Multiple Substituent Effects on 13C Chemical Shifts of N-Benzylideneanilines. Evidence for Substituent–Substituent Interactions and Their Implications of Conformational Changes with Substituents

Abstract

Abstract Carbon-13 chemical shifts are reported for series of N-benzylideneanilines; 4-YC6H4CH=NC6H4NMe2-4, 4-NO2C6H4CH=NC6H4X-4, 4-YC6H4CH=NC6H4NO2-4, and 4-YC6H4CH=NC6H3(CH3)2-2,6. Multiple substituent effects on the azomethine carbon chemical shifts were examined, and nonadditivity observed for 4-YC6H4CH=NC6H4NMe2-4 and 4-NO2C6H4CH=NC6H4X-4 is interpreted in terms of substituent–substituent interactions accompanied by conformational changes with substituents in these N-benzylideneanilines. Effects of the substituents Y (on the benzylidene benzene ring) on the azomethine carbon chemical shifts were also studied in detail. The results suggest that there are two competetive electronic factors responsible for chemical shift variation, and that the substituent effects should be treated separately between electron-releasing and electron-withdrawing groups. The latter groups always exert inverse (the chemical shifts undergo upfield shifts with increase in the electron-withdrawing properties of the substituents) substituent effects, whereas the former exhibit normal (the chemical shifts undergo upfield shifts with increase in the electron-releasing properties of the substituents) or inverse substituent effects depending on the degree of electron demand developed on the azomethine carbons.