Benzenoids kekulene

Diederich. F. Staab, H.A. Benzenoid versus annulenoid aromaticity Synthesis and properties of kekulene. Angew. Chem., Int. Ed. Engl. 1978. 17, 372-374. 

Fig. 8.14 Benzenoid coronoids kekulene and its EC values, followed by fibonacenic coronoids with EC = 4 for all their rings...

Fig. 8.18 Top row one algebraic Kekule structure (left) corresponds (right) to a pair of geometric Kekule structures of kekulene (a coronafusene) - note that these are not EC counts, and that there are many other geometric Kekule stmctures. Bottom row one algebraic Kekule structure (left) corresponds to four geometric Kekule structures (right) for the perifusene with 11 benzenoid rings combining pyrene and perylene moieties (left)...

Zhou and Navangul calculated MNDO hardnesses for 14 benzenoid hydrocarbons and showed it to be a good indicator for their aromaticity.Using the hardness criterion, kekulene, coronene, and the corannulene tetraanion were all predicted to be superaromatic, also confirmed by calculations of Babic and Trinajstic and Cioslowski, O Connor, and Fleischmann in the case of kekulene. " Jiao and Schleyer, however, concluded, based on geometrical, energetic, and magnetic criteria of aromaticity, that kekulene is not superaromatic. ... 

Aromaticity (interpreted or defined in different ways) is a classical property which has been attributed to certain organic compounds. Indeed, the benzenoid hydrocarbons which exist chemically, are frequently referred to as polycyclic aromatic hydrocarbons (PAHs). Also kekulene is a typical PAH. Balaban (1980) asked the rhetorical question " Is aromaticity outmoded In his treatise an extensive historical introduction (going back to 1608) is found. The question is of course answered in the negative. A modern treatment of aromaticity can hardly avoid reference to Clar (1972). 

In the first case (i) of the above diagram, the effective units are four benzenoids (two pyrenes and two benzenes) in case (ii) the effective units are two benzenoids (anthracenes) and one coronoid (kekulene) in case (iii) the effective units are two benzenoids (naphthalenes) and one special degenerate single coronoid (condensed [18]annulene, viz. hexabenzo[6cdc/,fcim7io][18]-annulene). 

Bowever, aU benzenoids with the hP values prescribed above need not be inspected (for h > 8). Take for instance h = 12, for which the maximum value is 7 in accordance with eqn. (1). Here only one out of the 331 benzenoids with seven hexagons is of interest, viz. coronene. The corresponding corona hole h = 7, = 6) occurs in kekulene h = 12), whUe all single... 

Exactly all the circular benzenoids 0 N S) with 5 > 18 can be perforated by a coronene hole to produce single coronoids 0(n 5). Then one has i T=n+6, 5=5 — 6. A detailed treatment on a complete enumeration of the isomers of the class 0 has been published (Cyvin BN, Brunvoll, Chen and Cyvin 1993). The smallest system of this class is kekulene. In the cited reference a coronoid with coronene hole(s) was referred to as "coronenic . We quote a nice sentence therefrom "A coronenic coronoid has a coronene corona hole. We shall not use the term "coronenic in the following, although it is analogous with our term "naphthalenic (Definition 3.3). A brief account on the title systems and their enumeration is given in the next two paragraphs. 

Diederich F, Staab HA (1978) Benzenoid versus Annulenoid Aromatidty — Synthesis and Properties of Kekulene. Angew Chem Inf Ed Engl 17 372... 

Circular phenylenes have the distinguishing characteristic of a resonance picture that includes forms that encompass both the irmer and outer peripheral loops, a phenomenon described as superdelocalization [81]. This class of circular phenylenes remains elusive [67, 82]. The simplest member of this series that does not suffer from additional circular strain is [6]phenylene 77d (Scheme 4.18), also christened antikekulene [57] to highlight its relationship to kekulene, its aU-benzenoid relative with an equal number of rings [83]. In antikekulene, avoidance of (benzo)cyclobutadienoid local circuits is expected to enhance the contribution of the potentially superdelocalized resonance form depicted for the structure in Scheme 4.18, albeit with the added and destabilizing feature that both inside and outside peripheries contain a (4n) electron count. 

The structures shown in Figure 63, which include kekulene as the last structure, which was prepared in 1978 by Staab, Diderich, and co-workers, have been excluded from both alternative definitions of benzenoids, although they are expected to show considerable similarities in their properties to ben-zenoid hydrocarbons. The reason for their exclusion from the class of benzenoids is that, although they can be viewed as derived by fusion of benzene rings, they also incorporate a larger (central) ring that does not represent benzene. Hence, such structures should not be taken as a standard for characterization of benzenoid hydrocarbons. Because of their considerable similarity to benzenoids, one may refer to these as pseudo-benzenoids. Such compounds have been of considerable theoretical interest, particularly in view of the intriguing notion of super-aromaticity that we will address later. 

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