Radical acceptors, amines

The inertia of (TMS)3SiH toward azides allows this functionality to be used as a radical acceptor. An example is given in Reaction (52) where the amine product was tosylated before work-up. 

Hydroperoxides oxidize aromatic amines more readily than analogous phenols. Thus, at 368 K cumyl hydroperoxide oxidizes a-naphthylamine and a-naphthol with ku = 1.4 x 10 4 and 1.7 x 10 5L mol-1 s 1, respectively [115,118], The oxidation of amines with hydroperoxides occurs apparently by chain mechanism, since the step of free radical generation proceeds much more slowly. This was proved in experiments on amines oxidation by cumyl hydroperoxide in the presence of /V,/V -diphcnyl-l, 4-phcnylcnediamine (QH2) as a radical acceptor [125]. The following reactions were supposed to occur in solution (80% decane and 20% chlorobenzene) ... 

A mixture of two antioxidants, one acceptor of the peroxyl radical (phenol or amine) and another alkyl radical acceptor (stable nitroxyl radical), causes the synergistic effect in autoxi-dation of hydrocarbons (ethylbenzene and nonene-1) [44-46]. 

Nitroxyl radicals as alkyl radical acceptors are known to be very weak antioxidants due to the extremely fast addition of dioxygen to alkyl radicals (see Chapter 2). They retard the oxidation of solid polymers due to specific features of free radical reactions in the solid polymer matrix (see Chapter 19). However, the combination of two inhibitors, one is the peroxyl radical acceptor (phenol, aromatic amine) and another is the alkyl radical acceptor (nitroxyl radical) showed the synergistic action [44-46]. The results of testing the combination of nitroxyl radical (>NO ) (2,2,6,6-tetramethyl-4-benzoylpiperidine-l-oxyl) + amine (phenol) in the autoxidation of nonene-1 at 393 K are given here ([>NO ]o + [InH]o = 1.5 x 10 4mol L 1 p02 98 kPa) [44]. 

Fig. 1 Disconnecting a C-C bond of a chiral amine suggests a chiral N-acylhydrazone radical acceptor and a radical precursor...

Styrene is one of the most important monomers produced by the chemical industry today. It is separated from the products of dehydrogenation of ethylbenzene by a multi-stage rectification. The process inhibitors for styrene are mainly nitroaromatic compounds or aromatic amines and phenolic compounds such as p-tertbutylcatechol or 2,6 ditertbutylhydroxytoluene mixed with other radical acceptor inhibitor. 

Inferior results were attained when utilising /j-PhDA as a radical acceptor the free aminic group from the o-position cannot react with the voluminous molecule of reactant, as due to the steric hindrances. 

As might be expected, radical acceptors (aromatic amine and phenolic compounds) can influence the solution rheology of degraded polymers [45] (Table 3.9). The best acceptors for polystyrene are hydroquinone and /7-aminophenol (which favors substitution in the nucleus) and the poorest are / -nitrophenol and pentachlorophenol (which hinder substitution). In general, the order of acceptor activity was the same as that for poly(methyl methacrylate). The relative efficiency of oxygen, however, is lower for polystyrene, as this polymer forms relatively stable peroxy radicals, which can react with polymer radicals, as follows, minimizing the molecular weight reduction ... 

The generation rate of free radicals can be measured from the consumption of the radical acceptor InH (inhibitor of the chain reaction, counter of free radicals). Stable flee radicals are most frequently used as alkyl radicals nitroxyl, phenoxyl, and diphenylpiciylhydrazyl. Inhibitors of oxidation, viz., phenol, naphthols, and aromatic amines, are applied for accepting peroxyl and alkoxyl radicals. 

Eullerene-based donor-acceptor complexes and ion-radical salts with tetrathia-fulvalenes, metalloporphyrins, and cyclic amines as donors 99UK23. 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

Acceptors of peroxyl radicals (phenols, hydroquinones, aromatic amines) retard hydrocarbon oxidation, terminating the chains (see Part II). 

At low temperature, this equilibrium shifts to the right, and only a small fraction of free peroxyl radicals participate in the reaction, whereas at a high temperature the equilibrium is shifted to the left, and the fraction of bound radicals is small. If we assume that at 210-260 K almost all R02 radicals are bound in complexes with amines, then it can be shown that the difference of low- and high-temperature Ee0 values is close to the enthalpy of this donor-acceptor interaction in the CTC complex AEe0 AHDA = 66.7 - 59.1 = 7.6 kJ mol-1. 

The ability of esters to form hydrogen bonds with polar reactants is especially important for the reactions of peroxyl radicals with antioxidants such as phenols and amines. Amines form hydrogen bonds with ester groups. The hydrogen bonding lowers the activity of antioxidants as acceptors of peroxyl radicals (see Chapters 14 and 15). 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

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