Preparative Scale Electrolysis Research Articles

TEMPO-Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity.

Homogeneous catalysts ("mediators") are frequently employed in organic electrosynthesis to control selectivity. Despite their advantages, they can have a negative influence on the overall energy and mass balance if used only once or recycled inefficiently. Polymediators are soluble redox-active polymers applicable as electrocatalysts, enabling recovery by dialysis or membrane filtration. Using anodic alcohol oxidation as an example, we have demonstrated that TEMPO-modified polymethacrylates (TPMA) can act as efficient and recyclable catalysts. In the present work, the influence of the molecular size on the redox properties and the catalytic activity was carefully elaborated using a series of TPMAs with well-defined molecular weight distributions. Cyclic voltammetry studies show that the polymer chain length has a pronounced impact on the key-properties. Together with preparative-scale electrolysis experiments, an optimum size range was identified for polymediator-guided sustainable reaction control.

Selective electrochemical C[sbnd]O bond cleavage of β-O-4 lignin model compounds mediated by iodide ion

The electrochemically oxidative cleavage of lignin β-O-4 model compounds mediated by iodide ion has been studied. The results indicate that electrolytic conditions play a predominant role in determining the distribution of cleavage products. The preparative-scale electrolysis proceeds in a simple undivided cell, employing a catalytic amount of NaI as the redox mediator and supporting electrolyte in methanol. Under these conditions, the CβO bond is selectively cleaved with 2,2-dimethoxy-2-arylacetaldehyde being the main product. In some cases, the reaction gives a good yield of cleavaged products. The results further demonstrate that the indirect electrolysis mediated by halide is a versatile approach for chemical transformation.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes.

The electrochemical behavior of fac-[Mn(pdbpy)(CO)3 Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO2 reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO2 -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO2 atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported MnI catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO2 reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)3 Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO2 .

Effect of Alkyl Chain Length of Imidazolium Cation on the Electroreduction of CO2 to CO on Ag Electrode in Acetonitrile

The effect of imidazolium-based ionic liquid on the electroreduction of CO2 to CO over a Ag electrode in acetonitrile catholyte was investigated. The voltage–current profiles clearly indicate that the electroreduction of CO2 is sensitive to the alkyl chain length at the N1-position in imidazolium cation (MIM+). Density functional theory computation suggests that the onset potential of CO2 reduction is related to the association degree between MIM+ and CO2•– species. More importantly, preparative scale electrolysis shows that the selectivity and output rate for the target product CO are also significantly affected by MIM+. With the elongation of the alkyl group in MIM+ from ethyl to octyl, the Faradaic efficiency for CO remarkably increases from 87 ± 4 % to 97 ± 2 % and then remains almost unchanged. However, the curve of the current density with respect to the chain length of alkyl group shows a convex style. These results indicate the dependence of CO2 reduction efficiency on the MIM+ adsorbed on the Ag electrode surface.

Anodic Oxidation of Nitrotriarylamines and Its Implications for Electrocatalysis

In 1980 Steckhan and Schmidt introduced the use of 4,4’,4”-tribromotriphenylamine (1a) and several more highly halogenated derivatives for the electrocatalytic anodic oxidation of organic substrates,1 and a sizeable number of such transformations have been reported.2 The relatively modest oxidation potential of 1 (+0.78 vs Ag/0.1 M AgNO3 reference) imposes restrictions, however, on the range of substrates that can treated in this manner. For this reason, we synthesized a series of triarylamines bearing several electron-withdrawing groups with the expectation that these would be useful for effecting catalytic oxidation of substrates that high oxidation potentials.3 We had found earlier that such substances can be difficult to oxidize anodically.4We have had some success using these more electronegatively substituted triarylamines.5 However, the situation is not as simple as it may appear. For example, the generalization is often made that mediated oxidation can be effected at potentials up to 500-600 mv positive of the oxidation potential of the substrate, but the latter depends, inter alia, upon the rate of further chemical reaction of the oxidized form of the substrate. Other assumptions are that electronegative substituents increase the oxidative power of the triarylamine catalyst, but not any of its other chemical properties and that the amine cation radical is stable on the time scale of the electrolysis. As we will report here, we find that both of these assumptions are often unwarranted for triarylamines. We recently carried out a series of controlled potential electrolyses of a 10:1 mole ratio of trans-stilbene and 4,4’,4”-trimethyl-2,2’,2”-trinitrotriphenylamine (1b) in aqueous acetonitrile. Under these conditions it was found that the initially produced dark blue solution of 1b+ (the cation radical of 1b) quickly turns first violet and then red and much unreacted stilbene remains. The color change demonstrated that 1 + is unstable under these conditions. A subsequent electrolysis of 1 in the absence of stilbene, followed by flash chromatography of the electrolysis mixture afforded a red solid whose formula (from exact mass measurement by mass spectrometry) is C21H16N4O8. We assign structure 2 (a substituted N-phenylphenoxazine) to this substance because a reasonable mechanistic path to it can be written involving three consecutive ECE processes involving nucleophilic attack by water upon 1 +to introduce the oxygen atoms onto the rings.One might conclude from this that the presence of three highly withdrawing nitro groups in 1 + is responsible for its instability by increasing its electrophilicity and that a less highly nitrated derivative would not take this path. This is not so; more recently, we have found that anodic oxidation of the mononitro triphenylamine 1c in aqueous acetonitrile under the same conditions also affords a red substance to which we again assign a phenoxazine structure. Although 1b and 1c both exhibit reversible cyclic voltammograms, this is misleading. In fact, the cation radicals from both 1b and 1c both undergo rather efficient nucleophilic attack by water in experiments of longer time scale. Phenoxazine formation is apparently efficient from 1c even though its oxidation potential is much lower than that of 1b. Decomposition of triphenylamine cation radicals may be a general pattern; the tribromide 1a has also been observed to decompose during preparative scale electrolysis.6 Finally, we note that instability of cation radicals, with its concomitant undesirable impact on electrosynthetic applications, may prove to be problematical in other organic electrocatalytic systems, particularly when they bear one or more strongly electron-withdrawing groups.

Reduction of 1,2-dicarbonyl compounds and of their N-phenylimine derivatives by sodium cyanide under aprotic conditions

Some aromatic 1,2-dicarbonyl compounds, i.e. 9,10-phenanthrenequinone, acenaphthenequinone and benzil, and their corresponding N -phenyl monoimines, have been reduced, using dry acetonitrile as the solvent, in the presence of sodium cyanide as a reducing agent. Comparative potentiostatic preparative-scale electrolysis is described.

ChemInform Abstract: Electrochemically Induced Ring‐Opening/Friedel—Crafts Arylation of Chalcone Epoxides Catalyzed by a Triarylimidazole Redox Mediator.

The indirect anodic oxidation of chalcone epoxides in the presence of electron-rich heteroarenes mediated by a triarylimidazole (Med) was investigated by cyclic voltammetry (CV) and controlled potential electrolysis. The CV results indicate that a homogeneous electron transfer between Med•+ and chalcone epoxides is facilitated by an electron-rich heteroarene that serves as an arylation reagent. The preparative scale electrolysis generated epoxide-ring-opened/Friedel–Crafts arylation products in moderate to good yields. The fact that only a catalytic amount of charge was required suggests that Med•+ initiates a chain reaction. In addition, overoxidation of the products is avoided even though their oxidation potential is less than that of the starting chalcone epoxides.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

Electrochemically induced ring-opening/Friedel–Crafts arylation of chalcone epoxides catalyzed by a triarylimidazole redox mediator.

The indirect anodic oxidation of chalcone epoxides in the presence of electron-rich heteroarenes mediated by a triarylimidazole (Med) was investigated by cyclic voltammetry (CV) and controlled potential electrolysis. The CV results indicate that a homogeneous electron transfer between Med•+ and chalcone epoxides is facilitated by an electron-rich heteroarene that serves as an arylation reagent. The preparative scale electrolysis generated epoxide-ring-opened/Friedel–Crafts arylation products in moderate to good yields. The fact that only a catalytic amount of charge was required suggests that Med•+ initiates a chain reaction. In addition, overoxidation of the products is avoided even though their oxidation potential is less than that of the starting chalcone epoxides.

Electrochemical synthesis of 3,5-disubstituted isoxazoles

The electrochemical oxidation of chalcone oximes has been explored using preparative scale electrolysis and cyclic voltammetry (CV). The results show that a constant current electrolysis of chalcone oximes in an undivided cell affords the corresponding isoxazoles in moderate to good yields, using graphite as the working electrode and NaClO4/CH3OH as the supporting electrolyte. The cyclic voltammograms for nearly all of the chalcone oximes investigated exhibit only one oxidation peak. On the basis of preparative electrolysis results and CV analysis, a reaction mechanism, involving a combination of electrochemically-generated base and an iminoxy radical intermediate, is proposed.

NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

The synthesis and characterization of the first catalytic manganese N-heterocyclic carbene complexes are reported: MnBr(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and MnBr(N-methyl-N'-2-pyridylimidazol-2-ylidine)(CO)3. Both new species mediate the reduction of CO2 to CO following two-electron reduction of the Mn(I) center, as observed with preparative scale electrolysis and verified with (13)CO2. The two-electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2'-bipyridine)(CO)3. Catalytic current enhancement is observed at voltages similar to MnBr(2,2'-bipyridine)(CO)3.

NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

AbstractThe synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)3 and MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)3. Both new species mediate the reduction of CO2 to CO following two‐electron reduction of the MnI center, as observed with preparative scale electrolysis and verified with 13CO2. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)3. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)3.

Electrochemistry of Acids on Platinum. Application to the Reduction of Carbon Dioxide in the Presence of Pyridinium Ion in Water

A detailed cyclic voltammetric investigation of the reduction of moderately weak acids on platinum reveals that they are reduced in two steps: one involving the hydrated protons initially present at equilibrium and the second the reduction of the acid through its prior conversion into hydrated protons. The reduction of pyridinium ions (protonated pyridine) follows this reaction scheme as does any other acid of similar pK (e.g., acetic acid). Rather than being catalytically reduced, CO2 plays a similar role through its prior conversion to carbonic acid. No trace of methanol or formate could be detected upon preparative-scale electrolysis of CO2 on the same electrode in the presence of pyridinium ions.

Electrochemically induced C[sbnd]H functionalization using bromide ion/2,2,6,6-tetramethylpiperidinyl-N-oxyl dual redox catalysts in a two-phase electrolytic system

The electrochemical oxidative functionalization of benzylic CH bonds, mediated by a dual bromide ion/2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) redox catalyst system in a two-phase electrolytic medium, has been explored using cyclic voltammetry (CV) and preparative electrolysis techniques. The results show that electron transfer between TEMPO+ and a neutral substrate occurs with an efficiency that depends upon the presence of a base. The preparative scale electrolysis led to the formation of dihydro-isoquinolinones, isochromanone and xanthenone in moderate to excellent yields. On the basis of the CV analysis and preparative electrolysis results, a reaction mechanism is proposed.

Correction to Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis

Correction to Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis

Turnover Numbers, Turnover Frequencies, and Overpotential in Molecular Catalysis of Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Electrolysis

The search for efficient catalysts to face modern energy challenges requires evaluation and comparison through reliable methods. Catalytic current efficiencies may be the combination of many factors besides the intrinsic chemical properties of the catalyst. Defining turnover number and turnover frequency (TOF) as reflecting these intrinsic chemical properties, it is shown that catalysts are not characterized by their TOF and their overpotential (η) as separate parameters but rather that the parameters are linked together by a definite relationship. The log TOF-η relationship can often be linearized, giving rise to a Tafel law, which allows the characterization of the catalyst by the value of the TOF at zero overpotential (TOF(0)). Foot-of-the-wave analysis of the cyclic voltammetric catalytic responses allows the determination of the TOF, log TOF-η relationship, and TOF(0), regardless of the side-phenomena that interfere at high current densities, preventing the expected catalytic current plateau from being reached. Strategies for optimized preparative-scale electrolyses may then be devised on these bases. The validity of this methodology is established on theoretical grounds and checked experimentally with examples taken from the catalytic reduction of CO(2) by iron(0) porphyrins.

Stereoselective Electrochemical Reduction of 8-Methylene-Oleandomycin

The electrochemical reduction of 8-methylene-oleandomycin was investigated by cyclic voltammetry and preparative scale electrolysis. The electrochemical reduction in the exocyclic double bond predominantly gives one epimer with the estimated diastereomeric ratio of 5:1. (8S)-methyl-oleandomycin was identified as the main product of the reaction. Stereoselectivity of the electroreduction was explained by the bottom-side adsorption of the 8-methylene-oleandomycin through its conjugated double bond to the electrode surface.

CO2 as a C1-organic building block: Electrocarboxylation of aromatic ketones. A quantitative study of the effect of the concentration of substrate and of carbon dioxide on the selectivity of the process

The purpose of this work is to establish and discuss quantitative relationships between the selectivity in preparative scale electrolysis and intrinsic and operational parameters for the electrocarboxylation of aromatic ketones. For the investigated ketones, under appopriate experimental conditions the selectivity of the process is mainly determined by the competition between carboxylation and protonation, de-halogenation reactions (when an halogenated ketone is involved), and possibly dimerizations involving the electrogenerated radical anion. A simple model was proposed to account for these unwanted side paths which allows to predict the dependence of the selectivity as a function of the [CO2]/[ketone] ratio and of the proton donor concentrations. Theoretical predictions fit excellently with experimental results. In particular, a dramatic increase of the selectivity of the target hydroxy acid can be achieved by performing the electrolysis at moderately high carbon dioxide pressure.

Electrochemical reduction of 4′-demicarosyl-10-hydro-11-dehydro-11-hydroxyimino-9-carbonyl-9-nor-8a,9 seco-8a-aza-8a-homorelomycin, the novel seco compound from the class of tylosin

The electrochemical reduction of 4′-demicarosyl-10-hydro-11-dehydro-11-hydroxyimino-9-carbonyl-9-nor-8a,9 seco-8a-aza-8a-homorelomycin was investigated by cyclic voltammetry and preparative scale electrolysis. It has been found that oxime group is reduced by ECEC mechanism in a four-electron process. The resulting products of the reduction consisted of the corresponding α,β-unsaturated amine and α,β-saturated ketone. The rate determining step was identified to be the cleavage of N–O bond. In the acidic media the actual leaving group is water and in neutral and moderate basic media hydroxyde ion.

Electrochemical oxidation of azithromycin and its derivatives

The electrochemical oxidation of azithromycin was investigated in order to elucidate the mechanism for possible oxidative metabolic pathways in humans. Electrochemical studies were carried out by cyclic voltammetry and preparative scale electrolysis at glassy carbon electrodes. It was found that azithromycin undergoes anodic oxidation at one or both amine groups with the rapid follow-up chemistry of intermediate radical cation. Main products of the oxidation were determined by HPLC analysis and were identified as a protonated azithromycin and products obtained by demethylation of the 3′-dimethylamino or macrolactone amino group.