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>i.e.</i>, 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 (<i>Angew. Chem. Int. Ed.</i>, 2020, <b>59</b>, 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.