Azulene

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Azulene
Azulen num.svg
Azulene 3d structure.png
Names
Preferred IUPAC name
Azulene [1]
Systematic IUPAC name
Bicyclo[5.3.0]decapentaene
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.005.449 OOjs UI icon edit-ltr-progressive.svg
KEGG
PubChem CID
UNII
  • InChI=1S/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H Yes check.svgY
    Key: CUFNKYGDVFVPHO-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H
    Key: CUFNKYGDVFVPHO-UHFFFAOYAT
  • c1cccc2cccc2c1
Properties
C10H8
Molar mass 128.174 g·mol−1
Melting point 99 to 100 °C (210 to 212 °F; 372 to 373 K)
Boiling point 242 °C (468 °F; 515 K)
-98.5·10−6 cm3/mol

g/L [2]

Thermochemistry
−1266.5 kcal/mol [3]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Azulene is an aromatic organic compound and an isomer of naphthalene. Naphthalene is colourless, whereas azulene is dark blue. The compound is named after its colour, as "azul" is Spanish for blue. Two terpenoids, vetivazulene (4,8-dimethyl-2-isopropylazulene) and guaiazulene (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton are found in nature as constituents of pigments in mushrooms, guaiac wood oil, and some marine invertebrates.

Contents

Azulene has a long history, dating back to the 15th century as the azure-blue chromophore obtained by steam distillation of German chamomile. The chromophore was discovered in yarrow and wormwood and named in 1863 by Septimus Piesse. Its structure was first reported by Lavoslav Ružička, followed by its organic synthesis in 1937 by Placidus Plattner.

Structure and bonding

The blue color of the mushroom Lactarius indigo is due to the azulene derivative (7-isopropenyl-4-methylazulen-1-yl)methyl stearate. Lactarius indigo 48568 edit.jpg
The blue color of the mushroom Lactarius indigo is due to the azulene derivative (7-isopropenyl-4-methylazulen-1-yl)methyl stearate.
The blue color of the mushroom Entoloma hochstetteri is also identified as another kind of azulene derivative: 7-acetyl-1,4-dimethylazulene. Entoloma hochstetteri.jpg
The blue color of the mushroom Entoloma hochstetteri is also identified as another kind of azulene derivative: 7-acetyl-1,4-dimethylazulene.

Azulene is usually viewed as resulting from fusion of cyclopentadiene and cycloheptatriene rings. Like naphthalene and cyclodecapentaene, it is a 10 pi electron system. It exhibits aromatic properties: (i) the peripheral bonds have similar lengths and (ii) it undergoes Friedel-Crafts-like substitutions. The stability gain from aromaticity is estimated to be half that of naphthalene.

Its dipole moment is 1.08  D , [6] in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron tropylium cation: one electron from the seven-membered ring is transferred to the five-membered ring to give each ring aromatic stability by Hückel's rule. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is nucleophilic.

Azulene resonance.png

The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds. [7] Another notable feature of azulene is that it violates Kasha's rule by exhibiting fluorescence from an upper-excited state (S2 → S0). [8]

Organic synthesis

Synthetic routes to azulene have long been of interest because of its unusual structure. [9] In 1939 the first method was reported by St. Pfau and Plattner [10] starting from indane and ethyl diazoacetate.

An efficient one-pot route entails annulation of cyclopentadiene with unsaturated C5-synthons. [11] The alternative approach from cycloheptatriene has long been known, one illustrative method being shown below. [12] [13]

Azulenesynthesis.png

Procedure:

  1. cycloheptatriene 2+2 cycloaddition with dichloro ketene
  2. diazomethane insertion reaction
  3. dehydrohalogenation reaction with DMF
  4. Luche reduction to alcohol with sodium borohydride
  5. elimination reaction with Burgess reagent
  6. oxidation with p-chloranil
  7. dehalogenation with polymethylhydrosiloxane, palladium(II) acetate, potassium phosphate and the DPDB ligand

Another synthesis route starts from the of pyridinium or pyrylium salts with cyclopentadienyl anion: [14]

Azulene synthesis 1.png

Azulene can also be synthesized via a Diels Alder and retro-Diels Alder reaction: [14]

Azulene synthesis 2.png

The starting material of the above reaction can be generated through the Flash Vacuum Pyrolysis of phenyl propiolate.

Organometallic complexes

In organometallic chemistry, azulene serves as a ligand for low-valent metal centers. Illustrative complexes are (azulene)Mo2(CO)6 and (azulene)Fe2(CO)5. [15]

Derivatives

1-Hydroxyazulene is an unstable green oil and it does not show keto–enol tautomerism. [16] 2-Hydroxyazulene is obtained by hydrolysis of 2-methoxyazulene with hydrobromic acid. It is stable and does show keto–enol tautomerism. [17] The pKa of 2-hydroxyazulene in water is 8.71. It is more acidic than phenol or naphthol. The pKa of 6-hydroxyazulenes in water is 7.38 making it also more acidic than phenol or naphthol. [17]

In naphth[a]azulene, a naphthalene ring is condensed at the 1,2-positions of azulene. In one such system [18] deviation from planarity is found, similar to that of tetrahelicene.

Guaiazulene (1,4-dimethyl-7-isopropylazulene) is an alkylated derivative of azulene with an almost identical intensely blue colour. It is commercially available to the cosmetics industry where it functions as a skin conditioning agent.

Related Research Articles

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

Aromatic compounds or arenes usually refers to 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:

Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule is a complex consisting of two cyclopentadienyl rings sandwiching a central iron atom. It is an orange solid with a camphor-like odor that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and Fc+ respectively.

<span class="mw-page-title-main">Hückel's rule</span> Method of determining aromaticity in organic molecules

In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.

The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.

Aromatization is a chemical reaction in which an aromatic system is formed from a single nonaromatic precursor. Typically aromatization is achieved by dehydrogenation of existing cyclic compounds, illustrated by the conversion of cyclohexane into benzene. Aromatization includes the formation of heterocyclic systems.

Cycloheptatriene (CHT) is an organic compound with the formula C7H8. It is a closed ring of seven carbon atoms joined by three double bonds (as the name implies) and four single bonds. This colourless liquid has been of recurring theoretical interest in organic chemistry. It is a ligand in organometallic chemistry and a building block in organic synthesis. Cycloheptatriene is not aromatic, as reflected by the nonplanarity of the methylene bridge (-CH2-) with respect to the other atoms; however the related tropylium cation is.

<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">Titanocene dichloride</span> Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

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

Fulvalene (bicyclopentadienylidene) is the member of the fulvalene family with the molecular formula C10H8. It is of theoretical interest as one of the simplest non-benzenoid conjugated hydrocarbons. Fulvalene is an unstable isomer of the more common benzenoid aromatic compounds naphthalene and azulene. Fulvalene consists of two 5-membered rings, each with two double bonds, joined by yet a fifth double bond. It has D2h symmetry.

<span class="mw-page-title-main">2-Naphthol</span> Chemical compound

2-Naphthol, or β-naphthol, is a fluorescent colorless (or occasionally yellow) crystalline solid with the formula C10H7OH. It is an isomer of 1-naphthol, differing by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homologues of phenol, but more reactive. Both isomers are soluble in simple alcohols, ethers, and chloroform. 2-Naphthol is a widely used intermediate for the production of dyes and other compounds.

1-Naphthol, or α-naphthol, is a organic compound with the formula C10H7OH. It is a fluorescent white solid. 1-Naphthol differs from its isomer 2-naphthol by the location of the hydroxyl group on the naphthalene ring. The naphthols are naphthalene homologues of phenol. Both isomers are soluble in simple organic solvents. They are precursors to a variety of useful compounds.

Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

<span class="mw-page-title-main">Birch reduction</span> Organic reaction used to convert arenes to cyclohexadienes

The Birch reduction is an organic reaction that is used to convert arenes to 1,4-cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent with an alkali metal and a proton source. Unlike catalytic hydrogenation, Birch reduction does not reduce the aromatic ring all the way to a cyclohexane.

<span class="mw-page-title-main">Rhodocene</span> Organometallic chemical compound

Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring.

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl dimer</span> Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

<span class="mw-page-title-main">1-Tetralone</span> Chemical compound

1-Tetralone is a bicyclic aromatic hydrocarbon and a ketone. In terms of its structure, it can also be regarded as benzo-fused cyclohexanone. It is a colorless oil with a faint odor. It is used as starting material for agricultural and pharmaceutical agents. The carbon skeleton of 1-tetralone is found in natural products such as Aristelegone A (4,7-dimethyl-6-methoxy-1-tetralone) from the family of Aristolochiaceae used in traditional Chinese medicine.

<span class="mw-page-title-main">Thermal rearrangement of aromatic hydrocarbons</span>

Thermal rearrangements of aromatic hydrocarbons are considered to be unimolecular reactions that directly involve the atoms of an aromatic ring structure and require no other reagent than heat. These reactions can be categorized in two major types: one that involves a complete and permanent skeletal reorganization (isomerization), and one in which the atoms are scrambled but no net change in the aromatic ring occurs (automerization). The general reaction schemes of the two types are illustrated in Figure 1.

<span class="mw-page-title-main">Borepin</span> Aromatic, boron-containing rings

Borepins are a class of boron-containing heterocycles used in main group chemistry. They consist of a seven-membered unsaturated ring with a tricoordinate boron in it. Simple borepins are analogues of cycloheptatriene, which is a seven-membered ring containing three carbon-carbon double bonds, each of which contributes 2π electrons for a total of 6π electrons. Unlike other seven-membered systems such as silepins and phosphepins, boron has a vacant p-orbital that can interact with the π and π* orbitals of the cycloheptatriene. This leads to an isoelectronic state akin to that of the tropylium cation, aromatizing the borepin while also allowing it to act as a Lewis acid. The aromaticity of borepin is relatively weak compared to traditional aromatics such as benzene or even cycloheptatriene, which has led to the synthesis of many fused, π-conjugated borepin systems over the years. Simple and complex borepins have been extensively studied more recently due to their high fluorescence and potential applications in technologies like organic light-emitting diodes (OLEDs) and photovoltaic cells.

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. 207. doi:10.1039/9781849733069. ISBN   978-0-85404-182-4.
  2. ↑ Sweet, L. I.; Meier, P. G. (1997). "Lethal and Sublethal Effects of Azulene and Longifolene to Microtox®, Ceriodaphnia dubia, Daphnia magna, and Pimephales promelas" (PDF). Bulletin of Environmental Contamination and Toxicology . 58 (2): 268–274. Bibcode:1997BuECT..58..268S. doi:10.1007/s001289900330. hdl: 2027.42/42354 . PMID   8975804.
  3. ↑ Salter, Carl; Foresman, James B. (1998). "Naphthalene and Azulene I: Semimicro Bomb Calorimetry and Quantum Mechanical Calculations". Journal of Chemical Education . 75 (10): 1341. Bibcode:1998JChEd..75.1341S. doi:10.1021/ed075p1341.
  4. ↑ Harmon, A. D.; Weisgraber, K. H.; Weiss, U. (1980). "Preformed azulene pigments of Lactarius indigo (Schw.) Fries (Russulaceae, Basidiomycetes)". Experientia . 36: 54–56. doi:10.1007/BF02003967. S2CID   21207966.
  5. ↑ Nicholas, Gillian May (1998). Australasian fungi: a natural product study (Thesis). p. 56. doi: 10.26021/9162 .
  6. ↑ Anderson, Arthur G.; Steckler, Bernard M. (1959). "Azulene. VIII. A Study of the Visible Absorption Spectra and Dipole Moments of Some 1- and 1,3-Substituted Azulenes". Journal of the American Chemical Society . 81 (18): 4941–4946. doi:10.1021/ja01527a046.
  7. ↑ Michl, Joseph; Thulstrup, E. W. (1976). "Why is azulene blue and anthracene white? A simple mo picture". Tetrahedron . 32 (2): 205. doi:10.1016/0040-4020(76)87002-0.
  8. ↑ Tétreault, N.; Muthyala, R. S.; Liu, R. S. H.; Steer, R.P. (1999). "Control of the Photophysical Properties of Polyatomic Molecules by Substitution and Solvation: The Second Excited Singlet State of Azulene". Journal of Physical Chemistry A . 103 (15): 2524–31. Bibcode:1999JPCA..103.2524T. doi:10.1021/jp984407q.
  9. ↑ Gordon, Maxwell (1 February 1952). "The Azulenes". Chemical Reviews . 50 (1): 127–200. doi:10.1021/cr60155a004.
  10. ↑ St. Pfau, Alexander; Plattner, Pl. A. (1939). "Zur Kenntnis der flüchtigen Pflanzenstoffe VIII. Synthese des Vetivazulens". Helvetica Chimica Acta . 22: 202–208. doi:10.1002/hlca.19390220126.
  11. ↑ Hafner, Klaus; Meinhardt, Klaus-Peter (1984). "Azulene". Organic Syntheses . 62: 134. doi:10.15227/orgsyn.062.0134.
  12. ↑ Carret, Sébastien; Blanc, Aurélien; Coquerel, Yoann; Berthod, Mikaël; Greene, Andrew E.; Deprés, Jean-Pierre (2005). "Approach to the Blues: A Highly Flexible Route to the Azulenes". Angewandte Chemie International Edition . 44 (32): 5130–5133. doi:10.1002/anie.200501276. PMID   16013070.
  13. ↑ Lemal, David M.; Goldman, Glenn D. (1988). "Synthesis of azulene, a blue hydrocarbon". Journal of Chemical Education . 65 (10): 923. Bibcode:1988JChEd..65..923L. doi:10.1021/ed065p923.
  14. 1 2 Shoji, Taku; Ito, Shunji; Yasunami, Masafumi (1 October 2021). "Synthesis of Azulene Derivatives from 2H-Cyclohepta[b]furan-2-ones as Starting Materials: Their Reactivity and Properties". International Journal of Molecular Sciences. 22 (19): 10686. doi: 10.3390/ijms221910686 . ISSN   1422-0067. PMC   8509482 . PMID   34639027.
  15. ↑ Churchill, Melvyn R. (2007). "Transition Metal Complexes of Azulene and Related Ligands". Progress in Inorganic Chemistry. Vol. 11. pp. 53–98. doi:10.1002/9780470166123.ch2. ISBN   9780470166123.
  16. ↑ Asao, Toyonobu; Shunji Ito; Noboru Morita (1989). "1-Hydroxyazulene and 3-hydroxyguaiazulene: Synthesis and their properties". Tetrahedron Letters . 30 (48): 6693–6696. doi:10.1016/S0040-4039(00)70653-8.
  17. 1 2 Takase, Kahei; Toyonobu Asao; Yoshikazu Takagi; Tetsuo Nozoe (1968). "Syntheses and some properties of 2- and 6-hydroxyazulenes". Chemical Communications (7): 368b–370. doi:10.1039/C1968000368B.
  18. ↑ Yamamura, Kimiaki; Kawabata, Shizuka; Kimura, Takatomo; Eda, Kazuo; Hashimoto, Masao (2005). "Novel Synthesis of Benzalacetone Analogues of Naphth[a]azulenes by Intramolecular Tropylium Ion-Mediated Furan Ring-Opening Reaction and X-ray Investigation of a Naphth[1,2-a]azulene Derivative". The Journal of Organic Chemistry . 70 (22): 8902–6. doi:10.1021/jo051409f. PMID   16238325.
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