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.

The addition of N‐methyltriazolinedione (M) to biadamantylidene (A) gives the [2+2] adduct P, but clearly does not proceed in one step beause an aminoaziridinium intermediate (I) can be observed to build up during the reaction. The overall rate of P formation correlates with the amount of I that builds up, but not with any solvent polarity parameter, despite the fact that charge‐separated intermediates are involved. NMR experiments established that the equilibrium constant for I formation is >1000 M−1 in CDCl3, ≥500 M−1 in CD2Cl2 and in the range ca 2–15 M−1 for C6D6 and C6D5Br. It is very unlikely that I is directly on the reaction pathway between M+A and P, and it is proposed that the zwitterionic intermediate with one CN bond, X, is the probable precursor of both I and P, although X does not build up in concentration enough to be observed. CH hydrogen bonding stabilizing both X and I in chloroform is suggested as a possible rationalization for the correlation between overall rate of P formation and stability of I relative to the starting materials. Formation of P in benzene is catalyzed by Meldrum's acid. M unfortunately reacts too rapidly with compounds containing OH bonds to study formation of P in the presence of strong hydrogen bonding reagents. © 1997 John Wiley & Sons, Ltd.

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ChemInform Abstract Reaction of cyclopentadiene (I) with the ketenes generated from the acyl chlorides (II) gives mainly the bicyclic adducts (III) which are selectively deprotonated and alkylated at the exo side with the iodides (IV) to form the isomeric 7-ethyl-7-methylbicycloheptenones (Va) or (Vb).