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 various types of ring strain destabilize an all-planar geometry.[1]: 121–122 

Cyclodecapentaene

all-cis isomer of cyclodecapentaene
Names
Preferred IUPAC name
Cyclodeca-1,3,5,7,9-pentaene
Other names
[10]Annulene
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/C10H10/c1-2-4-6-8-10-9-7-5-3-1/h1-10H/b2-1-,3-1-,4-2-,5-3+,6-4+,7-5+,8-6+,9-7-,10-8-,10-9-
    Key: ZYRKBGIIBMTQHN-HGJACCJQSA-N
  • C1=C\C=C\C=C/C=C/C=C/1
Properties
C10H10
Molar mass 130.190 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Conformation, strain, and non-aromaticity
SMILES: C1=CC=CC=CC=CC=C1; all cis-, all in a plane, making a regular decagon
(1): a hypothetical planar con­forma­tion for all-cis-​[10]annulene.
SMILES: C1=CC=CC=CC=CC=C1; all cis-, but the outer two double-bonds are tilted up, in the same direction, making a sort of bowl shape
(2): the lowest-energy con­forma­tion for all-cis-​[10]annulene
SMILES: C1=CC=CC=CC=CC=C1; isomer trans,cis,trans,cis,cis, making a pinched oval, with two hydrogens leaping out of the plane in the same direction at the pinch
(3): one hypothetical planar con­form­ation for trans,­cis,­trans,­cis,­cis-​[10]annulene
SMILES: C1=CC=CC=CC=CC=C1; isomer trans,cis,cis,cis,cis, making a "figure-8"
(4): one conformation for trans,­cis,­cis,­cis,­cis-​[10]annulene

Although not aromatic itself, [10]annulene can transition between different con­forma­tional isomers through aromatic or quasi­aromatic excited states, such that its con­forma­tional iso­mer­ism is fixed only at extreme cryogenic temperatures.[2] Under­stand­ing the com­posi­tion and react­ivity of these mix­tures com­put­ation­ally has proven difficult,[3] because a large number of conformations all minimize the energy locally.[4]

The all-cis isomer (1), a fully convex decagon, would have bond angles of 144°, which creates large amounts of angle strain relative to the ideal 120° in sp2 atomic hybridization. Instead, the all-cis isomer adopts a planar boat-like conformation (2) to relieve the angle strain,[5] although it, too, is less stable than the next planar isomer, trans,​cis,​trans,​cis,​cis-[10]annulene (3). Yet even isomer (3) is unstable, suffering from steric repulsion between the two internal hydrogen atoms,[6] and tends to distort into the perimeter of two fused circles, one larger and the other smaller, as in azulene.[2] The nonplanar trans,­cis,­cis,­cis,­cis isomer is the most stable of all possible isomers, although it is unclear whether it too has a boat-like configuration as in conformer (4), or the "heart" configuration produced if one internal hydrogen in conformer (3) were flipped inside-out.[2]

Synthesis

Cyclodecapentaene can undergo an electrocyclic rearrangement to[7] or from dihydronaphthalene. Photolysis of the latter generates [10]annulene, but it quickly reverts to the reactant, even at cryogenic temperatures.[1]: 122 

Aromatic derivatives
(5): an aromatic bridged [10]annulene
(6): An aromatic tricyclic [10]annulene

Aromaticity can be induced in compounds having a [10]annulene-type core if planarity is forcibly imposed by other substituents. Two methods to do so are known.

One method is to formally replace two hydrogen atoms by a methylene bridge (−CH2−); this gives the planar bicyclic 1,6-methano­[10]annulene (5). Indeed, 1,6-methano­[10]annulene has no bond length alternation in its X-ray structure and signs of a telltale diamagnetic ring current in its NMR spectrum.[3] Likewise, a tricyclic methine bridge gives an aromatic structure (6) similar to the stable oxonium ion oxatriquinacene.[8]

(7): a very acidic cyclo­deca­pentaene derivative

When de­proton­ated to form the anion this type of compound is even more stabilized. The central carbanion enhances the molecule's planarity and the number of resonance structures that can be drawn is extended to 7 included two resonance forms with a complete benzene ring. Computational chemistry suggests a tricyclic[10]annulene derivative with an annulated benzene ring and a full set of cyano substituents (7) would be one of the most acidic compounds known, with a computed pKa in DMSO of −30.4 (compared to for instance −20 for magic acid).[9]

The other method is to further remove hydrogens and develop triple bonds or cyclopropanes along the ring. Thus com­puta­tional studies suggest that cyclo­deca­tetraene­yne is (although formally a 12-π system) planar and aromatic,[10] as is bicyclo[8.1.0]undeca-1,3,7,9-tetraen-5-yne.[11] Predicting the aroma­ticity of these compounds is not always obvious: the polycyclic hydrocarbon tetra­dihydro­naphtho­[10]annulene, in which a valence isomer of [10]annulene is fused to two naphthalenes, does not exhibit aromaticity inside the central 10-π ring.[12]

  • Azulene is also a 10 π-electron system in which aromaticity is maintained by direct trans­annular bonding to form a fused 7–5 bicyclic molecule.
  • Cyclodecatetraene is a stable, non-aromatic 8 π-electron system with no ring strain.[1]: 131 

References
  1. Kemp-Jones, A. V.; Masamune S. (1973). "The monocyclic 10π-electron system". In Nozoe Tetsuo; Breslow, Ronald; Hafner, Klaus; Itô Shô; Murata Ichiro (eds.). Topics in Nonbenzenoid Aroma­tic Chemistry. Vol. I. Tokyo/New York: Hirokawa Publishing, on behalf of Halsted Press, a division of John Wiley & Sons. pp. 121–131. ISBN 0-470-65155-5 – via the Internet Archive.
  2. Castro, Claire; Karney, William L.; McShane, Colleen M.; Pemberton, Ryan P. (2006-04-01). "[10]Annulene: Bond Shifting and Conformational Mechanisms for Auto­meriz­ation". The Journal of Organic Chemistry. 71 (8): 3001–3006. doi:10.1021/jo0521450. ISSN 0022-3263.
  3. Slayden, Suzanne W.; Liebman, Joel F. (2001-05-01). "The Energetics of Aromatic Hydrocarbons: An Experimental Thermo­chemical Perspective". Chemical Reviews. 101 (5): 1545–1546. doi:10.1021/cr990324+. ISSN 0009-2665.
  4. Xie Yaoming; Schaefer, Henry F.; Liang Guyan; Bowen, J. Phillip (Feb 1994). "[10]Annulene: The Wealth of Energetically Low-Lying Structural Isomers of the Same (CH)10 Connectivity". Journal of the American Chemical Society. 116 (4): 1442–1449. doi:10.1021/ja00083a032. ISSN 0002-7863.
  5. Xie et al. 1994, though note that Kemp-Jones & Masamune 1973, pp. 126–7 instead proposes a "twist" conformation, with 6 atoms coplanar and the remaining 4 in a raised handle.
  6. Sulzbach, Horst M.; Schleyer, Paul v. R.; Jiao Haijun; Xie Yaoming; Schaefer, Henry F. (Feb 1995). "A [10]Annulene Isomer May Be Aromatic, After All!". Journal of the American Chemical Society. 117 (4): 1369–1373. doi:10.1021/ja00109a021. ISSN 0002-7863.
  7. Masamune S.; Seidner, R. T. (1969). "[10]Annulenes". Journal of the Chemical Society D: Chemical Communications (10): 542. doi:10.1039/c29690000542. ISSN 0577-6171.
  8. Gilchrist, Thomas L.; Rees, Charles W.; Tuddenham, David; Williams, David J. (1980). "7b-Methyl-7bH-cyclopent[cd]indene-1,2-dicarboxylic acid, a new 10-electron aromatic system; X-ray crystal structure". Journal of the Chemical Society, Chemical Communications (15): 691–692. doi:10.1039/C39800000691.
  9. Vianello, Robert; Maksi, Zvonimir B. (2005). "Extremal acidity of Rees polycyanated hydrocarbons in the gas phase and DMSO—a density functional study". Chemical Communications (27): 3412–3414. doi:10.1039/B502006A. PMID 15997281.
  10. Navarro Vázquez, Armando; Schreiner, Peter R. (2005-06-01). "1,2-Didehydro­[10]annulenes: Structures, Aromaticity, and Cycl­iz­ations". Journal of the American Chemical Society. 127 (22): 8150–8159. doi:10.1021/ja0507968. ISSN 0002-7863.
  11. Parmar, Karnjit; Blaquiere, Christa S.; Lukan, Brianna E.; Gengler, Sydnie N.; Gravel, Michel (Sep 2022). "Synthesis of a highly aromatic and planar dehydro­[10]annulene derivative". Nature Synthesis. 1 (9): 696–700. doi:10.1038/s44160-022-00135-z. ISSN 2731-0582.
  12. Umeda Rui; Hibi Daijiro; Miki Koji; Tobe Yoshito (2009-09-17). "Tetradehydro­dinaphtho­[10]annulene: A Hitherto Unknown Dehydro­annulene and a Viable Precursor to Stable Zethrene Derivatives". Organic Letters. 11 (18): 4104–4106. doi:10.1021/ol9015942. ISSN 1523-7060.
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