Schiff base

Not to be confused with Schiff test.

File:Imine general structure B.svg

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

File:Aldimine-(secondary)-skeletal.svg

General structure of an azomethine compound

In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure R¹R²C=NR³ (R³ = alkyl or aryl, but not hydrogen). 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. The term can be synonymous with azomethine which refers specifically to secondary aldimines (i.e. R−CH=NR' where R' ≠ H).

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:

File:Schiff Base.png

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.

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). 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. 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.

Schiff base ligands

• Copper(II) complex of the Schiff base ligand salicylaldoxime.
  Copper(II) complex of the Schiff base ligand salicylaldoxime.
• Salen is a common tetradentate ligand that becomes deprotonated upon complexation.
  Salen is a common tetradentate ligand that becomes deprotonated upon complexation.
• Jacobsen's catalyst is derived from a chiral salen ligand.
  Jacobsen's catalyst is derived from a chiral salen ligand.
• Generic diiminopyridine complex
  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. Schiff bases have also been incorporated into metal–organic frameworks (MOF).

File:AsymmetricSynthesisNoyori.png

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.