Electrochemical reduction of 2-bromothiazole at carbon cathodes in acetonitrile

Abstract

Abstract Cyclic voltammetry and controlled-potential electrolysis have been employed to examine the electrochemical reduction of 2-bromothiazole at glassy carbon cathodes in acetonitrile containing tetramethylammonium tetrafluoroborate. At a scan rate of 100 mV s−1, a cyclic voltammogram for the reduction of 2-bromothiazole exhibits two irreversible waves; the first wave is due to the cleavage of the carbon–bromine bond, and the second wave is attributed to the reduction of thiazole. Bulk electrolyses of 2-bromothiazole at a potential corresponding to its first voltammetric wave afford thiazole quantitatively. In the absence of any deliberately added proton donor, the coulometric n value of 1.3 reveals that the reduction of 2-bromothiazole involves both radical and carbanion intermediates. However, the n value increases to essentially 2 when proton donors of increasing acidity and at higher concentrations are introduced. When acetonitrile-d3 is employed as the solvent, thiazole can undergo a base-catalyzed hydrogen-deuterium exchange to yield thiazole-2,5-d2. Using homogeneous redox catalysis, we have established that the DISP mechanism predominates over the ECE mechanism for the reduction of 2-bromothiazole and that the rate constant for decomposition of the electrogenerated radical-anion of 2-bromothiazole is 1.8×104 s−1.