Barrelene | |
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Bicyclo[2.2.2]octa-2,5,7-triene |
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Identifiers | |
CAS number | 500-24-3 |
ChemSpider | 120100 |
Jmol-3D images | Image 1 Image 2 |
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Properties | |
Molecular formula | C8H8 |
Molar mass | 104.15 g mol−1 |
(verify) (what is: / ?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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Infobox references |
Barrelene is a bicyclic organic compound with chemical formula C8H8 and systematic name bicyclo[2.2.2]octa-2,5,7-triene. First synthesized and described by H. E. Zimmerman in 1960 the name derives from the obvious resemblance with a barrel, with the staves being three ethylene units attached to two methine groups. It is the formal Diels-Alder adduct of benzene and acetylene. Due to its unusual molecular geometry the compound is of considerable interest to theoretical chemists. Like benzene, barrelene has a set of 6 cyclic, but not planar, overlapping p-orbitals. Because it is not possible to avoid a destabilizing overlap of opposite-sign lobes, the structure represents Möbius aromaticity.
Triptycenes, with the alkene groups part of an arene, are related compounds. It is also a starting material for many other organic compounds such as semibullvalene.
The original Zimmerman synthesis modified in 1969[1] starts from coumalic acid:[2]
many alternative routes have been devised since then, one of them starting from oxepin:[3] [4]
An alternate route that allows synthesis of the parent barrelene system and a variety of substituted barrelenes has also been reported[5]
Barrelene is hydrogenated with hydrogen gas and Adams' catalyst in ethanol to the fully saturated bicyclo[2.2.2]-octane. Bromination with bromine in tetrachloromethane gives a di-bromo adduct because a coupling reaction intervenes:
Epoxidation of barrelene with oxone gives the trioxatrishomobarrelene[6] which on rearrangement with boron trifluoride (driving force:relief of strain energy) converts into the trioxatrishomocubane:[7]
This compound can be envisioned as a cubane with three oxygen atoms inserted into three opposite edges or as 9-crown-3 capped by two methine units. The molecule is chiral and the separate enantiomers have been isolated.
Certain barrelenes have been used as a monomer in a ring opening metathesis polymerization :[8] [9]
The catalyst is a Fischer carbene (a molybdenum bis-(hexafluoro-tert-butoxy) carbene catalyst) and the long alkyl chain attached to the monomer is required for solubility. Oxidation of the polymer with DDQ affords the naphthalene pendant of poly(p-phenylene vinylene).
Isopentane solutions of barrelene undergo photolytic isomerisation when acetone is added as a photosensitizer to produce semibullvalene. Prolonged irradiation results in further isomerisation to form cyclooctatetraene.[10]