Aromaticity/antiaromaticity electronic indicators

FIGURE 9. Hiickel and Mobius orbital systems for homoconjugated molecules. In each case, the number of participating electrons (e) is given and classification according to aromatic or antiaromatic character indicated... 

As the collection of recent reviews in the topic shows [12-20], a rather large consensus appears in the computation or experimental tests for the diagnosis of the aromaticity/antiaromaticity (energetic, structural, magnetic, chemical reactivity, and electronic diagnostic tools), whereas the mechanisms themselves still remain open to the debate. The primary controversy in the area involves the questions of whether aromaticity/antiaromaticity can be quantified and, if so, which of the methods commonly used to evaluate aromaticity/antiaromaticity is most appropriate. The literature on aromaticity and its measure is so vast that I must be content here with outlining briefly only the aromaticity indicators which have been extensively used to diagnose aromaticity/antiaromaticity in the domain of all-metal aromatics. 

When 1,3,5-cycloheptatriene is heated with bromine, a stable salt is formed, cycloheptatrienyl bromide. In this molecule, the organic cation contains six delocalized tt electrons, and the positive charge is equally distributed over seven carbons (as shown in the electrostatic potential map in the margin). Even though it is a carbocation, the system is remarkably unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pA"a = 39) compared with that of cyclopentadiene. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

In compounds in which overlapping parallel p orbitals form a closed loop of 4n -f 2 electrons, the molecule is stabilized by resonance and the ring is aromatic. But the evidence given above (and additional evidence discussed below) indicates that when the closed loop contains 4n electrons, the molecule is destabilized by resonance. In summary, 52, 59, and 60 and their simple derivatives are certainly not aromatic and are very likely antiaromatic. 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

Structural indices constructed in this fashion are, in essence, phenomenological, and one is entitled to ask whether the specific features in the geometry of the aromatic and antiaromatic molecules are indeed determined, and if so, to what degree, by the cyclic electron (bond) delocalization. 

Discussion of the subject matter centers primarily on such physicochemical properties as are deemed indicative of -electron mobility and the attendant development of aromatic or antiaromatic character. Although it is not entirely neglected, the description of synthetic procedure is limited for the most part to the crucial final step. It may also be well to note that, while a serious attempt has been made to provide as complete as possible coverage of the area, the main emphasis in this review is on proper representation rather than on exhaustive enumeration. Also, in order to achieve maximum effectiveness in the coverage of the literature, compounds belonging to a given size-class are described in terms of increasing molecular complexity rather than historical sequence. 

The electron spin resonance (ESR) spectra of the radical ions of 230 indicate there are no large deviations from the free-electron g value that would have been expected had the 3d orbitals of the sulfur atom played an important part in influencing the spin density of the molecule. Consequently, structure 230 may not be the main contributor to the electronic structure of the compound. Such stability in this compound could be attributed to the inertness of the NSN group and the presence of the aromatic naphthalene ring. However, the H-NMR chemical shifts (8 = 4.45 ppm) suggest the compound is antiaromatic. The compound is therefore referred to as an ambiguous aromatic compound (78JA1235). 

Another example where aromaticity plays an important role is the barrier to the rotation of amides (compound 18 is represented with N in the middle to indicate any azole) [31]. In classical amides, like dimethylformamide (15), the calculated barrier is 80.1-81.0 kJ mol1 (MP2/6-311++G ), which compares well with the experimental barriers of 91.2 (solution) and 85.8 kJ mol1 (gas-phase) [32], The cases of A-formylaziridine (16) and iV-formyl-2-azirine (17) are more complex due to the pyramidalization of the nitrogen atom and the presence of rotation and inversion barriers [32], The effect of the antiaromatic character of 2-azirine (four electrons) [18] on the barrier is difficult to assess due to changes in the ring strain. 

A final example of a bipyrrole-derived [2 + 2] Schiff base expanded porphyrin was reported by Johnson, et al. in 1995 in the form of a preliminary abstract. In this instance, macrocycle 9.34 was prepared via the condensation of hydrazine with the diformyl bipyrrole 9.23 (Scheme 9.1.6). This macrocycle was reported to exhibit spectral characteristics indicating that it is antiaromatic. Further, treatment with manganese dioxide was said to result in oxidation of 9.34 to an aromatic 22 7t-electron system. This latter system proved, however, to be much less stable than its 24 7i-electron parent , and this precluded isolation and characterization. In fact, with full experimental details not yet reported in the literature, the characterization, and structural assignment for 9.34 must also be considered tentative. 

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