Cyclobutadiene

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Cyclobutadiene also briefly CBD is an extremely unstable diene with the formula C4H4. In free form, it disintegrates within 5 seconds to 1,3- butadiene. Cyclobutadiene annulene is the smallest and, as such, only at low temperatures in solid matrices are intercepted from inert gases without it dimerizes. Is a ring system containing conjugated double bonds. Having an even number of pairs of electrons, it is according to the Hückel rule a Antiaromat and thus a low Mesomeriestabilisierungsenergie diradical. Therefore, it is highly reactive and dimerizes under almost all conditions by the Diels -Alder reaction, the reaction takes place even still at -78 ° C. In this case, preferably, the endo- isomer forms, while the exo- isomer is formed only as by-product.

After numerous failed attempts, it could be produced by Rowland Pettit 1965 for the first time at the University of Texas, but he could not isolate it. Cyclobutadiene can be prepared by reduction of its metal compounds.

Cyclobutadiene is a stable ligand in many metal complexes. This is due to the fact that two electrons are pushed from the metal to ligand orbitals, thereby hückelaromatisches 6π -electron system is formed. CBD can generally oxidatively (for example, cerium (IV ) nitrate ) to be removed from the metal center.

Cyclobutadiene can be implemented with electron-deficient alkynes after release from the metal complexes. It starts to form Dewar benzene, which then slowly rearranges to a benzene derivative. Cyclobutadiene further can be obtained from the decomposition of 2 -pyrone. After thermally controlled ring-closing diene obtained by evolution of CO2.