Cyclobutadiene

Cyclobutadiene is the smallest [n]-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 verified by infrared studies. This is in contrast to the square geometry predicted by simple Hückel theory. Though it has alternating single and double bonds, it fails Hückel'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. Cyclobutadiene dimerizes by a Diels-Alder reaction at 35 K.

Synthesis
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 443 with ammonium cerium(IV) nitrate. This cyclobutadieneiron tricarbonyl complex was prepared from 49 and cis-dichlorocyclobutene in a double dehydrohalogenation.

Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkynes to a Dewar benzene:


 * [[Image:Cyclobutadiene-DewarbenzeneConversion.png|600px|Cyclobutadiene to Dewarbenzene conversion]]

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

One cyclobutadiene derivative is also accessible through a [2+2]cycloaddition of a di-alkyne. In this particular reaction the trapping reagent is 2,3,4,5-tetraphenylcyclopenta-2,4-dienone and one of the final products (after expulsion of carbon monoxide) a cyclooctatetraene:


 * [[Image:CyclobutadienSynthDessyWhite.png|400px|Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006]]