In imines the five core atoms (C2C=NX, ketimine; and C(H)C=NX, aldimine; 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 (derived from an aldehyde) 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 (derived from a ketone) 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]
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 allows for the synthesis of secondary amines:[29][30]
R2C=NR' + R"Li → R2R"CN(Li)R'
R2R"CN(Li)R' + H2O → R2R"CNHR' + LiOH
This can be expanded to include enolisable carbons in the Mannich reaction, which is a straightforward and commonly used approach for producing β-amino-carbonyl compounds.[31]
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.[37] 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.[38][39]
Imines are common in nature.[41][42] The pyridoxal phosphate-dependent enzymes (PLP enzymes) catalyze myriad reactions involving aldimines (or Schiff bases).[43] Cyclic imines are also substrates for many imine reductase enzymes.[44]
↑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, Template:ISBN.
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