Kekulene

The kekulene macrocycle consists of twelve anellated benzene rings and may be considered as an [iSlaanulene (inside) or a 30]annulene (outside). H. Staab (F. Diederich, 1978) called it a superbenzene , since it has the same symmetry as benzene. 

Consider the two structures shown for kekulene, one suggesting inner and outer annulenes, and the other a series of phenanthrene-like units. Indicate properties that you would expect to be associated with each structure. Proton NMR and bond-length data are given. How do they compare with your expectations ... 

The proton NMR ( H NMR) spectrum of 90 (called kekulene) showed that in a case where electrons can form either aromatic sextets or larger systems, the sextets... 

Fig. 6. The contour maps of q3 of the two hexabenzocoronenes and kekulene showing the characteristic features of their Clar patterns...

For substrates with extremely low volatility, Staab et al. used a modification of the standard procedure [18], which has been successfully applied for the synthesis of kekulene [19]. A closed and evacuated valve filled with the substrate is introduced for 3-5 min in a preheated air-bath (ca. 500 °C). Pyrolysis does occur in the solid phase, not in the gas phase. However, the sulfur-free products sublime more easily than the starting material and after a short time they condensate at a cooling device outside the reaction vessel, where the work-up can be carried out as usual. Yields are low for this modification nevertheless, for substrates with extemely low volatility (and extremely low solubility of the sulfides as well) this often is the only choice available. 

A highlight in this field certainly is the successful pyrolysis of 124 to give 125 in 75% yield, an intermediate in the synthesis of kekulene [19]. 

There is a close relationship between polycyclic compounds such as coronene 81198) or Staab s kekulene 82199) and the macrocyclic cyclophanes such as 83200). They can be described as perimeter structures in which an outer and inner perimeter can be considered. 

The homogeneous bold connected cycles comprise the family named circu-lenesby Wynberg and Dopper [79].Corannulene (62) [4-6],coronene (63) [80], and pleidannulene (64) [81] represent the [5]-, [6]-, and [7]circulenes, respectively. A related 12-membered cycle, but not a circulene, can be seen in the structure of Kekulene (65) [82,83]. 

KAPA. See 1,3-Propanediamine, mono-K salt Katsuki-Sharpless oxn. See Sharpless epoxidation Kekulene = 15,23 16,22-dimethenobenzofl, 2-a 5,4-a ]dipentaphene, 338 Kemp s acid, 346-347... 

The molecule of 15,23 16,22-dimethenobenzo[l,2-iZ 4,5-fl ]dipentha-phene (C48H24, Figure 2), also known as kekulene or superbenzene, was studied at both the HF/STO-3G and HF/6-31G levels. The optimized geometry was found to be planar and have D f, symmetry, with the six inner hydrogen atoms separated by only 1.851 A. In addition, structures of five other ben-zenoid hydrocarbons (anthracene, phenanthrene, tetracene, benzanthracene, and chrysene) were optimized. The corresponding total energies were found to... 

Figure 2 The molecular skeleton of 15,23 16,22-dimethenobenzo[l,2-a 4,5-a ]di-penthaphene (kekulene).

The H-NMR spectrum and crystal structure of kekulene (4) disproved the concentric [18]annulene/ [30]annulene character implied by the resonance form shown in Fig. 1. Instead, kekulene is better represented by... 

Fig. 1 Known torands (1-3) and related molecules kekulene (4), cyclosexipyridine (5).Schiff-base pyridomacrocycles (6 and 7), and a molecular coil (8).

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