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

Abstract

The 13C nuclear magnetic resonance (NMR) chemical shifts δc 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‐XC6H4CH═CHCH═NC6H4Y‐p (X = NO2, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, CO2Et, Cl, F, H, Me, MeO, or NMe2) and were used to study the substituent effect. In the study on 13C NMR chemical shifts of the titled compounds with single substituent changed, for every bridge carbon δc, 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 13C NMR chemical shifts, and for C‐β, C‐α, and C═N, the choice of correlation equation depends on the ratio ρF(Y)/ρR(Y). When the ratio ρF(Y)/ρR(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 ρF(Y)/ρR(Y) stray from 1, the more suitable the corresponding δc values are to be correlated with the dual‐parameter equation. In the study on δc of model compounds with simultaneous variations of substituents X and Y, for δc(C═N), a multi‐parameter correlation equation is obtained, and the substituent cross‐interaction item Δσ2 is suitable to scale the interaction between substituents; however, for δc(C‐α and C‐β), the substituent cross‐interaction item Δσ2 is perhaps too small to be observed. The multi‐parameter correlation equations can be recommended to predict well the corresponding δc values of disubstituted cinnamyl anilines. Copyright © 2012 John Wiley & Sons, Ltd.

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 . …”

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

“…It is known that the length of the conjugated chain can affect the inductive and resonance effects and thus might lead to changes in the electron distribution. Cao et al [18] also investigated the substituent effects in M2 (Scheme 1). Our observations of the substituent effects in M2 encouraged us to prepare other type of benzylidene anilines, in which the conjugated chain in the aniline unit is elongated.…”