Reduction, electrochemical

Compared to oxidation, EC reduction has not been not widely used with HPLC. Reducible groups include hydroxylamines, nitrosamines, -oxides, peroxides, quinones, aromatic nitro compounds and disulfides. The antibiotic chloramphenicol can be assayed in blood by EC reduction using a mercury film electrode.  

A pendent mercury drop electrode has been used to reduce the 4,5-azomethine group in 1,4-benzodiazepines There were additional contributions from partial reduction of nitro moieties, as in nitrazepam, and A -oxides, as in chlordiazepoxide. The 5-lipoxygenase inhibitor methyl 2-[(3,4-dihydro-3,4-dioxo-l-naphthalenyl) aminojbenzoate (CGS 8515) has been measured in ethyl acetate extracts of plasma from animals treated with the drug using an ODS-modified silica column with methanol-aq. acetate buffer (0.15 mol L, pH 4.5) (60 -f 40) as eluent and EC 

EC reduction has been used extensively in the detection of residues of nitro-aromatic, nitramine, and nitrate explosives at pendent Hg, Au amalgam or glassy carbon electrodes - the forensic evaluation of this subject has been reviewed. The methodology described may be applicable to drugs and other compounds containing these functional groups. 

Electrochemical systems for the reduction of C02 require the application of an external bias or current to supply the electrons to reduce C02. Instead of a sacri- 

Although a wide variety of metal electrodes have been examined for the reduction of C02, only some of the more major advances in C02 reduction at metal electrodes are highlighted here. More extensive surveys of the electrochemical reduction of C02 are provided elsewhere [40-44], 

C02 reduction at metal electrodes in both aqueous and nonaqueous media, as well as in systems coupled with electron-mediating complexes are detailed. The faradaic efficiency of such a system can be used as a measure of efficiency and selectivity. For a specific, electrochemically generated product, the faradaic efficiency is the ratio of the actual and theoretical amounts of product formed within the same time interval, based on charge passed. An efficient and selective system will lead to a 100% faradaic yield for a single product in other words, all of the charge passed in the system has gone into the production of that product. 

Compared to physical vapor deposition, electrochemical deposition can be used to produce uniform and homogeneous distributions of Cu-NPs of similar size. Because electrochemical processes involve very few chemicals, they are known as 

The modification and functionabzation of metal electrodes with self-assembled thiol adlayers [66, 67] opens up many possibilities for electrochemical applications. At present, this very active field is centered on the studies of the electrochemical properties of self-assembled monolayers (SAMs) [68, 69]. In short, electrochemical deposition represents a promising technique for the preparation of Cu-NPs due to its ease of use, low cost, environmental friendliness and convenience of manufacture. In particular, the high sensitivity and low cost of the SPEs when using Cu-NPs should lead to their widespread use in a variety of scientific and medical fields, including medical diagnostics. 

ELECTRODEPOSITION AND ELECTROLYTIC REDUCTION (ELECTROWINNING) 6.8.1 Electrochemical reduction 

It consists in a deposition of ions from an electrolyte onto the cathode in an electrolytic cell, under the influence of an applied potential. Usually the process is accompanied by material dissolution from the anode. The electrowinning from aqueous solutions is an important commercial method for the production (and/or refinement) of many metals, including, for instance, chromium, nickel, copper, zinc. As for the electrodeposition from non-aqueous solutions, the primary production of aluminium, electrodeposited from a solution of A1203 in molten cryolite, is a typical example. Other metals which may be regularly reduced in a similar way are Li, Na, K, Mg, Ca, Nb, Ta, etc. 

Electrodeposition of alloys Electrolysis has also been used in order to obtain several metal compounds and alloys via the simultaneous co-deposition, from aqueous solutions or fused salts of the metal components. 

Electrolytic reduction of compounds In electrolytic cells the reduction of an element to low oxidation states may be performed, and compounds such as sulphides, phosphides, antimonides, etc. may be prepared. 

With certain carbonyl compounds, however, such as 3-keto steroids, the isotopic composition is poor due to the rapid exchange of the activated a-hydrogens in the substrate prior to reduction. The corresponding alcohols, in their thermodynamically more stable configuration, are usually found as 

This reduction is not as suitable for sterically hindered ketones, since in these cases the alcohol is the major product. The reduction of 11- and 12- keto steroids, for example, is usually very slow. Furthermore, the 11-keto steroid (76) yields only about 10% of the 11,1 l-d2 labeled analog (77), the main product being the 1 IjS-dj-l la-hydroxyl derivative (78).  

It should be noted that this reductive method compliments other reactions for the preparation of certain thermodynamically more stable labeled epimeric alcohols (see section III-B). 

The cell is most conveniently constructed from two pieces of 1.0 diameter Teflon rod about 1.5 long. A flat surface is milled on one side of each piece and a 3/8 hole is drilled through to the cell cavity. Each cavity 

The electrodes are 1 cm square with a small tab on one corner for the electrical connection. They are cut from 1.5 mm lead sheet. A variable voltage direct current source (0-12 VDC) capable of supplying a 0.1 amp current is necessary to carry out the electrolysis. 

As discussed, the aqueous waste with hexavalent chromium requires reduction of chromium to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate. Demonstrated reducing agents are sodium metabisulfite (Na2S205), sulfur dioxide (S02), ferrous sulfide (FeS), and other ferrous ion (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). The treatment processes using these are described below. 

The carbon-carbon double bond can be reduced by diimide prepared in solution in a number of ways.34 183,184 Oxidation of hydrazine with oxygen (air) or H202 in the presence of a catalytic amount of Cu(II) ion was the first method to generate and use diimide in hydrogenation.183-185 Acid-catalyzed decomposition of alkali azido-dicarboxylates,185,186 as well as thermal or base-catalyzed decomposition of aromatic sulfonyl hydrazides,183,184 are also useful methods for preparing the diimide reducing agent. 

The relative reactivity of alkenes toward reduction by diimide depends on the degree of substitution. Increasing alkyl substitution results in decreasing reactivity, and strained alkenes exhibit higher reactivity than nonstrained compounds 187 

The steric course of the addition of the two hydrogen atoms is generally syn. Accordingly, the reduction of cis- and frans-stilbenes with dideuterodiimide affords meso- and racem-1,2-dideutero-l,2-diphenylethanes, respectively, with 97% selectivity 188 

It was found that most synthetic processes that are employed to prepare the diimide reagent generate trans-diimide, but ds-diimide undergoes faster hydrogen atom transfer to a double bond than does the trans isomer. It follows that a fast trans-cis isomerization precedes reduction. The transfer of hydrogen atoms takes place in a synchronous process188 via the transition state 19  

Many studies demonstrated that in the approach of diimide to the double bond, steric hindrance is the decisive factor in determining stereochemistry. Highly selective additions of hydrogen (deuterium) from the sterically less hindered side189,190 are illustrated in Eqs. (11.48) and (11.49)  

Kaneko, T. Tsuchiya, and H. Igeta, Chem. Pharm. Bull. 22, 2894 (1974). 

Lund studied the electrochemical reduction of substituted pyridazines and reported that reduction of 3,6-diphenylpyridazine gave the 1,4,5,6-tetrahydro derivative 75.130 This is probably produced through the intermediacy of 4,5-dihydropyridazine 28a since, according to Lund, neither the 1,2- nor the 1,4-dihydro tautomers are expected to be reduced further under the reaction conditions used [Eq. (20)]. 

On the other hand, 3-phenyl-6-dimethylaminopyridazine 76a is reduced smoothly to a 4,5-dihydropyridazine (77a). 3-Phenyl-6-methoxypyridazine (76b) and 3-methyl-6-chloropyridazine (76c) are also reduced to dihydro compounds but these are hydrolyzed to dihydropyridazinones (77b,c) at the low pH used in the reduction [Eq. (211].131 

Electroreduction of pyridazines in the presence of acetic anhydride gives the acylated open-chain diamines.132 

6-Diphenylpyridazine is reduced with sodium and ethanol, affording the 1,2-dihydro derivative.133 Since 3,6-diphenyl-4,5-dihydropyridazine is more stable than the isomeric 1,2-dihydro compound, the latter is isomerized in the presence of alkali to the 4,5-dihydro compound.134 The 1,2-diethoxycarbonyl-l,2-dihydropyridazine is formed by selenium dioxide oxidation135 or via allylic bromination-dehydrobromination136137 of the corresponding 1,2,3,6-tetrahydro compound. 1-Ethoxycarbonyl- or 1,2-diethoxycarbonyl-l,2-dihydropyridazines were obtained similarly from alkali treatment of 1,2-diethoxycarbonylhexahydropyridazines.138 

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The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

The yield of hydroquinone is 85 to 90% based on aniline. The process is mainly a batch process where significant amounts of soHds must be handled (manganese dioxide as well as metal iron finely divided). However, the principal drawback of this process resides in the massive coproduction of mineral products such as manganese sulfate, ammonium sulfate, or iron oxides which are environmentally not friendly. Even though purified manganese sulfate is used in the agricultural field, few solutions have been developed to dispose of this unsuitable coproduct. Such methods include MnSO reoxidation to MnO (1), or MnSO electrochemical reduction to metal manganese (2). None of these methods has found appHcations on an industrial scale. In addition, since 1980, few innovative studies have been pubUshed on this process (3). 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

New Synthesis. Many attempts have been made to synthesize oxaUc acid by electrochemical reduction of carbon dioxide in either aqueous or nonaqueous electrolytes (53—57). For instance, oxaUc acid is prepared from CO2 as its Zn salt in an undivided ceU with Zn anodes and stainless steel cathodes ia acetonitrile containing (C4H2)4NC104 and current efficiency of >90% (53). Micropilot experiments and a process design were also made. 

For negative films, the electrochemical reduction properties of the reducing agents must be properly positioned to provide rapid amplification of exposed grains and much slower spontaneous amplifica tion of unexposed grains. The abiUty to discriminate between exposed and unexposed grains is a... 

Fig. 13. A speck of developing sdvei on the surface of a sdvei hahde crystal. The silver acts as an electrode for the electrochemical reduction of silver ions.

Electrolytic Reductions. Both nitro compounds and nitriles can be reduced electrochemically. One advantage of electrochemical reduction is the cleanness of the operation. Since there are a minimum of by-products, both waste disposal and purification of the product are greatiy simplified. However, unless very cheap electricity is available, these processes are generally too expensive to compete with the traditional chemical methods. 

In general, however, the electrochemical reduction of nitriles offers no significant advantages over traditional chemical methods and has not been widely used. 

In the reduction of nitro compounds to amines, several of the iatermediate species are stable and under the right conditions, it is possible to stop the reduction at these iatermediate stages and isolate the products (see Figure 1, where R = CgH ). Nitrosoben2ene [586-96-9] C H NO, can be obtained by electrochemical reduction of nitrobenzene [98-95-3]. Phenylhydroxylamine, C H NHOH, is obtained when nitrobenzene reacts with ziac dust and calcium chloride ia an alcohoHc solution. When a similar reaction is carried out with iron or ziac ia an acidic solution, aniline is the reduction product. Hydrazobenzene [122-66-7] formed when nitrobenzene reacts with ziac dust ia an alkaline solution. Azoxybenzene [495-48-7], C22H2QN2O, is... 

Other preparations of succinic acid mentioned in the Hterature are electrochemical reduction of maleic or fumaric acid (153,154), ultrasound-promoted Zn—acetic acid reduction of maleic or fumaric acid (155), reduction of maleic acid with H2PO2 at room temperature (156),... 

Tantalum. Numerous methods developed to extract tantalum metal from compounds included the reduction of the oxide with carbon or calcium the reduction of the pentachloride with magnesium, sodium, or hydrogen and the thermal dissociation of the pentachloride (30). The only processes that ever achieved commercial significance are the electrochemical reduction of tantalum pentoxide in molten K TaF /KF/KCl mixtures and the reduction of K TaF with sodium. 

One development involves the use of vitamin B 2 to cataly2e chemical, in addition to biochemical processes. Vitamin B 2 derivatives and B 2 model compounds (41,42) cataly2e the electrochemical reduction of alkyl haUdes and formation of C—C bonds (43,44), as well as the 2inc—acetic acid-promoted reduction of nitriles (45), alpha, beta-unsaturated nitriles (46), alpha, beta-unsaturated carbonyl derivatives and esters (47,48), and olefins (49). It is assumed that these reactions proceed through intermediates containing a Co—C bond which is then reductively cleaved. 

The commercial manufacture of sodium chlorite is based almost entirely on the reduction of chlorine dioxide gas in a sodium hydroxide solution containing hydrogen peroxide [7722-84-1] as the reducing agent. The chlorine dioxide is generated from the chemical or electrochemical reduction of sodium chlorate under acidic conditions. 

In a series of detailed studies, Armand and coworkers have examined the electrochemical reduction of pyrazines (72CR(C)(275)279). The first step results in the formation of 1,4-dihydropyrazines (85), but the reaction is not electrochemically reproducible. The 1,4-dihydropyrazine is pH sensitive and isomerizes at a pH dependent rate to the 1,2-dihydro compound (83). The 1,2-dihydropyrazine then appears to undergo further reduction to 1,2,3,4-tetrahydropyrazine (88) which is again not electrochemically reproducible. Compound (88) then appears to undergo isomerization to another tetrahydro derivative, presumably (8, prior to complete reduction to piperazine (89). These results have been confirmed (72JA7295). 

The fusion of a benzene ring to pyrazine results in a considerable increase in the resistance to reduction and it is usually difficult to reduce quinoxalines beyond the tetrahydroquinoxa-line state (91). Two possible dihydroquinoxalines, viz. the 1,2- (92) and the 1,4- (93), are known, and 1,4-dihydroquinoxaline appears to be appreciably more stable than 1,4-dihydropyrazine (63JOC2488). Electrochemical reduction appears to follow a course anzdogous to the reduction of pyrazine, giving the 1,4-dihydro derivative which isomerizes to the 1,2- or 3,4-dihydroquinoxaline before subsequent reduction to 1,2,3,4-tetra-hydroquinoxaline (91). Quinoxaline itself is reduced directly to (91) with LiAlH4 and direct synthesis of (91) is also possible. Tetrahydroquinoxalines in which the benzenoid ring is reduced are well known but these are usually prepared from cyclohexane derivatives (Scheme 30). 

Radical anions have recently been detected during electrochemical reductions of lumazines (80H(14)1603) and are also assumed to be reactive intermediates in the reductive acylation of 2,4-disubstituted pteridines to the corresponding 5,8-diacyl-5,8-dihydro derivatives. 

Hi) Electrochemical reactions and reactions with free electrons Electrochemical oxidation of 3-methyl-l-phenylpyrazole gave the 3-carboxylic acid whereas electrochemical reduction (Section 4.04.2.1.6(i)) of l,5-diphenyl-3-styrylpyrazole produced the A -pyrazoline (B-76MI40402) with concomitant reduction of the exocyclic double bond (343). 

The electrochemical reduction of 3-nitrophthalic acid at controlled potentials gave 2,1-benzisoxazole-3-carboxylic acid. Cyclization is presumed to proceed via an intermediate oxime (67AHC(8)277). Treating 5-iodoanthranilic acid with acetic anhydride gave 3-acetoxy-5-iodo-2,l-benzisoxazole (596) (65JMC550). 

Controlled hydrogenation over Ni or the electrochemical reduction of o -nitrobenzo itriles produced 3-amino-2,l-benzisoxazoles either as the major product or by-product, depending in part on the reaction media and ratio of reactants (72BSF2365, 65CB1562). Reduction of o-nitrobenzonitrile gave either 3-amino-2,l-benzisoxazole or 2-aminobenzonitrile. The benzisoxazole is presumed to arise via an intermediate hydroxylamine. The electrochemical reduction of o-nitrobenzonitrile at acid pH produced the hydroxylamine as the primary product. Reduction at neutral pH gave the amino-2,1-benzisoxazole and the hydroxylamine (72BSF2365). 

Electrochemical reduction of 2,3-diphenylthiirene 1-oxide yields acetylene (80%) and benzil (10%). Electrolysis of 2,3-diphenylthiirene 1,1-dioxide in DMF gives trans-stilbene (30%) but in the presence of acetic acid, 1,2-diphenylvinylmethyl sulfone (27%) is obtained in addition to the stilbene (40%) (81CC120). 

Benzofurazan, 7-chloro-4-nitro-, 6, 394 as fluorigenic agents, 6, 410, 426 Benzofurazan, 4-chloro-7-sulfo-ammonium salt properties, 6, 426 Benzofurazan, 4-nitro-synthesis, 6, 408 Benzofurazans, 6, 393-426 Beckmann fragmentation, 6, 412 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 diazo coupling, 6, 409 dipole moments, 6, 400 electrochemical reduction, 5, 73 electrophilic reactions, 6, 409-410 ESR spectroscopy, 6, 400... 

Benzo[a]quinoIizinium bromide, 7-methyl-electrochemical reductions, 2, 534 Benzo[c]quinolizinium bromide ring opening, 2, 533 Benzo[c]quinolizinium chloride hydrogenation, 2, 536 Benzo[c]quinoIizinium chloride, 6-amino-synthesis, 2, 554... 

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