Cyclodecapentaene

Cyclodecapentaene or [10]annulene is an annulene with molecular formula C₁₀H₁₀. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because of a combination of steric strain and angular strain. The planar all-cis isomer (1) (like benzene is) would have bond angles of 144° which creates large amounts of bond strain relative to the ideal 120° for sp² hybridization. A boat-like all-cis conformation (2) relieves much of the angular strain but is still unstable with respect to the next planar trans, cis, trans, cis, cis isomer (3). Yet this isomer suffers from steric repulsion between the two internal hydrogen atoms. The nonplanar trans, cis, cis, cis, cis isomer (4) is the most stable of all the possible isomers.

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The parent [10]annulene can be obtained by photolysis of cis-9,10-dihydronaphthalene as a mixture of isomers. Because of their lack in stability even at low temperatures the reaction products revert back to the original dihydronaphthalene.

Aromaticity can still be induced in [10]annulenes by fixation of the planar geometries and there exist two methods to bring it about. Replacing two hydrogen atoms by a methylene bridge gives the planar bicyclic 1,6-methanonaphthalene (5). This compound is aromatic as judged from lack in bond length alternation by x-ray crystallography and a telltale diamagnetic ring current from NMR spectroscopy.

A classical organic synthesis of this compound starts from a Birch reduction of naphthalene to tetrahydronaphthalene (or isodecalin), is followed by carbene addition of dichlorocarbene (prepared in-situ from chloroform and the potassium salt of tert-butanol) followed by a second organic reduction that removes the chloride substituents and is concluded by the organic oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).

Another way to restore aromaticity in [10]annulene by plane fixation is by incorporation of a methine bridge to a tricyclic[10]annulene core structure (6) ². When deprotonated to form the anion this type of compound is even more stabilized. The central carbanion makes the molecule even more planar and the number of resonance structures that can be drawn is extended to 7 included two resonance forms with a complete benzene ring. By computational chemistry it is demonstrated that the tricyclic[10]annulene derivative with an annulated benzene ring and a full set of cyano substituents (7) is one of the most acidic compounds known with a computed pKa in DMSO of - 30.4 (compared to for instance -20 for magic acid) ¹.

Azulene is also a 10 pi electron system in which aromaticity is maintained but in yet another way.

References

• ¹ Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO – a density functional study Robert Vianello and Zvonimir B. Maksi Chemical Communications, 2005, (27), 3412 - 3414 Abstract
• ² 7b-Methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylic acid, a new 10 -electron aromatic system; X-ray crystal structure Thomas L. Gilchrist, Charles W. Rees, David Tuddenham and David J. Williams Journal of the Chemical Society, Chemical Communications, 1980, (15), 691 - 692 Abstract
• ³ Advanced Organic Chemistry F.A. Carey, R.J. Sundberg Plenum Press 1984 ISBN 0-306-41087-7