Pyrylium is a
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
(positive
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
) with formula , consisting of a six-membered ring of five
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
atoms, each with one
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 ...
atom, and one positively charged
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
atom. The bonds in the ring are
conjugated as in
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
, giving it an
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
character. In particular, because of the positive charge, the oxygen atom is
trivalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules.
Description
The combining capacity, or affinity of an ...
. Pyrilium is a mono-
cyclic
Cycle, cycles, or cyclic may refer to:
Anthropology and social sciences
* Cyclic history, a theory of history
* Cyclical theory, a theory of American political history associated with Arthur Schlesinger, Sr.
* Social cycle, various cycles in s ...
and
heterocyclic
A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and ...
compound, one of the
oxonium ion
In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion ().
Alkyloxonium
Hydronium is one of a series of oxonium ions with the formula R''n' ...
s.
Salts
Pyrylium and its derivatives form stable
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
s with a variety of anions.
Derivatives
Many important cations are formally derived from pyrylium by substitution of various
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
s for some or all the hydrogens in the ring. The
2,4,6-triphenylpyrilium, referred to as the Katritzky salt, (after
Alan R. Katritzky
Alan Roy Katritzky Fellow of the Royal Society, FRS (18 August 1928 – 10 February 2014) was a British-born American chemist, latterly working at the University of Florida. He was a Heterocyclic compound, heterocyclic chemistry pioneer, who pl ...
) is an important example used in many modern examples of metal catalyzed
cross-couplings.
[
]
Chemical properties
Like other
oxonium ion
In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion ().
Alkyloxonium
Hydronium is one of a series of oxonium ions with the formula R''n' ...
s, pyrylium is unstable in neutral water. However, pyrylium is much less reactive than ordinary oxonium ions because of aromatic stabilization. The 2,4,6-triphenyl salt is commonly reacted with
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, ...
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
s at the 1 position, forming
pyridinium
Pyridinium refers to the cation . It is the conjugate acid of pyridine. Many related cations are known involving substituted pyridines, e.g. picolines, lutidines, collidines. They are prepared by treating pyridine with acids.
As pyridine is oft ...
salts and activating them towards
oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...
by metal complexes, most notably ones with
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
. Pyrylium cations also react with
nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s in the 2, 4, and 6 positions, which can induce a variety of reactions. The high electronegativity of the oxygen results in strong single perturbation by one
heteroatom
In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen.
Organic chemistry
In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...
in the six-membered ring.
Synthesis
Pyrylium salts are easily produced from simple starting materials through a
condensation reaction
In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
.
Pyrylium salts with aromatic substituents, such 2,4,6-triphenylpyrylium tetrafluoroborate, can be obtained from two moles of
acetophenone
Acetophenone is the organic compound with the formula C6H5C(O)CH3. It is the simplest aromatic ketone. This colorless, viscous liquid is a precursor to useful resins and fragrances.
Production
Acetophenone is formed as a byproduct of the cumene p ...
and one mole of
benzaldehyde
Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful.
It is a colorless liquid with a characteristic almond-like odor. ...
in the presence of
tetrafluoroboric acid
Fluoroboric acid or tetrafluoroboric acid (archaically, fluoboric acid) is an inorganic compound with the chemical formula +BF4−], where H+ represents the solvated proton. The solvent can be any suitably Lewis-basic entity. For instance, in w ...
and an oxidizing agent (Dilthey synthesis). For pyrylium salts with alkyl substituents, such as 2,4,6-trimethylpyrylium salts, the best method uses the
Alexandru Balaban, Balaban-
Nenitzescu-Praill synthesis from
tertiary butanol and
acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a col ...
in the presence of tetrafluoroboric, perchloric, or trifluoromethanesulfonic acids. 2,4,6-Triphenylpyrylium salts are converted by bases into a stable 1,5-enedione (pseudobase), but 2,4,6-trimethylpyrylium salts on treatment with hot alkali hydroxides afford an unstable pseudobase that undergoes an intramolecular condensation yielding 3,5-
dimethylphenol. In warm deuterium oxide, 2,4,6-trimethylpyrylium salts undergo isotopic exchange of 4-methyl hydrogens faster than for the 2- and 6-methyl groups, allowing the synthesis of regioselectively deuterated compounds.
Derivatives
The reactivity of pyrylium salts toward nucleophiles makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts afford
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
s with
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
,
pyridinium
Pyridinium refers to the cation . It is the conjugate acid of pyridine. Many related cations are known involving substituted pyridines, e.g. picolines, lutidines, collidines. They are prepared by treating pyridine with acids.
As pyridine is oft ...
salts with primary amines,
pyridine-''N''-oxides with
hydroxylamine
Hydroxylamine is an inorganic compound with the formula . The material is a white crystalline, hygroscopic compound.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Professional Publishing Ltd. pp. 431–43 ...
,
phosphabenzene
Phosphorine (IUPAC name: phosphinine) is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is m ...
s with
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
derivatives,
thiopyrylium
Thiopyrylium is a cation with the chemical formula C5H5S+. It is analogous to the pyrylium cation with the oxygen atom replaced by a sulfur atom.
Thiopyrylium salts are less reactive than the analogous pyrylium salts due to the higher polarizab ...
salts with
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
, and benzene derivatives with
acetonitrile
Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...
or
nitromethane
Nitromethane, sometimes shortened to simply "nitro", is an organic compound with the chemical formula . It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in ...
.
Pyrones
![Pyrones](https://upload.wikimedia.org/wikipedia/commons/9/94/Pyrones.png)
A pyrylium cation with a
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
anion
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...
in the 2-position is not the
zwitterion
In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups.
: With amino acids, for example, in solution a chemical equilibrium wil ...
ic aromatic compound (1), but the neutral
unsaturated lactone
Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring.
Lactones are formed by intramolecular esterification of the co ...
2-pyrone or pyran-2-one (2). Important representatives of this class are the
coumarin
Coumarin () or 2''H''-chromen-2-one is an aromatic organic chemical compound with formula . Its molecule can be described as a benzene molecule with two adjacent hydrogen atoms replaced by a lactone-like chain , forming a second six-membered h ...
s. Likewise a 4-hydroxyl pyrylium compound is a
γ-pyrone or pyran-4-one (4), to which group belong compounds such as
maltol
Maltol is a naturally occurring organic compound that is used primarily as a flavor enhancer. It is found in the bark of larch tree, in pine needles, and in roasted malt (from which it gets its name). It is a white crystalline powder that is ...
.
Chemical properties
![Pyronecycloaddition](https://upload.wikimedia.org/wikipedia/commons/7/70/Pyronecycloaddition.png)
2-Pyrones are known to react with
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s in a
Diels–Alder reaction
In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a peric ...
to form
arene
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
compounds with expulsion of
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
, for example:
Polycyclic pyrylium ions
Chromenylium ion
One
bicyclic
In chemistry, a bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all o ...
pyrylium ion is called benzopyrylium ion (
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
: chromenylium ion) (formula: , molar mass: 131.15 g/mol, exact mass: 131.04968983). It can be seen as a charged derivative of 2''H''-1-
benzopyran
Benzopyran is a polycyclic organic compound that results from the fusion of a benzene ring to a heterocyclic pyran ring.
According to current IUPAC nomenclature, the name chromene used in previous recommendations is retained; however, systemati ...
(IUPAC: 2''H''-chromene, ), or a (charged) substituted heterocyclic derivative of
naphthalene
Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromati ...
().
Flavylium ion
In biology, the 2-phenylbenzopyrylium (2-phenylchromenylium) ion is referred to as
flavylium
Pyrylium is a cation (positive ion) with formula , consisting of a six-membered ring of five carbon atoms, each with one hydrogen atom, and one positively charged oxygen atom. The bonds in the ring are conjugated as in benzene, giving it an arom ...
. A class of flavylium-derived compounds are
anthocyanidin
Anthocyanidins are common plant pigments, the sugar-free counterparts of anthocyanins. They are based on the flavylium cation, an oxonium ion, with various groups substituted for its hydrogen atoms. They generally change color from red through ...
s and
anthocyanin
Anthocyanins (), also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical compo ...
s, pigments that are responsible for the colors of many flowers.
Naphthoxanthenium cation
Higher
polycyclic derivatives of pyrylium also exist. One good example is
naphthoxanthenium. This dye is highly stable, aromatic, and planar. It absorbs in the UV and blue region and presents exceptional photophysical properties. It can be synthesized by chemical or photochemical reactions.
File:Benzopyryliumchlorid.svg, Benzopyrylium chloride (chromenylium chloride), a salt with chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
as the counterion
160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...
File:Flavylium cation.svg, Flavylium cation
File:Naphthoxanthenium.tif, Naphthoxanthenium cation
See also
* 6-membered aromatic rings with one carbon replaced by another group:
borabenzene
Borabenzene is a hypothetical organoboron compound with the formula C5H5B. Unlike the related but highly stable benzene molecule, borabenzene would be electron-deficient. Related derivatives are the boratabenzene anions, including the parent 5H ...
,
silabenzene
A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene (3 theoretical isomers), trisilaben ...
,
germabenzene
Germabenzene (C5H6Ge) is the parent representative of a group of chemical compounds containing in their molecular structure a benzene ring with a carbon atom replaced by a germanium atom. Germabenzene itself has been studied theoretically, and syn ...
,
stannabenzene
Stannabenzene (C5H6Sn) is the parent representative of a group of organotin compounds that are related to benzene with a carbon atom replaced by a tin atom. Stannabenzene itself has been studied by computational chemistry, but has not been isolat ...
,
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
,
phosphorine
Phosphorine (IUPAC name: phosphinine) is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is m ...
,
arsabenzene
Arsabenzene (IUPAC name: arsinine) is an organoarsenic heterocyclic compound with the chemical formula C5H5As. It belongs to a group of compounds called heteroarenes that have the general formula C5H5E (E= N, P, As, Sb, Bi).
This air sensitive li ...
,
stibabenzene
Stibinin, also known as stibabenzene, is an organic chemical compound. Stibinin has the chemical formula . The molecule, stibinin, is a derivative of benzene, with one of the carbon atoms in the 6-membered ring replaced by an antimony (Sb) atom. ...
,
bismabenzene,
pyrylium
Pyrylium is a cation (positive ion) with formula , consisting of a six-membered ring of five carbon atoms, each with one hydrogen atom, and one positively charged oxygen atom. The bonds in the ring are conjugated as in benzene, giving it an arom ...
,
thiopyrylium
Thiopyrylium is a cation with the chemical formula C5H5S+. It is analogous to the pyrylium cation with the oxygen atom replaced by a sulfur atom.
Thiopyrylium salts are less reactive than the analogous pyrylium salts due to the higher polarizab ...
,
selenopyrylium
Selenopyrylium is an aromatic heterocyclic compound consisting of a six-membered ring with five carbon atoms and a positively charged selenium atom.
Naming and numbering
Formerly it was named selenapyrylium. However, this is misleading as "selena ...
,
telluropyrylium
Telluropyrylium is an aromatic heterocyclic compound consisting of a six member ring with five carbon atoms, and a positively charged tellurium atom. Derivatives of telluropyrylium are important in research of infrared dyes.
Naming and numbering ...
*
Pyran
In chemistry, pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the ...
, (pyrones lacking the ketone group)
References
{{DEFAULTSORT:Pyrylium salt
Oxygen heterocycles
Six-membered rings