The ωB97X-D/def2-TZVP + 0.955(EZPV) theoretical model was used to determine if 2,3-secopyramidane (tricyclo2.1.0.01,3pentane) participates during the Skattebo̷l rearrangement of endo-2-ethenylcycloprop-1-ylidene. Intrinsic reaction coordinate animations show an almost barrierless attempt at 1,2-addition of the divalent C atom to the ethenyl double bond because the transition state is stabilized by intramolecular σ- and π-complexation. The formation of 2,3-secopyramidane is incomplete, however, and retro-1,2-addition takes over, leading to ring-puckered cyclopent-3-en-1-ylidene, a bishomoaromatic carbene. Ring-flattening is followed by a 1,2-H atom shift yielding cyclopenta-1,3-diene.
Rosenberg et al. (Wed,) studied this question.