TORSIONAL BARRIERS ABOUT N-C (ARYL) BOND AND ATROPISOMERISM - A H-1-NMR STUDY

Role of substituents at 3', 4', 5' and 6'-positions in N-arylsuccinimi dyl system on the torsional barrier about aryl-N bond has been reporte d with the help of asymmetric cage moiety through VT NMR studies of o- methyl resonances. Substituents at 3'-position raise the energy barrie r and chlorine is found to have a higher buttressing effect than a met hyl group. 5'-Substituents also influence the energy barrier which may be attributed to their interaction with the cage system. Methyl subst ituents at 2'-and 6'-positions provide a stable configuration about th e N-C phenyl bond (Delta G(double dagger) > 24 k cal/mol) and the phen yl ring is shown to be orthogonal to the succinimidyl plane. Atropisom ers of 2'-ethyl-6'-methylphenylsuccinimides have been isolated at room temperature and characterized. In case of 6'-chloro, 2'-methyl deriva tive, Delta G(double dagger) has been evaluated to be 23.2 k cal/mol; however, the rotamers are nonseparable at room temperature.