Electrochemistry of 2-bromo-2-methylpropanamides. Reduction mechanism and cyclocoupling reaction with amide solvents

Abstract

The electrochemical reduction of a series of secondary and teritary α-bromoisobutyramides has been studied in dipolar aprotic solvents. A carbanion is formed at the mercury electrode as a consequence of two-electron C–Br bond cleavage. Voltammetry and macroelectrolysis point to a self-protonation mechanism, the carbanion undergoing protonation by a parent molecule to yield the isobutyramide. Concurrently, tertiary 2-bromoamides undergo 1,2-elimination to yield an αβ-unsaturated amide, while secondary 2-bromoamides are deprotonated at the nitrogen atom, affording a bromo-containing anion. The decay of the latter is strongly dependent on the solvent and the substituent at nitrogen. In acetonitrile, elimination and fragmentation products are identified in the electrolysed solution. On the other hand, in N,N-dimethylformamide or N,N-dimethylacetamide, the bromo-containing anion is eventually cyclocondensed onto the carbonyl group of the amide solvent, to yield an oxazolidin-4-one derivative. Preliminary data suggest that an analogous cyclocoupling reaction takes place when the reduction is carried out in 1-methyl-2-pyrrolidone.