Antiaromaticity in relation to 1,3,5,7‐cyclooctatetraene structures

Abstract

An isodesmic energy analysis has been carried out at the MP2/6–31G*//HF/3–21G level for the nonplanar ground state (1) of 1,3,5,7‐cyclooctateraene and for two planar forms, one having complete π delocalization (2) and the other having alternating single and double bonds (3). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree–Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3. Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 John Wiley & Sons, Inc.

The ionic states of cyclooctatetraene: Analysis of a new experimental photoelectron spectrum by ab initio and density functional methods

“…The TS linking the two D 2d forms of neutral COT has 1 A 1g symmetry, in agreement with previous studies; 16 it is strongly alternating (D 4h ) rather than a regular octagon (D 8h ). This procedure applied to the lower ionic states of COT is shown in Table V. The TS for the A 2 A 1 state of COT is typical, corresponding to the structural interchange A 2 A 1 (D 2h ) ⇆ 2 B 2u (D 4h ).…”

“…However, the dynamic nature of the structure was demonstrated from 13 C satellites in the 1 H NMR spectrum, which showed a complex series of changes with temperature. [7][8][9] Two main structural processes occur simultaneously, namely, (a) C= =C ⇆ C− −C bond interchanges and (b) ring inversion D 2d ⇆ D 2d ; both are assumed to involve a planar transition state (TS), [12][13][14][15][16][17][18][19] which is discussed below. The equilibrium for the all-cis (Z) conformation, alternatively denoted as the Z,Z,Z,Z-conformer, is shown in Figs.…”

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…Since the inner angle of the planar COT (135°) is larger than the ideal bond angle for sp 2 hybridized carbon (120°), there are some strains in the planar D 8h and D 4h transition states, and hence the antiaromatic destabilizations in both D 8h and D 4h structures are at most several kcal mol -1 . In accord with this explanation, the hydrogen transfer energy for the planar D 4h COT going to cyclooctatriene was reported to be -8 kcal mol -1 (B3LYP/6-311+G*) and -9 kcal mol -1 (MP2/6-311+G*) [9], whereas isodesmic (27.8 kcal mol -1 MP2/6-31G*//HF/3-21G [29]; 28.4 kcal mol -1 MP4SDTQ/6-31G**//MP2/6-31G**+ZPE(HF/6-31G*) [30]) and homodesmic (-28.4 kcal mol -1 [30]) stabilization energies gave rather different results. The other important criteria for the study of aromaticity and antiaromaticity are related to the magnetic properties of compounds.…”

Hyperconjugation

“…Wu and Schleyer point out in their 2013 review, “despite being the standard unconjugated polyene model used in many organic textbooks, the tub‐shaped cyclooctatetraene minimum (D 2d , 56° CCCC dihedral angles) is far from being conjugation free.” Citing a report from Polizer et al, they continue, “Actually, the isodesmic bond separation stabilization energy of D 2d cyclooctatetraene (41 ± 1.5 kcal/mol) is only modestly less than that of the fully conjugated aromatic benzene (64.6 ± 1.1 kcal/mol) . (Figure ).…”