Cyclodecapentaene

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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).

Cyclodecapentaene or [10]annulene is an

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 

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

locally.[4]

The all-

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,[citation needed] 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

bridge gives an aromatic structure (6) similar to the stable oxonium ion oxatriquinacene.[8]

(7): a very acidic cyclo­deca­pentaene derivative

When

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]

Other related compounds

  • 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. ^
    ISBN 0-470-65155-5 – via the Internet Archive
    .
  2. ^
    ISSN 0022-3263
    .
  3. ^
    ISSN 0009-2665
    .
  4. 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. ISSN 0002-7863
    .
  7. ISSN 0577-6171
    .
  8. doi:10.1039/C39800000691
    .
  9. PMID 15997281
    .
  10. ISSN 0002-7863
    .
  11. ISSN 2731-0582
    .
  12. ISSN 1523-7060
    .
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