Long‐range transmission of substituent effects on 13 C NMR chemical shifts of imine carbon in benzylidene anilines

The 13 C NMR chemical shifts of six kinds of substituted benzylidene anilines, with different backbone conjugation length, have been used as a probe to investigate the long‐range transmission of substituent effects. In this context, it was found that for substituents Y at the aniline unit, the transmission of the inductive and conjugative effects depend on the chemical bond numbers n (Y) between Y and the imine carbon, and the parameters n (Y) −2 σ F (Y) and n (Y) −2 σ R (Y) are suitable to scale the corrected inductive and conjugative effects, respectively. However, for substituents X, the chemical bond numbers n (X) between X and the imine carbon influences only the transmission of inductive effects of X, and the n (X) −2 σ F (X) item is appropriate to evaluate the modified inductive effects of X. Similarly, Δσ (cor) 2 was proposed to describe the transmitted effect of the cross‐interaction effect. With the parameters n (X) −2 σ F (X), σ R (X), n (Y) −2 σ F (Y), n (Y) −2 σ R (Y), Δσ (cor) 2 , and δ C (parent), the δ C (C = N) values of 181 samples can be well correlated. The correlation coefficient is 0.9957, and the standard derivation is only 0.23 ppm. Moreover, the multi‐parameter correlation equation is predicted well the δ C (C = N) of other 25 samples of designed conjugated benzylidene anilines. Copyright © 2012 John Wiley & Sons, Ltd.