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(n)cyclacenes, 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(n)cyclacenes. 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(n)cyclacenes as fully conjugated systems has failed so far: they remain an inaccessible target despite some 25 years of synthetic effort. Concurrently, theoretical studies revealed that 6(n)cyclacenes as linearly annelated systems show small energy gaps between triplet and singlet states; they can thus be expected to be unstable species. Angularly annelated systems, on the other hand, were predicted to have large singlet-triplet splittings. Nakamura and co-workers and our laboratory have found ways around the obstacles to create cyclacenes. Nakamura and co-workers prepared the first angular annelated cyclacene by selective reduction of the north and south poles of C(60). We used a cyclic system that is fully conjugated and also adopts a boat conformation: cyclooctatetraene. By following this principle, we synthesized two types of linearly annelated cyclacenes: metal-stabilized four-membered rings alternating with eight-membered rings (4.8(n)cyclacenes), and six-membered rings alternating with eight-membered rings (6.8(n)cyclacenes). These new scaffolds, if extended along the molecular axis, would represent as-yet unknown forms of carbon nanotubes and are thus inviting synthetic targets. Calculations show that the incorporation of heteroatoms (S and NH) into these scaffolds is promising, encouraging the synthetic pursuit of thia- and azacyclacenes.