► A cyclic voltammogram for reduction of Lindane at glassy carbon exhibits one irreversible peak. ► Electroreduction of Lindane is a six-electron process that leads to benzene and a small amount of ...chlorobenzene. ► Computations based on density functional theory suggest possible intermediates involved in the reduction of Lindane. ► Reduction of Lindane is a sequence of six one-electron steps with loss of chloride after each step.
Direct reduction of Lindane (1R,2r,3S,4R,5r,6S-hexachlorocyclohexane, 1) at carbon cathodes in dimethylformamide (DMF) containing 0.10M tetra-n-butylammonium tetrafluoroborate (TBABF4) has been explored by means of cyclic voltammetry and controlled-potential (bulk) electrolysis. Cyclic voltammograms for reduction of 1 at a glassy carbon electrode exhibit two cathodic peaks at −1.40V and −2.10V as well as an anodic peak at −1.93V; the first cathodic peak is attributed to reduction of 1 itself, whereas the second cathodic peak is due to reduction of chlorobenzene that is derived from 1. Controlled-potential (bulk) electrolyses conducted with reticulated vitreous carbon electrodes held at −1.70 or −2.20V reveal that reduction of 1 is essentially a six-electron process that affords benzene as major product (80–100% yield) along with small amounts of chlorobenzene (3–10% yield). To account for these products, a mechanism is proposed that is supported by the results of theoretical computations based on density functional theory.
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.
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.
Cyclic voltammetry and controlled-potential (bulk) electrolysis have been used to investigate the direct reduction of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan,
1) in dimethylformamide (DMF) ...containing tetra-
n-butylammonium tetrafluoroborate (TBABF
4). Cyclic voltammograms for reduction of
1 at glassy carbon electrodes exhibit three irreversible peaks attributed to successive reductive cleavage of the three aryl carbon–chlorine bonds. Bulk electrolyses of
1 at reticulated vitreous carbon cathodes held at a potential between the first and second cathodic peaks afford only 5-chloro-2-(4-chlorophenoxy)phenol; however, at a more negative potential, a mixture of 5-chloro-2-(4-chlorophenoxy)phenol, 5-chloro-2-phenoxyphenol, and 2-phenoxyphenol is obtained. A scheme consisting of electron-transfer steps and accompanying chemical reactions is proposed that follows the classic mechanism for the reduction of aryl halides. To provide support for this mechanism, theoretical calculations based on density functional theory have been performed to model the electronic structures of
1 and the likely intermediates formed via its electroreduction. Catalytic reduction of
1 by nickel(I) salen and nickel(I) diethylsalen, each mediator electrogenerated at a glassy carbon electrode in DMF – 0.10
M TBABF
4, has been explored with the aid of cyclic voltammetry and controlled-potential electrolysis.
Cyclic voltammetry and controlled‐potential (bulk) electrolysis have been used to study the direct electrochemical reduction of cyclohexyl bromide (1) and cyclohexyl iodide (2) at glassy carbon ...cathodes in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4). Direct reduction of 1 is a one‐step process that affords a carbanion intermediate, whereas 2 undergoes stepwise reduction to a radical and then a carbanion intermediate. Mixtures of cyclohexane, cyclohexene, and bicyclohexyl arise from bulk electrolyses of both 1 and 2. Catalytic reduction of 1 and 2 by nickel(I) salen and cobalt(I) salen electrogenerated at glassy carbon cathodes in DMF‐TMABF4 has been investigated with the aid of both cyclic voltammetry and bulk electrolysis. Products arising from these catalytic reductions are cyclohexane, cyclohexene, and bicyclohexyl, although significant amounts of unreduced 1 are found when cobalt(I) salen is utilized as the catalyst. Mechanistic aspects of the direct and catalyzed reductions of 1 and 2 are discussed.
Radical ideas: Direct electrochemical reduction of cyclohexyl bromide and cyclohexyl iodide at glassy carbon cathodes in dimethylformamide (DMF) containing 0.10 M tetramethyl‐ammonium tetrafluoroborate (TMABF4) affords both free radical‐ and carbanion‐derived products – cyclohexane, cyclohexene, and bicyclohexyl. Reduction of these halides with electrogenerated nickel(I) salen or cobalt(I) salen leads to free‐radical‐derived products (cyclohexane, cyclohexene, and bicyclohexyl) in different amounts.
•Cyclic voltammograms for reduction of methyl triclosan at C exhibit three peaks.•Electrolysis of methyl triclosan at C affords chlorinated and unchlorinated products.•Overall reduction of methyl ...triclosan at glassy C is an eight-electron process.•Methyl triclosan is easier to dechlorinate electrolytically than triclosan.
Methyl triclosan 2,4-dichloro-1-(4-chloro-2-methoxyphenoxy)benzene, 1, which is now considered an environmental pollutant, has been shown to be formed via microbial methylation of triclosan 5-chloro-2-(2,4-dichlorophenoxy)phenol, 2. In the present work, cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct reduction of 1 at glassy carbon electrodes in dimethylformamide (DMF) containing 0.050M tetramethylammonium tetrafluoroborate (TMABF4). Cyclic voltammograms for the reduction of 1 at a glassy carbon electrode exhibit three irreversible cathodic peaks with peak potentials of −1.72, −2.07, and −2.35V (versus a cadmium-saturated mercury amalgam reference electrode). These three peaks correspond, respectively, to (a) scission of the ortho C–Cl bond, (b) cleavage of the two para C–Cl bonds, and (c) rupture of the C–O bond. Controlled-potential (bulk) electrolyses of 1 at reticulated vitreous carbon electrodes result in the formation of mixtures of 4-chloro-1-(4-chlorophenoxy)-2-methoxybenzene (3), 4-chloro-2-methoxy-1-phenoxybenzene (4a), 1-(4-chlorophenoxy)-2-methoxybenzene (4b), 1-methoxy-2-phenoxybenzene (5), anisole (6), and phenol.
Metal-to-ligand charge-transfer (MLCT) photolyses (lambda > or = 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), Cu(bpod)(2)PF(6) (2), and ...Cu(bpod)(2)(NO(3))(2) (3) yield Bergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)(2)(CH(3)COO)(2) compound (4) are photochemically inert under the same conditions. In the case of 4, sensitized photochemical generation of the lowest energy (3)pi-pi state, which is localized on the enediyne unit, leads to production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemical studies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between E(p) values of -1.75 and -1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E(1/2) = +0.15 and +0.38 V, respectively. Combined with the observed Cu(I)-to-pyridine(pi) MLCT and pyridine(pi)-to-Cu(II) ligand-to-metal charge transfer (LMCT) absorption centered near approximately 315 nm, the results suggest a mechanism for photo-Bergman cyclization that is derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediates produced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5'-phosphate and 3'-phosphoglycolate termini that are derived from H-atom abstraction from the 4'-position of the deoxyribose ring rather than redox-induced base oxidation.
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