Pyrylium: Difference between revisions
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[[Image:Pyryliumsalt.png|right|100px|pyrylium salt]]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 |
[[Image:Pyryliumsalt.png|right|100px|pyrylium salt]]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. |
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==Chemical properties== |
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Benzo-fused pyrylium compounds are also called '''flavylium''' compounds of which the pigment [[anthocyanin]] is an important representative. |
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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 and not known. Pyrylium cations react with [[nucleophile]]s in the 2,4 and 6 positions and may result in ring-opening reactions. |
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==Pyrones== |
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| ⚫ | 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 a '''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]]. |
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===Chemical properties=== |
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Likewise a 4-hydroxyl pyrylium compound is a '''φ-pyrone''' or '''2H-pyran-4-one''' ('''4''') to which group belongs a compound such like [[maltol]]. |
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==2-pyrones== |
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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>: |
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>: |
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[[Image:Pyronecycloaddition.png|400px|center|Pyrone cycloaddition]] |
[[Image:Pyronecycloaddition.png|400px|center|Pyrone cycloaddition]] |
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==Chromenylium ion== |
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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). |
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==Flavylium ion== |
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In biology the ion '''2-phenylchromenylium''' is referred to as '''flavylium'''. A class of flavylium derived compounds are [[anthocyanin]], i.e. pigments which are responsible for the colors of many flowers. |
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== See also == |
== See also == |
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Revision as of 21:31, 27 January 2007
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 [1]. 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 and not known. Pyrylium cations react with nucleophiles in the 2,4 and 6 positions and may result in ring-opening reactions.
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 a 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 [2]:
Chromenylium ion
The benzo-fused pyrylium ion is also called benzopyrilium or according to IUPAC chromenylium ion. This is the charged version of 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, 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, pyrylium salt
- 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). Article
References
- ^ Heterocyclic Chemistry T. L. Gilchrist ISBN 0-582-27843-0
- ^ 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