Radialene
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[n]Radialenes are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds. The double bonds are commonly alkene groups but those with a carbonyl (C=O) group are also called radialenes [1]. For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.
Radialenes are related to open-chain dendralenes and also to compounds like butadiene and benzene which also consist of a ring of sp2 hydridized carbon atoms.
Radialenes are investigated in organic chemistry for their unusual properties and reactivity but have not ventured outside the laboratory. Reported uses are as experimental building blocks for novel organic conductors and ferromagnets [2]. The first radialene called hexaethylidencyclohexane was synthesised in 1961 [3].
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[edit] Conformation
[3] and [4]radialenes are expected to have a planar molecular geometry with all carbon atoms in the same plane. This is verified experimentally in hexamethyl[3]radialene and cyclobutanetetraonetetrakis(hydrazone). Decamethyl[5]radialene has a twist envelope geometry with C2 symmetry while a chair conformation is calculated for [6]radialene and found experimentally for hexa-(ethylidene)cyclohexane
Due to their specific pi-electron distributions, hydrocarbons such as perylene and triphenylene are not considered radialenes. One study <name=Patra/> describes a [6]radialene composed of thiophene units [4]:
This compound is reported as planar with D3h symmetry (X-ray diffraction) but not aromatic: the carbon-carbon bond lengths are unusually long (145 pm vs. 140 pm for benzene) and the calculated NICS value is close to zero.
[edit] Synthesis & properties
The parent [3],[4],[5] and [6]radialenes polymerize when in contact with oxygen.
[edit] [4]Radialene
The unsubstituted [4]radialene has been prepared in an elimination reaction of cis,trans,cis-tetra(bromomethyl)cyclobutane with sodium methoxide in ethanol [5]
![[4]radialene synthesis](../../../../images/shared/thumb/a/ae/-4-radialeneSynthesis.png/400px--4-radialeneSynthesis.png)
Hydrogenation with platinum on carbon gives cis,cis,cis-tetramethylcyclobutane in accordance with the proposed structure. On standing in air at room temperature the compound accepts oxygen and polymerizes.
[edit] References
- ^ Effect of Overcrowding in [n]Radialenes on the Synthesis of Bis[4]radialenesMenahem Kaftory, Mark Botoshansky, Shunji Hyoda, Toshihiro Watanabe, and Fumio Toda J. Org. Chem.; 1999; 64(7) pp 2287 - 2292; (Article) doi:10.1021/jo9818
- ^ Planar [6]Radialenes: Structure, Synthesis, and Aromaticity ofBenzotriselenophene and Benzotrithiophene Asit Patra, Yair H. Wijsboom, Linda J. W. Shimon, and Michael Bendikov Angew. Chem. Int. Ed. 2007, 46, 8814 –8818 doi:10.1002/anie.200703123
- ^ (German) Zur Kenntnis der Hexaalkylbenzole. 3. Mitteilung. Über einen neuen Kohlenwasserstoff C18H24 (Hexaäthylidencyclohexan) Helvetica Chimica Acta Volume 44, Issue 2, Date: 1961, Pages: 380-386 H. Hopff, A. K. Wick
- ^ Coupling reaction reagent bis(1,5-cyclooctadiene)nickel(0). Equal amount of tetramer formed
- ^ The Chemistry of Photodimers of Maleic and Fumaric Acid Derivatives. IV.1 Tetramethylenecyclobutane Gary W. Griffin and Laurence I. Peterson J. Am. Chem. Soc.; 1962; 84(17) pp 3398 - 3400; doi:10.1021/ja00876a033
