Semibullvalenes and Related Molecules: Ever Closer Approaches to Neutral Homoaromaticity

Since the first synthesis of semibullvalene by Zimmerman et al. more than three decades ago, this molecular system has been the focus of extensive study. A major factor in the continuing fascination with semibullvalenes is the potential for realizing a neutral homoaromatic ground state species. Dewar-Hoffmann electronic stabilization of the transition state of barbaralanes and semibullvalenes has been very successfully applied, principally by Quast, to yield negligible barriers to their Cope rearrangement. In our work we are attempting to “eliminate” the barrier to the Cope rearrangement of semibullvalenes through strain mediated destabilization of the localized forms. We predict, from the results of high level quantum mechanical calculations, that small ring annelation of the semibullvalene nucleus will result in homoaromatic ground state species. Experimentally, our small series of bisannelated semibullvalenes have proved to be localized Cope systems with exceptionally low activation barriers to the Cope rearrangement in the condensed phases.