Electrochemical reduction of 2-halo-N-phenylacetamides at glassy carbon cathodes in dimethylformamide

Abstract

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct electrochemical reductions of 2-bromo-N-phenylacetamide (1a) and 2-iodo-N-phenylacetamide (1b) at glassy carbon cathodes in dimethylformamide containing tetramethylammonium tetrafluoroborate (TMABF4) as supporting electrolyte. Cyclic voltammograms for reduction of 1a and 1b each exhibit a pair of irreversible cathodic peaks; the first peak arises from a combination of one-electron and two-electron reductive cleavage of the respective carbon–halogen bond and the second peak is due to overall two-electron reductive cleavage of the carbon–halogen bond to produce a free halide ion together with 2-oxo-2-(phenylamino)ethan-1-ide. Controlled-potential (bulk) electrolysis of a 5mM solution of 1a or 1b at a potential corresponding to either the first or second cathodic peak affords only phenylacetamide. However, bulk electrolysis of a 10mM solution of 1a or 1b at a potential corresponding to either cathodic peak leads to a mixture of phenylacetamide and 1,4-diphenylpiperazine-2,5-dione. In the presence of a large excess of 1,1,1,3,3,3-hexafluoro-2-propanol (a proton donor), bulk electrolyses of 1a or 1b at a potential on the first peak give phenylacetamide exclusively. When either 1a or 1b is electrolyzed in the presence of 1M D2O, the resulting phenylacetamide is deuterated to a very significant extent (evidence for a carbanion intermediate), and a small quantity of 2-(dimethylamino)-N-phenylacetamide is seen as a side-product. Mechanisms to account for the behavior of 1a and 1b are proposed.