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

Abstract

The 13C 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)−2σF(Y) and n(Y)−2σR(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)−2σF(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)−2σF(X), σR(X), n(Y)−2σF(Y), n(Y)−2σR(Y), Δσ(cor)2, and δC(parent), the δC(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 δC(C = N) of other 25 samples of designed conjugated benzylidene anilines. Copyright © 2012 John Wiley & Sons, Ltd.