Electrochemical reduction hydroxylamines

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 benzylic nitro compounds (27) in an ethanolic aqueous acetic acid buffer (35 65) affords a mixture of the corresponding oxime and hydroxylamine (equation 6)48. The hydroxylamine can subsequently be oxidized back to the oxime (28) (via the intermediate nitroso compound) conversions as high as 90% can be obtained. 

As can be seen from the preceding discussion, the existence of nitroso compounds as intermediates in the electrochemical reduction of nitro compounds is mostly inferential nitroso compounds are easier to reduce than nitro compounds. Hence, they should be reduced as quickly as they are formed and would not be expected to be isolable. However, nitroso compounds have occasionally been isolated in unusual structural cases54 and the nitrosobenzene radical anion has been identified by ESR spectroscopy in at least one instance64. It is possible to prepare nitroso compounds by a two-step sequence one reduces the nitro compound electrochemically to the hydroxylamine, then electrochemi-cally oxidizes the hydroxylamine to the nitroso compound65. 

The mechanism of the global 4-electron electrochemical reduction of aromatic nitro compounds to hydroxylamines in aqueous medium shown in reaction 37 was investigated by polarography and cyclic voltametry. The nitro group is converted first to a dihy-droxylamine, that on dehydration yields a nitroso group the latter is further reduced to a... 

Furthermore, aromatic nitro compounds may be reduced to hydroxylamines by tita-nium(in) chloride or ammonium sulfide ", as well as by electrochemical reduction . 

Aliphatic hydroxylamines are not easily reduced by direct electrochemical reduction. However, they are reduced indirectly by electrochemically generated low-valent Ti(III) or Fe(II) to afford the corresponding amino compounds (equation 1). This electrochemical method has certain advantages it takes place at a less negative potential where hydrogen ions are not reduced and the low concentration of the metal salts facilitates the work-up compared to the reduction by chemical reagents where at least equivalent amounts of reagents must be used. 

In a similar way it is possible to perform the amination of aromatics by indirect electrochemical reduction of hydroxylamine using a combination of the Cu /Cu and redox pairs Thus, aniline and toluidine are formed from benzene... 

Catalytic reactions in electrochemistry — When the product of an electrochemical reduction reaction is regenerated by a chemical reoxidation, or when the product of an electrochemical oxidation is regenerated by a re-reduction, the regeneration reaction is called a catalytic reaction. For thermodynamic reasons the chemical oxidant (or the reductant) has to be electro-chemically irreversible in the potential range where the catalyst is electroactive. The reduction of Ti(IV) in the presence of hydroxylamine is an example for an oxidative regeneration [i, ii] ... 

Hydrazine hydrate can also be used to reduce nitroalkanes in the presence of a graphite catalyst. " The low cost of the catalyst serves to make this a particularly attractive reduction method. Electrochemical reduction of tertiary nitroalkanes such as the 2-substituted-I,l-dimethyl-l-nitroethanes (42a-42c) yields amines (44a-44c) as well as hydroxylamines (43a-43c equation 27). ... 

The biological reduction of nitrate is not heme-related a well-known family of Mo-oxo pterin enzymes facilitate O-atom transfers between nitrite and nitrate, sulfite and sulfate, and other substratesStill, the electrochemical reduction of nitrate on bare metal electrodes is well known, and can also be facilitated by addition of N4 chelates. For instance, Hobbs et al. have examined the electrochemical reduction of nitrate in basic solutions using iron and nickel electrodes coated with phthalocyanin yielding nitrite, hydroxylamine, and ammonia. The iron electrodes efficiency increased when coated with phthalocyanin while the nickel s activity decreased. A related study has been done by Shibata et al. to determine the synthetic applications of simultaneous reduction of nitrate and carbon dioxide using phthalocyanin complexes of most of the first-row transition metals and other metals with mixed success . 

Electrochemical reduction 5.2 10 of adsorbed X-nitro-9-fluorenone on GC yields the hydroxylamine... 

Electrochemical reduction of adsorbed 5,5 -dithiobis(2-nitrobenzoic acid) on Au yields the hydroxylamine... 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

Conversion of substituted nitrobenzenes to the arylhydroxylamine is easily achieved by reduction in neutral or slightly acid solution. In the first classical experiments, Haber [35] used a platinum cathode and ammonia ammonium chloride buffer and die process was improved by Brand [57] using either a nickel or silvered copper cathode in an acetate buffer. The hydroxylamine can also be obtained from reduction in dilute sulphuric acid provided tire temperature is kept below 15° C to suppress furtlier reduction [58]. This electrochemical route to arylhydroxylamines due to Brand is superior to the chemical reduction using zinc dust and ammonium chloride solution. The latter process is known to give variable yields depending on... 

Electrochemical studies in nonaqueous media [59] indicate that phenyl-r-butyl nitrone is not electroactive between +1.5 and -2.4 V vs. SCE. Studies of the reduction of phenyldiazonium salts in the presence of the nitrone gave EPR spectra of the phenyl adduct. However, even here, a two-electron reduction may give the phenyl anion, which can add to the nitrone to give phenyl-f-butyl hydroxylamine, which can be reoxidized to the nitroxide. Chemical tests were used to show that the phenyl radical, rather than the anion, was produced. 

The cathodic reductions of nitro compounds are among the most thoroughly investigated reactions of organic electrochemistry. At least on the laboratory scale, the reaction permits the synthesis of many intermediates with different oxidation states. However, most syntheses can now also be carried out more economically by catalytic reactions. Therefore, only a few electrochemical reactions are still of industrial interest, i.e. the single-step syntheses of hydroxylamines, aminophenols, or anisidines. 

This reaction scheme was later confirmed for the reduction of oximes in acidic solution by isolation of the intermediate ketimine in two cases 143-). However, in alkaline medium, reduction of the C=N bond in oximes and semicarbazones pre-ceds splitting of the N-Y bond, so that hydroxylamines and hydrazines may be obtained144 The stereochemistry of the electrochemical and chemical (H2/Pt, Na/EtOH) reduction of several imines to amines has been compared recently by Fry and Reed 144b ... 

Hydroxylamine (H2NOH). The electrochemical oxidation of H2NOH in dimethyl suloxide at a platinum electrode yields N20,u and as such is the reverse of the reduction of HON=0 under acidic conditions [Eq. (11.23)]. Because the primary electron-transfer step is H20/HO oxidation (2 H20 H30+... 

Electrochemical formation of Ti2+ allowing the reduction of hydroxylamines, sulfoxides, nitroso [60] ... 

The major use of cyclic voltammetry is to provide qualitative information about electrochemical processes under various conditions. As an example, consider the cyclic voltammogram for the agricultural insecticide parathion that is shown in Figure 23-25. Here, the switching potentials were about —1.2 V and -1-0.3 V. The initial forward scan was, however, started at 0.0 V and not -fO.3 V. Three peaks are observed. The first cathodic peak (A) results from a four-electron reduction of the parathion to give a hydroxylamine derivative... 

The most characteristic reaction of hindered nitroxyls is their reduction which results either in the corresponding hydroxylamine or amine. Hydroxylamines, as a rule, are readily oxidized to the corresponding radicals. Oxidation can be accomplished with atmospheric oxygen in the presence of catalytic amounts of heavy metal salts e.g., cupric salts. Pb02 and Kj[Fe(CN)g] can also be used as oxidizing agents. This easily occuring transformation-radical — hydroxylamine —> radical-is the basis for several important syntheses, in particular, electrochemical syntheses of the nitroxyl radicals (4) and (6) (39,40) ... 

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