NMR and DFT investigations of the substituent and solvent effect on amino—imino tautomerism in acridin‐9‐amines substituted at the exocyclic nitrogen atom

Abstract

AbstractThe 1H and 13C NMR spectra of 9‐(methoxyamino)acridine (1) and 9‐hydrazinoacridine (2) show that these compounds exist principally in the imino tautomeric form in CDCl3, acetone‐d6, CD3CN, DMSO‐d6 and Py‐d5, all solvents with different polarities and abilities to participate in specific interactions. The spectra of the other two compounds investigated—N‐(2‐chloroethyl)acridin‐9‐amine (3) and N‐(5‐methylpyridin‐2‐yl)acridin‐9‐amine (4)—indicate that they coexist in the amino and imino forms. The amino tautomer of compound 3 predominates in CDCl3, CD3CN and Py‐d5 and that of compound 4 in CDCl3 and Py‐d5. On the other hand, the amino and imino forms of compound 3 coexist in acetone‐d6 and probably DMSO‐d6, whereas those of compound 4 coexist in acetone‐d6 and DMSO‐d6. The positions of the signals in the NMR spectra compare qualitatively with those predicted computationally at the GIAO/DFT level of theory. The equilibrium constants predicted by the DFT(PCM) method are in agreement with the results of NMR spectral analysis. In general, both the data predicted at the DFT level of theory and x‐ray structural data show that the imino tautomers display a ‘butterfly’‐type geometry, whereas the amino forms are characterized by an almost flat acridine moiety. Electron‐attracting substituents at the exocyclic N atom improve the stability of the imino form, and electron‐withdrawing substituents do likewise for the amino form. The importance of tautomeric phenomena in the context of the ability of acridin‐9‐amines to participate in specific interactions is outlined in brief, as are the possible applications of these compounds as probes of environmental properties. Copyright © 2005 John Wiley & Sons, Ltd.