Substituent effect study on δ c values of the bridge group carbons in disubstituted cinnamyl aniline series

The 13 C nuclear magnetic resonance (NMR) chemical shifts δ c of bridge group carbons (C‐β, C‐α, and C═N) were measured in this work for a wide set of substituted cinnamyl anilines p ‐XC 6 H 4 CH═CHCH═NC 6 H 4 Y‐ p (X = NO 2 , Cl, H, Me, MeO, or NMe 2 ; Y = NO 2 , CN, CO 2 Et, Cl, F, H, Me, MeO, or NMe 2 ) and were used to study the substituent effect. In the study on 13 C NMR chemical shifts of the titled compounds with single substituent changed, for every bridge carbon δ c , the effect of cinnamyl substituent X is opposite to that of aniline substituent Y. That is, the action of the same substituent on different aromatic rings is different from the 13 C NMR chemical shifts, and for C‐β, C‐α, and C═N, the choice of correlation equation depends on the ratio ρ F (Y)/ ρ R (Y). When the ratio ρ F (Y)/ ρ R (Y) is close to 1, the chemical shifts of bridge carbons can be well correlated with the single‐parameter equation; otherwise, it is better to adopt the dual‐parameter equation for correlation, and the further the values of ρ F (Y)/ ρ R (Y) stray from 1, the more suitable the corresponding δ c values are to be correlated with the dual‐parameter equation. In the study on δ c of model compounds with simultaneous variations of substituents X and Y, for δ c (C═N), a multi‐parameter correlation equation is obtained, and the substituent cross‐interaction item Δ σ 2 is suitable to scale the interaction between substituents; however, for δ c (C‐α and C‐β), the substituent cross‐interaction item Δ σ 2 is perhaps too small to be observed. The multi‐parameter correlation equations can be recommended to predict well the corresponding δ c values of disubstituted cinnamyl anilines. Copyright © 2012 John Wiley & Sons, Ltd.