Pyrylium salt 4196982 216229586 2008-05-31T18:22:14Z Atbalaban 7163008 /* External links */ [[Image:Pyryliumsalt.png|right|100px|pyrylium salt]] [[Image:Pyrylium-3D-balls.png|right|100px|Ball-and-stick model of the pyrylium cation]] A '''pyrylium compound''' is a [[conjugated system|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.<ref>''Heterocyclic Chemistry'', T. L. Gilchrist, ISBN 0-582-27843-0</ref> 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 [[nucleophile]]s 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 tetrafluooborate can be obtained from two moles of acteophenone 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 tretment 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== [[Image:Pyrones.png|210 px|right|pyrones]] A pyrylium cation with a [[hydroxyl]] anion [[substituent]] in the 2-position is not the [[zwitterion]]ic aromatic compound ('''1''') but a neutral [[unsaturation|unsaturated]] [[lactone]] or an [[2-pyrone|α-pyrone]] or pyran-2-one ('''2'''). Important representatives of this class are the [[coumarin]]s. 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 [[alkyne]]s in a [[Diels-Alder reaction]] to form [[arene compound]]s with expulsion of [[carbon dioxide]], for example:<ref>''An alkynylboronic ester cycloaddition route to functionalised aromatic boronic esters'' Patrick M. Delaney, Jane E. Moore and Joseph P. A. Harrity [[Chem. Commun.]], '''2006''', 3323 - 3325, {{DOI|10.1039/b607322k}}</ref> [[Image:Pyronecycloaddition.png|400px|center|Pyrone cycloaddition]] ==Chromenylium ion== [[Image:Benzopyryliumsalz.png|frame|Chromenylium (benzopyrylium) salts with chloride as the counterion]] The benzo-fused pyrylium ion is also called benzopyrilium or according to [[IUPAC]] chromenylium ion. This is the charged version of [[benzopyran|1-benzopyran]] or [[chromene]] (IUPAC). ==Flavylium ion== In biology, the ion 2-phenylchromenylium is referred to as flavylium. A class of flavylium derived compounds are [[anthocyanin]]s, 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]] * [[Pyran]]s are pyrones lacking the ketone group. == References == <div class="references-small"><references/></div> == External links == * "[http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv7p0144 2,6-Di-tert-butyl-4-methylpyrylium trifluoromethanesulfonate]" ''[[Organic Syntheses]]'', Coll. Vol. 7, p.144 (1990); Vol. 60, p.34 (1981). [[Category:oxygen heterocycles]] 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.