Substituent Effects on Azomethine Proton Chemical Shifts and 13C–H Coupling Constants in N-Benzylideneanilines. Generality of Inverse Substituent Effects on the Azomethine Proton Chemical Shifts

Azomethine proton chemical shifts were measured for series of N-benzylideneanilines; C6H5CH=NC6H4X-4, 4-YC6H4CH=NC6H5, 4-NMe2C6H4CH=NC6H4X-4, 4-YC6H4CH=NC6H4NMe2-4, 4-NO2C6H4CH=NC6H4X-4, 4-YC6H4CH=NC6H4NO2-4, and 4-YC6H4CH=NC6H3(CH3)2-2,6. It was found that for N-benzylideneanilines carrying the variable substituents X on the anilino benzene ring the azomethine protons undergo upfield shifts with increasing electron-withdrawing property of X, whereas those for N-benzylideneanilines carrying the variable substituents Y on the benzylidene benzene ring exhibited expected lowfield shifts with increasing electron-withdrawing property of Y. 13C–H coupling constants for azomethine carbon–hydrogen bonds (J(13C–H)) were also measured for several derivatives of 4-NO2C6H4CH=NC6H4X-4, 4-YC6H4CH=NC6H4NO2-4, and 4-YC6H4CH=NC6H3(CH3)2-2,6. In contrast with the substituent effects on the azomethine proton chemical shifts, the coupling constant increases with increasing electron-withdrawing property of the substituent X or Y, regardless of the position of the substituents, indicating that a through-bond electronic effect of the substituent is a predominant factor to determine the magnitude of the coupling constant. These results show that the anomalous substituent effects on the azomethine proton chemical shifts are a general phenomenon in N-benzylideneanilines which carry variable substituents on the anilino benzene ring, and support our previous proposition that the anomaly is likely to originate primarily from the change of molecular conformation of these N-benzylideneanilines in solution.