Effect of substituents on the C-13 chemical shifts of the azomethine carbon atom of N-(phenyl substituted) pyridine-4-aldimines

Abstract

C-13 chemical shifts of the azomethine carbon atom for N-(substituted phenyl)pyridine-4-aldimines, 4-Py-CH=N-C6H4-X, X having a wide range of substituent effects, have been determined in CDCl3 solution. Exceptionally good Hammett correlations of C-13 NMR chemical shifts of azomethine carbons with electrophilic substituent constants sigma(+) points out to a strong resonance interaction of substituents on the aniline ring with the azomethine carbon atom of the investigated imines. The increased demand for electrons in the investigated system is being compared with that of the N-benzylidenaniline. The mode of transmission of substituent effects, both inductive and resonance, in both systems, is discussed.