Due to their unique architectural features, the cycloparaphenylenes (1, Figure 1) have attracted attention from many vantage points. 1 Their strained and distorted aromatic systems and radially oriented p orbitals (2) have intrigued synthetic chemists, theoreticians, supramolecular chemists, and materials scientists alike. Despite this widespread interest, the cycloparaphenylenes remain an unsolved synthetic challenge. 2-4 Here we describe the first synthesis of [9]-, [12]-, and [18]cycloparaphenylene. We also utilize computational methods to understand the structural and optical properties of these new macrocycles. As these structures correspond to the shortest-possible segment of an armchair carbon nanotube, 5 we refer to them as "carbon nanohoops."The heart of the synthetic challenge for the cycloparaphenylenes lies in the strain energy of the bent aromatic system. 6 Our strategy was to build up strain sequentially during the synthesis, using a 3,6-syn-dimethoxy-cyclohexa-1,4-diene moiety as a masked aromatic ring in a macrocyclic intermediate (see structure 4, Scheme 1). We reasoned that the cyclohexadiene unit would provide the curvature and rigidity necessary for macrocyclization to afford a marginally strained intermediate. Subsequent aromatization would then provide the driving force necessary to achieve the strain energies of target compounds, calculated to be 47, 28, and 5 kcal/ mol for [9]-, [12]-, and [18]cycloparaphenylenes, respectively (Vide infra).

The design and construction of non-natural products have fascinated and perplexed organic chemists for years. Their assembly, akin to what has been accomplished for the total synthesis of natural products, has stretched the limits of what can be prepared in the laboratory. Unlike many natural products, however, carbon-rich structures often lack heteroatoms, further complicating their construction. Consider some of the classical molecules in this genre: cubane and dodecahedrane. While highly symmetric, their assembly is far from trivial. These fascinating hydrocarbon targets have fueled the development of carbon-carbon bond-forming reactions, as new methods are needed to access these types of compounds. Among these carbon-rich structures, polycyclic aromatics such as helicenes, fullerenes, and some fullerenes share common ground due to the distortion of one or more aromatic rings out of planarity. Recently added to this group are the [n]cycloparaphenylenes ([n]CPPs), "carbon nanohoops". Here, a linear string of benzene rings connected at the para positions is wrapped back upon itself to form a cyclic structure. Clearly a simple linear p-oligophenylene cannot be cyclized in this manner without extremely harsh reaction conditions. In order to access these structures using solution-phase organic chemistry, clever synthetic strategies that can compensate for this severe distortion are required. Although cycloparaphenylenes can be considered the smallest possible fragment of an armchair carbon nanotube (CNT), they were envisioned as synthetic targets long before CNTs were discovered in 1991. CPP synthesis was first attempted in 1934, almost 70 years before Iijima's first report on CNTs. The long-forgotten targets reemerged in 1993 with a report from Vögtle, though he ultimately was unsuccessful in achieving their synthesis. More than a decade later, in 2008, CPPs succumbed to total synthesis by Jasti and Bertozzi, allowing access to three different-sized carbon nanohoops in milligram quantities. Since then, the Jasti group has embraced the smallest CPPs as inspiring synthetic targets, challenging us to develop new methodology to construct increasingly strained macrocycles. Having recently synthesized [5]-, [6]- and [7]CPP, the three smallest nanohoops synthesized to date, we have been able to realize a variety of new physical phenomena unique to these structures. Perhaps most significantly, unlike linear p-phenylenes and inorganic quantum dots, the HOMO-LUMO gaps of the CPPs narrow with decreasing CPP size. The smallest CPPs discussed in this Account illustrate this feature exceptionally well, as their HOMO-LUMO gaps become narrower than those of even the longest p-polyphenylenes. The smaller CPPs are fascinating from a structural standpoint as well because of the high amount of distortion in each benzene ring. From the synthesis of [7]CPP (84 kcal/mol of strain energy) to that of [5]CPP (119 kcal/mol of strain energy), our laboratory has been able to test the boundaries of synthetic and physical organic chemistry. In ...

[n]Cycloparaphenylenes (or "carbon nanohoops") are cyclic fragments of carbon nanotubes that consist of n para linked benzene rings. These strained, all sp(2) hybridized macrocycles, have size-dependent optical and electronic properties that are the most dynamic at the smallest size regime where n = 5-12. This review highlights the unique physical phenomena surrounding this class of polycyclic aromatic hydrocarbons, specifically emphasizing the novel structural, optical, and electronic properties of [5]-[12]CPPs.

No abstract