Antiaromatic destabilization energies Antiaromaticity

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

The estimation of aromatic stabilization (antiaromatic destabilization) energy based on thermodynamic characteristics of different reactions may yield for the same compound quite dissimilar values. As has already been pointed out, these discrepancies stem from the fact that the cyclic electron... 

Electrochemical evidence for the antiaromaticity of cyclobutadiene has been provided by Breslow and co-workers 17 The oxidation potentials for the hydro-quinone dianion 7 (—1.50 V, —0.68 V versus Ag — AgCl, at Pt electrode) are substantially more negative than the oxidation potentials of model 9 (—1.22 V, —0.45 V). In the 7 -8 conversion a dimethylenecyclobutene derivative, with only a small degree of possible cyclobutadiene character, is converted into a full cyclobutadiene, presumably partially stabilized by the ketofunctions. The data indicate that the cyclobutadiene resonance destabilization amounts to at least 12 kcal/mole, and an estimate of the true antiaromatic destabilization energy of 15—20 kcal/mole has been made17). 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

Dibenzoannelation of cyclobutadiene results in biphenylene (50), a long-known isolable hydrocarbon with an archival gas-phase enthalpy of 417.9 3.3 kj/mol. That its enthalpy of formation is essentially the same as the monobenzoannelated cyclobutadiene speaks to the antiaromaticity of the latter. An ab initio study of benzocyclobutadiene and biphenylene found that the intrinsic destabilization energy is ca. 33 kJ/mol greater in the former compound." This does not mean biphenylene is not destabilized relative to benzenoid expectations. The destabilization of biphenylene of more than 200 kJ/mol relative to triphenylene or biphenyl as explained earlier is huge, although not as large as for cyclobutadiene itself. The benzene rings clearly ameliorate the antiaromaticity of cyclobutadiene. 

In all cases shown in Figure 90, we have only two Kekule valence structures. However, fully antiaromatic systems can have more than two Kekule valence structures, as illustrated in Figure 91. The expressions for their anti-aromatic destabilization energy are listed in Table 38. We see that the presence of only An conjugated circuits is a necessary condition for a structure to be a candidate for being anti-aromatic, or fully anti-aromatic . However, this is not sufficient. For such compounds to be truly antiaromatic, they must be planar and have all CC bonds of approximately equal length. 

Antiaromatic Destabilization from the Energies of Isodesmic, Homodesmotic, and Hyperhomodesmotic Reactions... 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

As has been shown by DRE (72JA4941) and TRE (78BCJ1788) calculations, aromatic (antiaromatic) character is often inverted in the lowest excited state. Therefore, for a molecule with an aromatic ground state one may expect antiaromatic destabilization of the lowest excited state and a sizeable energy gap between them. Conversely, for molecules with the antiaromatic ground state this gap will be much smaller. 

The MNDO calculations on sila-, germa-, and stannacyclopentadienyli-denes have shown that whereas for cyclopentadienylidene (272) the energies of the antiaromatic 47t- and the aromatic 6ir-electron structures are close in value (89UK1067), in the (273)-(275) series the 67r-electron structures are quite noticeably destabilized (Table XXIII). Unlike (272), the electronic ground state of compounds (273)—(275) correspond to minima on the PES. These results point to the diminished role of antiaromatic destabilization in the 47r-electron structure (273)—(275), as opposed to (272). It should therefore be expected that these molecules would be more stable than (272). This has indeed been confirmed by our calculation on the heats of the isodesmic reaction (85) (Table XXIII). 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

There is, however, an important difference between examples 27 and 41. The later compound forms a Huckel-aromatic orbital system in 41b while the former compound adopts a Mobius orbital system with 4q + 2 electrons, i.e. 27 is Mobius antiaromatic although six electrons participate in cyclic delocalization (see Section III. B). This is in line with a destabilizing resonance energy of 9.9 kcalmol"1 (Table 2) calculated with the MM2ERW method41-42. 

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