Carbon-13 nuclear magnetic resonance spectra of ortho-substituted acetophenones: enhanced substituent effects on the carbonyl group

Abstract

The ¹³C carbonyl chemical shifts of substituted 2′-methyl-, 2′-methoxy-, and 2′, 6′-dimethyl-acetophenones are compared with those of corresponding acetophenones. The correlations indicate that the carbonyl chemical shifts are more sensitive to the 4-X induced substituent effect in both 2′-methyl- and 2′-methoxy-acetophenones than in acetophenones. The enhancements are in full agreement with i.r. data and have been attributed to intra-ring electronic interactions in 2′-methyl- and 2′-methoxy-acetophenones and to quasi-ring formation in the former. In contrast, δ_coremained nearly constant in the 2′,6′-dimethylacetophenones, owing to the large torsion in these compounds.