Super-aromaticity

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. 

Figure 118. Coronene and related benzenoids that have been thought to give rise to super-aromaticity .

The idea of super-aromaticity (just as was the case with the notion of parity of Kekule valence structures) attracted attention. However, the presence of large conjugated circuits makes a small contribution, if any, to molecular RE. So the question can be raised Is super-aromaticity a fact or an artifact In fact this question has been raised by Aihara in an article entitled, Is Super aromaticity a Fact or an Artifact The Kekulene Problem. Aihara has examined the kekulene problem on the basis of several theoretical approaches, such as the method of additive nodal increments of Cioslow-ski,749j5o method of conjugated circuits, the... 

As Aihara points out, This approach is general and applicable to any macrocyclic conjugated systems with an inner cavity Thus, the myth of super-aromaticity, just as has been the case with the myth of parity, has been demoted if not ended. 

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

In order to achieve high yields, the reaction usually is conducted by application of high pressure. For laboratory use, the need for high-pressure equipment, together with the toxicity of carbon monoxide, makes that reaction less practicable. The scope of that reaction is limited to benzene, alkyl substituted and certain other electron-rich aromatic compounds. With mono-substituted benzenes, thepara-for-mylated product is formed preferentially. Super-acidic catalysts have been developed, for example generated from trifluoromethanesulfonic acid, hydrogen fluoride and boron trifluoride the application of elevated pressure is then not necessary. 

If you like it drier, you need it drier, he said, as if it were an adjustment to medication. Mr. Conti uses a regular premium gin, like Boodles or Tanqueray, not one of the super-premiums like Ten or Wet, which he considers too aromatic for the drink. 

This strategy using super nonionic strong base is also applied to less reactive aromatic nitro compounds. The reaction of 1 -nitronaphthalene with ethyl isocyanoacetate proceeds very slowly in the presence of DBU, but this reaction is accelerated by the use of phosphazene base to give the corresponding pyrrole in reasonable yield (Eq. 10.36).40... 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

The reactions are based on the reduction of aromatic nitrocompds such as PA, TNT, Tetryl, etc into amines which give rise to several super-imposed reactions, such as formation of diazocompds or endothermic expls... 

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