Stone-Wales Rearrangements Involving Heptagonal Defects

Abstract

Three cases of Stone-Wales type rearrangements involving heptagonal rings have been studied by using AM1 semiempirial MO method. In all cases, transition states of the two distinct mechanisms known to pyracylene rearrangements, stepwise and concerted, have been found. Activation energies are estimated to be 110 to 150 kcal/mol.

Notes

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.

If an azulene unit is moved to and eventually fused with another azulene unit, a (5 5 7 7) defect is produced.

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).

Subscript to M or T refer to one of the six different possibilities of bond migration shown in Scheme 3.

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.

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).