Cyclobutadiene is the smallest href="http://articles.gourt.com/en/annulene">annulene ([4-annulene), an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula ₄₄ and a rectangular structure (with a square form hypothetically possible). Though it has alternating single and double bonds, it fails Huckel's rule, because its ring has 4 π electrons, and 4 is not twice an odd number. Some cyclobutadiene-metal compounds are stable because the metal atom provides 2 more electrons to the system,

The pi electron energy of cyclobutadiene is higher than that of its open-chain counterpart, 1,3-butadiene, and it is therefore said to be antiaromatic rather than aromatic. As a result, it is highly reactive and has a very short lifetime. In fact cyclobutadiene is so unstable and reactive that two cyclobutadiene molecules will instantaneously dimerize by a Diels-Alder reaction under nearly any conditions, even at -78°C.

After numerous attempts, it was first synthesized in 1965 by Rowland Pettit of the University of Texas, although he could not isolate it. Cyclobutadiene can be generated through degradation from a cyclobutadiene metal compounds for example ₄₄()₃ with ammonium cerium(IV) nitrate. This cyclobutadieneiron tricarbonyl complex was prepared from ₄()₉ and cis-dichlorocyclobutene Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes G. F. Emerson, L. Watts, R. Pettit; J. Am. Chem. Soc.; 1965; 87(1); 131-133. First Page Iron, tricarbonyl (η4-1,3-cyclobutadiene)- R. Pettit and J. Henery Organic Syntheses, Coll. Vol. 6, p.310 (1988); Vol. 50, p.21 (1970) Link.

Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkynes to a Dewar benzene Cyclobutadiene L. Watts, J. D. Fitzpatrick, R. Pettit J. Am. Chem. Soc.; 1965; 87(14); 3253-3254. Abstract:

The Dewar benzene converts to dimethyl phthalate on heating at 90°C.

See also

• Cyclobutene

References