Reductive electrosynthesis

Electrocatalysts for Carbon Dioxide Reduction Electrosynthesis in Supercritical Fluids Reactive Metal Electrode... 

Gonzalez-Garcia J, Banks CE, Sljukic B et al (2007) Electrosynthesis of hydrogen peroxide via the reduction of oxygen assisted by power ultrasound. Ultrason Sonochem 14 405 112... 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Vitamin B12 derivatives are also effective catalysts for the electroreductive cyclization of bromoalkenes in conductive microemulsions,299 300 or for ring-expansion reactions in cyclic a-(bromomethyl)-(3-keto esters in DMF.301 Vitamin Bi2 attached to an epoxy-polymer has been used in electrosynthesis of valeronitrile by reductive coupling of iodoethane and acrylonitrile.302... 

Except for deposition of Prussian blue from the mixture of ferric and ferricya-nide ions, its electrosynthesis from the single ferricyanide solution is reported [13]. Ferricyanide ions are not extremely stable even in aqueous solution, which is noticed in the change of color after a few days of storage. Thus, the coordination sphere can be destroyed also in the course of electrochemical reactions. The mentioned processes may lead to formation of ferric-ferricyanide complex or free ferric ions. The reduction of the resulting mixture leads to the formation of Prussian blue. 

Nano-electrode arrays can be formed through nano-structuring of the electrocatalyst on an inert electrode support. Indeed, if the current of the analyte reduction (oxidation) on a blank electrode is negligible compared to the activity of the electrocatalyst, the former can be considered as an insulator surface. Hence, for the synthesis of nanoelectrode arrays one has to carry out material nano-structuring. Recently, an elegant approach [140] for the electrosynthesis of mesoporous nano-structured surfaces by depositioning different metals (Pt, Pd, Co, Sn) through lyotropic liquid crystalline phases has been proposed [141-143],... 

Indirect electrosynthesis of reactive formyl transition metal compounds involves an initial step of reduction of metal carbonyls to radicals followed by transfer of a hydrogen atom from trialkyltin hydrides190. Electroreduction of metal carbonyls yields products of dimerization and loss of CO from the radical anion. Electroreduction in the presence of R3SnH yields the formylmetalcarbonyls ... 

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. 

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

The electrogeneration of [(TPP)Co] from (TPP)Co, and the reaction of this species with CHjI can be followed by cyclic voltammetry as shown in Figures lc and Id. In the absence of any added reagent, there are two reversible reduction waves which occur at Ei/2 = 0.85 jind -1.86 V (see Figure lc). These are due to the formation of [(TPP)CoJ and [(TPP)Co]2-, where the second reduction has occurred at the porphyrin ir ring system. The first reduction of (TPP)Co is not reversible in the presence of CH3I, and occurs at Ep = -0.86 V (see Figure Id). A new reversible reduction also appears at Ej/2 = -1.39 V. This process is due to (TPP)Co(CHj) which is formed as shown by Equation 8. The formation of (TPP)Co(CHj) as the final product of the electrosynthesis was confirmed by spectroelectrochemical experiments which were carried out under the same experimental conditions(26). 

The fact that most pulp and paper applications use dilute solutions of hydrogen peroxide in alkali has kindled interest in the electrosynthesis of alkaline peroxide solutions by cathodic reduction of oxygen. 

Na2S204 is usually produced by reduction of NaHS03with reducing agents such as sodium borohydride, zinc or sodium formate. The electroreduction of sulfur-ous solutions as a source of dithionite has been extensively studied by Oloman [129,215,216]. Olin Corp. has recently brought electrosynthesis of dithionite to commercial scale with plants of about 12 ton/day operating in Charleston, USA... 

A synthetically very potent and unique feature of organic electrosynthesis is the oxidative or reductive Umpolung of reactivity. Reactive acceptors are anodically available as radical cations in a wide variety by the oxidative Umpolung of donors. This way two donors can be coupled in one step if one of them is converted to an acceptor at the electrode. Chemically, at least two additional... 

Some of these processes have been developed for technical conversions and have been summarized in Ref. [228, 229]. The anodic technical production of t-butylbenzaldehyde has been coupled with the cathodic reduction of phthahc anhydride to phthalide in a paired electrosynthesis in a capillary gap cell [230]. Indirect oxidations with Mn +/Mn + or as mediators... 

Nitrogen heteroaromatics are expected to be useful probases. The cathodic reduction of phenazine, (31), resembles closely that of (29a) [70,71], and the kinetic basicity of (31) is comparable to that of (29a) [54]. However, application of (31) as a PB in electrosynthesis has not been reported, and there is only a single report concerning the use of the radical anion of acridine, (32), as an EGB [72]. 

The electrosynthesis of bicyclic ketones (198) has been performed by the reduction of bromoalkylcyclohexenones using Ni(II) complexes as mediators. The electrochemical Michael-type addition (197) (198) can be attained in a DMF-NH4Cl04/Et4NCl04-(C/C) system in the presence of Ni(cyclam)(Cl04)2 as a redox mediator at —1.8 V (SCE) (Scheme 77) [308]. 

The Barhier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Sml2, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barhier-type allylation of carbonyl compounds in an SmH-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amount of SmCfi [570]. 

The addition of carbanions, generated electrochemically by reduction of the carbon-halogen bond, to carbon dioxide has been examined under a variety of experimental conditions. Direct electrosynthesis of carboxylic acids in a divided cell using an aprotic solvent and a tetraalkylammonium salt as electrolyte is most sue-... 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

Electroorganic synthesis deals with conversion of organic compounds into useful products by anodic oxidation or cathodic reduction. Today there exist literally thousands of published examples of electrosynthesis reactions but only a very small number—certainly not more than several tens—are really exploited commercially, the best known example being the cathodic hydrodimerization of acrylonitrile to adipodinitrile, a precursor to hexam-ethylene diamine, which is the aminoconstituent of nylon 6,6 (779) ... 

The model had been substantiated by measuring the potential dependent electrosorption isotherms of all species involved that show that the proto-nated alcaloids are those species that are by far most strongly adsorbed, whereas the acetyl pyridines are least strongly adsorbed, especially at lower pH relative to the electrosynthesis, which is performed at pH 4 to 4.5. The optically inductive reduction of 2- and 4-acetyl pyridine to the optically active carbinols demands the formation of a dense but not too densely packed surface layer of adsorbed protonated alcaloid, which still allows for insertion of the oxo-compound or the ketyl radical, respectively. Performing the reaction with too high alcaloid concentrations leads to compaction on the adsorbate layer, the ketyl radical is squeezed out, and optimal induction is no longer observed. 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Apart from a change in the course of the reduction, which will be discussed below in the section on stereospecific electrosynthesis, the epimers of a,a -dibromosuccinic acid show differences in half-wave potentials (134,135). The free acid of the erythro dibromosuccinic acid is reduced at more positive potentials than the threo-ioxm. For the anions of these acids, however, the reduction at higher pH values occurs at more positive potentials for the threo-epimer than for the erythro-anion. For the esters of dibromosuccinic acids, the difference in half-wave potentials of the two epimers is too small to be significant. 

Another group of applications consists of electrosynthesis of substances that can be otherwise synthesized only with difficulty. The chemical reduction of p-aminoketones is complicated by the elimination of these compounds. A polarographic two-electron wave in acidic solution, corresponding to the reduction of carbonyl grouping to p-aminoalcohol... 

Using ultramicroelectrodes, it is possible to study reactions under the conditions of synthesis, including electrosynthesis. An example is the electrohydrodimerisation of acrylonitrile to adiponitrile (Scheme 6.11, top) mentioned in the introduction in industry this is typically carried out with an emulsion of acrylonitrile in an aqueous phosphate buffer as electrolyte. At substrate concentrations in the mM level, the reduction of acrylonitrile takes another route leading to saturation of the C—C double bond (Scheme 6.11, bottom). This precludes studies of the dimerisation using substrate concentrations at the mM level and thereby working electrodes of conventional sizes. The transition between the two mechanisms could be studied conveniently using an ultramicro electrode as the working electrode... 

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