Electronic substituent effect on intramolecular CH/pi interaction as evidenced by NOE experiments

In order to demonstrate the hydrogen-bond-like character of the CH/pi inter action, electronic substituent effects on the equilibria between the stretc hed and the folded conformers of series of compounds capable of forming CH/ pi interactions were examined by measurements of NOE enhancements of H-1 NM R signals. Nuclear Overhauser enhancement is shown to be useful to determin e the abundance of the CH/pi proximate folded conformer. The Hammett plots of all series of the compounds capable of having CH/pi interaction gave neg ative rho values. Together with other substituent effects (effects of elect ronegative substituents, on the CH donor, of ring size, and of alpha-alkyl substituent), the involvement of delocalization interaction and the hydroge n-bond-like character of the CH/pi interaction were established.