Editing Schiff base

{{Short description|1=Organic compound containing the group >C=N–}}
{{Distinguish|Schiff test}}
[[Image:Imine general structure B.svg|thumb|120px|right|General structure of an [[imine]]. Schiff bases are [[imine]]s in which {{chem2|R^{3} }}  is an [[alkyl]] or [[aryl group]] (not a hydrogen). {{chem2|R^{1} }}  and {{chem2|R^{2} }}  may be hydrogens]]
[[File:Aldimine-(secondary)-skeletal.svg|thumb|120px|General structure of an azomethine compound]]

In [[organic chemistry]], a '''Schiff base''' (named after [[Hugo Schiff]]) is a compound with the general structure {{chem2|R^{1}R^{2}C\dNR^{3} }} ({{chem2|R^{3} }} = [[alkyl]] or [[aryl]], but not [[hydrogen]]).<ref>{{GoldBookRef |title = Schiff base|file = S05498}}</ref><ref>{{March6th|page=1281}}</ref> They can be considered a sub-class of [[imines]], being either secondary [[ketimines#Nomenclature and classification|ketimines]] or secondary [[ketimines#Nomenclature and classification|aldimines]] depending on their structure.   '''Anil''' refers to a common subset of Schiff bases: imines derived from [[aniline]]s.<ref>{{GoldBookRef|title = anil|file = A00357}}</ref>  The term can be synonymous with '''azomethine''' which refers specifically to secondary aldimines (i.e. {{chem2|R\sCH\dNR'}} where R' ≠ H).<ref>{{GoldBookRef|title = azomethines|file = A00564}}</ref>

== Synthesis ==
Schiff bases can be synthesized from an [[aliphatic amine|aliphatic]] or [[aromatic amine]] and a [[carbonyl]] compound by [[nucleophilic addition]] forming a [[hemiaminal]], followed by a [[dehydration reaction|dehydration]] to generate an [[imine]]. In a typical reaction, [[4,4'-oxydianiline]] reacts with ''o''-[[vanillin]]:<ref>{{Cite journal| last = Jarrahpour| first = A. A.|author2=M. Zarei | title = Synthesis of 2-({[4-(4-<nowiki/>{[(E)-1-(2-hydroxy-3-methoxyphenyl)methylidene amino}phenoxy)phenyl imino}methyl)- 6 -methoxy phenol| journal = Molbank| volume = M352| date = February 24, 2004| url = http://www.mdpi.net/molbank/molbank2004/m0352.htm| issn =1422-8599| access-date = February 22, 2010}}</ref>

[[File:Schiff Base.png|700px|center|thumb|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.<ref>{{cite book|last1=Bajema|first1=Elizabeth A.   |last2=Roberts|first2=Kaleigh F. |last3=Meade|first3=Thomas J.  |editor1-last=Sigel|editor1-first=Astrid|editor2-last=Freisinger|editor2-first=Eva
|editor3-last=Sigel|editor3-first=Roland K. O. |editor4-last=Carver|editor4-first=Peggy L. |title=Essential Metals in Medicine:Therapeutic Use and Toxicity of Metal Ions in the Clinic
|series=Metal Ions in Life Sciences |volume=19 |date=2019 |publisher=de Gruyter GmbH|location=Berlin|isbn=978-3-11-052691-2|doi=10.1515/9783110527872-017|pmid=30855112|pages=267–301|chapter=Chapter 11. Cobalt-Schiff Base Complexes:Preclinical Research and Potential Therapeutic Uses|s2cid=73727460 }}</ref>

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).<ref>{{Cite journal|last1 = Eliot|first1 = A. C.|last2 = Kirsch|first2 = J. F.|doi = 10.1146/annurev.biochem.73.011303.074021|title = PYRIDOXALPHOSPHATEENZYMES: Mechanistic, Structural, and Evolutionary Considerations|journal = Annual Review of Biochemistry|volume = 73|pages = 383–415|year = 2004|pmid = 15189147|s2cid = 36010634}}</ref> Similarly, the cofactor [[retinal]] forms a Schiff base in [[rhodopsin]]s, 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 [[ligand]]s to form [[coordination complexes]] with [[metal ions]].<ref>{{cite journal |first1=Malgorzata T. |last1=Kaczmarek |first2=Michal |last2=Zabiszak |first3=Martyna |last3=Nowak |first4=Renata |last4=Jastrzab |title=Lanthanides: Schiff base complexes, applications in cancer diagnosis, therapy, and antibacterial activity |journal=[[Coordination Chemistry Reviews]] |year=2018 |volume=370 |page=42–54 |doi=10.1016/j.ccr.2018.05.012}}</ref> One example is [[Jacobsen's catalyst]]. The imine nitrogen is basic and exhibits [[Pi backbonding|pi-acceptor properties]]. Several, especially the [[diiminopyridine]]s are [[noninnocent ligand]]s. Many Schiff base ligands are derived from alkyl [[diamine]]s and aromatic aldehydes.<ref>{{cite book|doi=10.1016/B0-08-043748-6/01070-7|chapter=Acyclic and Macrocyclic Schiff Base Ligands|title=Comprehensive Coordination Chemistry II|pages=411–446|year=2003|last1=Hernández-Molina|first1=R.|last2=Mederos|first2=A.|isbn=9780080437484}}</ref>

{{Gallery|title=Schiff base ligands|width=200px|height=150px|align=center|File:Cu(Salox)2.png|Copper(II) complex of the Schiff base ligand [[salicylaldoxime]].|File:Salen structure.svg|[[Salen ligand|Salen]] is a common tetradentate ligand that becomes deprotonated upon complexation.|File:Jacobsen's catalyst (S,S).png|[[Jacobsen's catalyst]] is derived from a chiral [[salen ligand]].|File:DIPMLn.svg|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 [[Intermolecular metal-catalyzed carbenoid cyclopropanations|metal-carbenoid cyclopropanation]] of [[styrene]].<ref>{{cite journal|title = Homogeneous catalysis in the decomposition of diazo compounds by copper chelates: Asymmetric carbenoid reactions|journal = [[Tetrahedron (journal)|Tetrahedron]]|volume = 24|issue = 9|year = 1968|pages = 3655–3669|doi = 10.1016/S0040-4020(01)91998-2|last1 = Nozaki|first1 = H.|last2 = Takaya|first2 = H.|last3 = Moriuti|first3 = S.|last4 = Noyori|first4 = R.}}</ref>  Schiff bases have also been incorporated into [[metal–organic framework]]s (MOF).<ref>{{cite journal|title = A Crystalline Imine-Linked 3-D Porous Covalent Organic Framework|journal = J. Am. Chem. Soc.|volume = 131|issue = 13|year = 2009|pages = 4570–4571|doi = 10.1021/ja8096256|pmid=19281246|last1 = Uribe-Romo|first1 = Fernando J.|last2 = Hunt|first2 = Joseph R.|last3 = Furukawa|first3 = Hiroyasu|last4 = KlöCk|first4 = Cornelius|last5 = o'Keeffe|first5 = Michael|last6 = Yaghi|first6 = Omar M.}}</ref>

[[File:AsymmetricSynthesisNoyori.png|400px|center]]

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

==Historic references==
*{{cite journal|last1=Schiff|first1=Hugo|title=Mittheilungen aus dem Universitäts-laboratorium in Pisa: 2. Eine neue Reihe organischer Basen|journal=Annalen der Chemie und Pharmacie|date=1864|volume=131|pages=118–119|url=https://babel.hathitrust.org/cgi/pt?id=uva.x002457965;view=1up;seq=130|trans-title=Communications from the university laboratory in Pisa: 2. A new series of organic bases|language=de|doi=10.1002/jlac.18641310113}}
*{{cite journal|last1=Schiff|first1=Ugo|title=Sopra una nova serie di basi organiche|journal=Giornale di Scienze Naturali ed Economiche|date=1866|volume=2|pages=201–257|url=http://babel.hathitrust.org/cgi/pt?id=hvd.32044106232283;view=1up;seq=207|trans-title=On a new series of organic bases|language=it}}
*{{cite journal|last1=Schiff|first1=Hugo|title=Eine neue Reihe organischer Diamine|journal=Annalen der Chemie und Pharmacie, Supplementband|date=1866|volume=3|pages=343–370|url=https://babel.hathitrust.org/cgi/pt?id=uc1.b3483657;view=1up;seq=353|trans-title=A new series of organic diamines|language=de}}
*{{cite journal|last1=Schiff|first1=Hugo|title=Eine neue Reihe organischer Diamine. Zweite Abtheilung.|journal=Annalen der Chemie und Pharmacie|date=1866|volume=140|pages=92–137|url=https://babel.hathitrust.org/cgi/pt?id=uva.x002457971;view=1up;seq=488|trans-title=A new series of organic diamines. Second part.|language=de|doi=10.1002/jlac.18661400106}}

== References ==
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[[Category:Functional groups]]
[[Category:Schiff bases]]
[[Category:Organometallic chemistry]]
[[Category:Ligands]]