Electrochemical oxidation hydroxylamines

In electrochemical oxidation of l-hydroxy-3-imidazoline-3-oxides containing one to four H atoms at a-C, one observes in ESR-spectra not only triplet splitting of the nucleus 14N of the nitroxyl group (a v 15-16 G) but also splitting of the neighboring protons (a// 18-20 G), with multiplets corresponding to their number (from doublet to quintet) (101). Unlike spatially hindered hydroxylamines which show reversibility in electrochemical oxidation, hydroxylamines with H at a-C are oxidized irreversibly. Oxidation of hydroxylamines with nitroxyl radical proceeds easily and with quantitative yields (102). In the oxidation of asymmetric polylluorinated hydroxylamines with Mn02, isomeric polyfluorinated nitrones have been obtained (103). 

To undertake oxidation of both cyclic and acyclic hydroxylamines to nitrones, an electrochemical oxidative system has been developed, where WC>42-/WC>52-are used as cathodic redox mediators and Br /Br2 or I—/I2 as anodic redox mediators (129-131). 

Although electrochemical oxidation of Al-cyclohexyl-Al-hydroxylamine in the presence of pyridine afforded the corresponding dimeric nitroso compound with a low yield, A-hydroxy t-alkylamines were transformed into the corresponding nitroso compounds (equation 8) . Similarly, A-phenylhydroxylamine was transformed into nitrosobenzene under similar reaction conditions. ... 

V,/V-Bis(trifluoromethyl)hydroxylamine (5) is oxidized with potassium permanganate in acetic acid to an interesting free-radical compound, bis(trifluoromethyl)nitroxid-A7-yl(6), a pink-violet gas which condenses to a deep brown-violet liquid.246 Various oxidizing agents are effective in the oxidation of 5 to the corresponding nitroxyl 6.247 The best appears to be cerium(IV) salts either in the solid state or in aqueous acid solution.247 Efficient oxidation processes have been developed using aqueous potassium persulfate solutions, or electrochemical oxidation with cerium(III) nitrate and sodium nitrate in dilute nitric acid.247... 

Aminyl radicals also can be generated via electrochemical oxidation of amide bases or O-substituted hydroxylamines. Suginome has studied radical cyclizations involving oxidations of lithium alkenylamides as a route to ccs-l-methyl-2,5-disubstituted pyrrolidines (85TL6085). Electrolysis of lithium alkenylamide 17a, generated from the amine and butyllithium at - 78°C, led to the formation of 18a, exclusively cis, in 52% yield (Scheme 4). The reactions require 0.25 M UC104 in THF HMPA (30 1) as the supporting electrolyte. A variety of 2-substituted amines were studied. 

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+... 

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] ... 

The method of incorporation of ARs into the polymer matrix has been demonstrated by the polymerisation of pyrrole [118]. In the presence of protons, radical IV causes polymerisation of pyrrole to polypyrrole with the incorporated reduced (hydroxylamine) form and oxidised (nitrosonium ion) form of ARs in the polymer matrix. In electrochemical oxidation of pyrrole in the presence of ARs, a coupled electrochemical-chemical synthesis produces polypyrrole films with incorporated nitrosonium ions. They can by reduced to ARs by partial film reduction. 

Hydroxylamine is used as a reductant in the synthesis of coordination compounds.The complex, [RuCl(NO)(py)4][PF6], undergoes a one-electron electrochemical oxidation in aqueous solution and this complex is obtained by reduction of [RuCl(N0)(py)4][C104]2 with hydroxylamine, followed by addition of ammonium hexafluorophosphate. 8 Like hydrazine, hydroxylamine can act as both a reductant and as a ligand. Stetsenko has shown that Alexander s base, [Pt(NH20H)4][0H]2, is obtained in high yield from potassium tetrachloroplatinate by reduction with excess hydroxylamine hydrochloride in the presence of sodium carbonate. 

The nitroxyl radical serves as a catalyst precursor which is oxidized by using different primary oxidants (e.g. sodium chlorite [327], electrochemical oxidation [328-330] or periodic acid [331]). The resulting oxoammonium cation is considered to be the true catalytic oxidant which is reduced to the corresponding hydroxyl-amine during the oxidation of the alcohol. Subsequently, the hydroxylamine is re-oxidized with a suitable stoichiometric oxidant to the nitroxyl radical. 

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. 

However, an improved electrochemical redox methodology using a flow cell fitted with two consecutive porous electrodes of opposite polarities (cathode then anode), allows a rapid and total oxidation at the anode of the hydroxylamine intermediate produced at the cathode. Various nitroso compounds may be obtained in high yields without... 

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. 

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) ... 

Qi, X. and R.P. Baldwin (1994). Liquid chromatography/electrochemical detection of hydroxylamines by oxidation at a cobalt phthalocyanine chemically modified electrode. Electroanalysis 6(5-6), 353-360. 

HPLC separation with subsequent electrochemical detection has been employed to detect, by oxidation, ethanol-derived radical adducts to PBN and POBN and the corresponding EPR-silent hydroxylamines arising from biologi-... 

An electrochemically induced change of spherical micelles to larger aggregates was observed for coumarin-containing block copolymers [294]. Electrochemical addressability was also found for ferrocene-based micelles of block copolymers, which can undergo conjugation with hydroxylamines in terms of an oxime function [186]. After conjugation micelles are formed, whose size can be manipulated by the oxidation state of the ferrocene units (here only reported by chemical redox). 

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