Imine

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The general structure of an imine
"Ketimine" redirects here. It should not be confused with "Ketamine"

An imine (/ɨˈmn/ or /ˈɪmɪn/) is a functional group or chemical compound containing a carbonnitrogen double bond, with the nitrogen attached to a hydrogen atom (H) or an organic group. If this group is not a hydrogen atom, then the compound can sometimes be referred to as a Schiff base.[1] The carbon has two additional single bonds.[2][3][4]

Nomenclature and classification[edit]

Imines are related to ketones and aldehydes by replacement of the oxygen with an NR group. When R = H, the compound is a primary imine, when R is hydrocarbyl, the compound is a secondary imine. Imines exhibit diverse reactivity and are commonly encountered throughout chemistry.[4] When R3 is OH, the imine is called an oxime, and when R3 is NH2 the imine is called a hydrazone.

A primary imine in which C is attached to both a hydrocarbyl and a H is called a primary aldimine; a secondary imine with such groups is called a secondary aldimine.[5] A primary imine in which C is attached to two hydrocarbyls is called a primary ketimine; an imine with such groups is called a secondary ketimine.[6]

One way of naming aldimines is to take the name of the radical, remove final "e", and add "-imine", for example ethanimine. See the aldimine article for other naming conventions. Alternately, an imine is named as a derivative of a carbonyl, adding the word "imine" to the name of a carbonyl compound whose oxo group is replaced by an imino group, for example sydnone imine and acetone imine (an intermediate in the synthesis of acetone azine).

N-Sulfinyl imines are a special class of imines having a sulfinyl group attached to the nitrogen atom.

Synthesis of imines[edit]

Imines are typically prepared by the condensation of primary amines and aldehydes and less commonly ketones. In terms of mechanism, such reactions proceed via the nucleophilic addition giving a hemiaminal -C(OH)(NHR)- intermediate, followed by an elimination of water to yield the imine. (see alkylimino-de-oxo-bisubstitution for a detailed mechanism) The equilibrium in this reaction usually favors the carbonyl compound and amine, so that azeotropic distillation or use of a dehydrating agent, such as molecular sieves or magnesium sulfate, is required to push the reaction in favor of imine formation.

Several other methods exist for the synthesis of imines.

Imine reactions[edit]

The most important reactions of imines are their hydrolysis to the corresponding amine and carbonyl compound. Otherwise this functional group participates in many other reactions, many of which are analogous to the reactions of aldehydes and ketones.

Acid-base reactions[edit]

Somewhat like the parent amines, imines are mildly basic and reversibly protonate to give iminium salts. Iminium derivatives are particularly susceptible to reduction to the amines using transfer hydrogenation or by the stoichiometric action of sodium cyanoborohydride. Since imines derived from unsymmetrical ketones are prochiral, their reduction is a useful method for the synthesis of chiral amines.

As ligands[edit]

Imines are common ligands in coordination chemistry. The condensation of salicylaldehyde and ethylenediamine give families of imine-containing chelating agents such as salen.

Imine reductions[edit]

An imine can be reduced to an amine via hydrogenation for example in a synthesis of m-tolylbenzylamine:[10]

Imine hydrogenation

Other reducing agents are lithium aluminium hydride and sodium borohydride.[11]

The first asymmetric imine reduction was reported in 1973 by Kagan using Ph(Me)C=NBn and PhSiH2 in a hydrosilylation with chiral ligand DIOP and rhodium catalyst (RhCl(CH2CH2)2)2.[12] Many systems have since been investigated.[13][14]

Biological role[edit]

Imines are common in nature. Vitamin B6 promotes the deamination of amino acids via the formation of imines, for example.

See also[edit]

References[edit]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Schiff base".
  2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "imines".
  3. ^ March Jerry; (1985). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. ISBN 0-471-85472-7
  4. ^ a b Fletcher, Dermer, Fox, Nomenclature of Organic Compounds (1974) doi:10.1021/ba-1974-0126.ch023
  5. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "aldimines".
  6. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "ketimines".
  7. ^ W. J. Middleton and H. D. Carlson (1988), "Hexafluoroacetone imine", Org. Synth. ; Coll. Vol. 6: 664 .
  8. ^ Koos, Miroslav; Mosher, Harry S. (1993). "α-Amino-α-trifluoromethyl-phenylacetonitrile: A potential reagent for NMR determination of enantiomeric purity of acids". Tetrahedron 49 (8): 1541–1546. doi:10.1016/S0040-4020(01)80341-0. 
  9. ^ Moureu, Charles; Mignonac, Georges (1920). "Les Cetimines". Annales de chimie 9 (13): 322–359. Retrieved 18 June 2014. 
  10. ^ C. F. H. Allen and James VanAllan (1955), "m-Tolylbenzylamine", Org. Synth.: 827 ; Coll. Vol. 3 
  11. ^ For example: Ieva R. Politzer and A. I. Meyers (1988), "Aldehydes from 2-Benzyl-4,4,6-trimethyl-5,6-dihydro-1,3(4H)-oxazine: 1-Phenylcyclopentanecarboxaldehyde", Org. Synth. ; Coll. Vol. 6: 905 
  12. ^ Langlois, N (1973). "Synthese asymetrique d'amines par hydrosilylation d'imines catalysee par un complexe chiral du rhodium". Tet. Lett. 14 (49): 4865. doi:10.1016/S0040-4039(01)87358-5. 
  13. ^ Kobayashi, Shū; Ishitani, Haruro (1999). "Catalytic Enantioselective Addition to Imines". Chem. Rev. 99 (5): 1069. doi:10.1021/cr980414z. .
  14. ^ J. Martens: Reduction of Imino Groups (C=N) in (G. Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann) Houben-Weyl Stereoselective Synthesis, Workbench Edition E21 Volume 7, S. 4199-4238, Thieme Verlag Stuttgart, 1996, ISBN 3-13-106124-3.