Pyrylium

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Pyrylium
Pyrylium.svg
Pyrylium-3D-balls.png
Names
Preferred IUPAC name
Pyrylium [1]
Other names
Pyranium
Identifiers
3D model (JSmol)
1421881
ChEBI
ChemSpider
558560
PubChem CID
  • InChI=1S/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1 X mark.svgN
    Key: WVIICGIFSIBFOG-UHFFFAOYSA-N X mark.svgN
  • InChI=1/C5H5O/c1-2-4-6-5-3-1/h1-5H/q+1
    Key: WVIICGIFSIBFOG-UHFFFAOYAF
  • [o+]1ccccc1
Properties
C5H5O+
Molar mass 81.093 g·mol−1
Related compounds
Related compounds
 thiopyrylium, selenopyrylium, telluropyrylium
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Pyrylium is a cation (positive ion) with formula 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 as in benzene, giving it an aromatic character. In particular, because of the positive charge, the oxygen atom is trivalent. Pyrilium is a mono-cyclic and heterocyclic compound, one of the oxonium ions.

Contents

Synthesis

Pyrylium salts are easily produced from simple starting materials through a condensation reaction. [2]

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. [3] 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, [4] perchloric, [5] or trifluoromethanesulfonic acids. [6]

Hydroxide bases open and hydrolyze pyridine to 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 stable salts with a variety of anions. [9] [10] [11] [12] [13] [14]

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. [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 afford pyridines with ammonia, [16] 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. [15]

Pyrones

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.

pyrones Pyrones.png
pyrones

2-Pyrones are known to react with alkynes in a Diels–Alder reaction to form arene compounds with expulsion of carbon dioxide, for example: [17]

Pyrone cycloaddition Pyronecycloaddition.png
Pyrone cycloaddition

Polycyclic oxonium arenes

Chromenylium ion

Benzopyrylium chloride (chromenylium chloride), a salt with chloride as the counterion Benzopyryliumchlorid.svg
Benzopyrylium chloride (chromenylium chloride), a salt with chloride as the counterion

One bicyclic pyrylium 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 of naphthalene (C10H8).

Flavylium cation Flavylium cation.svg
Flavylium cation

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 ]

Naphthoxanthenium cation

Naphthoxanthenium cation Naphthoxanthenium.svg
Naphthoxanthenium cation

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. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Aromatic compound</span> Compound containing rings with delocalized pi electrons

Aromatic compounds or arenes are organic compounds "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation to their odor. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule. Aromatic compounds have the following general properties:

Pyrimidine is an aromatic, heterocyclic, organic compound similar to pyridine. One of the three diazines, it has nitrogen atoms at positions 1 and 3 in the ring. The other diazines are pyrazine and pyridazine.

<span class="mw-page-title-main">Pyridine</span> Heterocyclic aromatic organic compound

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom (=N−). It is a highly flammable, weakly alkaline, water-miscible liquid with a distinctive, unpleasant fish-like smell. Pyridine is colorless, but older or impure samples can appear yellow, due to the formation of extended, unsaturated polymeric chains, which show significant electrical conductivity. The pyridine ring occurs in many important compounds, including agrochemicals, pharmaceuticals, and vitamins. Historically, pyridine was produced from coal tar. As of 2016, it is synthesized on the scale of about 20,000 tons per year worldwide.

In chemistry, an oxonium ion is any cation containing an oxygen atom that has three bonds and 1+ formal charge. The simplest oxonium ion is the hydronium ion.

<span class="mw-page-title-main">Nitration</span> Chemical reaction which adds a nitro (–NO₂) group onto a molecule

In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid. The difference between the resulting molecular structures of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen atom, whereas in nitrate esters, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom.

In chemistry, pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds. In 2H-pyran, the saturated carbon is at position 2, whereas, in 4H-pyran, the saturated carbon is at position 4.

<span class="mw-page-title-main">Imidazole</span> Chemical compound

Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-adjacent nitrogen atoms in meta-substitution.

<span class="mw-page-title-main">Triazine</span> Aromatic, heterocyclic compound

Triazines are a class of nitrogen-containing heterocycles. The parent molecules' molecular formula is C3H3N3. They exist in three isomeric forms, 1,3,5-triazines being common.

Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.

1,3,5-Triazine, also called s-triazine, is an organic chemical compound with the formula (HCN)3. It is a six-membered heterocyclic aromatic ring, one of several isomeric triazines. s-Triazine —the "symmetric" isomer—and its derivatives are useful in a variety of applications.

<span class="mw-page-title-main">Carbenium ion</span> Class of ions

A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic carbon-based cations. In parallel with carbenium ions is another subset of carbocations, the carbonium ions with the formula R5+. In carbenium ions charge is localized. They are isoelectronic with monoboranes such as B(CH3)3.

The tropylium ion or cycloheptatrienyl cation is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropine from which cycloheptatriene (tropylidene) was first synthesized in 1881. Salts of the tropylium cation can be stable, even with nucleophiles of moderate strength e.g., tropylium tetrafluoroborate and tropylium bromide (see below). Its bromide and chloride salts can be made from cycloheptatriene and bromine or phosphorus pentachloride, respectively.

<span class="mw-page-title-main">Triethyloxonium tetrafluoroborate</span> Chemical compound

Triethyloxonium tetrafluoroborate is the organic oxonium compound with the formula [(CH3CH2)3O]+[BF4]−. It is often called Meerwein's reagent or Meerwein's salt after its discoverer Hans Meerwein. Also well known and commercially available is the related trimethyloxonium tetrafluoroborate. The compounds are white solids that dissolve in polar organic solvents. They are strong alkylating agents. Aside from the BF−4 salt, many related derivatives are available.

<span class="mw-page-title-main">Oxatriquinacene</span> Oxonium ion

Oxatriquinacene is an organic cation with formula C
9H
9O+
. It is an oxonium ion, with a tricoordinated oxygen atom with +1 charge connected to carbons 1,4, and 7 of a cyclononatriene ring, forming three fused pentagonal cycles. The compound may possess weak tris-homoaromatic character.

<span class="mw-page-title-main">Arsabenzene</span> Chemical compound

Arsabenzene (IUPAC name: arsinine) is an organoarsenic heterocyclic compound with the chemical formula C5H5As. It belongs to a group of compounds called heteroarenes that have the general formula C5H5E (E= N, P, As, Sb, Bi).

Thiopyrylium is a cation with the chemical formula C5H5S+. It is analogous to the pyrylium cation with the oxygen atom replaced by a sulfur atom.

<span class="mw-page-title-main">Alexandru Balaban</span> Romanian chemist

Alexandru T. Balaban is a Romanian chemist who made significant contributions to the fields of organic chemistry, theoretical chemistry, mathematical chemistry, and chemical graph theory.

A spiropyran is a type of organic chemical compound, known for photochromic properties that provide this molecule with the ability of being used in medical and technological areas. Spiropyrans were discovered in the early twentieth century. However, it was in the middle twenties when Fisher and Hirshbergin observed their photochromic characteristics and reversible reaction. In 1952, Fisher and co-workers announced for the first time photochromism in spiropyrans. Since then, there have been many studies on photochromic compounds that have continued up to the present.

<span class="mw-page-title-main">Selenopyrylium</span> Chemical compound

Selenopyrylium is an aromatic heterocyclic compound consisting of a six-membered ring with five carbon atoms and a positively charged selenium atom.

<span class="mw-page-title-main">Telluropyrylium</span> Chemical compound

Telluropyrylium is an aromatic heterocyclic compound consisting of a six member ring with five carbon atoms, and a positively charged tellurium atom. Derivatives of telluropyrylium are important in research of infrared dyes.

References

  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.
  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. Longman. ISBN   0-582-27843-0.
  10. ↑ Balaban, A. T.; Schroth, W.; Fischer, G. (1969). "Pyrylium Salts Part I. Syntheses". In Katritzky, A. R.; Boulton, A. J. (eds.). Advances in Heterocyclic Chemistry Volume 10. Vol. 10. New York: Academic Press. pp. 241–326. doi:10.1016/S0065-2725(08)60499-7. ISBN   978-0-12-020610-0.
  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.
  15. 1 2 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.
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