Radialene

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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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 describes a [6]radialene composed of thiophene units ^[4]:

This compound is reported as planar with D_3h 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.

Synthesis & properties

The parent [3],[4],[5] and [6]radialenes polymerize when in contact with oxygen.

[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]

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.

References