Tautomerism of azine derivatives. 6. Tautomerism of 2-pyrimidinylmethane derivatives
The existence of a pyrimidinyl-pyrimidinylidene tautomeric equilibrium in solutions of 2-pyrimidinylcyanoacetic acid esters in CDCl3 was observed. Unsymmetrically substituted compounds form two types of ylidene tautomers that differ with respect to the position of the NH proton, the ratio between which is controlled by the substituents in the 4 (6) position. The introduction of both donor and acceptor substituents into the 5 position of the pyrimidine ring increases the amount of the pyrimidine form. The same thing occurs when the polarity of the solvent is decreased. The addition of DMSO or DMF to CDCl3 leads to conversion of the intrachelate ylidene tautomers to unchelated tautomers. Protonation (CF3COOH) shifts the equilibrium to favor the ylidene tautomer that has higher basicity.