Tricyclo[2.1.0.0^2,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic...

Tricyclo[2.1.0.0^2,5]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane (c-C₄H₄)C, a highly strained molecule featuring an inverted C atom. Modeling of the carbene, using the (U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d) theoretical model, indicated a large singlet-triplet energy gap (ΔE_S-T = -45 kcal/mol), a high gas-phase proton affinity (PA = 258 kcal/mol), and a preference for electron-poor alkenes. These properties are consistent with those of nucleophilic carbenes. Structural differences between the C-symmetric singlet (ω_flap = ±44 deg) and C₂-symmetric triplet (ω_flap = 0 deg) stem from nonclassical electron delocalization in the former and the lack thereof in the latter. Degenerate bridge-flapping of the singlet's main bridge, which comprises the reactive divalent C3 atom, is computed to be slow due to a high activation barrier of the C₂-symmetric transition state (TS) (E_a = 17 kcal/mol). The position of the conformeric equilibrium is subject to stereoelectronic control. 1-Substituted derivatives of the carbene (R ≠ H) are sensitive to σ inductive effects. A proximal conformation is preferred when R is electron-donating and a distal one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The C₂-symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.