Restricted rotation about the carbon–nitrogen single bond of 1-aryl-4,6-dimethylpyrimidin-2(1H)-ones and the corresponding thiones

Abstract

By recrystallizing the salts (3) and (4) with D-camphor-10-sulphonic acid, the rotational isomers due to restricted rotation about the carbon-nitrogen single bond in ortho-substituted-1-aryl-4,6-dimethylpyrimidin-2(1H)-ones (1) and the corresponding thiones (2) were separated. The activation energies for the racemization of the pyrimidin-2(1H)-ones (1c and f) were found to be 31.8 and 34.0 kcal mol^–1, respectively and those of the pyrimidine-2(1H)-thiones (2c, d, and g) were 31.5, 30.1, and 31.1 kcal mol^–1, respectively.