Amidines. Part 34.15N NMR spectra of trisubstituted amidines. Substituent effects

Abstract

¹⁵ N NMR spectra of four series of amidines (R²R³N–CR¹NR⁴; 24 compounds) in CDCl₃ solutions have been recorded and the chemical shifts of both nitrogen atoms assigned. The relation between substitution at the three sites of the amidino [〉N–C(N–)–] group and the ¹⁵N NMR chemical shifts of both nitrogen atoms is discussed on the basis of the results and the data accessible in the literature for seven other nontautomerizing open-chain amidines.

It is shown that, due to a strong conjugation in the amidino group, a substituent at either of the two nitrogen atoms exerts opposite effects on the shielding and thus on the chemical shifts of the amino and imino nitrogen atoms.

The effect of substitution at either of the two nitrogen atoms depends on substituents at the other two sites i. e. at the second nitrogen and the amidino carbon atom. For amidines containing a substituted phenyl ring at the imino (N²) nitrogen atom the chemical shifts of both nitrogen atoms correlate with the Hammett type constants of substituents, but the slopes of the regression lines are opposite for both atoms, and depend on substituents at the amino (N¹) nitrogen atom.

The results indicate that calculation of the ¹⁵N NMR chemical shifts in tautomerizing amidines (i.e. containing a hydrogen atom at the amino nitrogen) on the basis of the shifts in the corresponding methylated model compounds may yield incorrect results, and that this is the most probable source of discrepancies in estimations of the tautomeric equilibria by the nitrogen NMR method.