Carbon-13 nuclear magnetic resonance spectra of strained molecules. Diamagnetic and paramagnetic shifts of sterically compressed carbon resonances in [2,2]metacyclophanes

Abstract

Carbon-13 nuclear magnetic resonance spectra of [2,2]metacyclophanes show two types of steric compression shifts (van der Waals shifts) operating in an opposite direction: a large diamagnetic shift of bridging methylene carbon resonances due to an ortho-alkyl group and an anomalous paramagnetic shift of the inner aryl carbon resonances caused by direct carbon–carbon compression.