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

Abstract

dLong-range electronic substituent effects were targeted using the substituent dependence of d 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 d 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 d 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 d 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 d 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. a The values of d C (C═N) M3 were taken from Neuvonen et al. [16] SUBSTITUENT EFFECTS IN BENZYLIDENE ANILINES

A QSPR correlation on the 13C NMR chemical shifts of bridge carbons for different series of aromatic Schiff bases

“…In previous investigation results of aromatic Schiff base, the coefficients in front of a substituent in the C‐terminal benzene ring are opposite to those in front of the substituent in the N‐terminal benzene ring. However, the coefficients in front of inductive effect parameters and conjugative effect parameters are positive in Eqn , which seems to be in conflict with the previous results.…”

“…Although the correlation result of Eqn is good, there is a large standard deviation. In many previous types of work, it is confirmed that there is an interaction between substituents, and it cannot be ignored. In the investigation of δ C values for p ‐XCAY‐ p , the substituent interaction item, Δ σ 2 was recommended to be divided into two parts: and .…”

“…Next, out of 280 δ C values in the pieces of literature Neuvonen et al ., Chen et al ., and Fang et al ., they are randomly divided into the training set and the cross‐validation set according to the method of Bosque . Some XBAOMe, XCAMe and XBNBACN (X = MeO, H, Cl, and NO 2 ) are selected randomly for the cross‐validation set, and δ C values of the remaining compounds comprise the training set.…”

“…In Neuvonen's studies on δ C values of bridge carbons for para ‐disubstituted benzylidene anilines ( p ‐XBAY‐ p , shown in Scheme ) and para ‐disubstituted phenyl benzoates, Eqn was recommended to correlate with the corresponding experimental dataset. Cao's group synthesized successively a series of para ‐disubstituted cinnamyl anilines ( p ‐XCAY‐ p ) and a series of para ‐disubstituted benzylidene N ‐(4‐substituted styryl)anilines ( p ‐XBSAY‐ p ), whose structures are shown in Scheme , and research results indicated that δ C values of specific bridge carbons can be correlated well with Eqn . …”

Comparison of the 13C (C = N) chemical shifts of substituted N‐(phenyl‐ethylene)‐anilines and substituted N‐(benzylidene)‐anilines

“…Neuvonen pointed out: ‘the presence of the specific cross‐interaction between X and Y could be verified’, but this specific cross‐interaction effect was not be quantified at that time. Recently, Cao et al developed Neuvonen's research by exploring the other properties of XBAYs and proposed the parameter Δ σ 2 to measure the substituent‐specific cross‐interaction effect between substituents X and Y. Here, Δ σ 2 = [ σ x − σ y ] 2 = {[ σ F ( X ) + σ R ( X )]‐[ σ F ( Y ) + σ R ( Y )]} 2 .…”