Circulene

Recently, remarkable progress by several groups has been obtained in the synthesis and structural characterization of a number of [8]circulene derivatives. Notably, in 2013, Wu and coworkers [62] reported the synthesis of a series of 

The overall yield for these two approaches was quite similar however, the latter methodology could lead to functionalization more easily. Compounds 132a,b are stable yellow solids under ambient conditions, and 132a was found to be more stable in an aerobic solution of dg-DMSO at 100 °C for 24 h [65]. The saddle shape of 132a was confirmed by X-ray crystallography analysis. Two symmetry-independent conformers A and B of 132a with symmetry 

HOMA [65] and NICS [64] values indicated that the two sextet-containing rings exhibited aromatic character and the empty ring was only moderately aromatic. Preliminary study also showed that 132b has potential application as a p-type semiconductor [64]. 

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Polynuclear aromatic hydrocarbons formed by the angular fusion of benzene rings may adopt either planar or nOnplanar minimum energy conformations, as demonstrated by the circulenes 8-10. [6]Circulene (coronene 9), the earliest known member of the family, prefers a planar structure of symmetry since the presence... 

Other molecules in which benzene rings are forced out of planarity are corannulene (20),45 (also called 5-circulene), 7-circulene (21),46 22,47 and 2348 (see also p. 161). 

Fig. 1. Structures of (a) [5]circulene, corannulene and (b) [6]circulene, coronene. Three corannulene (c) skeletons give the C60 structure (d).

In many cyclic aromatic compounds an element other than carbon (commonly N, O, or S) is also present in the ring. These compounds are called heterocycles. Figure 14.2.3 shows some nitrogen heterocycles commonly used as ligands, as well as planar, sunflower-like octathio[8]circulene, C Ss, which can be regarded as a novel form of carbon sulfide. 

In [m] circulenes, a family of polyaromatic hydrocarbons so named in 1975 by Wynsberg, in which m refers to the number of aromatic rings arranged in a circle, the total number of n electrons does not indicate aromaticity or anti-aromaticity according to the Hiickel rule. This rule is strictly only applicable to monocyclic systems. It is adequate, however, to consider the inner and the outer n electrons separately whose numbers obey the An + 2 Hiickel criterion for aromaticity, since both these circuits are monocyclic [49]. Coronene, a flat graphite frag-... 

Here the term circulene is used so that the coronoids form a subclass under circulenes. The smallest circulene which is not a coronoid, is [6]circulene, a system... 

The dualist representation, in contrast to the hexagon representation, does distinguish between coronene and [6]circulene the latter system is depicted below. 

Any (poly)circulene, which may be a (multiple) coronoid, is a multiply connected polyhex. 

It may seem unnecessary to bring in such artifacts as the above examples of non-polyhexes. Yet we find the discussion to be warranted. Ege and Vogler [28, 29], for instance, treated pyrene in a group together with coronene ([6]circulene) and six coronoids. Implicitly, the authors considered pyrene as [4]circulene and ascribed an inner perimeter of two edges to it. That leads to vertices of degree four as in one of the forbidden examples above. Therefore we claim that the interpretation of pyrene as [4]circulene is not justified. 

The distinction between catacondensed and pericondensed systems is applicable to all the classes of polyhexes treated above (Fig. 2). A catacondensed polyhex is defined by the absence of internal vertices. An internal vertex is a vertex shared by three hexagons. A pericondensed polyhex possesses at least one internal vertex. In terms of dualists a pericondensed polyhex reveals itself by the presence of at least one three-membered cycle (triangle). A dualist of a circulene has a cycle larger than a triangle. 

These numbers, when taken together, account for all the classes of polyhexes defined above, viz. benzenoids, helicenes, and circulenes including helicirculenes (Fig. 2). In Sect. 3 the distinction between catacondensed and pericondensed systems is the only one considered for the different classes in question (Fig. 2). Section 4 contains some additional definitions. 

In one of the classical papers on the enumeration of poly hexes, Lunnon [10] found the numbers of all geometrically planar polyhexes with h (number of hexagons) up to 12 see Table 4. In these numbers the helicenic systems are excluded, but all non-helicenic circulenes are included. Hence the numbers pertain to benzenoids plus planar circulenes. The contribution of Balaban and Harary [13] to the Lunnon numbers (cf. footnotes to Table 4) is commented in the next section. The Lunnon numbers have been extended to h = 13 and h = 14 by means of the data from Muller et al. [36]. 

The planar (non-helicenic) circulenes have also been considered separately, first by the Diisseldorf-Zagreb group [16,17, 35] see Table 5. He and He [33] depicted the forms of these systems for h <, 9 from these figures we have extracted separate numbers for the catacondensed and pericondensed systems up to this h value. For the catacondensed planar monocirculenes with h < 11 the numbers may be deduced from a work of Brunvoll et al. [54], who enumerated the benzenoid systems composed of coronene with catacondensed appendages up to h = 12. On... 

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