Corannulene bowl shape

If both ethylene subunits of a l,3-hexadien-5-yne derivative are members of benzene entities, the cycloaromatization, caused by flash vacuum thermolysis, may give rise to bowl-shaped molecules such as corannulene or semibuckminsterfullerene. However, in those cases, the initial step is not a Hopf cydization but an isomerization of the ethynyl to a vinylidenecarbene group [125]. 

From the assemblies of fused five- and six-membered rings identifiable on the surface of 1, corannulene (8) is the smallest fragment that retains a bowl-shaped conformation. However, there are other known hydrocarbons that adopt bowl (or... 

The preceding section indicated the importance of corannulene as the smallest member of the family of bowl-shaped, polynuclear aromatic hydrocarbons modeling the surface of buckminsterfullerene. Surprisingly, despite its obvious significance in relationship to the theory of aromaticity, 8 had not attracted the attention of chemists prior to the first report of its synthesis in 1966." As Barth and Lawton noted ... to the time of our first report, it appears this structure had neither adoriied the jacket or end cover of any book nor served as a symbol of an international symposium. More than that, corannulene does not appear to have been suggested in the chemical literature prior to 1966. 

Dihydtocorannulene (21) and tetrahydiocorannulene (22) were first produced as side products in the synthesis of corannulene (see Scheme 4). These structures retain the general bowl shape, and NMR inversion barriers were estimated to be 8.5 and 7 kcal/mol, respectively. Dihydrocorannulenes can also be produced by the addition of alkyllithiums to corannulene to afford 1-alky 1-1,2-dihydrocoran-nulenes (21 ). ... 

It is apparent from Table 1 that the cyclohexadiene ring in 1,2-dihydrocorannule-nes is significantly flatter than either 1,3-cyclohexadiene itself or 9,10-dihy-drophenanthrene due to the constriction imposed by the remainder of the bowl-shaped ring system. Nonetheless, the lack of planarity means that substituents at may adopt either pseudoaxial (pa) or pseudoequatorial (pe) positions. However, the surface of the corannulene system is curved, and, unlike 1,3-cyclohexadiene and 9,10-dihydrophenanthrene, neither the two pa positions nor the two pe positions are equivalent. Thus, substituents in 1 -/ -1,2-dihydrocorannulenes may be (1) endo-pseudoaxial (endo-pa), (2) e do-pseudoequatorial (e/ido-pe), (3) exo-pseudoaxial (exo-pa), and (4) exo-pseudoequatorial (exo-pe), where endo and exo are related to concave and convex orientations, respectively. 

Corannulene is the simplest example within the class of polynuclear aromatic compounds with curved surfaces that are related by carbon framework to the i fullerenes. These aromatic hydrocarbons possess bowl-shaped geometries, consist... 

All of the theoretical methods that allow for geometry optimization predict the bowl-shaped conformation of 8 to predominate over the planar alternative. However, the calculated energy differences between the two conformations differ drastically and span the range of 0.8 to 84 kcal/mol. Moreover, the calculated energy difference between the plants and bowl-shaped conformers is quite important since it represents the barrier for bowl-to-bowl inversion if (a) planar corannulene represents the transition state (TS) for the process, and (b) the entropic term in AG is negligible. Condition (a) seems to be fulfilled, since vibrational frequency calculations give one and only one imaginary frequency for the planar structure... 

MNDO calculations suggest a stacked-bowl arrangement of the two tetraan-ionic corannulene moieties in a convex-to-concave geometry with four lithiums inside the sandwich as the global energy minimum (Figure 11 Thus, in agreement with theoretical studies on the isolated tetraanions (see above), the corannulene tetraanion also prefers a bowl-shaped conformation when the countercations are included in the calculations. 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

Fullerenes can also be obtained by pyrolysis of hydrocarbons, preferably aromatics. The first example was the pyrolysis of naphthalene at 1000 °C in an argon stream [58, 59], The naphthalene skeleton is a monomer of the Cjq structure. FuUerenes are formed by dehydrogenative coupUng reactions. Primary reaction products are polynaphthyls with up to seven naphthalene moieties joined together. FuU dehydrogenation leads to both Cjq as well as C7Q in yields less than 0.5%. As side products, hydrofuUerenes, for example CjqHjj, have also been observed by mass spectrometry. Next to naphthalene, the bowl-shaped corannulene and benzo[k]fluoranthene were... 

Our goal in those days was to find a new Jt-system more aromatic than king benzene. Then came the news of the corannulene (6) synthesis by Barth Lawton (1966) and its bowl-shaped structure quickly aroused strong interest among us. As is mentioned below, analysis of its structure logically led us to conceive the extrapolation of its structure to a sphere so that we could envisage three-dimensional delocalization of the 7t-electrons. Fortuitously about that time my small son started... 

Corannulene, C2oH10, dibenzo y/ /, mno] fluoranthene, shown in Fig. 1(a), is a bowl-shaped PAH with 5-fold symmetry [6], and has a one-third skeleton of C60 (see Figs 1(c) and (d)). Recently, convenient synthesis for corannulene [7,8] has been developed. On the other hand, coronene, C24H12, hexabenzobenzene, shown in Fig. 1(b), has a flat structure with a 6-fold symmetry axis. 

We removed the top and bottom radialene units of the fullerene molecule to generate [10]cyclophenacene as described above. An alternative view of [60]fullerene may generate hemispherical bowl-shaped aromatic systems [lc, 37] for instance, fused corannulene C (C30H12, Chart 2.1), can be identified as half of fullerene [38]. 

In Chapter 2, Matsuo and Nakamura describe the synthetic developments of a subgroup of bowl-shaped it-conjugated corannulenes and expand the topic to the discussion of the synthesis of a novel class of organic fluorophores derived from C60, hoop-shaped condensed aromatic compounds. They are made by selective removal of ten sp2 carbon centers out of conjugation from the north and south pole regions of C60. The resulting 40-jr-electron [lOjcyclophenacenes excised... 

Corannulene (43), the smallest curved subunit of C6o, was first synthesized by Barth and Lawton in 196664. However, this bowl-shaped hydrocarbon remained relatively inaccessible prior to the studies of the groups of Scott65, Siegel66 and Zimmermann67. 

It has been shown by B3LYP/6-31IG" calculations that the characteristic bowl shape of corannulene 6 is flattened in the corresponding fully furo[r]-annelated derivative 7, which thus is by and large analogous to the decamethylidene derivative 8 <2004JOC8111>. 

Fig. 58. Circulenes a) corannulene 147, bowl-shaped) b) [7]circulene 149, saddle-shaped) c) coronene 148, planar)...

Numerous aromatic compounds including several bowl-shaped fullerene fragments have been prepared by this method, e.g. cyclopenta[z/]fluoranthene (22, Scheme8, [54d,56a]), cyclopenta[bc]corannulene (23, [55a]) and diace-naphtho[3,2,l,8-cdejg 3, 2, l, 8 -Zmnop]chrysene (24, see Scheme 9, [55b,c]). Of special importance is this approach for the synthesis of cyclopenta-annelated PAHs, e.g. and the three isomeric dicyclopentapyrenes (25-27, Scheme 9, [54e, 56b]). Using these reference samples, several cyclopenta-annelated PAHs could be identified as byproducts formed in the incomplete oxidation of hydrocarbons in fuel rich flames [57]. 

Bowl-shaped carbocations (33) were obtained by alkylation of the parent corannulene. ° The cations were obtained as stable salts and characterized by X-ray analysis. Phenalenyl cations (e.g. (34)) were obtained as air-stable BPh4 salts. Observations made in a selective synthesis of l,2-di(organo)fullerenes led to a mechanism whereby a cationic fullerene intermediate is generated by Cu(ll) oxidation of a fullerene radical (or anion). ... 

Stabilization of neutral radicals is another interesting property of bowl-shaped corannulene. The buckybowl neutral radicals 39 [52] and 40 [79] are stable as solids in air or as solutions in degassed toluene for long periods. Experimental results suggest spin delocalization of the radical species onto the corannulene moiety in 40 is more significant than 39. 

Sumanene (tricyclopenta[ /e/jl/,/t(7r]triphenylene, 2) is a C21-hydrocarbon containing three periphery methylene bridges, and it is also a basic bowl-shaped fragment of buckminsterfullerene. Sumanene (2) is named after suman , the Hindi and Sanskrit word for a type of flower [133], In contrast to corannulene, the chemistry of sumanene is relatively new because this compound was first successfully synthesized in the laboratory in 2003 [134]. 

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