Stone-Wales Rearrangements Involving Heptagonal Defects

REFERENCES

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• Remarkable low energy of 10 for an sp3 intermediate suggests that the azulene-type defects, if ever existed in fullerenes or nanotubes,3 would be readily removed during the rearrangement process.
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• If an azulene unit is moved to and eventually fused with another azulene unit, a (5 5 7 7) defect is produced.
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• To the authors' knowledge this molecule is still unknown, but AM1-RHF calculation produced a perfectly planar molecule having a closed shell electronic structure (HOMO -7.901 eV, LUMO -0.991 eV). The calculated heat of formation of 105.75 kal/mol as well as the HOMOLUMO positioins are intermediate between pyrene (67.16 kal/mol, HOMO -8.132 eV, LUMO -0.880 eV) and azupyrene (134.00 kcal/mol, HOMO -7.448 eV, LUMO -1.030 eV). Like its aromatic kins, 11 may also belong to the class of novel Hückel aromatic molecules containing a concentric node separating 14 π-electron periphery from 2 π-electron core (vide infra).
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• Subscript to M or T refer to one of the six different possibilities of bond migration shown in Scheme 3.
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• IUPAC name: dicyclopenta(ef,kl)heptalene. See Anderson A. G. Jr. Trends in Organic Chemistry 1992 3 315
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• Due to the possibility of a biradical intermediate along the in-plane route, we have performed UHF calculations as well. The first transition state turned out to have a low-energy structure (107.51 kcal/mol relative to azupyrene), but other stationary points have not reached SCF.
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• This reaction can be effected by heating azupyrene in a quartz tube at 450–460°C for 5–6 hours in 45% yield. Anderson16 proposed two mechanisms based on the labeling experiment (Schemes I and II): Scheme I Scheme II According to our AM1 calculations, the highest point in the bicyclobutane route (Scheme I) is not the bicyclobutane intermediate (99.74 kcal/mol above 12 by RHF) but a sp3 transition state (25, 119.1 kcaVmol above 12 by UHF). The barrier in the methylene-walk route (Scheme II) is located immediately after the start of methylene-walk (26, 101.75 kcdmol above 12 by UHF). The latter value is siginificantly lower than the lowest barrier computed for the in-plane route (12 1.2 kcdmol, Table I).
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