Pyrylium

Pyrylium

Names
IUPAC name Pyrylium
Identifiers
3D model (JSmol)	Interactive image
PubChem CID	9548819
CompTox Dashboard (EPA)	DTXSID40429539
SMILES C1=CC=[O+]C=C1
Properties
Chemical formula	C5H5O+
Molar mass	81.09 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

A pyrylium compound is a conjugated 6-membered carbon ring system with one carbon atom replaced by a positively charged oxygen atom forming a salt with a negatively charged counterion. It is the oxygen pendant of benzene and shares with it aromatic properties.

Chemical properties

The carbon to oxygen double bond is also an oxonium ion which due to aromatic stabilization is much less reactive than ordinary oxonium ions. However, the parent compound pyrylium is unstable in neutral water like oxonium salts. Pyrylium cations react with nucleophiles in the 2,4 and 6 positions and may result in ring-opening reactions. The high electronegativity of the oxygen results in the strongest single perturbation of one heteroatom in a six-membered ring.

Being aromatic, pyrylium salts are easily formed from simple starting materials, and their reactivity towards nuclephiles makes them useful materials for obtaining other compounds with stronger aromatic character. Thus, pyrylium salts afford pyridines with ammonia, pyridinium salts with primary amines, pyridine-N-oxides with hydroxylamine, phosphabenzenes with phosphine derivatives, thiopyrylium salts with hydrogen sulfide, and benzene derivatives with acetonitrile or nitromethane.

Pyrylium salts with aromatic substituents such 2,4,6-triphenylpyrylium tetrafluoroborate can be obtained from two moles of acetophenone and one mole of benzaldehyde in the presence of tetrafluoroboric acid and an oxidizing agent (Dilthey synthesis). For pyrylium salts with alkyl substituents such as 2,4,6-trimethylpyrylium salts, the best method uses the Balaban-Nenitzescu-Praill synthesis from tertiary butanol and acetic anhydride in the presence of tetrafluoroboric, perchloric, or trifluoromethanesulfonic acids. All these procedures are described in Organic Syntheses Collective Volume 5. 2,4,6-Triphenylpyrylium salts are converted by bases into stable a 1,5-enedione (pseudobase) but 2,4,6-trimethylpyrylium salts on treatment with hot alkali hydroxides afford an unstable pseudobase which 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.

Pyrones

pyrones

A pyrylium cation with a hydroxyl anion substituent in the 2-position is not the zwitterionic aromatic compound (1) but a neutral unsaturated lactone or an α-pyrone or pyran-2-one (2). Important representatives of this class are the coumarins. Likewise a 4-hydroxyl pyrylium compound is a φ-pyrone or pyran-4-one (4) to which group belongs a compound such like maltol.

Chemical properties

2-Pyrones are known to react with alkynes in a Diels-Alder reaction to form arene compounds with expulsion of carbon dioxide, for example:

Pyrone cycloaddition

Chromenylium ion

Chromenylium (benzopyrylium) salts with chloride as the counterion

The benzo-fused pyrylium ion is also called benzopyrilium or according to IUPAC chromenylium ion (Formula: C₉H₇O, molar mass: 131.15 g/mol, exact mass : 131.04968983). This is the charged version of 1-benzopyran or chromene (IUPAC).

Flavylium ion

Flavylium cation

In biology, the ion 2-phenylchromenylium is referred to as flavylium. A class of flavylium derived compounds are anthocyanidins and anthocyanins, i.e. pigments which are responsible for the colors of many flowers.

See also

• 6-membered aromatic rings with one carbon replaced by another group: borabenzene, benzene, silabenzene, germanabenzene, stannabenzene, pyridine, phosphorine
• Pyrans are pyrones lacking the ketone group.

External links

• "2,6-Di-tert-butyl-4-methylpyrylium trifluoromethanesulfonate" Organic Syntheses, Coll. Vol. 7, p.144 (1990); Vol. 60, p.34 (1981).

For the following bibliography, editorial help is needed).

• A.T. Balaban, W. Schroth and G. Fischer, “Pyrylium Salts. I. Synthesis”, in Advances in Heterocyclic Chemistry, (eds. A. R. Katritzky and A. J. Boulton), Academic Press, New York, 1969, vol. 10, pp. 241-326.
• A.T. Balaban, A. Dinculescu, G.N. Dorofeenko, G.W. Fischer, A.V. Koblik, V.V. Mezheritskii and W. Schroth, “Pyrylium Salts. Syntheses, Reactions and Physical Properties”, Advances in Heterocyclic Chemistry, Suppl. Vol. 2 (A. R. Katritzky, Ed.), Academic Press, New York, 1982.
• A.T. Balaban, “The pyrylium cation as a synthon in organic chemistry”, in “New Trends in Heterocyclic Chemistry”, (eds. R.B. Mitra, N.R. Ayyangar, V.N Gogte, R.M. Acheson and N. Cromwell), Elsevier, Amsterdam, 1979, pp. 79-111.
• A.T. Balaban, “Pyrylium salts as useful synthons”, in “Organic Synthesis : Modern Trends” (Proc. 6th IUPAC Internat. Symp. on Organic Synthesis, Moscow), (ed. O. Chizov), Blackwell, Oxford, 1987, pp. 263-274.
• T.S. Balaban and A.T. Balaban, “Pyrylium Salts”. In “Science of Synthesis; Houben-Weyl Methods of Molecular Transformations”, Georg Thieme Verlag, Stuttgart, 2003, Vol 14, pp. 11-200.
• A.T. Balaban and A.J. Boulton, Org. Synth., Coll. Vol.5, 1112-1113 (1973): 2,4,6-Trimethyl-pyrylium tetrafluoroborate.
• A.T. Balaban and A.J. Boulton, Org. Synth., Coll. Vol.5, 1114-1116 (1973): 2,4,6-Trimethyl-pyrylium trifluoromethanesulfonate.