Disproportionation of hydroxylamine

The Oxidation of Amines and Alcohols The Disproportionation of Hydroxylamine Miscellaneous Reactions... 

Depending on the type of iron catalyst, the reaction seems to take different mechanistic pathways. According to Johannsen and Jorgensen s results, the catalytic cycle starts with the formation of nitrosobenzene 32 either by disproportionation of hydroxylamine 29a to 32 and aniline in the presence of oxo iron(IV) phthalocyanine (PcFe4+=0) or by oxidation of 29a [131]. The second step, a hetero-ene reaction between the alkene 1 and nitrosobenzene 32, yields the allylic hydroxylamine 33, which is subsequently reduced by iron(II) phthalocyanine to afford the desired allylic amine 30 with regeneration of oxo iron(IV) phthalocyanine (Scheme 3.36). That means the nitrogen transfer proceeds as an off-metal reaction. The other byproduct, azoxybenzene, is probably formed by reaction of 29a with nitrosobenzene 32. 

Such a metal-assisted disproportionation of hydroxylamine may be initiated by a bimolecular reaction with uncoordinated hydroxylamine or by intramolecular electron transfer. The presence of a reducible metal allows a concurrent three electron oxidation of hydroxylamine to nitric oxide with the two electron reduction of hydroxylamine to ammonia, Eqs. (4.45) and (4.46). Ultimately, the reaction proved to be an efficient synthetic route to the ferrous nitrosyl adduct, which rapidly precipitates from solution in high yield. 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

Attention was paid to the reactivity of PP with aliphatic nitroso compounds too [235]. It was postulated that the stabilizing effect was based on formation of a PP bound nitroxide and its disproportionation into the respective derivatives of hydroxylamine and nitrone. 

Aromatic hydroxylamines are frequently produced during the hydrogenation of aryl nitro groups. Generally, these are undesired intermediates because when they are present in excessive amounts, a potentially explosive situation caused by the exothermic disproportionation of the hydroxylamine can result. 2 This is usually not a problem, but care should be exercised to prevent the accumulation of large amounts of the hydroxylamine, particularly when the... 

A less restrictive implementation of the procedure, find-all-pathways, on the initial chemical species set, would employ the. operators K or K and would generate several additional reactions of interest. Among these additional reactions of particular interest are, the decomposition reaction of nitrobenzene and the disproportionation of nitrobenzene with N-phenyl hydroxylamine forming aniline and nitrobenzene ... 

Under thermal conditions, hydroxylamine ethers can reversibly decompose (Reaction 15). The radicals formed disproportionate to eliminate olefins and yield hydroxylamine (Reaction 16). In the presence of sufficiently effective acceptors of alkyl radicals (e.g., oxygen), the reaction rate of peroxy radical formation is much higher than that of hydroxylamine formation. Thus, in the process of polymer photooxidation, nitroxyl radicals regenerate and can break multiple oxidative chains. 

The products of the superoxidized state may be explained if one considers the possibility that occasionally during turnover an active site at Mn is reduced by only one electron to Mn Mn . The following two-electron oxidation step would then yield the Mn Mn form of the enzyme, which is only slowly reduced back to an active Mn" form. Such chemistry was shown to be viable by the introduction of the one-electron reductant hydroxylamine to catalase followed by the addition of hydrogen peroxide (23). These catalases exhibit saturation kinetics and complete the disproportionation of hydrogen peroxide at a rate of 5.75 x 10 s for L. plantarum (23), 1.7 X 10 ... 

Scheme 14. pH-dependent disproportionation of nitroxides to oxoammonium salts and hydroxylamines... 

A side reaction in NMP is loss of nitroxide functionality by thermal elimination. This may occur by disproportionation of the propagating radieal with nitroxide or direct elimination of hydroxylamine as discussed in Section 9.3.6.3. In the case of methacrylate polymerization this leaves an unsaturated end group. " The chemistry has also been used to prepare macromonomers from PMMA prepared by ATRP (Section 9.7,2.1). 

In the case of porphyrin models (24), it was proposed that for [Fe (TPPS)] the formation of HNO fi om hydroxylamine occurred by a two-electron tra/is-disproportionation of a bis-coordinated NIhOH complex producing a low spin ferric intermediate [Fe Por)(NH3)(HNO)]. Based on the experimental kinetic evidence, and the obtained N2/N2O ratios from the reaction mixtures, a different mechanism was proposed for Fe MPl 1, in which the formation of [Fe (Por)(NHis)(HNO)] was suggested to result from the reaction of free HA toward iron-boimd HA. 

There is extensive history on the use of stoichiometric and catalytic organic nitroxyls for alcohol oxidation, wherein the key step involves a reaction between the alcohol and an JV-oxoammonium salt (Scheme 15.5, featuring TEMPO) [18, 20]. The JV-oxoammonium salt can be formed in situ from the corresponding nitroxyl radical using various oxidants, such as NaOCl or NO2 (Scheme 15.5, top left), or by acid-induced disproportionation of the nitroxyl into N-oxoammonium and hydroxylamine species (Scheme 15.5, bottom left) [21]. Stable Af-oxoammonium salts have also been isolated and used directly as reagents or catalysts for alcohol oxidation [22]. The pH-dependent mechanism of the reaction of the Af-oxoammonium salt with alcohols has been studied... 

The solution of hydroxylamine is a very weak base, it disproportionates slowly in addic solutions and rapidly in alkalis. 

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

Hydrogenation catalyst, Acid, Fuel Riesthuis, P. et al., J. Loss Prev. Process Ind., 1997, 10(10), 67 In the presence of precious metal hydrogenation catalyst, hydroxylamine salts may disproportionate and form dinitrogen monoxide. Such a mixture is present in a process whereby the hydroxyamine is formed by hydrogenation of nitrate. An explosion in the degassing line, after a period of abnormal operation, was attributed to nitrous oxide build-up. Fuel, in the form of hydrogen and methane diluent, was already present. 

In the reaction profile shown in Figure 1 (similar to that shown by Smith et al. (10)) the initial product was azoxybenzene. However this figure is deceptive firstly azoxybenzene may be produced by a non-catalyzed reaction between nitrosobenzene and phenyl hydroxylamine (10), secondly the figure does not show the mass balance. Indeed at 10 min when all nitrosobenzene has been removed from the solution the amount of azoxybenzene formed was 18.6 mmol, equivalent to 37.2 mmol of reacted nitrosobenzene. Therefore, 42.8 mmol of the original 80 mmol of nitrosobenzene (53.5 %) were unaccounted for. It is possible that the missing mass is in the form of phenyl hydroxylamine in solution, which continues to disproportionate to produce aniline and nitrosobenzene and subsequently azoxybenzene and azobenzene. However as we shall subsequently discover this interpretation is unsustainable. 

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