodide and Chloride". Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rd346.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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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