Collect. Czech. Chem. Commun. 2011, 76, 481-501
Published online 2011-04-20 11:35:49

Cooperative effects in the annelation of benzene by multiple etheno groups

Xiaoguang Bao, David A. Hrovat and Weston Thatcher Borden*

Department of Chemistry and the Center for Advanced, Scientific Computing and Modeling, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5070, USA


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20. Rather than being cooperative, linear bis-annelation of benzene would be expected to be competitive; and our calculations confirm this to be the case. However, two valence isomers of nearly the same energy can be formed by linear bis-etheno annelation of benzene21. The C–C bond lengths in one of these isomers are those anticipated for a molecule containing two benzocyclobutadiene rings, in which symmetry prevents the type of bond localization that occurs in the benzene ring of 3. The C–C bond lengths in the other valence isomer reveal that it has a [10]annulene periphery, perturbed by transannular bonds between C-1 and C-4 and C-6 and C-9. Our B3LYP/6-31G(d) calculations predict the latter isomer to be more stable than the former by 2.4 kcal/mol. The bis-annelation energy, calculated for forming the latter isomer, is –ΔE = 26.1 kcal/mol, which is 1.7 kcal/mol less than twice the benzene annelation energy of –ΔE = 14.9 kcal/mol for forming 3 and 14.7 kcal/mol less than –ΔE = 40.8 kcal/mol for angular bis-annelation of benzene to form 4.
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22. We could have constrained the lengths of the C=C bonds of the etheno groups and the C–C bonds joining them to the benzene rings in 4 and 5 to be the same as those in 3. Our calculations show that, including the energy for the optimization of the these C–C bond lengths in ΔEgeom for 5, would have increased the value of –ΔEgeom in 5 by 3.5 kcal/mol and decreased the value of –ΔE0 by the same amount.
23. There is, of course, one more filled π MO in 3, two more in 4, and three more in 5. In these MOs, which are not shown in Fig. 2, the interactions between the π orbitals of the benzene ring and the etheno bridges are bonding, so that these MOs do not contribute to the localization of the benzene ring π bonds, as do the MOs in Fig. 2.