Cyclacenes: Hoop-Shaped Systems Composed of Conjugated Rings

Abstract

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 C60. 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.

All‐Carbon Scaffolds by Rational Design

“…Routes to the total synthesis of [60] fullerene have been successfully pursued, [14] and cyclic, belt-like molecules have been constructed as models for carbon nanotubes. [15,16] Expanded polycyclic aromatic hydrocarbons, obtained by novel synthetic protocols, [17,18] are re-investigated as defined molecular models for graphene. [19,20] Actually, first interest into novel all-carbon allotropes dates back to 1966, when Hoffmann proposed that cyclo[n]carbons (cyclo-C n ), such as cyclo-C 18 (1, Fig.…”

Magnetically Induced Currents in [n]Cycloparaphenylenes, n = 6−11

“…These esthetically appealing structures have unique physical properties, and offer the possibility of creating novel supramolecular complexes. [12][13][14] The [n]CPs with five and six arene rings were synthesized in 1993, 15 whereas synthesis was attempted already in 1934 by Parekh and Guha. 16 The term nanohoops was coined by Jasti and co-workers, who in 2008 synthesized [9]CP, [12]CP, and [18]CP and studied their UV-vis spectra.…”

Nanostrukturierte Kohlenstoffmaterialien aus molekularen Vorstufen

“…Die Synthese solcher Verbindungen wurde bereits 1954 [114] erstmals angestrebt und wird auch gegenwärtig noch weiterentwickelt. [112,115] Ein grundlegender Durchbruch gelang unlängst Bertozzi und Mitarbeitern mit der ersten Synthese dreier Abkömmlinge der [n]-Paraphenylen-Makrocyclen, die formal einem Segment einer einwandigen Armchair-CNT entsprechen (Schema 2). [116] Weiterhin waren Bodwell und Mitarbeiter in der Lage, ein verbrücktes Teropyren auf einen bis dato unerreichten Winkel von 1678 zu biegen und somit ein großes Segment einer einwandigen (8,8) Armchair-CNT herzustellen.…”

“…Gemeinsames Problem bei der Herstellung von Derivaten der [n]-Cyclacene durch die Gruppen um Schlüter, [118] Stoddart [119,120] und Cory [121,122] war die ausbleibende finale Aromatisierung der vielversprechenden partiell gesättigten Vorstufenmolekü-le. [112] Theoretische Studien weisen darauf hin, dass die stabilisierende Aromatisierungsenergie im Falle der [n]-Cyclacene nicht ausreicht, um die hohe Ringspannung zu kompensieren. [112,123] Die berechnete Energiedifferenz zwischen dem Singulett-und dem reaktiven Triplettzustand ist sowohl für gerade als auch ungerade [n]-Cyclacene niedrig, weshalb vorhergesagt wurde, dass diese Moleküle einen (Di)radikalCharakter haben.…”

“…[112] Theoretische Studien weisen darauf hin, dass die stabilisierende Aromatisierungsenergie im Falle der [n]-Cyclacene nicht ausreicht, um die hohe Ringspannung zu kompensieren. [112,123] Die berechnete Energiedifferenz zwischen dem Singulett-und dem reaktiven Triplettzustand ist sowohl für gerade als auch ungerade [n]-Cyclacene niedrig, weshalb vorhergesagt wurde, dass diese Moleküle einen (Di)radikalCharakter haben. Bei massenspektrometrischen Untersuchungen einer Vorstufe des Gürtels eines C 84 -Fullerens durch Schlüter, Stanger und Mitarbeiter wurden ungewöhnliche Umlagerungen beobachtet, die anstelle der erhofften vollständigen Aromatisierung stattfanden.…”