Electrochemical Reduction of 1-Halo-2-butenes in Dimethylformamide

The electrochemical reduction of 1-bromo-2-butene and 1-chloro-2-butene in DMF at a Hg electrode has been studied by polarography, cyclic voltammetry (CV), a rotating ring-disc electrode and controlled-potential coulometry. A CV study using a gold electrode has also been carried out for these compounds to identify the detected intermediates. Two consecutive one-electron reduction processes are found for 1-bromo-2-butene in polarography and in CV using Hg electrode. The first process is initiated by the irreversible one-electron cleavage of the carbon-bromo bond to give the allyl radical and Br-, which is the rate-determining step. The second one follows a first-order EC mechanism, being initiated by generation of the allylmercury anion via a one-electron reduction of the allylmercury radical, previously formed by reaction of the allyl radical with Hg. A single irreversible two-electron process is found for 1-chloro-2-butene under all voltammetric conditions and for both compounds in CV using a gold electrode. Additional anodic peaks detected in CV, as well as anodic waves found at the rotating Hg ring electrode, are ascribed to oxidation of the allylmercury anion and the allyl anion.