Radialene

[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]

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^[2] 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 and properties

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

[3]Radialenes

[3]Radialene or trimethylenecyclopropane was synthesised in 1965. ^{[5] [6] [7]}. Reported derivatives are triquinocyclopropanes ^{[8] [9] [10]}, salts of trimethylenecyclopropane dianions ^[11], tris(thioxanthen-9-ylidene)cyclopropane ^[12], tris(fluoren-9-ylidene)cyclopropane ^[13] and hexakis(trimethylsilylethynyl)[3]radialene ^[14]

[4]Radialenes

The unsubstituted [4]radialene has been prepared in an elimination reaction of cis,trans,cis-tetra(bromomethyl)cyclobutane with sodium methoxide in ethanol ^[15]

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.

[6]Radialenes

The parent [6]radialene is unstable and polymerises immediately on formation. It has been synthesised from 1,5,9-cyclododecatriene, 1,3,6-tri(chloromethyl)mesitylene and tricyclobutabenzene ^{[16][17][18][19]}

Only substituted [6]radialenes exist as stable compounds. Stable derivatives are the hexamethyl substituted,^[3][20] dodecamethyl substituted ^[21] and hexabromo substituted ^[22] radialene.

A trisalkoxy-substituted radialene has also been also reported,^[23] the central ring adopting a non-planar twist-boat conformation:

References