Bowl-shaped aromatic

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]. 

The importance of this cycloaromatization in modern aromatic chemistry only became obvious when it was realized that it constitutes an optimal route to bowl-shaped aromatics and fullerene partial structures [9, 18, 19]. The key experiment, Scott s synthesis of cor-annulene (40) from 7,10-diethynylfluoranthene (37) [20], was quickly adapted to more extended systems, the examples 38 —> 41 [21] and 39 —> 42 [22] in Scheme 10 being just two of many ... 

Scheme 10. Bowl-shaped aromatics by high temperature pyrolysis of acetylenes.

The discovery of fullerenes has prompted a general interest in bowl-shaped aromatic hydrocarbons. Although corannulene 23 as the first bowl-shaped conjugated system had already been synthesized by Barth and Lawton [259], a simple... 

The bowl-shaped conj ugated hydrocarbons represent an attractive system for testing the theory of aromaticity since the curved surface of the carbon network represents a compromise between strain and conjugation. 

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. 

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... 

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... 

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... 

The deviation from the planar form of 7 is rather small. Since an aromatic structure is not easily destroyed by bending (cf. Section (5.2), [69]), the VB results obtained for the planar transition state are expected not to deviate much from those of bowl-shaped 7. Of course, the calculation on planar 7 is computationally much cheaper. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

A single crystal X-ray diffraction analysis has been carried out on both the chloro- and the phenyl-substituted subphthalocyanines 2.284 and 2.287 (Figures 2.3.2 and 2.3.3). These analyses served to show that subphthalocyanines lie in a bowl-shaped conformation. This is, of course, very different from the near-planar conformation of the parent phthalocyanines. Presumably, this bowl-shaped structure accounts, in part, for the decreased molar absorptivities of the subphthalocyanines relative to their phthalocyanine parents . Nevertheless, despite the non-planar nature of these macrocycles, the subphthalocyanines are capable of supporting an induced diamagnetic ring current (as judged by NMR spectroscopy). Thus, they may appropriately be considered as being aromatic. 

Compounds with the molecular formula (CgH4N2)3BX, where X = F or Cl, have been previously isolated from reactions of halogenoboranes and phthalodinitrile. The structure of the chlorine compound has now been determined and shows a phthalocyanine-like structure (31).179 The system is bowl-shaped, in contrast to the planar phthalocyanine structure. The boron atom is co-ordinated to three N atoms (at 1.467 A, i.e. short) and one Cl (1.863 A). The tt-electrons in the 14-membered ring form a quasi-aromatic conjugated Tr-system. 

Several functionalized trialkyltin hydrides have been prepared and used in organic synthesis. For example, an optically active organotin hydride with binaphthyl as chiral center underwent hydrostannylation with methyl methacrylate leading to a -stannyl ester diastereoselectivity, however, was not sufficient [234]. Although a bowl-shaped organotin hydride with bulky aromatic substituents was prepared, the structurally novel tin hydride resulted in quite high chemoselectivity. When tris(2,6-diphenylbenzyl)tin hydride (TDTH) was used for competitive reduction of carbonyls under the influence of a Lewis acid it was observed that unsaturated carbonyl compounds such as benzaldehyde and a,/ -enones are highly resistant to TDTH reduction (Scheme 12.131) [235]. 

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]. 

The calixarenes are a popular and versatile class of macrocycle formed from the condensation of a p-substituted phenol (e.g. p-tert-butylphenol) with formaldehyde. Since they contain bridged aromatic rings, they are formally members of the cyclophane family (Section 6.5). In cyclophane nomenclature they are termed substituted [l.l.l.ljmetacyclophanes . The descriptive name calixarene was coined by C. David Gutsche (Washington University, USA) because of the resemblance of the bowl-shaped conformation of the smaller calixarenes to a Greek vase called a calix crater (Figure 3.78). The number of phenolic residues is denoted by a number in square brackets. Thus the most common cyclic tetramer with p-f-butyl substituents is termed p-t-butyl-calix[4]arene (3.118). It is easy to understand why this appealing nomenclature has found wide acceptance within the field when it is compared to the Chemical Abstracts systematic name for 3.118, [19.3.1.U U l ]octacosa-l(25),3,5,7(28),9,ll,... 

Still and coworkers also described some remarkably selective peptide receptors. Tricyclic systems 40-42 represent an interesting sequence. These molecules adopt bowl-shaped conformations in which the central aromatic ring serves as the "base." and the outer rings of amide groups define the rim. The first example 40 was found to bind a range of N-Boc a-amino acid amides with excellent enantioselectivities, peaking at 130 7 for valine derivative 43 in Receptor 41 was then used to... 

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