Nitroxyl compounds

Moreover, when the decomposition is performed in the presence of nitroxyl compounds, the resulting radical/radical annihilation also involves grafting ... 

Gries H, Niedballa U, Weinmann H-J US Patent 4,925,652 Nitroxyl compounds, and diagnostic media based thereon useful for enhancing NMR imaging... 

Similar results have been reported by Felder and Schumacher (3, Bellus, Lind, and Wyatt (9). The results indicate that among the tested compounds, the most efficient quencher of singlet oxygen is the N-methyl-substituted piperidine derivative, followed by nickel chelates and nitroxyl compounds of the hindered amines. The 2,2,6,6-tetramethylpiperidines themselves were of relatively low efficiency. 

The hindered-amine stabilizers readily form nitroxyl compounds. Denisov (10,11) has found facile formation in polypropylene. Rozantsev (12,13) indicates that the mechanisms by which these sterically hindered amines act is via the generation of stable piperidinoxyl radicals, and he proposes their formation. 

Pentacyano(2-chloro 1-nitroxyl compound with pentacyano 300 N 1.72 ... 

The early recognition of the role of stable nitroxyl free radicals, e.g., 2,2,6,6-tetramethyl-4-oxopiperidine, and their hindered amine precursors, in polymer stabilization soon led to the development of the hindered amine light stabilizer (HALS) class of photoantioxidants. The first HALS, Tinuvin 770, AO-33, (commercialized in 1974) proved to offer much higher UV-stabil-ity to polymers than any conventional UV-stabilizer available at the time such as UV-absorbers, nickel compounds and benzoates. Table 3). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Another chemically more interesting spin labeled B12 derivative involves coordinate attachment of the nitroxyl function to the cobalt atom of a cobinamide. Fig. 22 shows a reaction in which an alkyl cobin-amide is mixed with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. The nitroxide displaces water from the 6th coordination position very slowly and therefore this reaction is usually allowed to proceed for a few days with a large excess of nitroxide. From the properties of the coordinated nitroxide derivative discussed below, it is certain that the cobalt is coordinated by the N—O functional group. An analogous compound to that shown in Fig. 22 can be made with a similar nitroxide in which the 4-hydroxyl-group is missing. In this case the N—O-function is the only basic site on the molecule and therefore must be the position of attachment to the cobalt 119). 

Nuclear magnetic resonance studies on spin labeled derivatives are not extremely useful due to the paramagnetism of the molecule. However, the NMR spectrum of spin labeled methylcobinamide confirms that the nitroxyl function is coordinated to the cobalt. It is possible in this compound to obtain good resolution of the methyl group resonance. 

Antioxidants that break chains by reactions with alkyl radicals. These are compounds, such as quinones, nitrones, iminoquinones, methylenequinones, stable nitroxyl radicals, and nitrocompounds that readily accept alkyl radicals. Such antioxidants are efficient at very low concentrations of dioxygen and in solid polymers. 

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

Some compounds, for example, quinones Q and nitroxyl radicals AmO , inhibit oxidation by accepting alkyl radicals [15]. 

Why are the activation energies of the reactions of nitroxyl radicals with O—H bonds lower than those in their reactions with C—H bonds As in the case of the reaction of R02 with quinones, the difference in E values occurs as a result of the different triplet repulsions in TS [23]. When a TS of the O H O type is formed (the AmO + H02 reaction), the triplet repulsion is close to zero because the O—O bond in the labile compound AmOOH is very weak. Conversely, the triplet repulsion in the reaction of AmO with the C—H bond is fairly great, due to the high dissociation energy of the AmO—R bond. This accounts for the difference between the activation energies and between the rate constants for the reactions considered above. Thus, the possibility of the realization of a cyclic chain termination mechanism in the reactions of nitroxyl radicals with peroxyl radicals, incorporating O—H groups, is caused by the weak triplet repulsion in the TS of such disproportionation reactions... 

A new cyclic mechanism of chain termination by nitroxyl radicals, including the formation of aminyl radicals as intermediate species, has been proposed by Korcek and coworkers [42,43]. It was shown that the addition of 4,4 -dioctyldiphenylnitroxyl radical to the hexa-decane that is oxidized (T = 433 K) leads to the formation of the corresponding diphenyl-amine as an intermediate compound during its transformations. The following cyclic mechanism of chain termination was suggested ... 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

Nitroxyl radicals, p-benzoquinone, and dinitrotoluene terminate chains only by the reaction with alkyl macroradicals. They form the following series according to their activity nitroxyl radical > quinone > nitro compound. 

The phenomena of nitroxyl radicals regeneration has been discovered in the study of the retarding effect of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-A-oxyl on PP initiated oxidation [51]. It has been shown that the limiting step of chain termination by the nitroxyl radical is the reaction with the alkyl macroradical of PP. The resulting compound AmOP is fairly reactive with respect to the peroxyl radical and nitroxyl radical is regenerated in this reaction. Thus, the cycle includes the following two reactions (mechanism I) [60-64] ... 

Therefore, such alkyl radical acceptors as quinones, nitroxyl radicals, and nitro compounds retard the oxidation of PP according to the following cyclic mechanism of chain termination ... 

Aliphatic and aromatic nitro compounds react with all three R3M radicals to generate intermediate nitroxyl radicals of general structure R3M—O—N(O )—R. For the tin series, such radicals are implicated in the denitration of nitroalkanes25. The persistence of these radicals decreases with the nature of R in the order Me (minutes) < Et < Bu (hours)28. 

TABLE 3. ESR spectral parameters for nitroxyl radicals formed from R3 M and nitro compounds... 

For the last 50 years many scientists have drawn special attention to nitrones due to their successful application as building blocks in the synthesis of various natural and biologically active compounds, of stable nitroxyl radicals, and of other important products for special purposes such as spin traps for the study of radical processes including those that take place in biological systems, and they also found use as both, modifiers and regulators of molecular weight in radical polymerization. 

In some cases, a-alkoxy-substituted nitroxyl radicals (276) and (281) turn out to be convenient starting compounds in the synthesis of a-alkoxynitrones. On reduction, they eliminate methanol affording a-alkoxy nitrones (Scheme 2.106). This method, leading to a-alkoxy nitrones makes it possible to generate these compounds when other methods are unsuccessful (514, 519). 

The reaction of organometalic compounds with nitrones can be applied not only to the synthesis of stable nitroxyl radicals but also to the preparation of optically active secondary amines (Scheme 2.162) (617, 618). 

N-Hydroxy-N-nitrosamines with an aliphatic group at O2 produce a compound stable to aqueous acid and base (Fig. 3.4, 29) [158], whereas all other N-hydroxy-N-nitrosamines are susceptible to hydrolysis and appropriate 02-derivatives also render these materials vulnerable. The hydrolysis endpoint is the formation of nitroxyl (HNO) [which dimerizes to form nitrous oxide (N20)] and a C-nitroso compound. These products are formed from aryl [159] and alkyl bound unsubstituted diazenium-diolates as well as Oralkylated derivatives [160]. Studies of the solvolysis of Oi-alkyl derivatives are complicated by their tendency to decompose via competing radical pathways [161], but the Oi-benzyl derivatives are unique in that they hydrolyze back to the original synthetic precursors (Scheme 3.14) [162]. 

C-Nitroso compounds, oximes, N-hydroxyguanidines and N-hydroxyureas each contain an N-O bond and release nitric oxide (NO) or one of its redox forms under some conditions. The nitrogen atom of a C-nitroso compound formally exists in the +1 oxidation state, the same oxidation state as nitroxyl (HNO), the one-electron reduced form of N O. The nitrogen atoms of oximes, N-hydroxyguanidines, and N-hydroxyureas each formally exist in the -1 oxidation state, the same oxidation state as hydroxylamine. Consequently, the direct formation of NO (formal oxidation state = +2) from any of these species requires oxidation, one electron for a C-nitroso compound and three electrons for an oxime, N-hydroxyguanidine or N-hydroxyurea. This chapter summarizes the syntheses and properties, NO-releasing mechanisms and the known structure-activity relationships of these compounds. 

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