Hydroxylamine nitrogen cycle

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

Figure 12 A diagram of the nitrogen cycle with catalyzing enzymes and metal requirements of each step. NIT, nitrogenase AMO, ammonium mono-oxygenase HAO, hydroxylamine oxidoreductase NAR, membrane-bound respiratory nitrate reductase NAP, periplasmic respiratory nitrate reductase NR, assimila-tory nitrate reductase NIR, respiratory nitrite reductase NiR, assimilatory nitrite reductase NOR, nitric oxide reductase N2OR, nitrous oxide reductase.

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. 

The ultimate loss of the HAS activity occurs by destruction of the heterocycle initiated thermally, photochemically, chemically or by high-energy radiation. An intramolecular H-abstraction from the p-carbon atom in thermolysis of 2,2,6,6-tetramethyl-4-oxo-piperidinyl-l-oxyl 133 via a general reaction (Eq. 8) was proposed as a pathway of thermal selfdestruction of the piperidine cycle [25] (Scheme 25). The respective hydroxylamine was isolated in the yield of 66.5%. The biradical intermediate 137 either dimerizes to nitroxide 138 or thermolyses via 139 to a nitrogen-free fragment 140 (phorone) and nitric oxide. 

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