1H and 13C dynamic nuclear magnetic resonance study of hindered internal rotation in (N,N-dimethylamino)pyrimidines

Abstract

The free energies of activation ΔG^‡ for hindered rotation around the C–N exocyclic bond in a series of 2- and 4-(N,N-dimethylamino)pyrimidines have been determined by ¹H and ¹³C n.m.r. line-shape analysis. Good linear correlations have been obtained between the coupling constant ¹J(C,H) for the dimethylamino group and the ΔG^‡ parameter. Such correlations have been used to estimate ΔG^‡ values of symmetrical or unsymmetrical pyrimidines for which an experimental determination of this thermodynamic parameter is either impossible or extremely difficult. In addition, substituent effects on the barrier heights indicate that: (i) the difference between the ΔG^‡₄ and ΔG^‡₂ values decreases with increasing electron-withdrawing power of the substituent, and (ii), substituent effects are larger through a ring nitrogen than a ring carbon atom. Correlation of the ΔG^‡ values with Hammett constants is discussed.