Protonation of the Pyrimidine Ring at the C(5) Position:  Formation of a Stable Cationic σ-Complex

NMR studies showed that, in addition to the expected N(1) protonation, 2,4,6-pyrimidinetriamine, N,N,N',N',N",N"-hexamethyl- (1) could also be protonated at the C(5) position in water, leading to an equilibrium between the C(5) and N(1) protonated forms. Analysis of the NMR titration data gives 6.87 and 6.89 for the pK(a) of the C(5) and N(1) protonation equilibria. Moreover, the reaction of 1 with chloroacetyl chloride leads to a novel 1,1-bis(pyrimidin-5-yl)-2-chloroethene type derivative (4) that is, peculiarly, fully monoprotonated at the C(5) position in either of the pyrimidine rings, forming a stable cationic sigma-complex.