Triphospha[3]radialene Makes Its Debut

Radialenes are an unusual class of unsaturated hydrocarbons that researchers in Japan have made even more unusual by incorporating phosphorus atoms (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201200374). The simplest member of the class, [3]radialene, is a cyclopropane ring with a double-bonded carbon jutting out from each vertex in the ring. It’s a structural isomer of benzene, but it comes with an important electronic difference: Benzene has a π-conjugated aromatic ring system, whereas [3]radialene has a π-cross-conjugated nonaromatic ring system, which instills useful optical and electronic properties in the molecule. Hideaki Miyake, Takahiro Sasamori, and Norihiro Tokitoh of Kyoto University became interested in increasing that difference by substituting phosphorus atoms for the external carbon atoms. They did so by combining tetrachlorocyclopropane with a bulky arylphosphine in the presence of a base, followed by oxidation with iodine, to produce 4,5,6-triphospha[3]radialene. They found that altering the cross-conjugated skeleton with the heavier phosphorus atoms red-shifts the ultraviolet absorption of radia­lene and increases its electron-accepting ability.