In-situ electrogeneration of [2,2′-ethylenebis(nitrilomethylidyne)diphenolato]nickelate(I) — nickel(I) salen — as a catalyst for reductive intramolecular cyclizations of 6-iodo- and 6-bromo-1-phenyl-1-hexyne

Abstract

In dimethylformamide containing tetraethylammonium perchlorate, the [2,2′-ethylenebis(nitrilomethylidyne)diphenolato]nickel(II) complex, more familiarly known as nickel(II) salen, undergoes reversible one-electron reduction at a carbon cathode to the corresponding nickel(I) species at a potential where 6-iodo- and 6-bromo-1-phenyl-1-hexyne are not electroactive. However, in the presence of the electrogenerated nickel(I) complex, these acetylenic halides are rapidly and catalytically reduced to form a radical intermediate that cyclizes to produce benzylidenecyclopentane in at least 84% yield. This approach to the electrosynthesis of the carbocyclic compound is highly efficient because direct electrolyses of 6-iodo- and 6-bromo-1-phenyl-1-hexyne at carbon cathodes lead to undesired acyclic side-products and to benzylidenecyclopentane in only modest yields.