Cyclodecapentaene

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Cyclodecapentaene
All-cis-cyclodecapentaene.svg
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
PubChem CID
  • 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 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 

Contents

Conformation, strain, and non-aromaticity

Planar cis-10annulene.svg
(1): a hypothetical planar con­forma­tion for all-cis-​[10]annulene.
Boat cis-10annulene.svg
(2): the lowest-energy con­forma­tion for all-cis-​[10]annulene
Planar alternant 10annulene.svg
(3): one hypothetical planar con­form­ation for trans,­cis,­trans,­cis,­cis-​[10]annulene
Figure-8 alternant 10annulene.svg
(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 ).[ citation needed ] Yet even isomer (3) is unstable, suffering from steric repulsion between the two internal hydrogen atoms, [6] and tends to distort into an 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 Bridged 10annulene.svg
(5): an aromatic bridged [10]annulene
( 6): An aromatic tricyclic [10]annulene Twice-bridged 10annulene.svg
(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 Cyano 10annulene derivative.svg
(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]

Related Research Articles

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

Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The word "aromatic" originates from the past grouping of molecules based on smell, before their general chemical properties were understood. The current definition of aromatic compounds does not have any relation with their smell.

<span class="mw-page-title-main">Aromaticity</span> Phenomenon of chemical stability in resonance hybrids of cyclic organic compounds

In chemistry, aromaticity means the molecule has cyclic (ring-shaped) structures with pi bonds in resonance. Aromatic rings give increased stability compared to saturated compounds having single bonds, and other geometric or connective non-cyclic arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability. The term aromaticity with this meaning is historically related to the concept of having an aroma, but is a distinct property from that meaning.

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

Azulene is an 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.

<span class="mw-page-title-main">Annulene</span> Completely conjugated monocyclic hydrocarbons

Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds ('mancude'). They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). The IUPAC accepts the use of 'annulene nomenclature' in naming carbocyclic ring systems with 7 or more carbon atoms, using the name '[n]annulene' for the mancude hydrocarbon with n carbon atoms in its ring, though in certain contexts (e.g., discussions of aromaticity for different ring sizes), smaller rings (n = 3 to 6) can also be informally referred to as annulenes. Using this form of nomenclature 1,3,5,7-cyclooctatetraene is [8]annulene and benzene is [6]annulene (and occasionally referred to as just 'annulene').

<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.

Antiaromaticity is a chemical property of a cyclic molecule with a π electron system that has higher energy, i.e., it is less stable due to the presence of 4n delocalised electrons in it, as opposed to aromaticity. Unlike aromatic compounds, which follow Hückel's rule and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.

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

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.

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

1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy.

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

Cyclooctadecanonaene or [18]annulene is an organic compound with chemical formula C
18H
18. It belongs to the class of highly conjugated compounds known as annulenes and is aromatic. The usual isomer that [18]annulene refers to is the most stable one, containing six interior hydrogens and twelve exterior ones, with the nine formal double bonds in the cis,trans,trans,cis,trans,trans,cis,trans,trans configuration. It is reported to be a red-brown crystalline solid.

<span class="mw-page-title-main">Cyclic compound</span> Molecule with a ring of bonded atoms

A cyclic compound is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon, none of the atoms are carbon, or where both carbon and non-carbon atoms are present. Depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be aromatic or non-aromatic; in the latter case, they may vary from being fully saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size numbers in the many billions.

<span class="mw-page-title-main">Homoaromaticity</span> Organic molecular structure

Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. Although this sp3 center disrupts the continuous overlap of p-orbitals, traditionally thought to be a requirement for aromaticity, considerable thermodynamic stability and many of the spectroscopic, magnetic, and chemical properties associated with aromatic compounds are still observed for such compounds. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a contiguous cycle of π electrons that is responsible for this preserved chemical stability.

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

Cyclododecahexaene or [12]annulene is a member of the series of annulenes with some interest in organic chemistry with regard to the study of aromaticity. Cyclododecahexaene is non-aromatic due to the lack of planarity of the structure. On the other hand the dianion with 14 electrons is a Hückel aromat and more stable.

<span class="mw-page-title-main">Möbius aromaticity</span>

In organic chemistry, Möbius aromaticity is a special type of aromaticity believed to exist in a number of organic molecules. In terms of molecular orbital theory these compounds have in common a monocyclic array of molecular orbitals in which there is an odd number of out-of-phase overlaps, the opposite pattern compared to the aromatic character to Hückel systems. The nodal plane of the orbitals, viewed as a ribbon, is a Möbius strip, rather than a cylinder, hence the name. The pattern of orbital energies is given by a rotated Frost circle (with the edge of the polygon on the bottom instead of a vertex), so systems with 4n electrons are aromatic, while those with 4n + 2 electrons are anti-aromatic/non-aromatic. Due to incrementally twisted nature of the orbitals of a Möbius aromatic system, stable Möbius aromatic molecules need to contain at least 8 electrons, although 4 electron Möbius aromatic transition states are well known in the context of the Dewar-Zimmerman framework for pericyclic reactions. Möbius molecular systems were considered in 1964 by Edgar Heilbronner by application of the Hückel method, but the first such isolable compound was not synthesized until 2003 by the group of Rainer Herges. However, the fleeting trans-C9H9+ cation, one conformation of which is shown on the right, was proposed to be a Möbius aromatic reactive intermediate in 1998 based on computational and experimental evidence.

Cyclotetradecaheptaene, often referred to as [14]annulene, is a hydrocarbon with molecular formula C14H14, which played an important role in the development of criteria (Hückel's rule) for aromaticity, a stabilizing property of central importance in physical organic chemistry. It forms dark-red needle-like crystals.

In organic chemistry, Baird's rule estimates whether the lowest triplet state of planar, cyclic structures will have aromatic properties or not. The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird at the University of Western Ontario in 1972.

<span class="mw-page-title-main">1,6-Methano(10)annulene</span> Chemical compound

1,6-Methano[10]annulene (also known as 1,6-methanonaphthalene or homonaphthalene) is an aromatic hydrocarbon with chemical formula C11H10. It was the first stable aromatic compound based on the cyclodecapentaene system to be discovered.

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

Cyclononatetraene is an organic compound with the formula C9H10. It was first prepared in 1969 by protonation of the corresponding aromatic anion (described below). It is unstable and isomerizes with a half-life of 50 minutes at room temperature to 3a,7a-dihydro-1H-indene via a thermal 6π disrotatory electrocyclic ring closing. Upon exposure to ultraviolet light, it undergoes a photochemical 8π electrocyclic ring closing to give bicyclo[6.1.0]nona-2,4,6-triene.

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

Bicalicene is polycyclic hydrocarbon with chemical formula C16H8, composed of two cyclopentadiene and two cyclopropene rings linked into a larger eight-membered ring. There are two isomers: cis-bicalicene and trans-bicalicene. It is a dimer of calicene.

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

Butalene is a polycyclic hydrocarbon composed of two fused cyclobutadiene rings. A reported possible synthesis of it involves an elimination reaction from a Dewar benzene derivative. The structure itself can be envisioned as benzene with an internal bridge, and calculations indicate it is somewhat less stable than the open 1,4-didehydrobenzene biradical, the valence isomer in which that bridged bond is broken.

<span class="mw-page-title-main">Bicyclo(6.2.0)decapentaene</span> Chemical compound

Bicyclo[6.2.0]decapentaene is a bicyclic organic compound and an isomer of naphthalene and azulene.

References

  1. 1 2 3 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. 1 2 3 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. 1 2 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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