Anti-aromatic compounds

In carbocyclic chemistry, rather firm dividing lines usually exist between aromatic, non-aromatic, and anti-aromatic compounds, while in heterocyclic chemistry enormous variations in the extent of aromatic character are displayed.52 Furthermore, there is an enormous number of potential heterocycles as compared to carbocycles, as will be detailed in section 3 of this review. The degree of aromaticity has classically been judged qualitatively in connection with the diene character of heterocycles manifested in Diels— Alder reactions or polymerizations. In this regard for instance, furan (42) is less aromatic than benzene (43), as is isoindole (44) compared to indole (45) (Scheme 18). Therefore, a quantitative aromaticity scale would be useful. 

The first calculations on benzene using optimised orbitals were done by Cooper et al. [52], using their spin-coupled VB method [20]. A review [53] has appeared with an overview of their work on aromatic and anti-aromatic compounds. 

Anti-aromatic compounds, on the other hand, have traditional single and double bonds and have as their canonical names ... 

Figure 2.2 Energy diagram for the anti-aromatic compound cyclobutadiene (formulated as a biradical and diene)...

The borderline between partially aromatic and partially anti-aromatic compounds may be a matter of convention and convenience because truly, from the point of view of structural chemistry, these two groups differ only in degree and not substantively. The fully anti-aromatic compounds, again, are well defined as the compounds that have only 4n conjugated circuits, in contrast to fully aromatic compounds that have been defined as the compounds that have only 4n + 2 conjugated circuits. 

Pentaphenylborole, a formally anti-aromatic compound that can be prepared by reaction of 1,1-dibutyl-2,3,4,5-tetraphenylstannole with dichloro(phenyl)-borane, is reduced by potassium to the formally aromatic dianion, and acts as a four-electron ligand in forming transition-metal complexes. ... 

Before each of the classes is fully described, let us explain why we have /outclasses and not two classes (aromatic and anti-aromatic), or why three classes (aromatic, non-aromatic, and anti-aromatic) will not suffice. There are no problems with the dichotomy aromatic—anti-aromatic based on the presence of only 4n + 2 or only An conjugated circuits in the set of Kekule valence structures of a compound, respectively. The problem is with the non-aromatic class, which would include structures having both An+ 2 and An conjugated circuits. Some structures in this class may show a greater similarity with benzene, and on the other hand they may show some similarity to anti-aromatic compounds. The problem is that the class of such neither aromatic nor anti-aromatic structures, that have both 4n -I- 2 and An conjugated circuits, is so large and so broad that it becomes of little use. 

In order to set a boundary between aromatic, nonaromatic, and anti-aromatic compounds, first we have to define a measure of the degree of aromaticity and then apply it to a series of structurally related compounds of apparently decreasing aromatic character in order to verify its adequacy. If we continue to consider RE as the prime indicator of aromatic characteristics of compounds, then we can define the degree of aromaticity to measure that portion of the total RE which is due to contributions from the aromatic 4/7+2 conjugated circuits. This leads to the following ... 

We are now in a position to establish numerically the boundary between the aromatic and anti-aromatic compounds. A non-benzenoid hydrocarbon will qualify as aromatic or not, depending on whether, in such non-benzenoid hydrocarbons, there is a domi-... 

The official boundary between aromatic/anti-aromatic classification is a matter of convention, agreed upon between the users. One possibility is to consider A = 1/2 as the boundary of aromatic/anti-aromatic classification for the following reasons If structures with A = 0 and A < 0 c/o not exist, because they would be unstable, while a structure close to A = 1/2 appears to hover at the borderline between olefinic, aromatic, and antiaromatic classification", then the boundary A = 1/2 appears a plausible alternative choice. However, from a theoretical point of view, the case A = 0, which occurs for RE = 0, appears as a natural boundary between the aromatic and the anti-aromatic species. In the case of the compounds shown in Figure 2, which gradually change from aromatic to anti-aromatic, we see from Table 34 that RE for the compound which is in the middle of the list, cycloocta[c/e/ biphenylene, is close to zero, as one would like to be the case. Although anti-aromatic compounds are elusive and often hypothetical, it may become possible to have a truly anti-aromatic compound by forcing a non-planar... 

An indirect proof that anti-aromatic compounds are elusive comes from data on inter-stellar compounds. In view of the low density of matter and extremely low temperatures in outer space, structures that would be difficult to observe in the laboratory may have long enough life in the interstellar space to be detected. Thus, for instance, among others, the smallest aromatic compound, cyclic CaHa" , has been identified in the inter-stellar space. The search for anti-aromatic compounds in the interstellar space thus appears to be an interesting project. However, as of today, no anti-aromatic compounds have been detected in outer space, although, as is well known, the inter stellar space is rich in hydrocarbons. 

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