Substituent effects in mono- and disubstituted 1,3,5,7-cyclooctatetraene derivatives in natural and planar conformations

Abstract

Substituent effect in cyclooctatetraene (COT) has been investigated by means of quantum-chemical (DFT-B3LYP) calculations. Two substituents, nitroso (p-electron-withdrawing) and hydroxy (p-electron-donating), have been examined and compared with monosubstituted and unsubstituted systems. Additionally, both the natural (tub-shaped) and the planar geometries of the COT have been taken into account. The behavior of COT ring, being the antiaromatic 4n p-electron system, has been compared with that of the benzene ring, being the aromatic 4n + 2 system. The 1,4-substitution differs significantly from the 1,3-and 1,5-substitution, which corresponds to an analogous situation found for benzene derivatives (para vs. meta substitution). Substituents influence COT more effectively in its planar geometry as compared with the tub-shaped geometry. This is due to the better overlapping of the p z orbitals of the carbon atoms within the ring in its planar conformation. As far as the p-electron communication between the substituents is concerned, the behavior of both 4n and 4n + 2 systems is in some way similar. Both systems tend to keep their p-electronic structures. Nevertheless, benzene ring, i.e. 4n + 2 system seems to be a clearly worse medium for transmission of the substituents communication than its 4n counterpart.

Cyclooctatetraene dianion—an artifact?

“…Interestingly, HOMA for tub-like structures is negative, similarly as for antiaromatic species, and it is close to that obtained for the free neutral COT ring. 3,28 This additionally supports the earlier conclusion according to which in these cases, we have a situation when the neutral COT ring interacts with free electrons located in its surroundings. The symmetry of starting geometries used in optimization procedure is given in square brackets.Values of average CC bond lengths, R av , are in angstroms.…”

“…Unlike cyclobutadiene and benzene, the unsubstituted COT ring adopts a nonplanar structure with D 2d symmetry. 2,3 (See Fig. 1 for graphical representation.)…”

“…Therefore, the substituent effect was also investigated for simple COT derivatives. 3,5 Planarization of the COT ring was a subject of several studies. It was found that the planar structure of COT can be reached by the annelation with rigid bridges leading to geometric constrains that result in bond delocalization in the COT ring.…”

Activation parameter changes as a mechanistic tool in SN2 reactions in solution

“…The effects of structural changes in the nucleophile, leaving and non-leaving groups on the activation parameters of the SN2 reactions are quantitatively described using the Hammett or Hammett-like substituent constants [16][17][18][19][28][29][30] for aromatic systems [31][32][33]. Substituent effects are among the most important concepts of structural effects [31].…”

Towards mechanisms of bimolecular nucleophilic reactions in solution—probing the variation of the activation parameters in the reactions of aromatic compounds

“…[3][4][5][6][7]73] In most cases, substituent effects are quantified by the use of the Hammett or the Hammett-like substituent constants [68][69][70][71][74][75][76] for aromatic system. [77][78][79] The activation parameters are widely used for characterizing the TS structures. [6,23,36,37,68,72,73] For example, it was found that solvolysis of benzyl-and benzhydryl halides follows the S N 2 and S N 1 mechanism, respectively, [80,81] and S N 2 reactions show more negative values of ΔS 6 ¼ .…”