EPR study of bridgehead-substituted bicycle [2.2.2]oct-1-yl and triptycyl radicals

• Published in: Magnetic resonance in chemistry Vol. 33; pp. S53 - S59
• Main Authors: Binmore, GT, Walton, JC, Adcock, W, Clark, CI, Krstic, AR
• Format: Journal Article
• Language: English
• Published: W SUSSEX Wiley 01.12.1995
• Subjects: Chemistry; Chemistry, Multidisciplinary; Chemistry, Physical; Physical Sciences; Science & Technology; Spectroscopy; Technology; radical addition; SN-119; ACIDS; EPR; NMR CHEMICAL-SHIFTS; BICYCLO<2.2.2>OCTANE RING-SYSTEM; THROUGH-BOND TRANSMISSION; F-19; SPACE; bicycle [2.2.2]octanes; triptycyl compounds; CUBYL; ESTERS; C-13 NMR
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260331310

4-Substituted bicyclo[2.2.2]oct-1-yl radicals were generated by bromine atom abstraction from the corresponding 1-bromobicyclo[2.2.2]octanes and observed in solution by EPR spectroscopy. A tendency towards lower a(HB) and a(H-gamma) values for inductively electron-withdrawing substituents such as OMe and F and towards higher values for electron-releasing groups such as Me(3)Ge and Me(3)Sn was observed. For a series of bridgehead radicals, the hfs of B-hydrogens showed a monotonic increase as the extent of flattening at the bridgehead increased, The EPR data indicated that 4-substituents exercised a significant effect at the radical centre, mainly by a through-bond mechanism. 10-Substituted triptycyl radicals were generated in a similar way but showed no hfs from magnetic nuclei of the substituents. Thus, the triptycyl cage transmitted 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 benzene, tert-butylbenzene and 1,3-di-tert-butylbenzene to give cyclohexadienyl 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 similar conditions. Both bicycle[2.2.2]oct-1-yl and adamant-l-yl radicals generated in tert-butylbenzene showed exclusive meta addition with formation of the corresponding 1-polycyclo-3-tert-butylcyclohexadienyl radical.