Catalytic Reduction of 1,6-Dihalohexanes by Nickel(I) Salen Electrogenerated at Glassy Carbon Cathodes in Dimethylformamide

Cyclic voltammetry (CV) and controlled-potential electrolysis (CPE) were employed to examine the reactions of electrogenerated ligand-reduced nickel(II) salen with benzyl bromide, 1-bromomethylnaphthalene, and α-bromodiphenylmethane. Cyclic voltammograms for nickel(II) salen in the presence of benzyl bromide or 1-bromomethylnaphthalene exhibit characteristic features for the catalytic reduction of substrates involving radical intermediates. Bulk electrolyses of benzyl bromide and 1-bromomethylnaphthalene at carbon cathodes catalyzed by nickel(II) salen were also carried out at selected potentials to afford various products. These results were compared with similar reaction involving 1-bromooctane as the substrate. Further comparison of the CVs for nickel(II) salen before and after reactions with the four different organic halides reveals that the steric effect could play an important role in the corresponding nucleophilic attack of the substrates by ligand-reduced catalyst (a radical−anion), which follows the sequence of 1-bromooctane > benzyl bromide > 1-bromomethylnaphthalene > α-bromodiphenylmethane in terms of reaction efficiency. Moreover, theoretical calculations using density functional theory were carried out to establish a proposed mechanism for the electrochemical reactions on the basis of previous and current studies.

Section: Methodsmentioning

confidence: 99%

Reaction of Electrogenerated Ligand-Reduced Nickel Salen with Benzyl Bromide, 1-Bromomethylnaphthalene, and α-Bromodiphenylmethane: A Study of Steric Effects

Nguyen

Tomasso

Easter

et al. 2015

“…[28][29][30] Cyclic voltammetry was performed with a planar, circular silver working electrode (with a geometric area of 0.071 cm 2 ) constructed from 3.0-mm diameter rod (99.9%, Alfa Aesar), as described elsewhere. 30 Working electrodes, with surface areas of approximately 20 cm 2 , for controlled-potential (bulk) electrolyses were fabricated from silver gauze (Alfa Aesar, 99.9%, 20 mesh woven from 0.356-mm diameter wire).…”

Section: Methodsmentioning

confidence: 99%

Electrochemical Reduction of 1-Bromomethyl-2-oxocycloalkane-1-carboxylates at Silver Cathodes in Dimethylformamide: One-Carbon Ring-Expansion Reactions

Wappes

Mubarak

Peters

2014

Self Cite

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the separate electrochemical reductions of methyl 1-bromomethyl-2-oxocyclopentane-1-carboxylate (1) and ethyl 1-bromomethyl-2-oxocyclohexane-1-carboxylate (2) at silver cathodes in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF 4 ). Oneelectron reductive cleavage of the carbon-bromine bond of each substrate yields a radical intermediate that undergoes a ring-expansion reaction, followed by hydrogen-atom abstraction from the solvent, to afford methyl 3-oxocyclohexane-1-carboxylate (3a) and ethyl 3-oxocycloheptane-1-carboxylate (3b), respectively, in good yield. Each substrate gives rise to three other products: (a) a debrominated analogue of each starting material, (b) a dimeric species formed via radical coupling, and (c) a species possessing an ester group extended by one carbon atom. Electrolyses of 1 and 2 done in the presence of D 2 O have revealed that carbanion intermediates result in small amounts from two-electron cleavage of carbon-bromine bonds. A mechanistic scheme, involving both radicals and carbanions, is proposed to account for the formation of the various products.

“…Information about the two-compartment (divided) electrolysis cell, as well as details about instrumentation and procedures for bulk electrolyses, can be found in earlier papers. 6,31 Separation, identification, and quantitation of electrolysis products.-Prior to the start of each controlled-potential (bulk) electrolysis, a known amount of an internal standard (n-undecane) was added to the solution so that the absolute yield of each product (with respect to the amount of starting material) could be determined; details pertaining to this quantitation procedure have been published elsewhere. 32 At the conclusion of a bulk electrolysis, the catholyte (ca.…”

Section: Methodsmentioning

confidence: 99%

Electrochemical Reduction of a Bromo Propargyloxy Ester at Silver Cathodes in Dimethylformamide

Henderson

Buehler

Pasciak

et al. 2014

Self Cite

Cyclic voltammograms for the reduction of ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)-3-(propargyloxy)propanoate (1) at a silver cathode in dimethylformamide (DMF) containing 0.10 M tetraethylammonium tetrafluoroborate (TEABF 4 ) exhibit several cathodic peaks, the first of which is attributed to reductive cleavage of the carbon-bromine bond. Controlled-potential (bulk) electrolyses of 1 at silver gauze electrodes in DMF-0.10 M TEABF 4 give rise to four products: cis-and trans-isomers of ethyl 3-(3 ,4 -dimethoxyphenyl)-prop-2-enoate (4), ethyl 3-(3 ,4 -dimethoxyphenyl)propiolate (7), and ethyl 3-(3 ,4 -dimethoxyphenyl)-3-(prop-2-yn-1-yloxy)propanoate (8). These products have been identified with the aid of mass spectrometry and nuclear magnetic resonance spectroscopy. We propose that reduction of 1 involves two-electron cleavage of the carbon-bromine bond to form a carbanion. Then the latter species eliminates − OCH 2 C≡CH to afford 4. In addition, − OCH 2 C≡CH can deprotonate 1 to yield (Z)-ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)acrylate (5), which is further deprotonated by − OCH 2 C≡CH to give 7. Alternatively, the carbanion resulting from the original two-electron reduction of 1 can gain a proton from the medium to form 8. In two previous projects 1,2 involving collaboration with our laboratory, electrochemical methods were probed as an avenue to the synthesis of building blocks for lignans, which comprise a class of phenylalanine-derived compounds that have attracted considerable attention due to their pharmacological activity and abundance in nature. In the first of those investigations, 1 we examined the reductive intramolecular cyclization of ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)-3-(propargyloxy)propanoate (1), catalyzed by electrogenerated nickel(I) tetramethylcyclam, to afford 2-(3 ,4 -dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methylenetetrahydrofuran (2) in good yield (72-85%); the latter species is a desired precursor for the formation of isogmelinol (a furofuran lignan). In addition, a second product, 2-(3 ,4 -dimethoxyphenyl)-3-ethoxycarbonyl-4-methyl-2,5-dihydrofuran (3), was obtained in significant yield (22-30%).A second paper 2 was concerned with the direct reduction of 1 at both glassy carbon and reticulated vitreous carbon cathodes in dimethylformamide (DMF) containing various tetraalkylammonium salts. Interestingly, although 2 was not obtained, 3 (which * Electrochemical Society Student Member.* * Electrochemical Society Fellow. z E-mail: peters@indiana.edu arises via base-promoted rearrangement of 2) was found in modest yield (18-26%). More importantly, the principal product was ethyl trans-3-(3 ,4 -dimethoxyphenyl)acrylate (4), although small amounts of (Z)-ethyl 2-bromo-3-(3 ,4 -dimethoxyphenyl)acrylate (5) and ethyl 3-(3 ,4 -dimethoxyphenyl)-2,3-bis(prop-2-yn-1-yloxy)propanoate (6) were detected.Recent publications from our laboratory 3-8 and by other research groups 9-24 have dealt with the electrochemistry of halogenated organic compounds at silver cathodes and have revealed the catalytic ef...