Electroreductive Radical Cyclization of Ethyl 2‐Bromo‐3‐allyloxy‐ and‐3‐(propargyloxy)propanoates Catalyzed by (Tetramethylcyclam)nickel(I) Electrogenerated at Carbon Cathodes in Dimethylformamide

Abstract

AbstractReductive intramolecular cyclization of ethyl 2‐bromo‐3‐(3′,4′‐methylenedioxyphenyl)‐3‐(propargyloxy)propanoate(1) and ethyl 3‐allyloxy‐2‐bromo‐3‐(3′,4′‐dimethoxyphenyl)propanoate (2) promoted by (1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane)nickel(I), [Ni(tmc)]+, electrogenerated at glassy‐carbon cathodes in dimethylformamide containing tetraalkylammonium salts, has been investigated. Cyclic voltammograms for reduction of [Ni(tmc)]2+ in the presence of 1 and 2 reveal that [Ni(tmc)]+ catalytically reduces these two compounds at potentials significantly more positive than those required for direct reduction of the bromo esters. During controlled‐potential electrolyses of solutions of [Ni(tmc)]2+ in the presence of 1 and 2, catalytic reduction of each substrate proceeds by one‐electron cleavage of the carbon–bromine bond to form a radical intermediate that undergoes cyclization to afford, respectively, ethyl 2‐(3′,4′‐methylenedioxyphenyl)‐4‐methylenetetrahydrofuran‐3‐carboxylate (3) and ethyl 2‐(3′,4′‐dimethoxyphenyl)‐4‐methyltetrahydrofuran‐3‐carboxylate (6). A mechanistic Scheme is proposed to account for the formation of each major product. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)