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

C-13 chemical shifts of the azomethine carbon atom for N-(substituted pheny l)pyridine-4-aldimines, 4-Py-CH=N-C6H4-X, X having a wide range of substitu ent effects, have been determined in CDCl3 solution. Exceptionally good Ham mett correlations of C-13 NMR chemical shifts of azomethine carbons with el ectrophilic 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 reson ance, in both systems, is discussed. (C) 1999 Elsevier Science B.V. All rig hts reserved.