Iminium

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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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

Iminylium ions[edit]

Iminylium ions have the general structure R2C=N+. They form a subclass of nitrenium ions.[8]

See also[edit]

References[edit]

  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.CS1 maint: uses authors parameter (link).
  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.CS1 maint: uses authors parameter (link)
  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