Investigation of the substituent specific cross‐interaction effects on ¹³C NMR of the CN bridging group in substituted benzylidene anilines

Abstract

The substituent effect on 13 C NMR of the C --N in benzylidene anilines XPhCH --NPhY was investigated, in which the substituents X and Y are in p-position or in m-position of the two aromatic rings. The substituent effects including the inductive effects of X and Y, the conjugative effects of X and Y, and the substituent specific cross-interaction effect were put into one model to quantify the 13 C NMR chemical shift d C (C --N) of the C --N in XPhCH --NPhY. A penta-parameter correlation equation with correlation coefficient 0.9975 and standard error 0.17 ppm was obtained for 80 samples of compounds. The result shows that the substituents X and Y have an opposite effect on the d C (C --N). The electron-withdrawing effects of X decrease the d C (C --N); while the electron-donating effects of X increase the d C (C --N). In contrast, the electron-withdrawing effects of Y increase the d C (C --N); while the electron-donating effects of Y decrease the d C (C --N). A new substituent specific cross-interaction effect parameter Ds 2 was proposed, which indicates that the most substituent specific cross-interaction effect exists in the pair of max electron-withdrawing group (EWG) and max electron-donating group (EDG) or the pair of max EDG and max EWG. Further to verify the obtained correlation equation, 15 samples of model compounds were prepared and their d C (C --N) was measured in this work. The predicted d C (C --N) values with the obtained equation are in good agreement with the measured ones for these prepared compounds, which confirmed the reliability of the obtained equation.By fixing the substituent X (or Y) at one end, Neuvonen [1,7] recently investigated the substituent cross-interaction effects on the electronic character of the C --N bridging group in substituted benzylidene anilines XPhCH --NPhY with the 13 C NMR spectra of C --N carbon. He obtained the valuable results:(1) for the X substituents: electron-withdrawing group (EWG) cause shielding, while electron-donating group (EDG) behave oppositely; (2) in contrast, for the Y substituents: EWG cause deshielding, while EDG cause shielding of the C --N carbon. Also, (wileyonlinelibrary.com)

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

“…Equation has also good correlation, because the molecular structure of XPEAYs has certain similarity with that of XBAYs. Its standard error 0.20 ppm is larger than that of Eqn ; maybe there are some others factors affecting the δ C (C = N) of XPEAYs. To seek out this difference, we plot the experimental δ C (C = N) of XBAYs against that of corresponding XPEAYs (e.g., p ‐MeBAF‐ p versus p ‐MePEAF‐ p ), and obtained Fig.…”

“…Cao proposed Eqn to correlate the δ C (C = N) of XBAYs, in which the standard error is only 0.17 ppm. …”

“…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 .…”

“…In XPEAYs, a group Me replaces the atom H in CH = N of XBAYs, and results in more complicated cross‐interactions of the substituted groups. For the compounds XBAYs, there are four situations of interactions among the substituents X and Y, while in the XPEAYs, Me is an EDG and will act with X and Y through the conjugate chain, respectively. It is a very attractive topic whether the change regularity of the δ C (C = N) in XPEAYs is as the same as that in XBAYs or not.…”

Crystal structure of N-(3,4-dichlorobenzylidene)-4-methylaniline, C14H11Cl2N

“…The product was synthesized by solvent-free method [1,2]. The raw materials 3,4-dichlorobenzaldehyde (98%) and pmethylaniline (98%) were purchased from J&K SCIENTIFIC LTD. Equimolar 3,4-dichlorobenzaldehyde and p-nitroaniline were stirred uniformly at rst.…”