Nitroxide with alkyl radicals, reaction

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

Decomposition of azo compounds and peroxides provides the alkoxyamine by the nitroxide-trapping of the primary radicals [29]. The radicals produced by hydrogen abstraction with oxy radicals are also trapped by the nitroxide [242, 243]. In the photoreaction, alkoxyamines were isolated with high yields [244]. The reactions of Grignard reagents with nitroxides [215] and the coupling reaction of sodium nitroxides with bromo compounds [234,235] are also used. The hydrolysis of 56 followed by the reaction with acyl or alkyl halides afforded alkoxyamines with various functional groups,63 (Eq. 68) [245-251] ... 

In a reducing environment, conditions may allow for the same type of mechanism to occur, but with the radical anion of the spin trap as the intermediate. Actually, the possibility of radical ion-mediated spin trapping was first discussed in a study of a reductive system, namely in the search for radical intermediates in the reaction between alkanethiolates and alkyl halides conducted in the presence of TBN [2] (Crozet et al., 1975). TBN is known to trap primary radicals with formation of nitroxides (attack of R at N), and it was therefore anomalous to find alkoxyaminyl radicals (attack of R at O) in the above reaction. It was suggested that the alkanethiolate or some other reductant reduces TBN to its radical anion, which attacks the alkyl halide via oxygen in an SN2 fashion, as in equations (8) and (9) (see p. 129). 

The pro-fluorescent nitroxide TMDBIO when it reacts with an alkyl radical generates a fluorescent alkoxy amine so the fluorescence then becomes an integral measure of the number of such reactions that have taken place (2). This is seen in Figure 5 below for a sample of PP before and after oxidation at 150°C. 

In all the nitroxide regeneration mechanisms the nitroxide reacts with an alkyl radical, this reaction is in competition with the extremely fast reaction between an alkyl radical and oxygen (see Scheme 17.14)... 

Open-ring nitroso- [224] 144 and nitrocompounds [126] 145 were reported as the ultimate products of ROO reaction with HAS derived nitroxides. A-Oxide 146 was mentioned among the ultimate products of HAS as well [225]. Some alkyl radical trapping and 102 quenching activities should be preserved in 146. 

Aromatic nitroxides were found to attack both alkyl and alkylperoxy radicals, but aliphatic nitroxides attacked only alkyl radicals, the rate obeying equation (IX) above. The kinetics of reaction involving nitroxide B were close to predictions made on the basis of equation (VII) and it is interesting to note that the rate of reaction was affected by solvent, possibly via the formation of a loose solvent—nitroxide complex. Hydroxyl-amines A and B reacted only with peroxy radicals, but hydroxylamine C was involved in a complex reaction sequence producing first strong inhibition and then equally strong catalysis of autoxidation ... 

This technique would, therefore, appear to provide a simple method for studying reactions between alkyl radicals and transient nitroxides and also, possibly, for studying other "cross radical-radical reactions. The value obtained for 2k was found to be in reasonable agreement with that obtained by a direct measurement of the bimolecular decay of n-hexyl-tert-butyl nitroxide, viz.,... 

In this mechanism, P-X is thermally or photochemically dissociated into P and X", where a stable (persistent) radical X is assumed to be stable enough to undergo no reaction other than the combination with P" (and other alkyl radicals, if any present), namely, an ideal stable free radical (SFR) does not react among themselves, does not initiate polymerization, and does not undergo disproportionation with P". The best known examples of SFR are nitroxides such as TEMPO (2,2,6,6- tetra-methylpiperidinyl-l-oxy) (Figure 1), even though they are not perfectly ideal in the mentioned sense. The rate constants of... 

Closely related routes to alkoxyamines have also been described by Studer and Grubbs research groups. They both used nitroso compounds as radical traps. Studer et used the reaction between hydrazine and lead oxide to promote the alkyl radical formation, whereas Grubbs et generated the alkyl radical by reaction of an alkyl bromide with copper(I) complex. The advantage of this technique relies on the in situ production of the nitroxide. However, yields for alkoxyamines are often moderate and the method is limited to alkoxyamines beating two identical alkyl groups in the stracture. 

Matyjaszewski ° reported a simple and versatile method for the preparation of several TEMPO-derivative alkoxyamines based on atom-transfer radical addition (ATRA). This reaction involves halogen transfer from organic halides to Gu(I)com-plexes to produce the alkyl radical, which is then trapped by the nitroxide to afford the targeted alkoxyamine. Matyjaszewski et al and Matyjaszewski and Greszta have shown that 1.05 equiv. of Gu(0) with respect to the alkyl halide is necessary for a quantitative reaction and a slight excess of nitroxide (1.2 equiv.) is needed to minimize alkyl radical termination reactions (Figure 11(a)). 

The reaction between nitroxides and carbon-centered radicals occurs at near (but not at) diffusion controlled rates. Rate constants and Arrhenius parameters for coupling of nitroxides and various carbon-centered radicals have been determined.508 311 The rate constants (20 °C) for the reaction of TEMPO with primary, secondary and tertiary alkyl and benzyl radicals are 1.2, 1.0, 0.8 and 0.5x109 M 1 s 1 respectively. The corresponding rate constants for reaction of 115 are slightly higher. If due allowance is made for the afore-mentioned sensitivity to radical structure510 and some dependence on reaction conditions,511 the reaction can be applied as a clock reaction to estimate rate constants for reactions between carbon-centered radicals and monomers504 506"07312 or other substrates.20... 

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