Radical interception

This equation seems to be a key reaction point in the antioxidant action of these amines—>N radicals in the presence of oxygen are transformed via peroxy radical intermediates into nitroxyl radicals. The nitroxyl radicals are very persistent and react efficiently with radicals produced on polyolefine degradation. Such radical interception blocks the chain in radical oxidation and therefore causes the antioxidant activity of sterically hindered amines (Brede et al. 1998). 

Reaction sequences involving halogen transfer, followed by non-radical interception of the alkyl iodide or bromide formed can allow for the trapping of products arising from less exothermic or even endothermic atom transfer additions and are exemplified in Scheme 11. Yoon [30] and Curran [31] have demonstrated that the a-halo ethers formed upon addition to vinyl ethers can be trapped with alcohols, leading to formation of acetals. Substitution reactions on the heteroaromatics pyrrole and indole have been carried out through a sequence of steps involving I- or Br-... 

Reactions Involved in Radical Interception. Some of the reactions which have been considered in the radical interceptor technique are shown in Reactions 1-8. Reactions 1 and 2 are schematic and do not describe the mechanism of formation of secondary free radicals in proteins by ionizing radiation. The reactions of the carbon radicals with tritiated scavengers (Reactions 3 and 3a) lead to the formation of carbon-tritium bonds. 

Black, H.S., Radical interception by carotenoids and effects on UV carcinogenesis, Nutr. 

The selectivity to alcohol in LPO may be significantly increased when boric acid, meta-hotic acid, or boric anhydride is present in stoichiometric amounts (2). The boron compounds appear to convert alkyUiydroperoxides to alkyl borates and may also intercept alkylperoxy radicals, converting them to alkylperoxyboron compounds these are later converted to alkyl borates. The alkyl borates are resistant to further oxidation they are hydrolyzed to recover alcohols. 

Decaitxjxylallon of a mixed anhydride (Ihlohydtoxamlc-cartwxylic) and interception of radicals as a sulfide, selenide or bromo denvative. 

The scope of Barton s thiohydroxamate ester chemistry has been significantly expanded by the finding that the intermediate alkyl radicals (R ) can be intercepted by a host of neutral molecules (see Scheme 25).42b 49c,52>53 Several different classes of compounds can thus be prepared from a common thiohydroxamate ester precursor. 

In a series of experiments, the radical concentrations are varied by attenuating the light intensity, with wire screens for example. One then makes a plot of 1/[R2 ] versus [R2 ]/lR ]. The result is a straight line with an intercept of 2k,2/kc and a slope of... 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

There is no indication as to whether these compounds are formed by hot or thermal reactions. Many of the products e.g. the vinyl compounds and the polymers) are explainable as resulting from free radical reactions. The virtual disappearance of the parent compound at high radiation doses is attributable to the interception of the stepwise reformation by competing radical reactions. The decrease in vinyl compounds is explained as being due to increased polymerisation. 

The slopes, Y-intercepts and squares of correlation coefficients for the linear regression analyses of the T versus AE(ir) plots (equation 7) for reactions 1-4 for one-hour and ten-hour half life rates of decomposition to form free radical products are given in Table II. 

It is also possible that the last step may occur during dissolution just before separation, or in the solid as the sample is warmed up to room temperature. The first of these can, in fortunate cases, be intercepted, so as to form a selected compound from a radical or other incomplete molecule in the solid (46). There are several examples of the second—room-temperature annealing—and it is useful when possible to dissolve the target at low temperature. 

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

A corresponding correlation is obtained for the rate constants of a,a -phenyl substituted alkanes 26 (R1 = C6H5, R2 = H, R3 = alkyl) (see Fig. 1 )41). It has, however, a different slope and a different axis intercept. When both correlations are extrapolated to ESp = 0, a difference of about 16 kcal/mol in AG is found. This value is not unexpected because in the decomposition of a,a -phenyl substituted ethanes (Table 5, no. 22—27) resonance stabilized secondary benzyl radicals are formed. From Fig. 1 therefore a resonance energy of about 8 kcal/mol for a secondary benzyl radical is deduced. This is of the expected order of magnitude54. ... 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

Samarium(II) iodide also allows the reductive coupling of sulfur-substituted aromatic lactams such as 7-166 with carbonyl compounds to afford a-hydroxyalkylated lactams 7-167 with a high anti-selectivity [74]. The substituted lactams can easily be prepared from imides 7-165. The reaction is initiated by a reductive desulfuration with samarium(ll) iodide to give a radical, which can be intercepted by the added aldehyde to give the desired products 7-167. Ketones can be used as the carbonyl moiety instead of aldehydes, with good - albeit slightly lower - yields. 

Primary anti-oxidants. These are sterically hindered phenols and amines (HALS) and aromatic secondary amines. They intercept the radicals as they are formed ... 

On the basis of their results the authors postulated two distinct pathways for the electroreduction of C02 by the rhenium catalyst, with the intermediates being (a) (Bipy)Re[CO]3 and (b) the radical (Bipy)Re[CO]3 (see Scheme 3.1). The former is indicated by the fact that production of the green dimer is completely suppressed in CO2-saturated electrolyte, as a result of the C02 intercepting the active species present. 

Both of these general anaesthetics have long been known to oxidize in the presence of air and sunlight. Chloroform was shown to photo-oxidize to a peroxide of uncertain structure which collapsed to the poison gas phosgene. Anaesthetic chloroform is protected by the addition of ethanol, which intercepts the initial free radical and prevents the photodegradation [17]. Diethyl ether... 

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