A Schiff base, named after Hugo Schiff, is a compound with a functional group that contains
a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group.
Schiff bases in a broad sense have the general formula R1R2C=NR3, where R is an organic side
chain. In this definition, Schiff base is synonymous with azomethine. Some restrict
the term to the secondary aldimines, thus with the general formula RCH=NR'. Schiff bases
can also be referred to as imines. The chain on the nitrogen makes the Schiff
base a stable imine. A Schiff base derived from an aniline, where R3 is a phenyl or a
substituted phenyl, can be called an anil.
Synthesis Schiff bases can be synthesized from an aliphatic
or aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal,
followed by a dehydration to generate an imine. In a typical reaction, 4,4'-diaminodiphenyl
ether reacts with o-vanillin:
Biochemistry Schiff bases are common enzymatic intermediates
where an amine, such as the terminal group of a lysine residue reversibly reacts with
an aldehyde or ketone of a cofactor or substrate. The common enzyme cofactor PLP forms a Schiff
base with a lysine residue and is transaldiminated to the substrate(s). Similarly, the cofactor
retinal forms a Schiff base in rhodopsins, including human rhodopsin, which is key in
the photoreception mechanism. An example where the substrate forms a Schiff
base to the enzyme is in the fructose 1,6-bisphosphate aldolase catalyzed reaction during glycolysis
and in the metabolism of amino acids. Coordination chemistry
Schiff bases are common ligands in coordination chemistry. The imine nitrogen is basic and
exhibits pi-acceptor properties. The ligands are typically derived from alkyl diamines
and aromatic aldehydes. Chiral Schiff bases were one of the first
ligands used for asymmetric catalysis. In 1968 Ryōji Noyori developed a copper-Schiff
base complex for the metal-carbenoid cyclopropanation of styrene. For this work he was later awarded
a share of the 2001 Nobel Prize in Chemistry.
References ^ IUPAC, Compendium of Chemical Terminology,
2nd ed.. Online corrected version: (2006–) "Schiff base".
^ IUPAC, Compendium of Chemical Terminology, 2nd ed.. Online corrected version: (2006–)
"azomethines". ^ IUPAC, Compendium of Chemical Terminology,
2nd ed.. Online corrected version: (2006–) "anils".
^ Jarrahpour, A. A.; M. Zarei. "Synthesis of 2-({[4-(4-{[(E)-1-(2-hydroxy-3-methoxyphenyl)methylidene
amino}phenoxy)phenyl imino}methyl)- 6 -methoxy phenol". Molbank M352. ISSN 1422-8599. Retrieved
February 22, 2010. ^ Eliot, A. C.; Kirsch, J. F.. "PYRIDOXALPHOSPHATEENZYMES:
Mechanistic, Structural, and Evolutionary Considerations". Annual Review of Biochemistry
73: 383–415. doi:10.1146/annurev.biochem.73.011303.074021. PMID 15189147.
^ R. Hernández-Molina, A. Mederos "Acyclic and Macrocyclic Schiff Base Ligands" in Comprehensive
Coordination Chemistry II 2003, Pages 411–446. doi:10.101601070-7
^ H. Nozaki, H. Takaya, S. Moriuti, R. Noyori. "Homogeneous catalysis in the decomposition
of diazo compounds by copper chelates: Asymmetric carbenoid reactions". Tetrahedron 24: 3655–3669.
doi:10.1016/S0040-4020(01)91998-2.