Pyrylium

Pyrylium salts are easily produced from simple starting materials through a condensation reaction.{{cite journal |last1=Balaban |first1=A. T. |last2=Wray |first2=V. |date=1977 |title=¹³C 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}}

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.{{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}} 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,{{OrgSynth|year=1969|volume=49|p=121|first1=A. T.|last1=Balatan|first2=A. J.|last2=Boulton|title=2,4,6-Trimethylpyrylium tetrafluoroborate|collvol=5|collvolpages=1112–1113|prep=CV5P1112}} perchloric,{{OrgSynth|year=1968|volume=44|p=98|title=2,4,6-Trimethylpyrylium perchlorate|first1=Alexandru T.|last1=Balaban|author-link1=Alexandru Balaban|first2=Costin D.|last2=Nenitzescu|author-link2=Costin Nenitescu|collvol=5|collvolpages=1088,1106,1114,1135|prep=CV5P1106}} or trifluoromethanesulfonic acids.{{OrgSynth|first1=Alexandru T.|last1=Balaban|author-link1=Alexandru Balaban|last2=Boulton|first2=A. J.|collvol=5|collvolpages=1112,1114–1116|year=1973|title=2,4,6-Trimethylpyrylium trifluoromethanesulfonate|prep=CV5P1114}}

Hydroxide bases open and hydrolyze pyridine to an enedione base that cyclizes in very strong acids to a pyrylium cation.{{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-BF₄)|volume=100|pages=361-381|prep=v100p0361}}

Enolizing conditions (strong acid) force pyrones to their pyrylium tautomer.{{OrgSynth|first1=Nana B.|last2=Murelli|last1=Agyemang|first2=Ryan P.|year=2019|title=Synthesis of 5-hydroxy-4-methoxy-2-methylpyrylium trifluoromethanesulfonate from Kojic acid|volume=96|pages=494-510|prep=v96p0494}}

Pyrylium and its derivatives form stable salts with a variety of anions.{{cite book |author=Gilchrist, T. L. |title=Heterocyclic Chemistry |year=1997 |publisher=Longman |isbn=0-582-27843-0}}{{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.}}{{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}}{{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}}{{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}}{{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}}

Like other oxonium ions, 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 nucleophiles at the ortho and para positions, typically through ANRORC.{{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|url-access=subscription }}

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-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}}

Pyrylium's electrophilicity makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts afford pyridines with ammonia,{{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 }} 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.

Many important cations are formally derived from pyrylium by substitution of various functional groups 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.

A pyrylium cation with a hydroxyl anion substituent in the 2-position is not the zwitterionic aromatic compound (1), but the neutral unsaturated lactone 2-pyrone or pyran-2-one (2). Important representatives of this class are the coumarins. 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

2-Pyrones are known to react with alkynes in a Diels–Alder reaction to form arene compounds with expulsion of carbon dioxide, for example:{{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 }}File:Pyronecycloaddition.png

File:Benzopyryliumchlorid.svg as the counterion]]

One bicyclic pyrylium ion is called benzopyrylium ion (IUPAC: chromenylium ion) (formula: {{Chem2|C9H7O+}}, molar mass: 131.15 g/mol, exact mass: 131.04968983). It can be seen as a charged derivative of 2H-1-benzopyran (IUPAC: 2H-chromene, {{Chem2|C9H8O}}), or a (charged) substituted heterocyclic derivative of naphthalene ({{Chem2|C10H8}}).

File:Flavylium cation.svg

In biology, the 2-phenylbenzopyrylium (2-phenylchromenylium) ion is referred to as flavylium. A class of flavylium-derived compounds are anthocyanidins and anthocyanins, pigments that are responsible for the colors of many flowers.{{Citation needed|date=July 2022}}

File:Naphthoxanthenium.svg

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.{{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 }}

{{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}}

{{Reflist}}

{{DEFAULTSORT:Pyrylium salt}}

Category:Oxygen heterocycles

Category:Six-membered rings

Category:Cations

Category:Oxonium compounds