
In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, a Schiff base (named after
Hugo Schiff) is a compound with the general structure ( =
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloalk ...
or
aryl
In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...
, but not
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
). They can be considered a sub-class of
imines, being either secondary
ketimines
In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...
or secondary
aldimines depending on their structure. The term is often synonymous with azomethine which refers specifically to secondary aldimines (i.e. where R' ≠ H).
A number of special naming systems exist for these compounds. For instance a Schiff base derived from an
aniline
Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine
In organic chemistry, an aromatic amine is an organic compound consisting of an aroma ...
, where is a
phenyl or a substituted phenyl, can be called an ''anil'', while bis-compounds are often referred to as
salen-type compounds.
The term Schiff base is normally applied to these compounds when they are being used as
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
s to form
coordination complexes
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many m ...
with
metal ions
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
. Such complexes occur naturally, for instance in
corrin, but the majority of Schiff bases are artificial and are used to form many important catalysts, such as
Jacobsen's catalyst.
Synthesis
Schiff bases can be synthesized from an
aliphatic
In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...
or
aromatic amine and a
carbonyl compound by
nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions di ...
forming a
hemiaminal
In organic chemistry, a hemiaminal (also carbinolamine) is a functional group or type of chemical compound that has a hydroxyl group and an amine attached to the same carbon atom: . R can be hydrogen or an alkyl group. Hemiaminals are intermediat ...
, followed by a
dehydration
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
to generate an
imine. In a typical reaction,
4,4'-oxydianiline
4,4′-Oxydianiline (ODA) is an organic compound with the formula O( C6 H4 NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and cross-linking agent for polymers, especially the polyimides, such as Kapton.
Us ...
reacts with ''o''-
vanillin
Vanillin is an organic compound with the molecular formula . It is a phenolic aldehyde. Its functional groups include aldehyde, hydroxyl, and ether. It is the primary component of the extract of the vanilla bean. Synthetic vanillin is now used ...
:
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
Amyloid beta (Aβ or Abeta) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid precursor protein (APP), which is ...
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
Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The International Union of Biochemistry and Molecular Biology has catalogued more than 140 PLP-dependent ac ...
(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
rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
s, including human rhodopsin (via Lysine 296), which is key in the photoreception mechanism.
Coordination chemistry
Schiff bases are common ligands in
coordination chemistry
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
. The imine nitrogen is basic and exhibits
pi-acceptor properties. The ligands are typically derived from alkyl
diamine
A diamine is an amine with exactly two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to primary diamines, as those are the most reactive.
In terms of quantities p ...
s and aromatic aldehydes.
Chiral Schiff bases were one of the first ligands used for
asymmetric catalysis. In 1968
Ryōji Noyori
is a Japanese chemist. He won the Nobel Prize in Chemistry in 2001, Noyori shared a half of the prize with William S. Knowles for the study of chirally catalyzed hydrogenations; the second half of the prize went to K. Barry Sharpless for his ...
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
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
. Schiff bases have also been incorporated into
metal–organic framework
Metal–organic frameworks (MOFs) are a class of compounds consisting of metal ions or cluster compound, clusters coordinated to organic compound, organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included ar ...
s (MOF).
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.
References
Further reading
*
*
organic field-effect transistor (OFET)
*{{cite journal, title = On-Substrate Preparation of an Electroactive Conjugated Polyazomethine from Solution-Processable Monomers and its Application in Electrochromic Devices, journal = Adv. Funct. Mater., volume = 23, issue = 8, year = 2013, pages = 3549–3559, author1=L. Sicard, author2=D. Navarathne, author3=T. Skalski, author4=W. G. Skene, doi = 10.1002/adfm.201203657, s2cid=94829368
Functional groups
Imines
Organometallic chemistry
Ligands