View source for Pyrylium

{{chembox
|Verifiedfields = changed
|Watchedfields = changed
|verifiedrevid = 443022610
|Name=Pyrylium
|ImageFileL1 = Pyrylium.svg
|ImageFileR1 = Pyrylium-3D-balls.png
|PIN = Pyrylium<ref>{{cite book |author=International Union of Pure and Applied Chemistry |author-link=International Union of Pure and Applied Chemistry |date=2014 |title=Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 |publisher=[[Royal Society of Chemistry|The Royal Society of Chemistry]] |page=1097 |doi=10.1039/9781849733069 |isbn=978-0-85404-182-4}}</ref>
|OtherNames = Pyranium
|Section1={{Chembox Identifiers
|CASNo_Ref = {{cascite|correct|??}}
| index1_label=salt
| index2_label=family
|CASNo = 289-67-8
| Beilstein = 1421881
|PubChem = 9548819
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 36120
|ChEBI1 = 59658
|ChEBI2 = 59657
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 7827742
| Gmelin = 558560
|SMILES = [o+]1ccccc1
|InChI = 1/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1
|InChIKey = WVIICGIFSIBFOG-UHFFFAOYAF
|StdInChI_Ref = {{stdinchicite|changed|chemspider}}
|StdInChI = 1S/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1
|StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
|StdInChIKey = WVIICGIFSIBFOG-UHFFFAOYSA-N
}}
|Section2={{Chembox Properties
|Formula=C<sub>5</sub>H<sub>5</sub>O<sup>+</sup>
|MolarMass=81.09 g/mol
}}
|Section8={{Chembox Related
| OtherCompounds = [[thiopyrylium]], [[selenopyrylium]], [[telluropyrylium]]
}}
}}
'''Pyrylium''' is a [[cation]] (positive [[ion]]) with formula {{chem2|C5H5O+}}, 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 system|conjugated]] as in [[benzene]], giving it an [[aromaticity|aromatic]] character. In particular, because of the positive charge, the oxygen atom is [[valence (chemistry)|trivalent]].  Pyrilium is a mono-[[cyclic compound|cyclic]] and [[heterocyclic compound|heterocyclic]] compound, one of the [[oxonium ion]]s.
==Synthesis==
Pyrylium salts are easily produced from simple starting materials through a [[condensation reaction]].<ref name="bala1977">{{cite journal |last1=Balaban |first1=A. T. |last2=Wray |first2=V. |date=1977 |title=<sup>13</sup>C n.m.r. spectra of some pyrylium salts and related compounds |journal=Organic Magnetic Resonance |volume=9 |issue=1 |pages=16–22 |doi=10.1002/mrc.1270090105}}</ref>
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]].<ref>{{OrgSynth|year=1969|volume=49|p=121|first1=Karl|last1=Dimroth|first2=C.|last2=Reichardt|first3=K.|last3=Vogel|title=2,4,6-Triphenylpyrylium tetrafluoroborate|collvol=5|collvolpage=1128|prep=CV5P1135}}</ref> For pyrylium salts with alkyl substituents, such as 2,4,6-trimethylpyrylium salts, the best method uses the [[Alexandru Balaban|Balaban]]-[[Costin Nenițescu|Nenitzescu]]-Praill synthesis from [[tertiary butanol]] and [[acetic anhydride]] in the presence of tetrafluoroboric,<ref>{{OrgSynth|year=1969|volume=49|p=121|first1=A.&nbsp;T.|last1=Balatan|first2=A.&nbsp;J.|last2=Boulton|title=2,4,6-Trimethylpyrylium tetrafluoroborate|collvol=5|collvolpages=1112–1113|prep=CV5P1112}}</ref> perchloric,<ref>{{OrgSynth|year=1968|volume=44|p=98|title=2,4,6-Trimethylpyrylium perchlorate|first1=Alexandru&nbsp;T.|last1=Balaban|author-link1=Alexandru Balaban|first2=Costin&nbsp;D.|last2=Nenitzescu|author-link2=Costin Nenitescu|collvol=5|collvolpages=1088,1106,1114,1135|prep=CV5P1106}}</ref> or trifluoromethanesulfonic acids.<ref>{{OrgSynth|first1=Alexandru&nbsp;T.|last1=Balaban|author-link1=Alexandru Balaban|last2=Boulton|first2=A.&nbsp;J.|collvol=5|collvolpages=1112,1114–1116|year=1973|title=2,4,6-Trimethylpyrylium trifluoromethanesulfonate|prep=CV5P1114}}</ref>
Hydroxide bases open and hydrolyze [[pyridine]] to an enedione base that cyclizes in very strong acids to a pyrylium cation.<ref>{{OrgSynth|last1=Gómez-Palomino|first1=Alejandro|last2=Ghiazza|first2=Clément|last3=Busch|first3=Julia|last4=Wagner|first4=Lucas|last5=Cornella|first5=Josep|year=2023|title=Preparation of Pyrylium tetrafluoroborate (Pyry-BF<sub>4</sub>)|volume=100|pages=361-381|prep=v100p0361}}</ref>
Enolizing conditions (strong acid) force pyrones to their pyrylium tautomer.<ref>{{OrgSynth|first1=Nana&nbsp;B.|last2=Murelli|last1=Agyemang|first2=Ryan&nbsp;P.|year=2019|title=Synthesis of 5-hydroxy-4-methoxy-2-methylpyrylium trifluoromethanesulfonate from Kojic acid|volume=96|pages=494-510|prep=v96p0494}}</ref>
==Chemical properties==
Pyrylium and its derivatives form stable [[salt (chemistry)|salt]]s with a variety of anions.<ref>{{cite book |author=Gilchrist, T. L. |title=Heterocyclic Chemistry |year=1997 |publisher=Longman |isbn=0-582-27843-0}}</ref><ref>{{cite book |author1=Balaban, A. T. |title=Advances in Heterocyclic Chemistry Volume 10 |author2=Schroth, W. |author3=Fischer, G. |chapter=Pyrylium Salts Part I. Syntheses |publisher=Academic Press |year=1969 |editor1=Katritzky, A. R. |volume=10 |location=New York |pages=241–326 |doi=10.1016/S0065-2725(08)60499-7 |isbn=978-0-12-020610-0 |editor2=Boulton, A. J.}}</ref><ref>{{cite book |author1=Balaban, A. T. |title=Pyrylium Salts. Syntheses, Reactions and Physical Properties |author2=Dinculescu, A. |author3=Dorofeenko, G. N. |author4=Fischer, G. W. |author5=Koblik, A. V. |author6=Mezheritskii, V. V. |author7=Schroth, W. |publisher=Academic Press |year=1982 |isbn=978-0-12-020652-0 |editor=Katritzky, A. R. |series=Advances in Heterocyclic Chemistry: Supplement |volume=2 |location=New York}}</ref><ref>{{cite book |author=Balaban, A. T. |title=New Trends in Heterocyclic Chemistry |publisher=Elsevier |year=1979 |isbn=978-0-444-41737-4 |editor1=Mitra, R. B. |series=Studies in Organic Chemistry |volume=3 |location=Amsterdam |pages=[https://archive.org/details/comprehensivecar0000unse/page/79 79–111] |chapter=The Pyrylium Cation as a Synthon in Organic Chemistry |editor2=Ayyangar, N. R. |editor3=Gogte, V. N. |editor4=Acheson, R. M. |editor5=Cromwell, N. |chapter-url=https://archive.org/details/comprehensivecar0000unse/page/79}}</ref><ref>{{cite book |author=Balaban, A. T. |title=Organic Synthesis: Modern Trends |publisher=Blackwell |year=1987 |isbn=0-632-02014-8 |editor=Chizov, O. |location=Oxford |pages=263–274 |chapter=Pyrylium Salts as Useful Synthons}}</ref><ref>{{cite book |author1=Balaban, T. S. |title=Hetarenes and Related Ring Systems, Six-membered Hetarenes with one Chalcogen |author2=Balaban, A. T. |publisher=Georg Thieme Verlag |year=2003 |isbn=978-3-13-118641-6 |series=Science of Synthesis; Houben-Weyl Methods of Molecular Transformations |volume=14 |location=Stuttgart |pages=11–200 |chapter=Pyrylium Salts}}</ref><!---Pyrylium [[perchlorate]] is soluble in [[acetonitrile]].
Trimethylpyrylium perchlorate has a melting point of 246–247 °C.
2,4,6-Triphenylpyrylium tetrafluoroborate, yellow needles, mp 214-215°
2-Methyl-4,6-diphenyl pyrylium chloride: light yellow, mp 125–126.-->
Like other [[oxonium ion]]s, 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 [[nucleophile]]s at the [[Arene substitution pattern|''ortho'' and ''para'' positions]], typically through [[ANRORC mechanism|ANRORC]].<ref name=":0">{{Cite journal |last1=Pang |first1=Yue |last2=Moser |first2=Daniel |last3=Cornella |first3=Josep |date=2020 |title=Pyrylium Salts: Selective Reagents for the Activation of Primary Amino Groups in Organic Synthesis |url=https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-0039-1690703 |journal=Synthesis |volume=52 |issue=4 |pages=489–503 |doi=10.1055/s-0039-1690703 |s2cid=208705148}}</ref>
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|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.{{Cn|date=August 2024}}
==Derivatives==
Pyrylium's electrophilicity makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts afford [[pyridine]]s with [[ammonia]],<ref>{{OrgSynth |author= Anderson, A. G. |author2= Stang, P. J. |title= 2,6-Di-''tert''-Butyl-4-Methylpyridine |collvol= 7 |collvolpages= 144 |year= 1981 |volume= 60 |pages= 34 |prep= cv7p0144 |url= http://www.orgsyn.org/orgsyn/pdfs/CV7P0144.pdf}}</ref> [[pyridinium]] salts with primary amines, [[pyridine-N-oxide|pyridine-''N''-oxides]] with [[hydroxylamine]], [[phosphabenzene]]s 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 [[functional group]]s for some or all the hydrogens in the ring.  [[2,4,6-Triphenylpyrylium]] reacts with [[primary amines]] to give [[pyridinium]] derivatives called "[[Alan R. Katritzky|Katritzky]] salts"; they are commonly used in metal-catalyzed [[nucleophilic displacement]] of the amine.<ref name=":0" />
=== Pyrones ===
A pyrylium cation with a [[hydroxyl]] anion [[substituent]] in the 2-position is not the [[zwitterion]]ic aromatic compound ('''1'''), but the neutral [[Saturated and unsaturated compounds|unsaturated]] [[lactone]] [[2-pyrone]] or pyran-2-one ('''2'''). Important representatives of this class are the [[coumarin]]s. Likewise a 4-hydroxyl pyrylium compound is a [[γ-pyrone]] or pyran-4-one ('''4'''), to which group belong compounds such as [[maltol]].[[File:Pyrones.png|210 px|pyrones|center]]
2-Pyrones are known to react with [[alkyne]]s in a [[Diels–Alder reaction]] to form [[arene]] compounds with expulsion of [[carbon dioxide]], for example:<ref>{{cite journal |author1=Delaney, P. M. |author2=Moore, J. E. |author3=Harrity, J. P. A. |title= An Alkynylboronic Ester Cycloaddition Route to Functionalised Aromatic Boronic Esters |journal= [[Chemical Communications]] |year= 2006 |volume= 2006 |issue= 31 |pages= 3323–3325 |doi= 10.1039/b607322k|pmid=16883424 }}</ref>[[File:Pyronecycloaddition.png|400px|center|Pyrone cycloaddition]]
=== Polycyclic oxonium arenes ===
==== Chromenylium ion ====
[[File:Benzopyryliumchlorid.svg|thumb|left|upright=0.75|Benzopyrylium chloride (chromenylium chloride), a salt with [[chloride]] as the [[counterion]]]]
One [[bicyclic molecule|bicyclic]] pyrylium ion is called benzopyrylium ion ([[IUPAC]]: chromenylium ion) (formula: {{Chem2|C9H7O+}}, molar mass: 131.15&nbsp;g/mol, exact mass: 131.04968983). It can be seen as a charged derivative of 2''H''-1-[[benzopyran]] (IUPAC: 2''H''-chromene, {{Chem2|C9H8O}}), or a (charged) substituted heterocyclic derivative of [[naphthalene]] ({{Chem2|C10H8}}).
[[File:Flavylium cation.svg|thumb|right|Flavylium cation]]
In biology, the 2-phenylbenzopyrylium  (2-phenylchromenylium) ion is referred to as [[flavylium]]. A class of flavylium-derived compounds are [[anthocyanidin]]s and [[anthocyanin]]s, pigments that are responsible for the colors of many flowers.{{Citation needed|date=July 2022}}
==== Naphthoxanthenium cation ====
[[File:Naphthoxanthenium.svg|thumb|left|Naphthoxanthenium cation]]
Higher [[polycyclic compound|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.<ref>{{cite journal |author1=Bucher, G. |author2=Bresolí-Obach, R. |author3=Brosa, C. |author4=Flors, C. |author5=Luis, J. L. |author6=Grillo, T. A. |author7=Nonell, S. |title= β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications |journal= [[Physical Chemistry Chemical Physics]] |year= 2014 |volume= 16 |issue=35 |pages= 18813–18820 |doi= 10.1039/C4CP02783C|pmid=25079707 |bibcode=2014PCCP...1618813B }}</ref>
==See also==
{{Wiktionary|pyrylium}}
* 6-membered aromatic rings with one carbon replaced by another group: [[borabenzene]], [[silabenzene]], [[germabenzene]], [[stannabenzene]], [[pyridine]], [[phosphorine]], [[arsabenzene]], [[stibabenzene]], [[bismabenzene]], [[thiopyrylium]], [[selenopyrylium]], [[telluropyrylium]]
*[[Pyran]], {{Chem2|C5H6O}} (pyrones lacking the ketone group)
{{clear}}
==References==
{{Reflist}}
{{DEFAULTSORT:Pyrylium salt}}
[[Category:Oxygen heterocycles]]
[[Category:Six-membered rings]]
[[Category:Cations]]
[[Category:Oxonium compounds]]