Synthesis of 1,4‐Butanediol by Catalytic Reduction of 2‐Bromo‐ and 2‐Iodoethanol with Homogeneous‐Phase Nickel (I) Salen Electrogenerated at Carbon and Mercury Cathodes

Cvclic voltammetry reveals that, at a glassy carbon or hanging mercury drop cathode in dimethylformamide or acetonitrile containing a tetraalkylammonium salt, nickel(II) salen undergoes a reversible one-electron reduction to the corresponding nickel(I) species. When 2-bromo- or 2-iodoethanol is added to the nickel(II) salen solution, the cyclic voltammogram shows an enhancement in the cathodic current and a disappearance of the anodic current, which are attributed to the catalytic reduction of the 2-haloethanol at a potential more positive than those required for direct reduction of these species. Controlled-potential catalytic reduction of 2-bromo- or 2-iodoethanol with nickel(I) salen electrogenerated at a reticulated vitreous carbon cathode gives rise to the formation of 1,4-butanediol in as much as 41% yield, along with ethanol (11 to 31%), and ethylene (13 to 55%); when a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol or acetic acid) is introduced into the system, ethane in yields as high as 30% (as well as the preceding three compounds) is obtained. If a mercury pool electrode is used and no proton donor is added, the products of the catalytic reduction of 2-bromoethanol are 1,4-butanediol (8 to 26%), ethanol (14 to 51%), and ethylene (33 to 62%)