Aminoxyl radicals Stable

A spin trap is a diamagnetic compound that reacts with a radical by addition of the radical functionality typically to a double bond in the trap, thus forming a new radical that is more stable (better, less unstable) than the original radical. By far the most common class of spin traps are nitrone compounds that, upon addition of the primary radical, produce a stable aminoxyl radical (Figure 10.1). The compound DMPO is the paradigmatic spin trap it is readily available, widely used, and its EPR spectra are relatively easy to interpret. Some of its radical adducts have unpractically short lifetimes. 

H2NO is a typical reactive intermediate endowed with short hfetime, aminoxyl radicals deriving from secondary amines may instead be stable whenever complying with the general structure 1 (R H), in which there are no hydrogens attached to the a-carbon atoms. 

Persistent or moderately stable aminoxyl radicals (1, in Scheme 6) lend themselves to one-electron oxidation to yield an oxoammonium ion (4) at a redox potential that depends on the structure of the hydroxylamine precursor Calculation methods have... 

As a rule of thumb, oue would expect that the more stable the aminoxyl radical, the lower its overall reactivity. This matches the above reported expectation (cf. Scheme 1) that the lower the BDEno-h value, the less efficiently the aminoxyl radical wiU abstract H-atom from a substrate. These two statements are indeed corroborated by a few literature examples. TEMF 0 (Chart 1), a persistent aminoxyl radical , upon H-abstraction forms one of the weakest NO—H bonds among those reported in Table l . It is no wonder... 

Table 1 shows that iV-hydroxyphthalimide (HPI) presents features almost opposite to those of TEMPO. In fact, the dissociation energy of the NO—H bond in HPI (88 kcal moR ) , accessible through EPR measurements" , is much higher than that of the corresponding NO—H bond in the hydroxylamine moiety of TEMPO (viz. TEMPOH, 69 kcalmoR ) . TEMPO is a persistent aminoxyl radical , whereas the aminoxyl radical of HPI, dubbed PINO (phthaUmide-N-qxyl)", is not stable but endowed with a half-life of ca 8000 s in AcOH solution at 25 °C . By using an oxidant such as a Pb(OAc)4 , or a Co(III) salt " " , HPI can be oxidized into PINO, and the H-abstraction reactivity of the latter subsequently exploited in the catalytic oxidation of appropriate H-donor substrates (RH) (Scheme 7). 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

The nitroso compounds are effective spin traps and a source of stable aminoxyl radicals ... 

Hindered amine stabilizers are transparent to visible and terrestrial UV light (300-400 nm). In polymeric matrices, they are oxidized in a sacrificial reaction by way of a not yet fully understood mechanism to stable nitroxyl (aminoxyl) radicals >N-0. A mechanism based on the reaction of HASs with alkyl hydroperoxides and alkyl peroxyl radicals is presented in Scheme 9.14 [87]. 

The hydrogen-bonded intermediate complex of a hindered phenolic antioxidant with a stable aminoxyl radical (TEMPO), used as a model for a hydrocarbon oxidant, has been isolated and its structure determined, as confirmation of the radical scavenging mechanism (79). 

Free radical reactions of ketenes are of importance because of their occurrence in hydrocarbon combustion and for their influence on air pollution, and some examples are already given elsewhere in this review. The subject has been briefly reviewed,and one approach that has been exploited to moderate these reactions is the use of stable aminoxyl radicals, which have been found to add to ketenes, as in the original example of the addition of the nitroxyl radical (CF3)2NO to Ph2C=C=0 forming the 1,2-diaddition product 207 (Eqn (4.132)). ° ... 

Solvents essentially influence the decay rate. Rate constants in polar solvents are less because of AR blocking due to formation of hydrogen bonds with solvents. In conditions where hydrogen-atom abstraction from surrounding molecules is difficult, aminoxyls are stable up to 200-220 °C [18]. ARs can accept one radical with the formation of diamagnetic compounds ... 

As a consequence of primary reactions of nitrogen dioxide radicals with the isolated double bonds, stable aminoxyl radicals can be generated. Such transformations are characteristic for rubbers. 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

Other teams worked on the functionalization of the aminoxyl group situated at the co position. For instance, the method of Ding et al. [342] is original for the synthesis of a novel series of poly(sodium styrenesulfonate) (PSSNa) macromonomers (compound 3 in Scheme 74) based on stable free radical polymerization in the presence of TEMPO. 

A technique related to spin labeling is that of using stable free radicals as spin probes to monitor molecular motions. In this technique a radical, usually an aminoxyl, is synthesized that mimics as closely as possible the structure of the molecule under investigation. The major difference compared with spin labeling is that the radical is not chemically attached to the molecule under investigation. 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

From the ESR spectrum shown in Figure 4.2, it is clear that stable radicals of an aminoxyl type are formed during the initial generation of NO3 in PVP. However, strong anisotropy of the hyperfine structure caused by hindered molecular mobility does not allow us to draw conclusions about the nature of the substituents at the radical centre. This spectrum gives only the prerequisites to represent the mechanism of the formation of stable radicals by the reactions (Equation 4.89)-(Equation 4.92). According to this mechanism, ARs are cross-links for macromolecules, and hence the radical concentration is cormected with the yield of a gel-fraction /Mq) in the course of a sample photolysis. 

Spin labels are almost invariably di-t-alkyl-substituted nitroxide (nitroxyl or aminoxyl) free radicals. These are used because they are quite stable to heat and light, and nitroxides with a wide variety of structures and functional groups can be readily synthesised, so facilitating the labelling of a range of polymers [4]. Many labels are functionalised piperidine and pyrroline derivatives (Table 9.1), although other types are also sometimes used. 

Eventually, alkyl radicals can be randomly produced along the polymer chain after its exposure to irradiations (electron beam or °Co y-ray irradiation). This was demonstrated with a N-phthaloylchitosan (the soluble intermediate for the modification of chitosan) followed by the grafting of either S or Ss s 9.59o stmcture and/or the location of the aminoxyl function was, however, unclear. Holmberg et irradiated poly(vinylidene fluoride) (PVDF) in the presence of TEMPO. The macroinitiator was then used to graft styrene or a styrene/ N-phenylmaleimide mixture onto the PVDF membranes. Such a result was surprising since the alkoxyamine formed should be very stable and not prone to decomposition. 

The chemical mechanisms involved in the action of antioxidants have been discussed in a number of reviews [8,11-18] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor process, (CB-D) summarised in reaction (3). The relatively stable radicals (A) produced (e.g. phenoxyl from phenols and aminoxyl from aromatic amines) carmot continue the kinetic chain and disappear from the system by coupling with other or the same free radicals. However, it should be noted that this process is stoichiometric and hydroperoxides... 

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