Generation of Secondary Radicals

Understandably, most workers who use radiolysis, photoionization, CTFS, or CTTS as the means for generation of (secondary) radical ions pay little attention to the nature of short-lived precursors of these ions. After all, the subject of interest is a secondary rather than a primary ion. This ad hoc approach is justifiable because radiolytic production is just another means of obtaining a sufficient yield of the radical ion. Quite often in such studies, the radiolysis is complemented by other techniques for radical ion generation, such as plasma oxidation, electron bombardment-matrix deposition, and chemical and electrochemical reduction or oxidation. While the data obtained in these studies are useful, there is little radiation chemistry in such—nominally, radiation chemistry—studies. 

An extension of a method developed for the preparation of butyl radicals from t-butyIhydrazine and lead dioxide, allowed synthesis of hindered chiral amines via generation of secondary radicals from bornylhydrazine and menthyIhydrazine. The radicals were trapped with nitroso-tert-octane and the resultant hydroxyl-amine mixtures reduced to give readily separable mixtures of borny1-tert-octyl amines (2),(3) and menthyl-tert-octylamines... 

Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions (73). Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

The increased crosslinking may be attributed to the preferential generation of secondary polymer radicals due to the presence of excited MAH and therefore an increase in the amount of coupling of secondary polymer radicals rather than of pendant MAH radicals. 

A general method for the generation of aminyl radicals is by treatment of sulphenamides 340, prepared from secondary amines and A-benzenesulphenylphthalimide, with tributyltin hydride in the presence of AIBN (2,2/-azobisisobutyronitrile). The cyclopropyl derivative... 

As observed, aromatic hydrocarbons gave products of protonation on dissolution in hydrofluoric acid. Oxidation into aromatic cation-radicals did not take place (Kon and Blois 1958). Trifluoro-acetic acid is able to transform aromatics into cation-radicals. This acid is considered a middle-powered one-electron oxidant (Eberson and Radnor 1991). Its oxidative ability can be enhanced in the presence of lead tetraacetate. This mixture, however, should be used carefully to avoid oxidation deeper than the one-electron removal. Thus, oxidation of 1,2-phenylenediamine by the system Pb(OCOCH3)4 -I- CE3COOH -P CH2CI2 leads to the formation of either primary or secondary cation-radicals. The primary product is the cation radical of initial phenylenediamine, whereas the secondary product is the cation radical of dihydrophenazine (Omelka et al. 2001). Sulfuric acid is also used as an one-electron oxidant, especially for aromatic hydrocarbons. In this case, generation of cation radicals proceeds simultaneously with the hydrocarbon protonation and sulfonation (Weissmann et al. 1957). 

Other approaches to synthesize highly substituted cyclopentanes, including amino and hydroxy groups, from y- and lactones, has been radical and anionic Michael cyclizations of the a-iodo-y- and -<5-lactones [94]. Likewise, methods using radical cyclization to oxime ethers have been reported to give amino substituted cyclopentanes [95,103,104]. It should be noted that although only one isomer is often obtained [103], such cyclizations generating a secondary radical may not be stereospecific [95,100,101]. 

There are many excellent books and reviews on the structure and reactions of secondary radical ions generated in radiolytic and photolytic reactions. Common topics include the means and kinetics of radical ion production, techniques for matrix stabilization, electronic and atomic structure, ion-molecule reactions, structural rearrangements, etc. On the other hand, the studies of primary radical ions, viz. solvent radical ions, have not been reviewed in a systematic fashion. In this chapter, we attempt to close this gap. To this end, we will concentrate on a few better-characterized systems. (There have been many scattered pulse radiolysis studies of organic solvents most of these studies are inconclusive as to the nature of the primary species.)... 

Analogous addition of a sugar radical to an unsaturated sulfone has been described by the same authors in the course of pseudomonic acid synthesis. For this, UV irradiation of the iodosugar was used to generate a secondary radical [120],... 

A versatile protocol for the generation and cyclization of secondary radicals from hexo-pyranose sugars is shown in Scheme 3 [10], The Wittig reaction of reducing sugars with two eq of an alkylidene phosphorane readily provide hex-5-ene-l-ols, which were converted into hex-5-enyl radicals by the l-ff-imidazole-l-carbothioate. The cyclization reaction is carried out in refluxing benzene or toluene with tributyltin hydride and AIBN, according to... 

In mechanistic matters, it has been demonstrated that co-alkenyl iodides undergo cyclization onto the vinyl function upon treatment with Me2CuLi, in competition with direct substitution. This, as well as the generation of trityl radical in the reaction of Me2CuLi with trityl chloride, constitutes evidence for single electron transfer in reactions of cuprates with iodides (and, to a lesser extent, bromides)16. The intermediacy of alkyl radicals in the substitution process (equation 12) is likely the source of the aforementioned racemization in reactions of secondary iodides4. 

The crosslinking of EPR in the presence of radicals from peroxide decomposition (benzoylperoxide, dicumylperoxide etc.) by light or heat, is attributed to the attack on the secondary CH2 moieties and the generation of polymer radicals which couple either with similar secondary polymer radicals or polymer radicals generated by attack on the tertiary CH moieties on the propylene units in the chain. 

It is necessary that an antioxidant protects cells at all stages of oxidative stress, and therefore an antioxidant should be able to scavenge the secondary radicals produced by the reaction of primary radicals with biomolecules. Radiation chemists designed methods to study reactions of secondary radicals from amino acids of proteins and base and sugar radicals of DNA with antioxidants.The most commonly employed aromatic amino acid radicals generated by radiation chemical experiments are the indolyl radicals of tryptophan (TRP ), the... 

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