Tricyclobutabenzene

Tricyclobutabenzene is an aromatic hydrocarbon consisting of a benzene core with three cyclobutane rings fused onto it. This compound and related compounds are studied in the laboratory because they are often display unusual conformations and because of their unusual reactivity. Tricyclobutabenzenes are isomers of radialenes with which they are able to interconvert.

The parent tricyclobutabenzene (C₁₂H₁₂) was first synthesized in 1979 ["Tricyclobutabenzene" Wutichai Nutakul, Randolph P. Thummel, Austin D. Taggart J. Am. Chem. Soc.; 1979; 101(3); 770-771. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1979/101/i03/f-pdf/f_ja00497a064.pdf Abstract] ] via a procedure depicted in "Scheme 1" [Reaction sequence: coupling reaction of cyclobutene Grignard reagents followed by Diels-Alder reaction with dimethylcyclobutene-1,2-dicarboxylate, followed by ester hydrolysis to dicarboxylic acid with potassium hydroxide in methanol followed by decarboxylation and aromatization with lead tetraacetate ] ["Poly-Oxygenated Tricyclobutabenzenes via Repeated [2 + 2] Cycloaddition of Benzyne and Ketene Silyl Acetal" Toshiyuki Hamura, Yousuke Ibusuki, Hidehiro Uekusa, Takashi Matsumoto, and Keisuke SuzukiJ. Am. Chem. Soc.; 2006; 128(11) pp 3534 - 3535; [http://dx.doi.org/10.1021/ja0602647 Abstract] ] . This compound is stable up to 250°C.

A polyoxygenated tricyclobutabenzene with an extraordinary bond length of 160 pm for the bond connecting two carbonyl groups Ref|Hamura was synthesized according to "scheme 2" [ Reaction sequence: the key step is a [2+2] cycloaddition between an aryne formed from the iodotriflate by action of "n"-butyllithium and a ketene silyl acetal. The silyl acetal is then converted to a ketone group by hydrofluoric acid and the remaining acetal groups by reaction with boron trifluoride] .

An ordinary bond of this type is only 148 pm and for comparison the C-C bond in isatin is 154 pm long. On the other hand, no change is recorded in the aromatic bond length alternation.

Applying similar chemistry derivatives are now known with 6 carbonyl groups or 12 methoxy groups. ["Dodecamethoxy- and Hexaoxotricyclobutabenzene: Synthesis and Characterization" Toshiyuki Hamura, Yousuke Ibusuki, Hidehiro Uekusa, Takashi Matsumoto, Jay S. Siegel, Kim K. Baldridge, and Keisuke Suzuki J. Am. Chem. Soc.; 2006; 128(31) pp 10032 - 10033; (Communication) DOI|10.1021/ja064063e] . A key starting material is the iodo triflate depicted below which is a benzotriyne synthon.

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