Schiff base

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General structure of an azomethine
General structure of a Schiff base (narrow definition)

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.[1] 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 (azomethines where the carbon is connected to a hydrogen atom), thus with the general formula RCH=NR'.[2] 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.[3]

Synthesis[edit]

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:[4]

A mixture of 4,4'-diaminodiphenyl ether 1 (1.00 g, 5.00 mmol) and o-vanillin 2 (1.52 g, 10.00 mmol) in methanol (40.00 mL) is stirred at room temperature for one hour to give an orange precipitate and after filtration and washing with methanol to give the pure Schiff base 3 (2.27 g, 97.00%)

Biochemistry[edit]

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).[5] Similarly, the cofactor retinal forms a Schiff base in rhodopsins, including human rhodopsin (via Lysine 296), 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[edit]

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.[6]

Schiff base ligands
Copper(II) complex of the Schiff base ligand salicylaldoxime
Salen is a common tetradentate ligand, it becomes deprotonated upon complexation. 
Jacobsen's catalyst is derived from a chiral salen ligand

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.[7] For this work he was later awarded a share of the 2001 Nobel Prize in Chemistry.

AsymmetricSynthesisNoyori.png

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–) "azomethines".
  3. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "anils".
  4. ^ Jarrahpour, A. A.; M. Zarei (February 24, 2004). "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. 
  5. ^ Eliot, A. C.; Kirsch, J. F. (2004). "PYRIDOXALPHOSPHATEENZYMES: Mechanistic, Structural, and Evolutionary Considerations". Annual Review of Biochemistry 73: 383–415. doi:10.1146/annurev.biochem.73.011303.074021. PMID 15189147.  edit
  6. ^ R. Hernández-Molina, A. Mederos "Acyclic and Macrocyclic Schiff Base Ligands" in Comprehensive Coordination Chemistry II 2003, Pages 411–446. doi:10.1016/B0-08-043748-6/01070-7
  7. ^ H. Nozaki, H. Takaya, S. Moriuti, R. Noyori (1968). "Homogeneous catalysis in the decomposition of diazo compounds by copper chelates: Asymmetric carbenoid reactions". Tetrahedron 24 (9): 3655–3669. doi:10.1016/S0040-4020(01)91998-2.