Abnormal Benzo Substituent Effect on Free Radical Formation Process. A Local Symmetry Effect

Relative stabilities of 1-bicyclo[2.2.2]octyl (9), 1-benzobicyclo[2.2.2]-octenyl (10), 1-dibenzobicyclo[2.2.2]octadienyl (11), and 1-tribenzobicyclo[2.2.2]octatrienyl(12) radicals were obtained by means of the relative rates of the free radical bromine abstraction from the corresponding bridgehead bromides by a silyl radical. It was found that 9 and 12 were more stable than expected. The observed order of stability 9>10>12>11 can not be explained by the usual concepts alone. The extra stabilization of 9 or 12 (or destabilization of 10 or 11) was interpreted in terms of the local symmetry (or local asymmetry) of these radicals.