Antiaromaticity cyclooctatetraene

One must be careful not to equate the concept of antiaroraaticity with the tendency to be non-planar. If the molecule were very antiaromatic the system would be non-planar because the delocalisation energy overwhelmed the tendency to planarity. In cyclooctatetraene antiaromaticity is too small, and the framework would prefer to be planar and delocalised, but the relative magnitude of the antiaromaticity is so much less than the energy required to twist about... 

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

Section 11 19 An additional requirement for aromaticity is that the number of rr elec Irons m conjugated planar monocyclic species must be equal to An + 2 where n is an integer This is called Huckel s rule Benzene with six TT electrons satisfies Huckel s rule for n = 1 Square cyclobutadiene (four TT electrons) and planar cyclooctatetraene (eight rr electrons) do not Both are examples of systems with An rr electrons and are antiaromatic... 

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. 

A thiepin is formally isoelectronic with the 8ic-electron 1,3,5,7-cyclooctatetraene and 1,3,5-cycloheptatrienide ion and, if planar, may actually be antiaromatic. Recently, the question of the antiaromaticity of thiepin has been the subject of interest for both synthetic and theoretical chemists. The apparent instability of the thiepin ring system is in good agreement with theoretical calculations. Dewar and Trinajstic 68) have reported that the thiepin is considered to be weakly antiaromatic (RE = — 1.45 kcal mol-1) based on PPP SCF MO calculations. On the other hand, Hess Jr. and Schaad 69) have found it to be substantially antiaromatic (RE = —0.232 J) by using the Huckel MO method. This result was also supported by a graph-theoretical treatment by Aihara 70). 

We can then say that planar cyclooctatetraene is antiaromatic because two antiaromatic unions dominate a single aromatic union. Alternatively, we can say that cyclooctatetraene is antiaromatic because the crucial union, Le. the union involving the greatest spatial overlap, is the (A + B + C) + D union which is antiaromatic. 

Problem 10.7 Cyclooctatetraene (CgH ), unlike benzene, is not aromatic it decolorizes both dil. aq. KMnO and Brj in CCI4. Its experimentally determined heat of combustion is -4581 kJ/mol. (a) Use the Hiickel rule to account for the differences in chemical properties of CgHg from those of benzene, (b) Use thermochemical data af Problem 10.4 to calculate the resonance energy, (c) Why is this compound not antiaromatic (d) Styrene, CgH5CH==CH2, with heat of combustion —4393 kJ/mol, is an isomer of cyclooctatetraene. Is styrene aromatic ... 

Hovis, J. S. and Hamers, R. J. Structure and bonding of ordered organic monolayers of 1,3,5,7-cyclooctatetraene on the Si(001) surface Surface cycloaddition chemistry of an antiaromatic molecule. Journal of Physical Chemistry 102, 687 (1998). 

Cyclooctatetraene and the 1,5- and 1,3-cyclooctadienes on Pt(lll) also gave a common product at higher temperatures, which, from the simplicity of the spectrum, is postulated to be flat-lying C8H8. This possibly involves 77-electron donation to the surface in order to overcome antiaromaticity. 

In contrast to aromatic molecules which have An + 2 n electrons, cyclobutadiene and cyclooctatetraene do not have An + 2 7r electrons and are not aromatic. In fact, diese molecules, which contain An jt electrons (n is an integer), are less stable than die planar model compounds and are termed antiaromatic. Bodi of these molecules adopt shapes that minimize interactions of die n orbitals. 

Twin Excited States of Cyclobutadiene, Cyclooctatetraene, and Antiaromatic Annulenes... 

Figure 18. (a) jt—a-Interplay diagrams for the twin states of /7-electron—/7-center antiaromatic species. (b,c) Calculated geometries for the twin states of cyclobutadiene and cyclooctatetraene (from refs 164a, 177, 188, and 219). 

However, 1,3,5,7-cyclooctatetraene, which has 4 n bonds or 8 n electrons, is not only nonaromatic but is actually considered antiaromatic because it is even less stable than the open-chain hexatriene. 

Hence heptafulvene is non-aromatic but cyclooctatetraene and pentalene (XXI) are antiaromatic (aromatic energy —26/5). 

More sophisticated calculations indicate that cyclic An systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17, 46]. 

Because of its nonplanar geometry, cyclooctatetraene is not antiaromatic and its hydrogens appear at 5.75 8, a value typical for alkenes. However, the triple bonds of the compound called benzo-l,5-cyclooctadiene-3,7-diyne force this molecule to assume a nearly planar geometry. The pi system of its eight-membered ring contains eight electrons. (Only two of the electrons of each triple bond are part of the conjugated system.)... 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

Cyclooctatetraene would be antiaromatic if Htickel s rule applied, so the conjugation of its double bonds is energetically unfavorable. Remember that Htickel s rule applies to a compound only if there is a continuous ring of overlapping p orbitals, usually in a planar system. Cyclooctatetraene is more flexible than cyclobutadiene, and it assumes a nonplanar tub conformation that avoids most of the overlap between adjacent pi bonds. Hiickel s rule simply does not apply. 

Large-Ring Annulenes Like cyclooctatetraene, larger annulenes with (4A0 systems do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations. Even though [12]annulene, [16]annulene, and [20]annulene are (4A0 systems (with N = 3,4, and 5, respectively), they all react as partially conjugated polyenes. 

Spin-coupled calculations at the idealized Dm geometry of cyclooctatetraene reveal a description dominated by triplet coupling of pairs of electrons [12], as anticipated earlier. Expressing the total spin function in the Serber basis [29], we find that the mode made up only of triplet-coupled pairs is responsible for 75% of the total. We find that the n orbitals for this antiaromatic system (see Figure 9) adopt localized forms that resemble closely those shown in Figure 4 for benzene, rather than the antipair representation shown for cyclobutadiene in Figure 6. 

K. Antiaromaticity and reactivity of a planar cyclooctatetraene fully annelated with bicyclo[2.1.1]hexane units, Chem. Em. J. 2008,14, 2067-2074. 

Ohmae, T Nishinaga, T Wu, M. lyoda, M. Cyclic tetrathiophenes planarized by silicon and sulfur bridges bearing antiaromatic cyclooctatetraene core syntheses, structures, and properties, J. Am. Chem. Soc. 2009,132, 1066-1074. 

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