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 aromaticity character. In particular, because of the positive charge, the oxygen atom is trivalent. Pyrilium is a mono-cyclic compound and heterocyclic compound, one of the .
Synthesis
Pyrylium salts are easily produced from simple starting materials through a condensation reaction.
Pyrylium salts with aromatic substituents, such 2,4,6-triphenylpyrylium tetrafluoroborate, can be obtained from two moles of acetophenone, one mole of benzaldehyde, and excess tetrafluoroboric acid.
Hydroxide bases open and hydrolyze pyridine to an enedione base that cyclizes in very strong acids to a pyrylium cation.
Enolizing conditions (strong acid) force pyrones to their pyrylium tautomer.
Chemical properties
Pyrylium and its derivatives form stable salts with a variety of anions.
Like other , pyrylium is unstable in neutral water. However, pyrylium is much less reactive than ordinary oxonium ions because of aromatic stabilization. The highly electronegative oxygen strongly perturbs the orbitals in the aromatic ring, and pyrylium derivatives are extremely resistant to electrophilic aromatic substitution. Pyrylium cations react with at the ortho and para positions, typically through ANRORC mechanism.
2,4,6-Triphenylpyrylium salts are converted by hydroxide 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-xylenol. 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
Pyrylium's electrophilicity makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts afford
with
ammonia,
salts with primary amines,
pyridine-N-oxide with
hydroxylamine,
with
phosphine derivatives,
thiopyrylium salts with
hydrogen sulfide, and benzene derivatives with
acetonitrile or
nitromethane.
Many important cations are formally derived from pyrylium by substitution of various for some or all the hydrogens in the ring. 2,4,6-Triphenylpyrylium reacts with primary amines to give pyridinium derivatives called "Katritzky salts"; they are commonly used in metal-catalyzed nucleophilic displacement of the amine.
Pyrones
A pyrylium cation with a
hydroxyl anion
substituent in the 2-position is not the
aromatic compound (
1), but the neutral unsaturated
lactone 2-pyrone or pyran-2-one (
2). Important representatives of this class are the
. Likewise a 4-hydroxyl pyrylium compound is a γ-pyrone or pyran-4-one (
4), to which group belong compounds such as
maltol.
2-Pyrones are known to react with
in a Diels–Alder reaction to form
arene compounds with expulsion of
carbon dioxide, for example:
Polycyclic oxonium arenes
Chromenylium ion
One bicyclic pyrylium ion is called benzopyrylium ion (
IUPAC: 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 (IUPAC: 2
H-chromene, ), or a (charged) substituted heterocyclic derivative of
naphthalene ().
Flavylium ion
In biology, the 2-phenylbenzopyrylium (2-phenylchromenylium) ion is referred to as
flavylium. A class of flavylium-derived compounds are
and
, 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 as the counterion
File:Flavylium cation.svg|Flavylium cation
File:Naphthoxanthenium.svg|Naphthoxanthenium cation
See also
-
6-membered aromatic rings with one carbon replaced by another group: borabenzene, silabenzene, germabenzene, stannabenzene, pyridine, phosphorine, arsabenzene, stibabenzene, bismabenzene, thiopyrylium, selenopyrylium, telluropyrylium
-
Pyran, (pyrones lacking the ketone group)
