Radialene


Radialenes are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds. The double bonds are commonly alkene groups but those with a carbonyl group are also called radialenes. 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 hybridized 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. The first radialene called hexaethylidencyclohexane was synthesised in 1961.

Conformation

and radialenes are expected to have a planar molecular geometry with all carbon atoms in the same plane. This is verified experimentally in hexamethylradialene and cyclobutanetetraonetetrakis. Decamethylradialene has a twist envelope geometry with C2 symmetry while a chair conformation is calculated for radialene and found experimentally for hexa-cyclohexane
Due to their specific pi-electron distributions, hydrocarbons such as perylene and triphenylene are not considered radialenes. One study describes a radialene composed of thiophene units:
This compound is reported as planar with D3h symmetry but not aromatic: the carbon-carbon bond lengths are unusually long and the calculated NICS value is close to zero.

Synthesis and properties

The parent ,, and radialenes polymerize when in contact with oxygen.

3Radialenes

Radialene or trimethylenecyclopropane was synthesised in 1965. Reported derivatives are triquinocyclopropanes, salts of trimethylenecyclopropane dianions, triscyclopropane, triscyclopropane and hexakisradialene. Phosphorus derivatives have also been reported. Phospharadialenes have been investigated as quantum efficiency improvers in solar cells Hexakisradialene derivatives have been investigated for their low oxidation potentials.

4Radialenes

The unsubstituted radialene has been prepared in an elimination reaction of cis,trans,cis-tetracyclobutane with sodium methoxide in ethanol.
Image:-4-radialeneSynthesis.png|400px|center|radialene synthesis
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.

5Radialenes

Successful low-temperature synthesis of the parent compound radialene was reported in 2015.

6Radialenes

The parent radialene is unstable and polymerises immediately on formation. It has been synthesised from 1,5,9-cyclododecatriyne, 1,3,6-trimesitylene and tricyclobutabenzene.
Only substituted radialenes exist as stable compounds. Stable derivatives are the hexamethyl substituted, dodecamethyl substituted and hexabromo substituted radialene.
A trisalkoxy-substituted radialene has also been reported, the central ring adopting a non-planar twist-boat conformation:

Uses

Radialenes have been researched as a potential way to access complex synthetic molecules and in polymer synthesis.