Synthesis, structural analysis, and properties of [8]circulenes.

Polygons: [8]Circulenes were easily prepared by Pd-catalyzed annulations of tetraiodotetraphenylenes with alkynes. Their saddle-shaped structure with an [8]radialene character was identified by X-ray crystallography. Similar to 1,3,5,7-cyclooctatetraene, they have a tub-shaped eight-membered ring, but all of the bond lengths and bond angles are almost equal. Variable-temperature NMR investigations showed interesting dynamic behavior.