Substituent effects on the ¹³C NMR chemical shifts of the imine carbon in <i>N</i>‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines

Abstract

Long‐range electronic substituent effects were targeted using the substituent dependence of δ C (C═N), and specific cross‐interactions were explored extendedly. A wide set of N ‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines, p ‐X–C 6 H 4 CH═NC 6 H 4 CH═CHC 6 H 4 – p ‐Y (X = NMe 2 , OMe, Me, H, Cl, F, CN, or NO 2 ; Y = NMe 2 , OMe, Me, H, Cl, or CN) were prepared for this study, and their 13 C NMR chemical shifts δ C (C═N) of C═N bonds were measured. The results show that both the inductive and resonance effects of the substituents Y on the δ C (C═N) of p ‐X–C 6 H 4 CH═NC 6 H 4 CH═CHC 6 H 4 – p ‐Y are less than those of the substituents Y in p ‐X–C 6 H 4 CH═NC 6 H 4 – p ‐Y. Moreover, the sensitivity of the electronic character of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the conjugated chain is elongated. It was confirmed that the substituent cross‐interaction is an important factor influencing δ C (C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long‐range transmission of the specific cross‐interaction effects on δ C (C═N) decreases with increasing conjugated distance between X and Y. The results of this study suggest that there is a long‐range transmission of the substituent effects in p ‐X–C 6 H 4 CH═NC 6 H 4 CH═CHC 6 H 4 – p ‐Y. Copyright © 2012 John Wiley &amp; Sons, Ltd.