The alternating of substituent effect on the 13C NMR shifts of all bridge carbons in cinnamyl aniline derivatives

Abstract

Bridge carbon 13C NMR shifts of a wide set of substituted cinnamyl anilines p-XC6H4CHCHCHNC6H4Y-p (XNO2, Cl, H, Me, MeO, or NMe2; YNO2, CN, CO2Et, Cl, F, H, Me, MeO or NMe2) had been used as a probe to study the change of substituent effect in the conjugated system. The goal of this work was to study the difference among the substituent effect on SCS of all bridge carbons, and find the alternating of substituent effect in this model compounds. In this study, it was found that the change of the inductive effect and the conjugative effect on different bridge carbons is related to the bond number (m) from the substituent to the corresponding carbon, and the adjusted parameters σF(rel)∗ and σR(ver)∗ can be suitable to scale the difference of the inductive effect and the conjugative effect on bridge carbons. Moreover, because of the difference of substituent effect on bridge carbons, the substituent cross-interaction item Δσ2Δσ2=σF(rel)∗(X)+σR(ver)∗(X)-σF(rel)∗(Y)-σR(ver)∗(Y)2 was not suitable simply to scale the interaction between substituents X and Y for all bridge carbons, so the Δσ2 was recommended to be divided into two parts: ΔσF(rel)2ΔσF(rel)2=σF(rel)∗(X)-σF(rel)∗(Y)2 and ΔσR(ver)2ΔσR(ver)2=σR(ver)∗(X)-σR(ver)∗(Y)2. With σF(rel)∗, σR(ver)∗, ΔσF(rel)2, ΔσR(ver)2, and δC,parent, the obtained correlation equation can be used to correlate with the 159 sorts of SCS of the different bridge carbon in cinnamyl aniline derivatives. The correlation coefficient is 0.9993, and the standard deviation is only 0.53ppm. The multi-parameter correlation equation can be recommended to predict well the corresponding bridge carbons SCS of disubstituted cinnamyl anilines.