A Comparison of the Radical-Stabilizing Ability of Aromatic Groups. γ. Values for Aromatic Groups

Article abstract of DOI:10.1021/jo00273a025

A series of 15 2-aryl-3,3-dimethyl-1-methylenecyclopropanes, 1, have been prepared and thermally rearranged in C6D6 to the corresponding 2-aryl-1-isopropylidenecyclopropanes, 3.Rearrangement rates give a measure of the ability of various aromatic groups to stabilize the transition state leading to a biradical intermediate.The 4-pyridine N-oxide group was found to be the most effective of the aromatic groups in stabilizing the radical intermediate.This has been rationalized by considering the mode of spin delocalization in such radicals.Resonance interactions result in an intermediate which is stabilized due to nitroxide radical character.The 2-furanyl and 2-thienyl groups are also very effective radical stabilizing groups.Rearrangement rates of 1 were converted to γ^. values, which are a quantitative measure of the relative abilities of various groups to stabilize free radicals.