Electrochemical behavior of some new pyrimidine derivatives

Electrochemical reduction of two recently synthesized pyrimidine compounds 1-amino-5-benzoyil-4-phenyl-1H-pyrimidine-2-one (I), and 1-amino-5-benzoil 4-phenyl-1H-pyrimidine-2-thione (II) were investigated by cyclic volatmmetry at a hanging mercury drop electrode in aqueous methanol (36 % v/v) and in non-aqueous methanol. Aseries of cathodic peaks without the corresponding anodic peaks were observed for I. As the pH of the solution was increased some of the cathodic peaks overlapped resulting in the loss of the previously observed peaks. For II, three cathodic peaks and one anodic peak were observed in addition to those observed for I. The peak potentials shifted in the negative direction with increasing pH. This shift was measured over a large pH range (1.80 ? 12.30) to determine the pKa values of the compounds. The acidity constants related to the amino groups were 4.80 and 9.80 for I and 5.50 and 9.80 for II. Athiol-thione tautomerization was observed for II which was more pronounced in the non-aqueous methanol medium. The pK values for both protonation and deprotonation of the thiocarbonyl group were also determined. The pK values were 5.80 and 9.80 for protonation and deprotonation in aqueous methanol and 6.80 and 10.80 in non-aqueous methanol. 1-amino-5-benzoil-4-fenil-1H-pirimidine-2-ona (I) i 1-amino-5-benzoil-4-fenil- 1H-pirimidin-2-tiona (II) ciklicnom voltametrijom na visecoj zivinoj kapi u vodenom rastvoru metanola (36 % v/v) i u bezvodnom metanolu. Za jedinjenje I dobijen je niz katodnih maksimuma bez odgovarajucih anodnih. Sa povecanjem pH rastvora neki od katodnih maksimuma su se preklapali uz nestanak ranije opazenih maksimuma. Kod jedinjenja II pored maksimuma opazenih kod I dobivena su i tri nova katodna i jedan anodni maksimum. Povecanjem pH potencijali maksimuma pomerali su se u negativnom smeru. Ovaj pomeraj meren je u opsegu pH 1,80 ? 12,30 rad idredjivanja pKa vrednosti ovih jedinjenja. Konstante kiselosti amino grupa su 4,80 za jedinjenje I i 9,80 za jedinjenje II. Kod jedinjenja II zapazena je tiol-tion tautomerizacija koja je bila izrazenija u nevodenoj metanolskoj sredini. Odredjene su pK vrednosti protonacije i deprotonacije tiokarbonilne grupe. Ove pK vrednosti su 5,80 i 9,80 za vodeni rastvor metanola, a 6,80 i 10,80 za nevodeni metanol.