13C NMR spectra of some symmetrically 2,2′-disubstituted stilbenes: conjugational and conformational effects

Abstract

In the 13C NMR spectra of symmetrically 2,2′-disubstituted cis -stilbenes the decrease of α-effects and increase of γ-effects at ortho -carbons with respect to tarns -isomers were observed. However the most significant differences in substituent effects between trans- and cis -isomers were found for γ-effects, at unsubstituted ortho -C-6 and C-4 atoms. The one bond C-H splitting of olefinic carbons is greater in cis -than trans -stilbenes. The differences in substituent effects and apparent coupling constants are assumed to originate mainly from the different conjugational interactions in cis -and trans -stilbenes. the conformational and configurational differences of these molecules were substantiated by molecular mechanics calculations.