ELECTROCHEMICAL REDUCTION OF 2-BROMOTHIAZOLE AT CARBON CATHODES IN ACETONITRILE

Cyclic voltammetry and controlled-potential electrolysis have been emp loyed to examine the electrochemical reduction of 2-bromothiazole at g lassy carbon cathodes in acetonitrile containing tetramethylammonium t etrafluoroborate. At a scan rate of 100 mV s(-1), a cyclic voltammogra m for the reduction of 2-bromothiazole exhibits two irreversible waves ; the first wave is due to the cleavage of the carbon-bromine bond, an d the second wave is attributed to the reduction of thiazole. Bulk ele ctrolyses of 2-bromothiazole at a potential corresponding to its first voltammetric wave afford thiazole quantitatively. In the absence of a ny deliberately added proton donor, the coulometric n value of 1.3 rev eals that the reduction of 2-bromothiazole involves both radical and c arbanion intermediates. However, the n value increases to essentially 2 when proton donors of increasing acidity and at higher concentration s are introduced. When acetonitrile-d(3) is employed as the solvent, t hiazole can undergo a base-catalyzed hydrogen-deuterium exchange to yi eld thiazole-2,5-d(2). Using homogeneous redox catalysis, we have esta blished that the DISP mechanism predominates over the ECE mechanism fo r the reduction of 2-bromothiazole and that the rate constant for deco mposition of the electrogenerated radical-anion of 2-bromothiazole is 1.8 x 10(4) s(-1). (C) 1998 Elsevier Science S.A. All rights reserved.