Substituent effects in the reduction behaviour of thio- and oxopyrimidines in non-aqueous solvents

The electrochemical reduction of a series of thio- and oxopyrimidine derivatives has been investigated in organic solvents on mercury electrodes. In all cases the electrochemical process gave a dimeric species as the major product. The overall reduction mechanism is the same for oxo and thio derivatives, and is found to be dependent only on the nature of the ring nitrogen substituent. A 'father-son reaction' is observed when hydrogen is bound to the ring nitrogen atom: the radical anion obtained from the first electron transfer draws out the nitrogen proton of a nonreduced molecule and this then dimerizes. In the presence of a protonating agent as well as for the N-substituted derivatives, the 'father-son reaction' is not observed. Theoretical calculations have been performed to gain insight into the proposed mechanisms: the LUMO energy and the vertical electron affinity show a linear correlation with the reduction potentials. Analysis of the theoretical parameters has allowed step-by-step determination of the electrochemical reduction process. The manner in which solvent properties influence electrochemical behaviour has been examined, and the role of the acceptor number (AN) has been discussed.