[3,3] and [5,5] Sigmatropic Shifts of Polycyclic Dibutadienylcyclopropanes: Computational Explorations of Concerted and Stepwise Mechanisms and “Negative Activation Energies”

We report a computational analysis of [3,3] and [5,5] sigmatropic shifts in 1,3,7,9-decatetraene and polycyclic bridged <i>cis</i>-1,2-dibutadienylcyclopropanes. While the activation barrier for unsubstituted decatetraene is 40 kcal/mol, the dibutadienylcyclopropanes are predicted to undergo room-temperature sigmatropic shifts. The 10-electron analogues of the 6-electron barbaralane, semibullvalene, bullvalene, and dihydrobullvalene scaffolds are predicted to have remarkably low sigmatropic shift activation barriers. For each case, we explore both concerted and stepwise transformations. Using ωB97X-D density functional theory (DFT), we demonstrate that substitution of these scaffolds can generate ambimodal transition states with post-transition state bifurcations (PTSBs). In addition, installing heteroatoms as bridgehead linkers─including boron, silicon, and phosphorus─produces interbridge distances which afford stable products from [3,3] and [5,5] sigmatropic shifts. Furthermore, substitution of the fused cyclopropyl units in the 10-electron semibullvalene analog leads to stable intermediates lacking interpolyene σ-bonds, so-called "pancake bonded" radical pairs, including examples of negative activation barriers.