Electrochemical reduction of halogenated pyrimidines at mercury cathodes in acetonitrile

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate the reduction of some mono-, di-, tri-, and tetrahalopyrimi dines at mercury cathodes in acetonitrile containing tetramethylammonium te trafluoroborate. Two irreversible cyclic voltammetric waves are observed fo r reduction of 2-bromo-, 5-bromo-, and 2-chloropyrimidine; the first wave i s due to cleavage of the carbon-halogen bond, and the second wave is attrib utable to reduction of pyrimidine. Cyclic voltammograms for 2,4-dichloro- a nd 4,6-dichloropyrimidine exhibit three cathodic waves, whereas that for 2, 4,6-trichloropyrimidine shows four cathodic waves, arising from sequential cleavage of carbon-chlorine bonds as well as the reduction of pyrimidine. F or the reduction of 2,4,5,6-tetrachloropyrimidine, a cyclic voltammogram ex hibits four major irreversible cathodic waves corresponding to the cleavage of carbon-chlorine bonds, but the wave for reduction of pyrimidine is poor ly defined. Bulk electrolyses of halopyrimidines at potentials for differen t stages of reduction lead to products that are consistent with expectation s based upon cyclic voltammetry. In addition, our findings agree well with theoretical calculations of the relative stabilities of the various reducti on intermediates. Mechanistic aspects of the reduction of halopyrimidines a re discussed and, using homogeneous redox catalysis, we have determined the lifetimes of the electrogenerated radical-anions of 2-bromo- and 2-chlorop yrimidine. (C) 2001 Elsevier Science B.V. All rights reserved.