Polycyclic corannulene

V. REDUCTION OF POLYCYCLIC BOWLS-THE CASE OF CORANNULENE A. Reduction of Corannulene... 

In 1995, the diffusion coefficients of a series of polycyclics and their respective dianions as well as corannulene (64) and its tetraanion (64 ) were measured in [DgjTHF solutions [58a]. Upon the formation of the charged species, a large decrease in the diffusion coefficients was observed and, partially, assigned to the increased solvation ofthe charged systems in the [DgjTHF solutions. However, some of the systems showed too large a decrease of their diffusion coefficients upon... 

Condensed polycyclic benzenoid aromatic hydrocarbons are customarily regarded as planar molecular structures because of the geometrical constraints of carbon atoms in a state of sp2 hybridization. A well-known exception is the class of compounds called the helicenes (18) for which the nonbonded overlap of two terminal benzenoid rings in a cata-condensed structure, as in structure 1, forces a molecule into a nonplanar helical structure. A second exceptional class of compounds is related to corannulene (2) and other an-nulenes of this type (19, 20). In corannulene, strain associated with the pericondensed five- and six-membered rings requires adoption of a bowlshaped structure (20, 21). For both structures 1 and 2 the aromatic character of the benzenoid rings is retained to an appreciable extent. 

The heterohelicenes can be used for the synthesis of a new class of polycyclic aromatic compounds, similar to coronene or corannulene, for which Wynberg suggests the name heterocirculenes. Thus the hetero-hexahelicene (446) gives (453) in 95% yield upon heating for several minutes at 140 ° with an AlCls-NaCl melt, and upon reaction with maleic anhydride and chloranil (453) yields (454). Decarboxylation of the latter or the corresponding acid with copper powder in quinoline furnishes the hetero-circulene (455). ... 

In 2011, Wu [43] applied intramolecular C-H arylation to the synthesis of highly curved buckybowls, which contain corannulene and sumanene fragments. Several polycyclic aromatic hydrocarbons (PAHs) to prepare less strained bowls have aheady been synthesized using palladium-catalyzed intramolecular arylation reactions [45]. 

In a treatise entitled "New Dimensions in Polynudear Aromatic Compounds", Herndon (1988) has described a number of polycyclic conjugated compounds, induding corannulene, a cydacene and CgQ buckminsterfullerene (Osawa 1970 Kroto et al. 1985 Kratschmer et al. 1990) see Fig. 10. The reader understands presumably very well that a full survey of the literature on this interesting molecule (and other fullerenes) is too voluminous (counting thousands of references) to be induded here. In the following small extract the number of Kekule structures K = 12500) for C q is reported (not necessarily as an original finding). 

Dinadayalane, T. C., Sastry, G. N. (2002a). Structure-energy relationships in curved polycyclic aromatic hydrocarbons Study of benzo-corannulenes. Journal of Organic Chemistry, 67,... 

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. 

Corannulenes (Figure 2.2) were first obtained in 1966, and described as bowl-shaped polycyclic aromatic hydrocarbons (PAHs). Almost simultaneously fullerenes were discovered in graphite vapour (Figure 2.2) [162], Fullerenes comprise 20 six-membered and 12 five-membered rings. The five-memhered rings (corannulene fragment) are bent. In 1991 nanotubes were discovered. Corannulene is a potential candidate for the synthesis of nanotuhes and fullerenes and was first synthesized in 17 steps [162]. 

Two intramolecular substitutional arylation reactions are catalyzed by Pd(0) and lead to the spherical polyaromatic hydrocarbon 3.616. X-ray diffraction study of 3.616 showed significant distortions in the structure. Such bowl-shaped polycyclic aromatic hydrocarbons are the building blocks for preparing fullerene structures [162, 291]. The first syntheses of the fullerene precursors were carried out by vacuum pyrolysis. Corannulene 3.617, diindeno[l,2,3,4-d /g l, 2, 3, 4 -mnop]chrysene 3.618, and semibuckminsterfullerene 3.619 were obtained, which creates a basis for the development of the synthesis of Cgo molecules (Figure 3.5) [300, 162]. 

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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