Extended Corannulenes

The PAHs indenocorannulene (39), dibenzo[a,g]corannulene (40), and diben-zo[o,g]cyclopenta[h,I]corannulene (41) are three examples of extended corannulenes, which contain a central corannulene system fused to five- and six-membered rings. The reduction of these systems [119, 120] focused on the following issues. First, what is the aromaticity of these curved PAHs anions, will they behave like large polycyclic systems, or have aimulenic character Secondly, what is the possibility of aggregation and dimerization in these systems The third subject of interest was the effect of different alkali metals on the reduction process. 

The reduction of 39 with K yields a unique four-step reduction process in which alternate dimerization and bond cleavage occurs [119]. Four different diamagnetic 

Since the bowl shape of 39 does not undergo a dynamic process on the NMR timescale, each half of the dimer imparts chirality. This property leads to the interesting possibility of multiple diasteromeric reduced species, depending on the position of the connection as shown in Fig. 13.4. Although it was not possible to fully assign the stereochemistry of the dimers, it is apparent from the Jh3, h3 coupling constant (10.0 and 10.5 respectively, for 39a and 39c) that they adopt anti conformation. 

The reductive dimerization/bond-cleavage of 39 represents the first case in which a large nonplanar PAH undergoes such a process. In addition, this is the 

Compounds 40 and 41 do not contain the dibenzofulvene subunit, which is responsible for the dimerization process in 39 therefore, their reduction produces only monomeric anions. The LUMO level in 40 and 41 is not doubly degenerate like that of corannulene, but there is rather a small energy gap between the LUMO and the NLUMO, which indicates that the formation of tetraanions of 40 and 41 should be possible [120]. 

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D. Corannulene with Extended jr-Systems—From Bowls to Balls. 501... 

Highly substituted corannulenes are not only important building blocks for new organic materials, they also provide the possibility to extend the aromatic system of the corannulene core. Direct multiple electrophilic aromatic substitution oti corannulene has shown limited but important successes. In contrast, functiOTial-ization of multihalocorannulenes is a general route. 

Rabinovitz and co-workers also used the Haworth phenanthrene synthesis to form complex fused aromatic ring systems." The bowl-like shape of corannulene 57 was extended through the acylation reaction with 58 to produce 59. Treatment of 59 with red phosphorus under strongly acidic conditions resulted in 60 in good yield. 

The 4/2+2 rule solved the mystery of the profound difference between benzene, [10]annulene, [14]-annulene, and [ISjannulene on one side and the 4/2 monocyclic systems, like elusive cyclobutadiene and puckered cyclooctatetraene, on the other side. Attempts were made to extend the 4/2+2 rule to polycyclic systems, for which it was not initially designed. Of numerous attempts in this direction, we will mention only that of Platt,who proposed that the 4/2+2 rule be applied to molecular periphery. It turns out that Platt s generalization of the Hiickel 4/2+2 rule is correct when one restricts attention to benzenoid hydrocarbons. For example, the perimeter rule correctly classifies pyrene (which has 16 Jt-elec-trons), perylene (which has 20 //-electrons), and coronene (which has 24 //-electrons) as aromatic as they have 14 or 18 //-electrons on the perimeter. But the perimeter rule does not give a correct answer for the non-benzenoid systems illustrated in Figure 10. The structure on the left, which has 14 //-electrons on the periphery, instead of being aromatic, as will be seen later, is in fact fully anti-aromatic . On the other hand, the structure on the right (corannulene), which has 15 //-electrons on the periphery, is not... 

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