N.M.R. Studies of Bridged Ring Systems: Ii. Unusual Magnetic Deshielding Effect on the Bridge Methylenes of Norbornadiene and Benzonorbornadiene

Proton magnetic resonance spectra of some norbornane derivatives having double bonds and (or) a benzene ring in the skeleton were studied. Differences in the chemical shifts between the bridge methylenes of norbornene and benzonorbornene and that of norbornane result from the diamagnetic anisotropy effect of a double bond or a benzene ring being in good agreement with the values calculated theoretically. However, in the norbornadiene derivatives which have two double bonds or one olefinic double bond with a benzene ring in a part of the skeleton, the signals of their bridge methylenes show extraordinarily larger downfield shifts by about 0.8 p.p.m. than those expected from the additive shielding due to each π-electron system. This anomaly is not observed in bicyclo[2.2.2]octane ring series. A large transannular interaction between two π-electron systems in that system would be a major contribution to the above anomaly.