Use of Silver Cathodes to Promote the Direct Reduction and Intramolecular Cyclization of ω-Halo-1-phenyl-1-alkynes in Dimethylformamide

Abstract

Electrochemical reduction of 6-bromo- and 6-iodo-1-phenyl-1-hexyne and of 7-bromo- and 7-iodo-1-phenyl-1-heptyne at a silver cathode in dimethylformamide (DMF) containing 0.050 M tetramethylammonium perchlorate (TMAP) has been investigated with the aid of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for each bromo compound show a single irreversible cathodic peak for scission of the carbon-bromine bond, whereas two irreversible cathodic peaks are associated with reduction of the carbon-iodine bond of each iodo species. Radical intermediates arising from one-electron reduction of these ?-halo-1-phenyl-1-alkynes at silver undergo intramolecular cyclization to afford benzylidenecycloalkanes; other products include alkynes, enynes, and dimeric species. Electrolyses of 6-iodo-1-phenyl-1-hexyne and 7-iodo-1-phenyl-1-heptyne afford the desired benzylidenecycloalkane in higher yield than do electrolyses of the corresponding brominated compounds. Benzylidenecyclopentane is obtained in up to 85% yield when 6-iodo-1-phenyl-1- hexyne is reduced at a silver electrode held at a potential that corresponds to the first cathodic peak for this substrate; moreover, if the solvent-electrolyte is dried with activated alumina, one can achieve 99% conversion of 6-iodo-1-phenyl-1-hexyne to benzylidenecyclopentane. Studies involving the use of a deuterium atom donor (DMF-d7) or a deuterium ion donor (D2O) offer evidence that radicals and carbanions are both involved as intermediates in the formation of products. © 2013 The Electrochemical Society.