Effect of substituents on the C-13 chemical shifts of the azomethine carbon atom of N-(phenyl substituted)pyridine-3-and-2-aldimines

Abstract

C-13 chemical shifts of the azomethine carbon atom for N-(phenyl substituted)pyridine-3-aldimines, 3-Py-CH=N-C6H4-X, and N-(phenyl substituted)pyridine-2-aldimines, 2-Py-CH=N-C6H4-X, having a wide range of substituent effects, were determined in CDCl3 solution. Exceptionally good Hammett correlation of the C-13 NMR chemical shifts of azomethine carbons with electrophilic substituent constants sigma(+) indicate strong resonance interaction of the substituents on the aniline ring with the azomethine carbon atom of the investigated series of imines. The demand for electrons in the investigated systems may be compared to that of the N-benzylidenanilines and N-(phenyl substituted)pyridine-4-aldimines. The mode of transmission of the substituent effects, both inductive and resonance, in all four systems, was discussed.