The Role of Strain in the Homoaromatization of Semibullvalenes

The low activation barrier to the Cope rearrangement of semibullvalenes has been attributed to the inherent ring-strain of this nucleus. Appropriate, Dewar–Hoffmann, substitution of semibullvalene results in the stabilization of the transition state and a further lowering of the Cope barrier. An alternative proposal for lowering/eliminating this barrier is the use of strain to destabilize the localized semibullvalene. Using density functional and Hartree–Fock calculations, we predict that additionally straining the semibullvalene nucleus, by small ring annelations, will lead to a lowering of the Cope barrier and ultimately to ground state neutral homoaromatics.