Electroreductive radical cyclization of ethyl 2-bromo-3-allyloxy- and -3-(propargyloxy)propanoates catalyzed by (tetramethylcyclam)nickel(I) electrogenerated at carbon cathodes in dimethylformamide

Prova tipográfica (In Press)Reductive intramolecular cyclization of ethyl 2-bromo-3-(3′,4′-methylenedioxophenyl)-3-(propargyloxy)propanoate (1) and ethyl 2-bromo-3-(3′,4′-dimethoxyphenyl)-3-(allyloxy)propanoate (2) promoted by (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetra-decane)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 bromoesters. During controlled-potential electrolyses of solutions of [Ni(tmc)]2+ in the presence of 1 and 2, catalytic reduction of each substrate proceeds via one-electron cleavage of the carbon–bromine bond to form a radical intermediate that undergoes cyclization to afford, respectively, 2-(3′,4′-methylenedioxophenyl)-3-ethoxycarbonyl-4-methylenetetrahydrofuran (3) and 2-(3′,4′-dimethoxyphenyl)-3-ethoxycarbonyl-4-methyltetrahydrofuran (6). A mechanistic scheme is proposed to account for the formation of each major product.Fundação Calouste GulbenkianCRUPFundação para a Ciência e Tecnologia (FCT)Government Assistance in Areas of National Need (GAANN)Fellowship awarded by Indiana Universit

Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(I) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate and characterize the reductive intramolecular cyclization of ethyl 2-bromo-3-(3′,4′-dimethoxyphenyl)-3-propargyloxy-propanoate (1) 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. Cyclic voltammograms for reduction of [Ni(tmc)]2+ in the presence of 1 reveal that [Ni(tmc)]+ catalytically reduces 1 at potentials more positive than those required for direct reduction of 1. During controlled-potential electrolyses of solutions containing [Ni(tmc)]2+ and 1, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon–bromine bond to form a radical intermediate that undergoes cyclization to afford 2-(3′,4′-dimethoxyphenyl-3-ethoxycarbonyl-4-methylene-tetrahydrofuran (2). In the presence of a base (either electrogenerated or deliberately added as potassium tert-butoxide), 2 rearranges to give 2-(3′,4′-dimethoxyphenyl-3-ethoxycarbonyl-4-methyl-2,5-dihydrofuran (3). A mechanistic scheme is proposed to explain the results obtained by means of cyclic voltammetry and controlled-potential electrolysis.Fundação Calouste Gulbenkian