Pyrylium

Pyrylium
Names
	Preferred IUPAC name Pyrylium
	Other names Pyranium
Identifiers
CAS Number	289-67-8;
3D model (JSmol)	Interactive image;
Beilstein Reference	1421881
ChEBI	CHEBI:36120; salt:CHEBI:59658; family:CHEBI:59657;
ChemSpider	7827742;
Gmelin Reference	558560
PubChemCID	9548819;
CompTox Dashboard(EPA)	DTXSID40429539;
	InChI InChI=1S/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1 Key: WVIICGIFSIBFOG-UHFFFAOYSA-N; InChI=1/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1 Key: WVIICGIFSIBFOG-UHFFFAOYAF;
	SMILES [o+]1ccccc1;
Properties
Chemical formula	C5H5O+
Molar mass	81.09 g/mol
Related compounds
Related compounds	thiopyrylium,selenopyrylium,telluropyrylium
	Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa). verify(what is?) Infobox references

Pyryliumis acation(positiveion) with formulaC₅H₅O⁺,consisting of a six-membered ring of fivecarbonatoms, each with onehydrogenatom, and one positively chargedoxygenatom. The bonds in the ring areconjugatedas inbenzene,giving it anaromaticcharacter. In particular, because of the positive charge, the oxygen atom istrivalent.Pyrilium is a mono-cyclicandheterocycliccompound, one of theoxonium ions.

Synthesis

Pyrylium salts are easily produced from simple starting materials through acondensation reaction.^[2]

Pyrylium salts with aromatic substituents, such 2,4,6-triphenylpyrylium tetrafluoroborate, can be obtained from two moles ofacetophenone,one mole ofbenzaldehyde,and excesstetrafluoroboric acid.^[3]For pyrylium salts with alkyl substituents, such as 2,4,6-trimethylpyrylium salts, the best method uses theBalaban-Nenitzescu-Praill synthesis fromtertiary butanolandacetic anhydridein the presence of tetrafluoroboric,^[4]perchloric,^[5]or trifluoromethanesulfonic acids.^[6]

Hydroxide bases open and hydrolyzepyridineto an enedione base that cyclizes in very strong acids to a pyrylium cation.^[7]

Enolizing conditions (strong acid) force pyrones to their pyrylium tautomer.^[8]

Chemical properties

Pyrylium and its derivatives form stablesaltswith a variety of anions.^[9]^[10]^[11]^[12]^[13]^[14]

Like otheroxonium 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 toelectrophilic aromatic substitution.Pyrylium cations react withnucleophilesat theorthoandparapositions,typically throughANRORC.^[15]

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.^{[citation needed]}

Derivatives

Pyrylium's electrophilicity makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts affordpyridineswithammonia,^[16]pyridiniumsalts with primary amines,pyridine-N-oxideswithhydroxylamine,phosphabenzeneswithphosphinederivatives,thiopyryliumsalts withhydrogen sulfide,and benzene derivatives withacetonitrileornitromethane.

Many important cations are formally derived from pyrylium by substitution of variousfunctional groupsfor some or all the hydrogens in the ring.2,4,6-Triphenylpyryliumreacts withprimary aminesto givepyridiniumderivatives called "Katritzkysalts "; they are commonly used in metal-catalyzednucleophilic displacementof the amine.^[15]

Pyrones

A pyrylium cation with ahydroxylanionsubstituentin the 2-position is not thezwitterionicaromatic compound (1), but the neutralunsaturated lactone 2-pyroneor pyran-2-one (2). Important representatives of this class are thecoumarins.Likewise a 4-hydroxyl pyrylium compound is aγ-pyroneor pyran-4-one (4), to which group belong compounds such asmaltol.

2-Pyrones are known to react withalkynesin aDiels–Alder reactionto formarenecompounds with expulsion ofcarbon dioxide,for example:^[17]

Polycyclic oxonium arenes

Chromenylium ion

Benzopyrylium chloride (chromenylium chloride), a salt withchlorideas thecounterion

Onebicyclicpyrylium ion is called benzopyrylium ion (IUPAC:chromenylium ion) (formula: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,C₉H₈O), or a (charged) substituted heterocyclic derivative ofnaphthalene(C₁₀H₈).

In biology, the 2-phenylbenzopyrylium (2-phenylchromenylium) ion is referred to asflavylium.A class of flavylium-derived compounds areanthocyanidinsandanthocyanins,pigments that are responsible for the colors of many flowers.^{[citation needed]}

Naphthoxanthenium cation

Higherpolycyclicderivatives of pyrylium also exist. One good example isnaphthoxanthenium.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.^[18]

References

^International Union of Pure and Applied Chemistry(2014).Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013.The Royal Society of Chemistry.p. 1097.doi:10.1039/9781849733069.ISBN 978-0-85404-182-4.
^Balaban, A. T.; Wray, V. (1977). "¹³C n.m.r. spectra of some pyrylium salts and related compounds ".Organic Magnetic Resonance.9(1): 16–22.doi:10.1002/mrc.1270090105.
^Dimroth, Karl; Reichardt, C.; Vogel, K. (1969)."2,4,6-Triphenylpyrylium tetrafluoroborate".Organic Syntheses.49;Collected Volumes,vol. 5.
^Balatan, A. T.; Boulton, A. J. (1969)."2,4,6-Trimethylpyrylium tetrafluoroborate".Organic Syntheses.49;Collected Volumes,vol. 5, pp. 1112–1113.
^Balaban, Alexandru T.; Nenitzescu, Costin D. (1968)."2,4,6-Trimethylpyrylium perchlorate".Organic Syntheses.44;Collected Volumes,vol. 5, pp. 1088, 1106, 1114, 1135.
^Balaban, Alexandru T.; Boulton, A. J. (1973)."2,4,6-Trimethylpyrylium trifluoromethanesulfonate".Organic Syntheses;Collected Volumes,vol. 5, pp. 1112, 1114–1116.
^Gómez-Palomino, Alejandro; Ghiazza, Clément; Busch, Julia; Wagner, Lucas; Cornella, Josep (2023)."Preparation of Pyrylium tetrafluoroborate (Pyry-BF₄) ".Organic Syntheses.100:361–381.
^Agyemang, Nana B.; Murelli, Ryan P. (2019)."Synthesis of 5-hydroxy-4-methoxy-2-methylpyrylium trifluoromethanesulfonate from Kojic acid".Organic Syntheses.96:494–510.
^Gilchrist, T. L. (1997).Heterocyclic Chemistry.ISBN 0-582-27843-0.
^Balaban, A. T.; Schroth, W.; Fischer, G. (1969). Katritzky, A. R.; Boulton, A. J. (eds.).Pyrylium Salts. I. Synthesis.Advances in Heterocyclic Chemistry. Vol. 10. New York: Academic Press. pp. 241–326.doi:10.1016/S0065-2725(08)60499-7.
^Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer, G. W.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. (1982). Katritzky, A. R. (ed.).Pyrylium Salts. Syntheses, Reactions and Physical Properties.Advances in Heterocyclic Chemistry: Supplement. Vol. 2. New York: Academic Press.ISBN 978-0-12-020652-0.
^Balaban, A. T. (1979)."The Pyrylium Cation as a Synthon in Organic Chemistry".In Mitra, R. B.; Ayyangar, N. R.; Gogte, V. N.; Acheson, R. M.; Cromwell, N. (eds.).New Trends in Heterocyclic Chemistry.Studies in Organic Chemistry. Vol. 3. Amsterdam: Elsevier. pp.79–111.ISBN 978-0-444-41737-4.
^Balaban, A. T. (1987). "Pyrylium Salts as Useful Synthons". In Chizov, O. (ed.).Organic Synthesis: Modern Trends.Oxford: Blackwell. pp. 263–274.ISBN 0-632-02014-8.
^Balaban, T. S.; Balaban, A. T. (2003). "Pyrylium Salts".Hetarenes and Related Ring Systems, Six-membered Hetarenes with one Chalcogen.Science of Synthesis; Houben-Weyl Methods of Molecular Transformations. Vol. 14. Stuttgart: Georg Thieme Verlag. pp. 11–200.ISBN 978-3-13-118641-6.
^^a ^bPang, Yue; Moser, Daniel; Cornella, Josep (2020)."Pyrylium Salts: Selective Reagents for the Activation of Primary Amino Groups in Organic Synthesis".Synthesis.52(4): 489–503.doi:10.1055/s-0039-1690703.S2CID 208705148.
^Anderson, A. G.; Stang, P. J. (1981)."2,6-Di-tert-Butyl-4-Methylpyridine "(PDF).Organic Syntheses.60:34;Collected Volumes,vol. 7, p. 144.
^Delaney, P. M.; Moore, J. E.; Harrity, J. P. A. (2006). "An Alkynylboronic Ester Cycloaddition Route to Functionalised Aromatic Boronic Esters".Chemical Communications.2006(31): 3323–3325.doi:10.1039/b607322k.PMID 16883424.
^Bucher, G.; Bresolí-Obach, R.; Brosa, C.; Flors, C.; Luis, J. L.; Grillo, T. A.; Nonell, S. (2014). "β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications".Physical Chemistry Chemical Physics.16(35): 18813–18820.Bibcode:2014PCCP...1618813B.doi:10.1039/C4CP02783C.PMID 25079707.

[1] International Union of Pure and Applied Chemistry(2014).Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013.The Royal Society of Chemistry.p. 1097.doi:10.1039/9781849733069.ISBN 978-0-85404-182-4.

[bala1977-2] Balaban, A. T.; Wray, V. (1977). "¹³C n.m.r. spectra of some pyrylium salts and related compounds ".Organic Magnetic Resonance.9(1): 16–22.doi:10.1002/mrc.1270090105.

[3] Dimroth, Karl; Reichardt, C.; Vogel, K. (1969)."2,4,6-Triphenylpyrylium tetrafluoroborate".Organic Syntheses.49;Collected Volumes,vol. 5.

[4] Balatan, A. T.; Boulton, A. J. (1969)."2,4,6-Trimethylpyrylium tetrafluoroborate".Organic Syntheses.49;Collected Volumes,vol. 5, pp. 1112–1113.

[5] Balaban, Alexandru T.; Nenitzescu, Costin D. (1968)."2,4,6-Trimethylpyrylium perchlorate".Organic Syntheses.44;Collected Volumes,vol. 5, pp. 1088, 1106, 1114, 1135.

[6] Balaban, Alexandru T.; Boulton, A. J. (1973)."2,4,6-Trimethylpyrylium trifluoromethanesulfonate".Organic Syntheses;Collected Volumes,vol. 5, pp. 1112, 1114–1116.

[7] Gómez-Palomino, Alejandro; Ghiazza, Clément; Busch, Julia; Wagner, Lucas; Cornella, Josep (2023)."Preparation of Pyrylium tetrafluoroborate (Pyry-BF₄) ".Organic Syntheses.100:361–381.

[8] Agyemang, Nana B.; Murelli, Ryan P. (2019)."Synthesis of 5-hydroxy-4-methoxy-2-methylpyrylium trifluoromethanesulfonate from Kojic acid".Organic Syntheses.96:494–510.

[9] Gilchrist, T. L. (1997).Heterocyclic Chemistry.ISBN 0-582-27843-0.

[10] Balaban, A. T.; Schroth, W.; Fischer, G. (1969). Katritzky, A. R.; Boulton, A. J. (eds.).Pyrylium Salts. I. Synthesis.Advances in Heterocyclic Chemistry. Vol. 10. New York: Academic Press. pp. 241–326.doi:10.1016/S0065-2725(08)60499-7.

[11] Balaban, A. T.; Dinculescu, A.; Dorofeenko, G. N.; Fischer, G. W.; Koblik, A. V.; Mezheritskii, V. V.; Schroth, W. (1982). Katritzky, A. R. (ed.).Pyrylium Salts. Syntheses, Reactions and Physical Properties.Advances in Heterocyclic Chemistry: Supplement. Vol. 2. New York: Academic Press.ISBN 978-0-12-020652-0.

[12] Balaban, A. T. (1979)."The Pyrylium Cation as a Synthon in Organic Chemistry".In Mitra, R. B.; Ayyangar, N. R.; Gogte, V. N.; Acheson, R. M.; Cromwell, N. (eds.).New Trends in Heterocyclic Chemistry.Studies in Organic Chemistry. Vol. 3. Amsterdam: Elsevier. pp.79–111.ISBN 978-0-444-41737-4.

[13] Balaban, A. T. (1987). "Pyrylium Salts as Useful Synthons". In Chizov, O. (ed.).Organic Synthesis: Modern Trends.Oxford: Blackwell. pp. 263–274.ISBN 0-632-02014-8.

[14] Balaban, T. S.; Balaban, A. T. (2003). "Pyrylium Salts".Hetarenes and Related Ring Systems, Six-membered Hetarenes with one Chalcogen.Science of Synthesis; Houben-Weyl Methods of Molecular Transformations. Vol. 14. Stuttgart: Georg Thieme Verlag. pp. 11–200.ISBN 978-3-13-118641-6.

[:0-15] Pang, Yue; Moser, Daniel; Cornella, Josep (2020)."Pyrylium Salts: Selective Reagents for the Activation of Primary Amino Groups in Organic Synthesis".Synthesis.52(4): 489–503.doi:10.1055/s-0039-1690703.S2CID 208705148.

[16] Anderson, A. G.; Stang, P. J. (1981)."2,6-Di-tert-Butyl-4-Methylpyridine "(PDF).Organic Syntheses.60:34;Collected Volumes,vol. 7, p. 144.

[17] Delaney, P. M.; Moore, J. E.; Harrity, J. P. A. (2006). "An Alkynylboronic Ester Cycloaddition Route to Functionalised Aromatic Boronic Esters".Chemical Communications.2006(31): 3323–3325.doi:10.1039/b607322k.PMID 16883424.

[18] Bucher, G.; Bresolí-Obach, R.; Brosa, C.; Flors, C.; Luis, J. L.; Grillo, T. A.; Nonell, S. (2014). "β-Phenyl quenching of 9-phenylphenalenones: a novel photocyclisation reaction with biological implications".Physical Chemistry Chemical Physics.16(35): 18813–18820.Bibcode:2014PCCP...1618813B.doi:10.1039/C4CP02783C.PMID 25079707.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]