Selective electrosynthesis

This work aims to investigate the electrocatalytic oxidation of sucrose on smooth and upd-lead modified platinum electrodes in order to find experimental conditions for selective electrosynthesis of high value added products. The preliminary results obtained show that the oxidation of sucrose on Pt-Pb electrodes leads mainly to C12 products, such as the I -monoacid and the 6-monoacid of sucrose. 

The direct electrooxidation of aqueous E>-g]uconic acid to l>arabinose on graphite has been performed in a very simple apparatus which may be suitable for practical application. The electrocatalytic oxidation of sucrose on smooth, lead-modified platinum electrodes has been examined with a view to finding experimental conditions for the selective electrosynthesis of value-added compounds. A paper in Bulgarian on the electrooxidation of diacetone-L-sorbose at low current densities in a nickel oxide electrolizer has been publi ed. The influence of the rize of palladium particles and their location on the support on their activity in the oxidation of glucose has been examined. An investigation of the effect of tonperature and pH on the platinum-catalysed oxidation of sucrose showed that changes in temperature affect mainly the reaction rate, where changes in pH alter the selectivity. ... 

Kirste A, Hayashi S, Schnakenburg G et al (2011) Highly selective electrosynthesis of biphenols on graphite electrodes in fluorinated media. Chem Eur J 17(50) 14164-14169... 

Another pioneering electrosynthesis was the soft fluorination developed by I.N. Rozhkov, I.L. Knunyants, etc. [64, 65]. Unlike the known hard fluorination in liquid HF [66], which leads to perfluorinated compounds, often accompanied by side reactions, the soft fluorination is a selective electrosynthesis. The process is performed on a Pt anode in MeCN at the oxidation potential of the substrate RH. The -eajv-e-p mechanism is suggested for this reaction [67-69] (Figs. 9.4 and 9.5) ... 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

Electrosynthesis of polymers compares favorably with the thick film method providing addressable and controlled deposition. In terms of selectivity to hydrogen peroxide in the presence of interferents, the most promising results were obtained with poly-l,2-diaminobenzene (poly-1,2-DAB)-modified electrodes [125],... 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Mechanistic studies of homogenous chemical reactions involving formation of (P)Rh(R) from (P)Rh and RX demonstrate a radical pathway(9). These studies were carried out under different experimental conditions from those in the electrosynthesis. Thus, the difference between the proposed mechanism using chemical and electrochemical synthetic methods may be due to differences related to the particular investigated alkyl halides in the two different studies or alternatively to the different reaction conditions between the two sets of experiments. However, it should be noted that the electrochemical method for generating the reactive species is under conditions which allow for a greater selectivity and control of the reaction products. 

Selected examples of indirect electrosynthesis which have found technical or pilot plant scale applications are discussed in the following ... 

There is a large variety of polar and radical reactions, transition metal-catalyzed and pericyclic conversions, that have been carefully developed with regard to scope, selectivity, and yield. They are compiled in large compendia, for example, in [16-19], and in series, for example [20, 21], and are continuously improved and extended in timely research papers. This literature should be consulted in parallel with suggestions taken from electrosynthesis. Electrosynthesis is a clear alternative to chemical synthesis, when reactive intermediates (see Sect. 3.3) such as radical ions, radicals, carbanions, or carboca-tions are involved. The more advantageous are summarized in the following sections. 

In this section we will focus on electroreductive and electrooxidative synthesis and touch briefly on the electrosynthesis of selected organometallics and electroinitiated chain polymerisation, previously introduced in Chapter 5. 

One of the present authors has investigated the importance of the nature of the electrode/electrolyte interface for the yields and selectivities of some anodic electrosynthesis reactions. A series of four successive reviews reports on the gathered information and improved understanding of the chemical kinetics of reactive intermediates generated at the interface carbon elec-trode/nonaqueous solvent (208-212) and citations of detailed investigations therein. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

Successful chemical and electrochemical synthesis of Pc from 1,3-D in aprotic solvents in comparison with those using PN shows that the highest influence of a solvent s nature on a reaction course takes place in the first stage of the process (1,3-D formation). For further reactions (cyclization and reduction of 1,3-D), a solvent s nature is not very important, as the results presented in Table 5.4 show. The formation of Pc from 1,3-D takes place in all the solvents used higher yields can be achieved by optimization of the process (variation of concentration of 1,3-D, use of electrosynthesis, and/or selection of the best solvent applied) [32]. 

When instead of platinum sheets as electrodes BDD material was employed the 13 14 ratio improved tremendously. Surprisingly, this direct electrochemical conversion of 12 gave the desired biphenol 13 in almost exclusive selectivity of 18 1 for 13 14. The typical pentacyclic by-product 15 could not be detected in the crude product. Inspired by the conditions for the electrosynthesis of trimethyl orthoformate (see Sect. 4.3.1 in Chap. 44) we applied to our conversion alcohols as common solvents for organic transformations at BDD electrodes. 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

The three kinds of reactors already described in this section are all traditional cross-flow reactors with permeable plates or membranes. The electrochemical filter-press cell reactors used, e.g., for electrosynthesis, are equipped with cation-selective membranes to prevent mixing of the anolyte and the catholyte. These cell reactors are therefore good examples of the extended type of cross-flow reactors according to the definition transferred from the filtration field. The application of the electrochemical filter-press cell reactor technique... 

Some 4-substituted aromatic aldehydes, such as anisic aldehyde, toluic aldehyde, or t-butylbenzaldehyde, are produced commercially. The electrosynthesis can be accomplished by several different methods and is a good example of the versatility of selective electrochemical oxidations [91]. 

Porous materials continue to attract considerable attention because of their wide variety ot scientific and technological applications, such as catalysis, shape- and size-selective absorjition and adsorption, gas storage, and electrode materials. Roth research and applications of porous materials—via electroanalysis, electrosynthesis, sensing, fuel cells, capacitors, electro-optical devices, and other means—heavily rely on electrochemistry. 

Ushito, M. and Seo, M., Selective ion permeability of manganese oxides prepared with an electrosynthesis. Electrochemistry, (Cl, 377, 1999. 

In the second part, we want to report some significant procedures of electrosynthesis, carried out in RTILs as well as in VOC-supporting electrolyte systems, including a comparison between the results obtained according to the two different procedures. The synthesis of P-lactams, the utilization of CO as renewable carbon source and the carbon-carbon bond formation via umpolung of aldehydes (benzoin condensation, Stetter reaction) and via Hemy reaction have been selected as typical procedures. 

Finally, the electrosynthesis of P-nitroalcohols has been performed, under mild conditions and in high yields and selectivity, by stirring nitromethane and aldehydes in previously electrolyzed RTILs in total absence of VOCs and supporting electrolyte. The effects of the number of Faradays per mole of aldehyde supplied to the electrode, the reaction time, temperature and the stracture of the RTILs on the yield and selectivity have been extensively investigated. After the workup of the catho-lyte, RTILs were recovered and reused. In every case, P-nitroalcohols were isolated in good yields (81-92%) (Scheme 16.29) [171]. 

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