σ,π Interaction in Halogen-Substituted Biadamantylidene Radical Cations

The order of E °‘ and vIP for 4-eq-halogenated-biadamantylidene is F > Cl ≅ Br, and the 5-F-substituted compound is harder to ozidize than the 4-eq-F-substituted one. The former result is most consistent with a detectable resonance contribution through the σ-framework, and the latter with σ-hyperconjugative destablilization proceeding through two pathways being more than double the same effect through one pathway (the Whiffen effect). AM1 calculations predict these results. The facial selectivity for epoxidation and diazetidine formation from 4-eq-halogenated 3 ( 4(X) ) is in the order Cl > F > Br, and the 5-fluoro compound ( 8 ) is less selective than 4(F) for both reactions. Steric as well as electronic factors might well contribute to these results, neither of which was expected from consideration of σ,π interaction. Cation radical catalyzed chain dioxetane formation from 4(F) and 3(Cl) is significantly more face selective than epoxidation or diazetidine formation, as expected on electronic grounds; σ,π interaction should be larger in the radical cation.