Hydrogen exchange, base-catalyzed pyrimidines

In pyrimidine, the most likely metallation site for lithiation is C-5 (cf. base-catalyzed hydrogen exchange, Section 2), but even two methoxy groups in the 4- and 6-positions failed to allow lithium-hydrogen exchange at -40°C in this system (65JCS6695). At lower temperature (<-70°C), however, 5-pyrimidinyllithium derivatives can be prepared by... 

In pyridazines, base-catalyzed hydrogendeuterium exchange takes place at positions 4 and 5 more easily than at positions 3 and 6. Pyridazine 1-oxide reacts first at positions 5 and 6 and then at C(3) and C(4). Pyrimidine exchanges most readily at the S-position, next at the 4-position, and least readily at the 2-position. In pyrimidine 1-oxide, the reactivity order is 2>6>4>>5. 1,2,4-Triazines easily undergo base-catalyzed hydrogen exchange at the 2-position. 

Second-Order Rate Coefficients for Base-Catalyzed Deuterium-Hydrogen Exchange in 5-Substituted Pyrimidine A/-Oxides"... 

---