Barrelene

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 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.


 * [[Image:BarreleneMO representation.png|100px|Barrelene with set of p-orbitals]]

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 starts from coumalic acid :


 * [[Image:BarreleneSynthesisZimmermann.png|600px|Barrelene Synthesis Zimmermann 1969]]

many alternative routes have been devised since then, one of them starting from oxepin :


 * [[Image:Barrelene1996Synthesis.png|700px|Barrelene Synthesis 1997]]

barrelene Reactions
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:


 * [[Image:BarreleneBromination.png|400px|Barrelene Bromination]]

Epoxidation of barrelene with oxone gives the trioxatrishomobarrelene which on rearrangement with boron trifluoride (driving force:relief of strain energy) converts into the trioxatrishomocubane :


 * [[Image:TrioxatrishomocubaneSynthesis.png|700px|Trioxatrishomocubane Synthesis]]

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 polymerisation


 * [[Image:BarrelenePolymerization.png|500px|Benzobarrelene polymerization]]

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).