EPR STUDY OF BRIDGEHEAD-SUBSTITUTED BICYCLE [2.2.2]OCT-1-YL AND TRIPTYCYL RADICALS

4-Substituted bicyclo[2.2.2]oct-1-yl radicals were generated by bromin e atom abstraction from the corresponding 1-bromobicyclo[2.2.2]octanes and observed in solution by EPR spectroscopy. A tendency towards lowe r a(HB) and a(H-gamma) values for inductively electron-withdrawing sub stituents such as OMe and F and towards higher values for electron-rel easing groups such as Me(3)Ge and Me(3)Sn was observed. For a series o f bridgehead radicals, the hfs of B-hydrogens showed a monotonic incre ase as the extent of flattening at the bridgehead increased, The EPR d ata indicated that 4-substituents exercised a significant effect at th e radical centre, mainly by a through-bond mechanism. 10-Substituted t riptycyl radicals were generated in a similar way but showed no hfs fr om magnetic nuclei of the substituents. Thus, the triptycyl cage trans mitted spin density much less effectively than the bicycle[2.2.2]octyl cage. Bicyclo[2.2.2]oct-1-yl and adamant-1-yl radicals added to benze ne, tert-butylbenzene and 1,3-di-tert-butylbenzene to give cyclohexadi enyl radicals which were characterized by EPR spectroscopy, Triptycyl radicals and strained bridgehead radicals such as cubyl and bicyclo[1. 1.1]pent-1-yl gave no detectable cyclohexadienyl radicals under simila r conditions. Both bicycle[2.2.2]oct-1-yl and adamant-l-yl radicals ge nerated in tert-butylbenzene showed exclusive meta addition with forma tion of the corresponding 1-polycyclo-3-tert-butylcyclohexadienyl radi cal.