Cyclobutadiene

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Cyclobutadiene
Cyclobutadiene
Chemical name 1,3-Cyclobutadiene
Other names Cyclobutadiene
[4]Annulene
Chemical formula C4H4
Molecular mass 52.07 g/mol
CAS number [1120-53-2]
SMILES C1=CC=C1
Disclaimer and references

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 C4H4 and a rectangular structure verified by infrared studies. This is in constrast 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.

[edit] 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 C4H4Fe(CO)3 with ammonium cerium(IV) nitrate. This cyclobutadieneiron tricarbonyl complex was prepared from Fe4(CO)9 and cis-dichlorocyclobutene in a double dehydrohalogenation [1] [2].

Cyclobutadiene when liberated from the iron complex reacts with electron-deficient alkynes to a Dewar benzene [3]:

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 [4]:

Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee 2006

[edit] See also

[edit] References

  1. ^ Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes G. F. Emerson, L. Watts, R. Pettit; J. Am. Chem. Soc.; 1965; 87(1); 131-133. First Page
  2. ^ 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
  3. ^ Cyclobutadiene L. Watts, J. D. Fitzpatrick, R. Pettit J. Am. Chem. Soc.; 1965; 87(14); 3253-3254. Abstract
  4. ^ Revisit of the Dessy-White Intramolecular Acetylene-Acetylene [2 + 2] Cycloadditions Chung-Chieh Lee, Man-kit Leung, Gene-Hsiang Lee, Yi-Hung Liu, and Shie-Ming Peng J. Org. Chem.; 2006; 71(22) pp 8417 - 8423; (Article) DOI:10.1021/jo061334v
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