Six pyrimidonium salts: determination of their crystal structures and studies of their prototropic behaviour in solution

Abstract

Six methyl-substituted 2-pyrimidonium chloride salts have been prepared and the rates of hydrogen exchange of their reactive methyl groups (at the 4- and 6-positions) have been determined in DCl/D2O solution. Adjacent methyl groups, whether on nitrogen or carbon, activate the exchanging centres, whereas more distant methyl groups have a deactivating effect. The molecular geometry of the salts has been determined by X-ray crystallography with the view to determining whether the presence of strain in the pyrimidonium ring can account for the activating effect of adjacent methyl. The most reactive compound, the 1,5,6-trimethylpyrimidonium chloride, has a geometry that is consistent with high reactivity, viz. a non-planar ring and short H … H intermethyl distance. However, the 1,4,5,6-tetramethyl compound, which also is non-planar and has an extremely short 5-6 intermethyl distance (1.99 Å), is not highly reactive; that is, the heightened strain is unable to overcome the deactivating inductive effect of the additional methyl group. We conclude that deviations from ring planarity and short intermethyl distances are insufficient to account for the activation produced by adjacent methyl though they appear to be contributing factors to this effect.