SIGMA,PI INTERACTION IN HALOGEN-SUBSTITUTED BIADAMANTYLIDENE RADICAL CATIONS

The order of E degrees' and vIP for 4-eq-halogenated-biadamantylidene is F > Cl congruent to 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 sigma- framework, and the latter with sigma-hyperconjugative destablilization proceeding through two pathways being more than double the same effec t through one pathway (the Whiffen effect). AM1 calculations predict t hese results. The facial selectivity for epoxidation and diazetidine f ormation from 4-eq-halogenated 3 (4(X)) is in the order Cl > F > Br, a nd the 5-fluoro compound (8) is less selective than 4(F) for both reac tions. Steric as well as electronic factors might well contribute to t hese results, neither of which was expected from consideration of sigm a,pi interaction. Cation radical catalyzed chain dioxetane formation f rom 4(F) and 3(Cl) is significantly more face selective than epoxidati on or diazetidine formation,as expected on electronic grounds; sigma,p i interaction should be larger in the radical cation.