Schiff base

Short description: Organic compound containing the group >C=N–

Not to be confused with Schiff test.

General structure of an imine. Schiff bases are imines in which R3
is an alkyl or aryl group (not a hydrogen). R1
and R2
may be hydrogens General structure of an azomethine compound

In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure R1

C=NR3
(R3
= alkyl or aryl, but not hydrogen).[1][2] They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimines depending on their structure. Anil refers to a common subset of Schiff bases: imines derived from anilines.[3] The term can be synonymous with azomethine which refers specifically to secondary aldimines (i.e. R–CH=NR' where R' ≠ H).[4]

## Contents

* 1 Synthesis
* 2 Biochemistry
* 3 Coordination chemistry
* 4 Conjugated Schiff bases
* 5 Historic references
* 6 References

## 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'-oxydianiline reacts with o-vanillin:[5]

A mixture of 4,4'-oxydianiline 1 (1.00 g, 5.00 mmol) and o-vanillin 2 (1.52 g, 10.0 mmol) in methanol (40.0 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%)

Schiff bases can also be synthesized via the Aza-Wittig reaction.

## Biochemistry

Schiff bases have been investigated in relation to a wide range of contexts, including antimicrobial, antiviral and anticancer activity. They have also been considered for the inhibition of amyloid-β aggregation.[6]

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 pyridoxal phosphate (PLP) forms a Schiff base with a lysine residue and is transaldiminated to the substrate(s).[7] Similarly, the cofactor retinal forms a Schiff base in rhodopsins, including human rhodopsin (via Lysine 296), which is key in the photoreception mechanism.

## Coordination chemistry

The term Schiff base is normally applied to these compounds when they are being used as ligands to form coordination complexes with metal ions.[8] One example is Jacobsen's catalyst. The imine nitrogen is basic and exhibits pi-acceptor properties. Several, especially the diiminopyridines are noninnocent ligands. Many Schiff base ligands are derived from alkyl diamines and aromatic aldehydes.[9]

Schiff base ligands

* Copper(II) complex of the Schiff base ligand salicylaldoxime.

* Salen is a common tetradentate ligand that becomes deprotonated upon complexation.

* Jacobsen's catalyst is derived from a chiral salen ligand.

* Generic diiminopyridine complex

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.[10] Schiff bases have also been incorporated into metal–organic frameworks (MOF).[11]

## Conjugated Schiff bases

Conjugated Schiff bases absorb strongly in the UV-visible region of the electromagnetic spectrum. This absorption is the basis of the anisidine value, which is a measure of oxidative spoilage for fats and oils.

## Historic references

* Schiff, Hugo (1864). "Mittheilungen aus dem Universitäts-laboratorium in Pisa: 2. Eine neue Reihe organischer Basen" (in de). Annalen der Chemie und Pharmacie 131: 118–119. doi:10.1002/jlac.18641310113. https://babel.hathitrust.org/cgi/pt?id=uva.x002457965;view=1up;seq=130.
* Schiff, Hugo (1866). "Sopra una nova serie di basi organiche" (in it). Giornale di Scienze Naturali ed Economiche 2: 201–257. http://babel.hathitrust.org/cgi/pt?id=hvd.32044106232283;view=1up;seq=207.
* Schiff, Hugo (1866). "Eine neue Reihe organischer Diamine" (in de). Annalen der Chemie und Pharmacie, Supplementband 3: 343–370. https://babel.hathitrust.org/cgi/pt?id=uc1.b3483657;view=1up;seq=353.
* Schiff, Hugo (1866). "Eine neue Reihe organischer Diamine. Zweite Abtheilung." (in de). Annalen der Chemie und Pharmacie 140: 92–137. doi:10.1002/jlac.18661400106. https://babel.hathitrust.org/cgi/pt?id=uva.x002457971;view=1up;seq=488.

## References

1. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "Schiff base". doi:10.1351/goldbook.S05498
2. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 1281, ISBN 978-0-471-72091-1, https://books.google.com/books?id=JDR-nZpojeEC&printsec=frontcover
3. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "anil". doi:10.1351/goldbook.A00357
4. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "azomethines". doi:10.1351/goldbook.A00564
5. ↑ 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. http://www.mdpi.net/molbank/molbank2004/m0352.htm. Retrieved February 22, 2010.
6. ↑ Bajema, Elizabeth A.; Roberts, Kaleigh F.; Meade, Thomas J. (2019). "Chapter 11. Cobalt-Schiff Base Complexes:Preclinical Research and Potential Therapeutic Uses". in Sigel, Astrid; Freisinger, Eva; Sigel, Roland K. O. et al.. Essential Metals in Medicine:Therapeutic Use and Toxicity of Metal Ions in the Clinic. Metal Ions in Life Sciences. 19. Berlin: de Gruyter GmbH. pp. 267–301. doi:10.1515/9783110527872-017. ISBN 978-3-11-052691-2.
7. ↑ 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.
8. ↑ Kaczmarek, Malgorzata T.; Zabiszak, Michal; Nowak, Martyna; Jastrzab, Renata (2018). "Lanthanides: Schiff base complexes, applications in cancer diagnosis, therapy, and antibacterial activity". Coordination Chemistry Reviews 370: 42–54. doi:10.1016/j.ccr.2018.05.012.
9. ↑ Hernández-Molina, R.; Mederos, A. (2003). "Acyclic and Macrocyclic Schiff Base Ligands". Comprehensive Coordination Chemistry II. pp. 411–446. doi:10.1016/B0-08-043748-6/01070-7. ISBN 9780080437484.
10. ↑ Nozaki, H.; Takaya, H.; Moriuti, S.; Noyori, R. (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.
11. ↑ Uribe-Romo, Fernando J.; Hunt, Joseph R.; Furukawa, Hiroyasu; KlöCk, Cornelius; o'Keeffe, Michael; Yaghi, Omar M. (2009). "A Crystalline Imine-Linked 3-D Porous Covalent Organic Framework". J. Am. Chem. Soc. 131 (13): 4570–4571. doi:10.1021/ja8096256. PMID 19281246.

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