For ketimines and aldimines, respectively, the five core atoms (C2C=NX and C(H)C=NX, X = H or C) are coplanar. Planarity results from the sp2-hybridization of the mutually
double-bonded carbon and the nitrogen atoms. The C=N distance is 1.29-1.31 Å for nonconjugated imines and 1.35 Å for conjugated imines. By contrast, C-N distances in
amines and
nitriles are 1.47 and 1.16 Å, respectively.[4] Rotation about the C=N bond is slow. Using
NMR spectroscopy, both
E- and Z-isomers of aldimines have been detected. Owing to steric effects, the E isomer is favored.[5]
Nomenclature and classification
The term "imine" was coined in 1883 by the German chemist
Albert Ladenburg.[6]
Usually imines refer to compounds with the general formula R2C=NR, as discussed below.[7] In the older literature, imine refers to the aza-analogue of an
epoxide. Thus, ethylenimine is the three-membered ring species
aziridine C2H4NH.[8] The relationship of imines to amines having double and single bonds can be correlated with
imides and
amides, as in
succinimide vs
acetamide.
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. If this group is not a hydrogen atom, then the compound can sometimes be referred to as a
Schiff base.[9] 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.[10] A primary imine in which C is attached to two hydrocarbyls is called a primary ketimine; a secondary imine with such groups is called a secondary ketimine.[11]
Rarer than primary amines is the use of ammonia to give a primary imine.[17] In the case of hexafluoroacetone, the
hemiaminal intermediate can be isolated.[18]
Somewhat like the parent amines, imines are mildly basic and reversibly protonate to give iminium salts:
R2C=NR' + H+ [R2C=NHR']+
Alternatively, primary imines are sufficiently acidic to allow N-alkylation, as illustrated with
benzophenone imine:[28]
(C6H5)2C=NH + CH3Li → (C6H5)2C=NLi + CH4
(C6H5)2C=NLi + CH3I → (C6H5)2C=NCH3 + LiI
Lewis acid-base reactions
Imines are common
ligands in
coordination chemistry. Particularly popular examples are found with Schiff base ligands derived from
salicylaldehyde, the
salen ligands. Metal-catalyzed reactions of imines proceed through such complexes. In classical
coordination complexes, imines bind metals through nitrogen. For low-valent metals, η2-imine ligands are observed.
Nucleophilic additions
Very analogous to ketones and aldehydes, primary imines are susceptible to attack by carbanion equivalents. The method allow for the synthesis of secondary amines:[29][30]
Owing to their enhanced electrophilicity, iminium derivatives are particularly susceptible to reduction to the amines. Such reductions can be achieved by
transfer hydrogenation or by the stoichiometric action of
sodium cyanoborohydride. Since imines derived from unsymmetrical ketones are
prochiral, their reduction defines a route to chiral amines.
Imine polymers (
polyimines) can be synthesised from multivalent
aldehydes and
amines.[36] The polymerisation reaction proceeds directly when the aldehyde and amine monomers are mixed together at room temperature. In most cases, (small) amounts of solvent may still be required. Polyimines are particularly interesting materials because of their application as
vitrimers. Owing to the dynamic covalent nature of the imine bonds, polyimines can be recycled relatively easily. Furthermore, polyimines are known for their self-healing behaviour.[37][38]
Imines are common in nature.[40][41] The
pyridoxal phosphate-dependent enzymes (PLP enzymes) catalyze myriad reactions involving aldimines (or Schiff bases).[42] Cyclic imines are also substrates for many
imine reductase enzymes.[43]
^C. Sandorfy (1970). "General and theoretical aspects". In Saul Patai (ed.). Carbon–Nitrogen Double Bonds. PATai's Chemistry of Functional Groups. John Wiley & Sons. pp. 1–60.
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^Bjørgo, Johannes; Boyd, Derek R.; Watson, Christopher G.; Jennings, W. Brian; Jerina, Donald M. (1974). "E–Z-isomerism in Aldimines". J. Chem. Soc., Perkin Trans. 2 (9): 1081–1084.
doi:
10.1039/P29740001081.
^Ladenburg, A. (1883).
"Ueber die Imine" [About imines]. Berichte der Deutschen Chemischen Gesellschaft (in German). 16: 1149–1152.
doi:
10.1002/cber.188301601259. From p. 1150: Denn offenbar gehört auch das Piperidin in die Klasse der von mir gesuchten Verbindungen, für welche der Name Imine durch die bestehende Nomenklatur angezeigt ist. [For obviously piperidine also belongs in the class of compounds that are sought by me, for which the name "imines" is indicated by the prevailing nomenclature.]
^Bigelow, Lucius A.; Eatough, Harry (1928). "Benzalaniline". Organic Syntheses. 8: 22.
doi:
10.15227/orgsyn.008.0022.
^Reeves, Jonathan T.; Visco, Michael D.; Marsini, Maurice A.; Grinberg, Nelu; Busacca, Carl A.; Mattson, Anita E.; Senanayake, Chris H. (2015-05-15). "A General Method for Imine Formation Using B(OCH2CF3)3". Organic Letters. 17 (10): 2442–2445.
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10.1021/acs.orglett.5b00949.
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^Morales, Sara; Guijarro, Fernando G.; García Ruano, José Luis; Cid, M. Belén (2014-01-22). "A General Aminocatalytic Method for the Synthesis of Aldimines". Journal of the American Chemical Society. 136 (3): 1082–1089.
doi:
10.1021/ja4111418.
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PMID24359453.
^Collados, Juan F.; Toledano, Estefanía; Guijarro, David; Yus, Miguel (2012-07-06). "Microwave-Assisted Solvent-Free Synthesis of Enantiomerically Pure N-(tert-Butylsulfinyl)imines". The Journal of Organic Chemistry. 77 (13): 5744–5750.
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ISSN0022-3263.
PMID22694241.
^Verardo, G.; Giumanini, A. G.; Strazzolini, P.; Poiana, M. (1988). "Ketimines From Ketones and Ammonia". Synthetic Communications. 18 (13): 1501–1511.
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10.1080/00397918808081307.
^
abMiddleton, W. J.; Carlson, H. D. (1970). "Hexafluoroacetone imine". Org. Syntheses. 50: 81–3.
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^"Moureau-Mignonac Ketimine Synthesis". Comprehensive Organic Name Reactions and Reagents. Hoboken, NJ, USA: John Wiley & Sons, Inc. 2010-09-15. pp. 1988–1990.
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^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.
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^Moureu, Charles; Mignonac, Georges (1920).
"Les Cetimines". Annales de Chimie. 9 (13): 322–359. Retrieved 18 June 2014.
^Pickard, P. L.; Tolbert, T. L. (December 1961). "An Improved Method of Ketimine Synthesis". The Journal of Organic Chemistry. 26 (12): 4886–4888.
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^Mandler, Michael; Truong, Phong; Zavalij, Peter; Doyle, Michael (Jan 14, 2014). "Catalytic Conversion of Diazocarbonyl Compounds to Imines: Applications to the Synthesis of Tetrahydropyrimidines and β-Lactams". Organic Letters. 16 (3): 740–743.
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