Catalysis with a silver lining: Mechanisms, reactivity, and adsorption phenomena for electrochemical reduction of organic halides at silver cathodes.

摘要

To advance our understanding of the electrochemical reduction of organic halides at silver cathodes, often referred to as a heterogeneous electrocatalyst for cleavage of carbon-halogen bonds, mono and dihaloalkanes have been studied with electrochemical, spectrochemical, and microscopy techniques. First, reduction of &ohgr;-halo-1-phenyl-1-alkynes was conducted at silver cathodes in order to promote intramolecular cyclization. It was found that the amount of desired carbocycle obtained is highly dependent on reaction conditions; highest yields were attained via reduction of 6-iodo-1-phenyl-1-hexyne at a potential corresponding to its least cathodic peak in solvent-electrolyte that had been dried over activated alumina. Second, reactivity was probed for primary, secondary, and tertiary alkyl monoiodides, bromides, and chlorides at silver cathodes by means of cyclic voltammetry, controlled-potential (bulk) electrolysis, and theoretical calculation of molecular dipole moment and polarizability. Dependence on the position of the iodide was observed in product distributions that arise from electrolysis; however, this trend was not seen for bromides and chlorides. On the other hand, dependence on the identity of the halogen for primary, secondary, and tertiary alkyl monohalides was revealed by the number of cathodic peaks and their potentials in addition to product distributions. Third, reduction mechanisms of 1,2- and 1,6-dibromohexane were found to be dependent on the relative position of the two bromines. Furthermore, by means of electrolysis of 1,6-dibromohexane in a thin-layer cavity, it was shown that adsorbed substrate experiences a four-electron transfer, whereas diffusion-controlled species undergo a two-electron process. Lastly, reduction mechanisms for the primary iodide and bromide previously investigated were probed for the presence of adsorbed alkyl halide and radical-anion intermediate by means of electrochemical surface-enhanced Raman spectroscopy and surface interrogation mode of scanning electrochemical microscopy.