Homogeneous redox catalysis of electrochemical reactions: Part I. Introduction

In redox homogeneous catalysis the catalyst couple merely plays the role of an electron carrier by contrast with chemical catalysis which involves the transitory formation of catalyst-substrate adduct. A detailed kinetic analysis of redox catalysis in the case of an EC-type electrode reaction is given involving the following reaction sequence

$\begin{array}{l}B\stackrel{k}{\to }C\\ \begin{array}{c}A+1e?B\\ B\to C\end{array}\}directelectrodereductionofthesubstrate\end{array}$ The Hush-Marcus relationship between heterogeneous and homogeneous electron transfer rate constants is used to select the range of the parameter variations and to predict the magnitude of the catalytic efficiency as a function of the potential separation between the catalyst reduction and the direct substrate reduction, the ratio of the catalyst over the substrate concentration and the electrochemical standard rate constant of the substrate. Two situations are of particular interest for the kinetic analysis of the experimental data: (a) kinetic control of the direct substrate reduction by the follow-up chemical reaction with diffusion by charge transfer with activation or diffusion control of the solution electron exchange, the rate determining step of the catalytic sequence being A+Q→P+B. In these conditions, kinetic information on the substrate reduction process is obtainable which could not have been derived from a direct electrochemical analysis.     

Homogeneous redox catalysis of multielectron electrochemical reactions: Part II. Competition between homogeneous electron transfer and addition on the catalyst

Homogeneous redox catalytic processes in which catalysis competes with partial destruction of the catalyst are investigated. The kinetics are shown to depend upon three parameters: the excess factor (concentration of substrate over concentration of the catalyst), a dimensionless kinetic parameter representing the rate of the initial homogeneous electron transfer step and a dimensionless parameter representing the competition between the second electron transfer and the addition on the catalyst. Procedures are described that allow the rate constant of the initial electron transfer step and the ratio of the rate constants of the second electron transfer vs. the addition step to be extracted from the experimental data. The reduction of n-butyl chloride and bromide, mediated by aromatic anion radicals, is taken as an example illustrating the application of the proposed procedures to experimental systems.     

Investigations on the redox character of dithizone by voltammetric methods : Part I.The reduction of dithizone in aqueous solutions

The mechanism of the electrochemical reduction of dithizone to the corresponding hydrazo compound, diphenylthiocarbazide, has been examined in detail by polarographic and voltammetric techniques over a wide pH range. The reaction is reversible and dithizone can be determined polarographically in the range 10^-3–10^-5M. This polarographic behaviour suggests new applications of dithizone as an electroanalytical reagent.     

An electrochemical study of redox indicators : Part I. Substituted chrysoidines

The electrochemical oxidation of various substituted chrysoidines was studied by cyclic voltammetry, to determine which have stable oxidation products. Only 4-hydroxy-chrysoidine has a stable product; the apparent oxidation potential is 0.779 V vs. NHE. 4-Methoxy- and 4-ethoxy-chrysoidine rapidly lose methanol or ethanol, respectively, so that the 4-hydroxychrysoidine wave appears on subsequent scans. All the other chrysoidines studied are irreversible. The results indicate that a hydroxy group in the 4-position is necessary for stability; the 4-alkoxychrysoidines can achieve the stable quinoidal structure by cleavage of the alkoxy group after nucleophilic attack.     

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis

Homogeneous electrochemical catalysis of N₂O reduction to N₂ is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes. An activation-driving force correlation is revealed with the organic species characteristic of a redox catalysis involving an outer-sphere electron transfer from the radical anions or dianions of the reduced catalyst to N₂O. Taking into account the previously estimated reorganization energy required to form the N₂O radical anions leads to an estimation of the N₂O/N₂O˙⁻ standard potential in acetonitrile electrolyte. The direct reduction of N₂O at a glassy carbon electrode follows the same quadratic activation driving force relationship. Our analysis reveals that the catalytic effect of the mediators is due to a smaller reorganization energy of the homogeneous electron transfer than that of the heterogeneous one. The physical effect of “spreading” electrons in the electrolyte is shown to be unfavorable for the homogeneous reduction. Importantly, we show that the reduction of N₂O by low valent rhenium and manganese bipyridyl carbonyl complexes is of a chemical nature, with an initial one-electron reduction process associated with a chemical reaction more efficient than the simple outer-sphere electron transfer process. This points to an inner-sphere mechanism possibly involving partial charge transfer from the low valent metal to the binding N₂O and emphasizes the differences between chemical and redox catalytic processes.

Homogeneous electrochemical catalysis of N₂O reduction to N₂ is investigated with a series of organic catalysts and rhenium and manganese bipyridyl carbonyl complexes.     

Efficient homogeneous catalysis in the reduction of CO2 to CO

The well-defined copper(I) boryl complex [(IPr)Cu(Bpin)] [where IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, and pin = pinacolate: 2,3-dimethyl-2,3-butanediolate] deoxygenates CO2 rapidly and quantitatively, affording CO and the borate complex [(IPr)Cu(OBpin)]. The boryl may be regenerated by treatment with the diboron compound pinB-Bpin, giving the stable byproduct pinB-O-Bpin. The use of a copper(I) alkoxide precatalyst and stoichiometric diboron reagent results in catalytic reduction of CO2, with high turnover numbers (1000 per Cu) and frequencies (100 per Cu in 1 h) depending on supporting ligand and reaction conditions.     

Mapping concentration profiles within the diffusion layer of an electrode: application to redox catalysis

Catalytic reductions of some aromatic halides were performed at a millimetric electrode with several redox mediators. The resulting concentration profiles were monitored amperometrically by placing an ultramicroelectrode inside the diffusion layer produced at the former electrode. The features of redox catalysis and the subsequent structuring of the diffusion layer were investigated experimentally under steady-state conditions imposed by the spontaneous convection of the solution. The concentration profiles established from the probe measurements were in agreement with our theoretical predictions, based on fast kinetics of redox catalysis. Under these conditions, very similar to preparative electrosynthesis, the diffusion layer separates into two domains where pure diffusion takes place and the concentration profiles therein are mainly linear. We demonstrate that the limit between these two zones does not depend on kinetics, but is rather fixed by the product of the ratio of the bulk concentrations of each species and the ratio of their diffusion coefficients.     

The synthesis of amines by the homogeneous hydrogenation of secondary and primary amides

Amides can be hydrogenated to amines using a catalyst prepared in situ from [Ru(acac)(3)] and 1,1,1-tris(diphenylphosphinomethyl)ethane; water is required to stabilize the catalyst and primary amines can only be formed (selectivity up to 85%) if ammonia is also present.     

Enantioselective catalysis. Part 133: Conformational analysis of amides of 9-amino(9-deoxy)epicinchonine

Amides of 9-amino(9-deoxy)epicinchonine have been used as chiral base catalysts to induce asymmetry in enantioselective decarboxylation reactions. To understand the reaction mechanism and to optimize the bases by variation of the amide substituents, a detailed knowledge of their preferred conformation is necessary. The conformations were investigated by ¹H NMR spectroscopy, X-ray analysis and semi-empirical molecular orbital calculations. Principally, cinchona alkaloids may adopt four different conformations (Fig. 1

1. Download : Download high-res image (168KB)
2. Download : Download full-size image

Figure 1. Schematic drawings illustrating the two closed and open conformations of 9-amino(9-deoxy)epicinchonine derivatives

), of which open b and open b′ are preferred by 2-ethoxy-N-(9-deoxyepicinchonine-9-yl)benzamide 2 in solution. AM1 Calculations confirm these results, and the small energy barrier between the two open conformations explains their simultaneous existence in solution. Crystal structure analyses of various amides show open conformations in the solid state. Local minimum energy conformations were found for open conformations of the compound 2 protonated at the quinuclidine system, which is the active species in the enantioselective decarboxylation reaction.     

A rapid radiochemical method for antimony and arsenic : Part I. Formation of stibine and arsine by flash electrolysis

The formation of stibine and arsine affords a clean, one-step, method of separating antimony and arsenic from fission products. The development of a high-current electrolytic cell for the rapid formation of these hydrides from a hydrochloric acid electrolyte, is described. A chemical yield of 45% was achieved for both antimony and arsenic in 10 sec. The effects of pH and antimony and arsenic oxidation states were also studied. In 5 N hydrochloric acid, the chemical yield is independent of the antimony oxidation state. For arsenic however, the yield of arsine is 7 times greater for As(III) than for As(V).     

A multilayer model for the study of space distributed redox modified electrodes: Part I. Description and discussion of the model

A multilayer model for the study of space distributed redox modified electrodes (redox polymer electrodes, or adsorption of an electroactive substance in several layers) is described and discussed in detail. It is shown in particular that it is equivalent to a system in which the electrons diffuse in the coating.     

The study of redox reactions of bisarenechromium complexes by the rotating disk electrode technique: I. Oxidation of bisarenechromium(O) complexes and reduction of bisarenechromium(I) cations

The redox properties of twenty bisarenechromium complexes have been studied by the rotating disk electrode technique in an aprotic solvent (DMSO). The half-wave potentials, E_1/2, have been found to correlate well with the meta-substituent constants. The conclusion is drawn that the electronic effects of substituents are mainly transferred to the reaction centre (the metal atom) by the inductive mechanism. The conjugation of substituents with the coordinated ring is weaker in bisarenechromium complexes than in ferrocene derivatives.     

Telomerization kinetics by redox catalysis: A study of reactions leading to telomers with low degrees of polymerization

An adaptation of the David and Gosselain formula for redox catalyzed telomerization was made. An equation was developed for transfer reactions between oligomer radicals and metallic ions. This equation was applied to data obtained in chlorotrifluoroethylene (CTFE)—carbon tetrachloride telomerizations catalyzed by a ferric chloride—benzoin mixture. It was found that the metal ion transfer constants (C_Meⁱ) were the predominant parameters influencing the distribution of products obtained. These were found constant for degrees of polymerization greater than 2.     

Decarbonylative C-h coupling of azoles and aryl esters: unprecedented nickel catalysis and application to the synthesis of muscoride a

A nickel-catalyzed decarbonylative C-H biaryl coupling of azoles and aryl esters is described. The newly developed catalytic system does not require the use of expensive metal catalysts or silver- or copper-based stoichiometric oxidants. We have successfully applied this new C-H arylation reaction to a convergent formal synthesis of muscoride A.     

Halogen-free process for the conversion of carbon dioxide to urethanes by homogeneous catalysis

Carbon dioxide is converted to urethanes (carbamic acid derivatives) through reaction with amine and alcohol catalyzed by tin complexes; th addition of acetals as a dehydrating agent under high CO2 pressure is the key to achieve high yields.