Geminal hyperconjugation as a driving force for C-C bond shortening in heavy-atom tunnelling

Quantum mechanical tunnelling significantly influences the reactivity of strained ring systems, yet strategies for controlling such reactivity remain largely unexplored. Here, we identify geminal hyperconjugation, i.e., electron delocalization between σ-bonds attached to a common atom, as a decisive electronic factor in governing heavy-atom tunnelling reactions involving three-membered rings. We illustrate this through a case study of the oxepin (1') ⇌ benzene oxide (1) equilibrium, recently shown to undergo solvent-controlled tunnelling at 3 K (Angew. Chem. Int. Ed., 2020, 59, 20318). Natural bond orbital analyses reveal that coordination of ICF₃, H₂O, or H⁺ to the oxygen atom of 1 enhances geminal C-O σ → O-C σ* delocalization in the oxirane ring, strengthening the C(1)-C(6) bond, and facilitating the 1' to 1 rearrangement. Tunnelling-inclusive Arrhenius plots show increasingly large deviations from a linear relationship for the electrocyclizations of 1' → 1, 1'-2H₂O → 1-2H₂O, and 1'-H⁺ → 1-H⁺. Related cases, including benzene sulfide (1S), oxirane (2), and benzazirine (3), are examined.