Azulene

Azulene
IUPAC name	bicyclo[5.3.0]decapentaene
Identifiers
CAS number	275-51-4 Y
PubChem	9231
SMILES	c1cccc2cccc2c1
InChI	1/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H
InChI key	CUFNKYGDVFVPHO-UHFFFAOYAT
ChemSpider ID	8876
Properties
Molecular formula	C10H8
Molar mass	128.17 g mol−1
Melting point	99–100 °C
Boiling point	242 °C
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

An azulene is an organic compound and a isomer of naphthalene. Whereas naphthalene is colourless, azulene is dark blue. Its name is derived from the Spanish word azul, meaning "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.

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 and first synthesis were reported by Lavoslav Ružička, followed 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.^[1]

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 0.8 debye, in contrast with naphthalene, which has a dipole moment of zero. This polarity can be explained by regarding azulene as the fusion of the aromatic 6 π-electron cyclopentadienyl anion and aromatic 6 π-electron tropylium cation. In order to achieve the stable aromatic sextet in both rings, one electron from the seven-membered ring is transferred to the five-membered ring. The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds. Reactivity studies confirm that seven-membered ring is electrophilic and the five-membered ring is nucleophilic.

Organic synthesis

Synthetic routes to azulene have long been of interest because of its unusual structure. An efficient one-pot route entails annulation of cyclopentadiene with unsaturated C₅-synthons.^[2] The alternative approach from cycloheptatriene has long been known, one illustrative method being shown below.^[3]

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

Organometallic complexes

In organometallic chemistry, azulene serves as a ligand for low-valent metal centers, which otherwise are known to form π-complexes with both cyclopentadienyl and cycloheptatrienyl ligands. Illustrative complexes are (azulene)Mo₂(CO)₆ and (azulene)Fe₂(CO)₅.^[4]

Related compounds

In naphth[a]azulenes, a naphthalene ring is condensed at the 1,2-positions of azulene. In one such system^[5] deformation from planarity is found similar to that of tetrahelicene.

External links

• MSDS Website
• MSDS Website

References

1. ^ Harmon AD, Weisgraber KH, Weiss U. (1979). "Preformed azulene pigments of Lactarius indigo (Schw.) Fries (Russulaceae, Basidiomycetes)". Cellular and Molecular Life Sciences 36 (1): 54–56. ISSN 1420-682X. 
2. ^ Klaus Hafner and Klaus-Peter Meinhardt (1990), "Azulene", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv7p0015 ; Coll. Vol. 7: 15 
3. ^ Approach to the Blues: A Highly Flexible Route to the Azulenes Sébastien Carret, Aurélien Blanc, Yoann Coquerel, Mikaël Berthod, Andrew E. Greene, Jean-Pierre Deprés Angewandte Chemie International Edition Volume 44, Issue 32 , Pages 5130 - 5133 2005 Abstract
4. ^ Melvyn R. Churchill “Transition Metal Complexes of Azulene and Related Ligands” Progress in Inorganic Chemistry 1970, volume 11. doi:10.1002/9780470166123.ch2
5. ^ 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 Kimiaki Yamamura, Shizuka Kawabata, Takatomo Kimura, Kazuo Eda, and Masao Hashimoto J. Org. Chem.; 2005; 70(22) pp 8902 - 8906; (Article) DOI: 10.1021/jo051409f Abstract