Rearrangement of 2-Aryl-3,3-dimethylmethylenecyclopropanes. Substituent Effects on a Nonpolar Radical-Like Transition State

Article abstract of DOI:10.1021/jo01290a015

The effect of substituents on the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, 4, to 2-arylisopropylidenecyclopropanes, 5, has been determined.This has allowed an evaluation of substituent effects on a free-radical-like transition state devoid of polar character.In general, no correlation of all the data could be obtained with Hammett ? values.Electronegative meta substituents slightly decreased rates (ρ = -0.16).Conjugating substituents such as CN and CO2CH3 in the para position increased rates.The effect of the p-CF3 group was rate enhancing.The potential for C-F hyperconjugative stabilization is discussed as a radical-stabilizing feature.Thiomethyl and trimethylsilyl substituents in the para position increased rates.The possibility of radical-stabilizing interactions with vicinal ?^* and ? orbitals is discussed. p-Methoxy substitution increased rates while p-fluoro substitution slowed rates.Resonance delocalization involving charge-separated forms 10a and 11 was suggested to be less favorable in the case of p-fluoro substitution.Delocalization involving an expanded octet accounted for increased rearrangement rates of p-Cl and p-Br-substituted 4.