Antiaromatic destabilization

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. ... 

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). 

Since the aromaticity is defined as the stabilization due to cyclic electron (bond) delocalization, the data on thermodynamics and kinetics of various reactions leading to removal of cyclic delocalization system (or, conversely, to its formation) may in principle be used for assessing aromatic stabilization or antiaromatic destabilization. 

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... 

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). 

Alternatively, we can deduce some homoaromatic stabilization of bicyclo[4.1.0]hepta-2,4-diene and homo antiaromatic destabilization of bicyclo[2.1.0]pent-2-ene from enthalpies associated with hydrogenation to form the saturated bicycloalkanes. The bicy-cloheptadiene hydrogenation enthalpy (i.e. 39 —> 18, n = 6) is ca 200 kJ mol-1, to be compared with (229.6 1.3) kJ mol1 for the hydrogenation of the corresponding monocyclic... 

The antiaromatic destabilization of singlet 126 was estimated (using equation 44) to be 49.1 kcalmoG1 (HF/DZ160 53.5 kcalmor1 at HF/6-31G7/HF/3-2IG159), by ca 17 kcalmol-1 smaller than the destabilization calculated for cyclobutadiene at... 

The X-ray structural analysis of (65) reveals some interesting features. The length ofthe C—N amide bond is 1.482 A compared to 1.41 A in an azetidinone. In addition the nitrogen has a tetrahedral structure, it is even more tetrahedral than ammonia. These parameters indicate that the conceivable azacyclobutadiene structure (66) does not contribute to (65), apparently as a consequence of an antiaromatic destabilization of (66) 217). 

When the phosphorus-carbon double bond is reacted with selenium, the saturated product could be isolated. These systems seem to be more stable than their unsaturated counterparts. This behavior can be explained by the fact that more protecting groups are placed about the three-membered ring by saturation also, the antiaromaticity destabilizes the unsaturated systems. 

Molecular structures of the cyclopropylcarbinyl cation salts lOa-c have been determined by X-ray. Both 10a and 10b show a bisected geometry that optimizes cyclopropyl conjugation with the cationic center, whereas in 10c such conjugation could lead to antiaromatic destabilization, and it was suggested that the structure reflected geometrical distortion to avoid interaction of the cyclopropyl group with the allyl cation portion of 10c. 

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). 

Four-center, four-electron processes do not occur thermally. The transition states for these unfavorable reactions have been described as having antiaromatic destabilization because they have four electrons in a normal closed loop. There are three exceptions that go by this path all have some unusual orbital arrangement to allow them to bypass the problem of antiaromatic destabilization of their transition states. 

In contrast to the Hiickel aromatic systems with 4n -I- 2 /r-electrons, cyclic systems with 4n n-clcctrons are destabilized by cyclic conjugation, and are termed antiaromatic" systems. The extent of such antiaromatic destabilization is estimated from the p A3 values of the corresponding cyclopropcncs, which arc obtained from the thermodynamic cycle shown below using the empirical data of pAi[, and the reduction potentials (E, j and... 

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