•At carbon or silver in DMF and DMSO, decabromodiphenyl ether undergoes multistep reduction.•Decabromodiphenyl ether undergoes 20–25% total debromination at carbon or silver in DMF.•Reduction of ...decabromodiphenyl ether affords diphenyl ether, dibenzofuran, benzene, and phenol.•Electrochemical debromination of decabromodiphenyl ether is more complete in DMF than in DMSO.
An investigation of the electrochemical reduction of the flame-retardant, decabromodiphenyl ether (DBDE), at carbon and silver cathodes has been undertaken with the aid of cyclic voltammetry and controlled-potential (bulk) electrolysis in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), each solvent containing 0.10M tetra-n-butylammonium tetrafluoroborate (TBABF4). Cyclic voltammograms for reduction of DBDE at a glassy carbon electrode exhibit five irreversible and comparably sized cathodic peaks that are associated with a sequence of debromination processes, whereas cyclic voltammograms acquired with a silver cathode show fewer discrete stages of irreversible reduction and are accentuated by one very prominent peak. Bulk electrolyses of DBDE at either a reticulated vitreous carbon or a silver cathode in DMF lead to approximately 20–25% conversion of DBDE to a mixture of diphenyl ether, dibenzofuran, benzene, and phenol, and the remaining products are congeners of dibromodiphenyl ether. When DMSO is employed as solvent for bulk electrolyses, diphenyl ether is formed in approximately 20% yield, and a mixture of congeners ranging from decabromo- to monobromodiphenyl ether is found. Bulk electrolyses of DBDE in DMF–TBABF4 at carbon held at a potential near the cathodic limit indicate that complete debromination can be achieved.
Cyclic voltammetry and controlled-potential (bulk) electrolyses have been employed to probe the electrochemical reductions of 1-bromo-6-chlorohexane and 1‐chloro-6-iodohexane at silver cathodes in ...dimethylformamide (DMF) containing 0.050M tetra-n-butylammonium tetrafluoroborate (TBABF4). A cyclic voltammogram for reduction of 1-bromo-6-chlorohexane shows a single major irreversible cathodic peak, whereas reduction of 1-chloro-6-iodohexane gives rise to a pair of irreversible cathodic peaks. Controlled-potential (bulk) electrolyses of 1-bromo-6-chlorohexane at a silver gauze cathode reveal that the process involves a two-electron cleavage of the carbon–bromine bond to afford 1-chlorohexane as the major product, along with 6-chloro-1-hexene, n‐hexane, 1‐hexene, and 1,5-hexadiene as minor species. In contrast, bulk electrolyses of 1-chloro-6-iodohexane indicate that the first voltammetric peak corresponds to a one-electron process, leading to production of a dimer (1,12-dichlorododecane) together with 1-chlorohexane and 6-chloro-1-hexene as well as 1‐hexene and 1,5-hexadiene in trace amounts. At potentials corresponding to the second cathodic peak, reduction of 1-chloro-6-iodohexane is a mixture of one- and two-electron steps that yields the same set of products, but in different proportions. Mechanistic schemes are proposed to explain the electrochemical behavior of both 1‐bromo-6-chlorohexane and 1-chloro-6-iodohexane.
Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the electrochemical reduction of 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene), commonly ...known as the pesticide methoxychlor, at glassy carbon and silver cathodes in dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4). Reduction of methoxychlor at both glassy carbon and silver shows four voltammetric peaks, the first three of which are associated with cleavage of carbon-chlorine bonds; the fourth peak is assigned to reduction of 4,4′-(ethene-1,1-diyl)bis(methoxybenzene). Bulk electrolyses of methoxychlor at reticulated vitreous carbon and silver mesh cathodes at potentials corresponding to each of the first three voltammetric peaks were conducted; coulometric n values and product distributions (determined by means of GC and GC-MS techniques) depend on potential. In particular, two completely dechlorinated products, namely 4,4′-(ethane-1,1-diyl)bis(methoxybenzene) and 4,4′-(ethene-1,1-diyl)bis(methoxybenzene) have been identified and quantitated. A mechanistic scheme is proposed to account for the formation of the various products.
Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct electrochemical reduction of 2-chloro-N-methyl-N-phenylacetamide (1a), ...2-chloro-N-ethyl-N-phenylacetamide (1b), and 2-chloro-N-phenylacetamide (1c) at carbon and silver cathodes, as well as the catalytic reduction of these compounds by electrogenerated nickel(I) salen, in dimethylformamide (DMF) containing 0.050M tetramethylammonium tetrafluoroborate (TMABF4). Cyclic voltammograms for reduction of 1a and 1b show a single irreversible cathodic peak for cleavage of the carbon–chlorine bond, but two irreversible cathodic peaks are observed in cyclic voltammograms for reduction of 1c. Controlled-potential reduction of 1a and 1b gives mixtures of dechlorinated amide and N-alkyl-N-phenylaniline, whereas bulk electrolyses of 1c afford N-phenylacetamide in almost quantitative yield. In addition, bulk electrolyses of 1a and 1b result in the formation of very small amounts of dimeric species that arise from coupling of the radical intermediate formed by one-electron cleavage of the carbon–chlorine bond. On the basis of the coulometric n values and product distributions, together with computations based on density functional theory, we propose mechanistic pictures for the reduction of 1a and 1b that involve radical intermediates, whereas reduction of 1c involves carbanion intermediates.
► Direct reductive dechlorination of CFC-113 occurs at a carbon cathode in DMF. ► Electrogenerated nickel(I) salen catalyzes the dechlorination of CFC-113. ► Electrochemical reduction of CFC-113 ...involves carbanion intermediates. ► Nickel(I) salen is deactivated upon alkylation by a fragment from CFC-113.
Cyclic voltammetry, controlled-potential (bulk) electrolysis, gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and high-performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) have been employed to investigate the direct reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) at a carbon cathode in dimethylformamide (DMF) containing 0.10M tetra-n-butylammonium tetrafluoroborate (TBABF4) as well as the nickel(I) salen-mediated reduction of CFC-113 in the same medium. Cyclic voltammograms for direct reduction of CFC-113 show two irreversible cathodic peaks attributable to formation of, first, 1-chloro-1,2,2-trifluoroethene (CFC-1113) and, second, trifluoroethene (HFC-1123); bulk electrolyses of CFC-113 at potentials corresponding to each of the cathodic peaks afford CFC-1113 and HFC-1123, respectively, in essentially quantitative yield. Cyclic voltammograms for reduction of nickel(II) salen in the presence of CFC-113 provide evidence for the catalytic formation of CFC-1113 and HFC-1123, and bulk electrolyses involving electrogenerated nickel(I) salen lead to mixtures of CFC-1113 and HFC-1123, although 1,1,1,2-tetrafluoroethane (HFC-134a) has been detected as a minor product. It has been determined with the aid of HPLC–ESI–MS that the salen ligand of the nickel(I) catalyst is modified during the catalytic reduction of CFC-113. A mechanistic scheme is proposed for the direct reduction of CFC-113.
Several nickel(II) complexes of cyclams bearing aryl groups on the carbon backbone were prepared and evaluated for their propensity to catalyze the electrochemical reduction of CO2 to CO and/or H+ ...to H2, representing the first catalytic analysis to be performed on an aryl–cyclam metal complex. Cyclic voltammetry (CV) revealed the attenuation of catalytic activity when the aryl group bears the strong electron-withdrawing trifluoromethyl substituent, whereas the phenyl, p-tolyl, and aryl-free derivatives displayed a range of catalytic activities. The gaseous-product distribution for the active complexes was determined by means of controlled-potential electrolysis (CPE) and revealed that the phenyl derivative is the most active as well as the most selective for CO2 reduction over proton reduction. Stark differences in the activity of the complexes studied are rationalized through comparison of their X-ray structures, absorption spectra, and CPE profiles. Further CV studies on the phenyl derivative were undertaken to provide a kinetic insight.
Electrochemical reduction of primary (1‐halodecane), secondary (2‐halohexane and cyclohexyl halide), and tertiary halides (tert‐butyl halide) at a silver cathode in dimethylformamide (DMF) containing ...0.050 M tetramethylammonium perchlorate (TMAP) was investigated with the aid of cyclic voltammetry and controlled‐potential (bulk) electrolysis for iodides, bromides, and chlorides. Selected reductions were probed in dried DMF–TMAP and DMF containing tetra‐n‐hexylammonium perchlorate (THAP). Cyclic voltammograms reveal that the number of cathodic peaks and their potentials are highly dependent on the identity and position of the halogen as well as the composition of the solvent–electrolyte. Intermediates arising from bulk electrolysis of these halides at silver undergo both radical and carbanion reactions. Electrolysis products depend on the identity and position of the halogen, amount of residual water, and size of the electrolyte cation in the solvent–electrolyte. Theoretical calculations of molecular dipole moments and polarizabilities were performed and compared to experimental observations.
Catalysis with a silver lining: Results from cyclic voltammetry and controlled‐potential (bulk) electrolysis of alkyl halides at silver cathodes in dimethylformamide are highly dependent on the identity and position of the halogen atom (see picture).
► Cobalt(I) salen-catalyzed reduction of CFC-113 occurs in a CO2-saturated medium. ► Interaction between cobalt(I) salen and CO2 preserves the fidelity of this catalyst. ► Chlorotrifluoroethene is ...reduced by cobalt(I) salen in a CO2-saturated medium. ► CFC-113 reduction by cobalt(I) salen involves four electrons in CO2-saturated DMF.
A comparative study of the cobalt(I) salen-catalyzed electrochemical reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) in both carbon dioxide- and argon-saturated dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) has been carried out by means of cyclic voltammetry with a glassy carbon electrode. Catalytic reduction of CFC-113 by cobalt(I) salen appears to be more efficient in a carbon dioxide-saturated medium. Chlorotrifluoroethene (a degradation product of CFC-113) is also more efficiently reduced by electrogenerated cobalt(I) salen in the presence of carbon dioxide. No evidence for the cobalt(I) salen-catalyzed reduction of trifluoroethene (another degradation product of CFC-113) has been seen. Preliminary bulk electrolyses of cobalt(II) salen–CFC-113 mixtures in a carbon dioxide-saturated medium have indicated the formation of chlorotrifluoroethene, trifluoroethene, and 1,1,1,2-tetrafluoroethane. However, these three compounds do not account for all of the original CFC-113; therefore, future work will focus on the possible production of 3,3-dichloro-2,2,3-trifluoropropanoic acid and 2,3,3-trifluoroacrylic acid that might arise via carboxylation of anionic intermediates derived from the reduction of CFC-113. Furthermore, ultraviolet–visible spectrophotometry has confirmed that cobalt(I) salen and carbon dioxide interact to preserve the fidelity of the catalyst.
Dimethyl-, diethyl-, and diphenyl-substituted analogues of nickel(II) salen have been synthesized, and the cyclic voltammetric behavior of each compound at a glassy carbon electrode in ...dimethylformamide (DMF) containing tetra-
n-butylammonium tetrafluoroborate (TBABF
4) has been compared with that of nickel(II) salen itself. Differences in the cathodic peak potentials for these species have been rationalized with the aid of theoretical computations based on density functional theory. Cyclic voltammograms for reduction of dimethylated nickel(II) salen reveal that placing a methyl group on the carbon atom of each imino (C
N) bond of the ligand improves the performance of electrogenerated dimethylated nickel(I) salen as a catalyst for reduction of 1-iodobutane. This conclusion is supported by experiments that combine controlled-potential electrolysis with high-performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) to detect and analyze the post-electrolysis nickel-containing species arising from the bulk catalytic reduction of 1-iodooctane. Additional studies have been made of the use of electrogenerated dimethylated nickel(I) salen for the catalytic reduction of alkyl halides and for the reductive intramolecular cyclization of acetylenic halides.
Catalytic reduction of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) by nickel(I) salen electrogenerated at a carbon cathode has been investigated by means of cyclic voltammetry and controlled-potential ...(bulk) electrolysis in dimethylformamide (DMF) containing 0.10M tetramethylammonium tetrafluoroborate (TMABF4). Cyclic voltammograms for reduction of nickel(II) salen in the presence of HBCD exhibit the characteristics of a catalytic process: (a) at low concentrations of HBCD, a tiny cathodic prepeak is observed that merges with the peak associated with reduction of nickel(II) salen and (b) as the substrate concentration increases, the cathodic peak current rises and the anodic peak current falls for the nickel(II) salen–nickel(I) salen redox couple. Bulk electrolyses at a reticulated vitreous carbon cathode of solutions containing 1.0mM nickel(II) salen and less than 20.0mM HBCD result in conversion of the starting material to 1,5,9-cyclododecatriene (CDT, 88–95%), along with small amounts of cyclododeca-1,5-dien-9-yne (CDY, 3–4%), in a six-electron process. Incomplete catalytic reduction occurs for concentrations of HBCD at or above 20.0mM; CDT is produced in only 35% yield, and traces of unreduced starting material remain. In addition, at this higher concentration of HBCD, other products (pentabromocyclododecene, tetrabromocyclododecene, tribromocyclododecadiene, dibromocyclododecadiene, and bromocyclododecatriene) have been detected, but not quantitated.
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