ChemInform Abstract: Cyclacenes: Hoop-Shaped Systems Composed of Conjugated Rings

A carbon nanotube, if it could be cut sideways at the end like a pipe, would yield beltlike structures. These subunits of carbon nanotubes, in their simplest manifestation as the [6]ncyclacenes, are composed of conjugated six-membered rings that are annelated, that is, made of a series of consecutively fused ring structures. These seemingly simple “one-benzene-thick” slices are of considerable interest as models—for electronic structure and spectroscopic properties, for example—for carbon nanotubes. In the late 1980s Stoddart and co-workers, soon to be followed by other groups, embarked on the synthesis of [6]ncyclacenes. The necessary curved shape of the belt was achieved with boat-shaped precursors for Diels−Alder reactions, namely, 7-oxanorbornene derivatives. The preferred endo-addition in Diels−Alder reactions assured an efficient synthesis of beltlike systems containing 12 to 18 six-membered rings. However, the removal of the auxiliary oxygen centers to achieve [6]ncyclacenes as fully conjugated sys...