Introduction to Semibullvalenes and Azasemibullvalenes

Abstract

Semibullvalene (SBV) as well as azasemibullvalene (NSBV) is a class of organic molecules featuring unique polycyclic skeleton. Both SBV and NSBV have attracted much attention to their structures and reactions. SBV and NSBV both feature strained ring skeleton and the bis(allyl) system shows rapid degenerate Cope rearrangement in both solution-phase and gas-phase, with very low activation barrier. The transition-state of Cope arrangement in SBV or NSBV features symmetrical structure, and relatively low energy close to the value of localized structure. Structural derivation by both theoretical/computational propose and experimental synthesis was expected to lower the activation barrier and stabilize the delocalized structure to become a homoaromatic minimum in energy. The following strategies have been applied: (1) electronic stabilization by substituents (Dewar–Hoffmann SBV); (2) small ring annulation; (3) introduction of heteroatom into skeleton; (4) coordination with metal ion; (5) solvation or other methods. Based on these strategies, the SBV or NSBV derivatives synthesized have lower activation barrier of Cope rearrangement than unsubstituted SBV.