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)
ChEBI
ChemSpider
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 81.09 g/mol
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

Salts

Pyrylium and its derivatives form stable salts with a variety of anions. [2] [3] [4] [5] [6] [7]

Derivatives

Many important cations are formally derived from pyrylium by substitution of various functional groups for some or all the hydrogens in the ring. The 2,4,6-triphenylpyrilium, referred to as the Katritzky salt, (after Alan R. Katritzky) is an important example used in many modern examples of metal catalyzed cross-couplings. [8]

Chemical properties

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 2,4,6-triphenyl salt is commonly reacted with aliphatic amines at the 1 position, forming pyridinium salts and activating them towards oxidative addition by metal complexes, most notably ones with nickel. [9] Pyrylium cations also react with nucleophiles in the 2, 4, and 6 positions, which can induce a variety of reactions. The high electronegativity of the oxygen results in strong single perturbation by one heteroatom in the six-membered ring.

Synthesis

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

Pyrylium salts with aromatic substituents, such 2,4,6-triphenylpyrylium tetrafluoroborate, can be obtained from two moles of acetophenone and one mole of benzaldehyde in the presence of tetrafluoroboric acid and an oxidizing agent (Dilthey synthesis). 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, perchloric, or trifluoromethanesulfonic acids. [10] [11] 2,4,6-Triphenylpyrylium salts are converted by 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.

Derivatives

The reactivity of pyrylium salts toward nucleophiles makes them useful materials for producing other compounds with stronger aromatic character. Pyrylium salts afford pyridines with ammonia, [12] 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.

Pyrones

pyrones Pyrones.png
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.

Chemical properties

Pyrone cycloaddition Pyronecycloaddition.png
Pyrone cycloaddition

2-Pyrones are known to react with alkynes in a Diels–Alder reaction to form arene compounds with expulsion of carbon dioxide, for example: [13]

Polycyclic pyrylium ions

Chromenylium ion

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 ion

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

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. [14]

See also

Related Research Articles

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

Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. 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 with their odor.

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

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. 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, 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">Phosphorine</span> Chemical compound

Phosphorine is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is mainly of interest in research.

<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.

Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds that consist only of a conjugated planar ring system. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex molecules. Typical simple aromatic compounds are benzene, indole, and pyridine.

Thiazole, or 1,3-thiazole, is a 5-membered heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS. The thiazole ring is notable as a component of the vitamin thiamine (B1).

<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.

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. But IUPAC confuses coordination number with valence, incorrectly considering carbon in carbenium as trivalent.

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">Cyclic compound</span> Molecule with a ring of bonded atoms

A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.

In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen, chalcogen, or halogen. The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH3.

<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.

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. ↑ Gilchrist, T. L. (1997). Heterocyclic Chemistry. ISBN   0-582-27843-0.
  3. ↑ 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.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ Balaban, A. T.; Boulton, A. J. (1973). "2,4,6-Trimethyl-Pyrylium Tetrafluoroborate" (PDF). Organic Syntheses .; Collective Volume, vol. 5, pp. 1112–1113
  11. ↑ Balaban, A. T.; Boulton, A. J. (1973). "2,4,6-Trimethyl-Pyrylium Trifluoromethanesulfonate" (PDF). Organic Syntheses .; Collective Volume, vol. 5, pp. 1114–1116
  12. ↑ Anderson, A. G.; Stang, P. J. (1981). "2,6-Di-tert-Butyl-4-Methylpyridine" (PDF). Organic Syntheses . 60: 34.; Collective Volume, vol. 7, p. 144
  13. ↑ 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.
  14. ↑ 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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