Radical migration radicals

Many other cases of free-radical migration of aryl groups have been found. ... 

In summary then, 1,2 free-radical migrations are much less prevalent than the analogous carbocation processes, and are important only for aryl, vinylic, acetoxy, and halogen migrating groups. The direction of migration is normally toward the... 

The existence of closely spaced radical pairs can be identified by spin-spin interactions in organic materials irradiated at low temperature [38] and these coupled spins disappear as the temperature is raised, because of both termination and radical migration. 

The stabilization of the radical cation by forming a polaron is a trade-off between its delocalization and the energy required to distort the DNA structure. The former lowers the kinetic energy of the intrinsically quantum mechanical migrating radical cation, and the latter will be determined by factors that are independent of specific base sequence, such as the force constants of bonds in the sugar diphosphate backbone. 

Radical migration of hydrogen is also known, though only over longer distances than 1,2-shifts, e.g. a 1,5-shift to oxygen via a 6-membered cyclic T.S. in the photolysis of the nitrite ester (129)—an example of the Barton reaction ... 

Cation-radicals, stabilized in zeolites, are excellent one-electron oxidizers for alkenes. In this bimolecular reaction, only those oxidizable alkenes can give rise to cation-radicals, which are able to penetrate into the zeolite channels. From two dienes, 2,4-hexadiene and cyclooctadiene, only the linear one (with the cylindrical width of 0.44 nm) can reach the biphenyl cation-radical or encounter it in the channel (if the cation-radical migrates from its site toward the donor). The eight-membered ring is too large to penetrate into the Na-ZSM-5 channels. The cyclooctadiene can be confined if the cylindrical width is 0.61 nm, however the width of the channels in Na-ZSM-5 is only 0.55 nm. No cyclooctadiene reaction with the confined biphenyl cation-radical was detected despite the fact that, in solution, one-electron exchange between cyclooctadiene and (biphenyl) proceeds readily (Morkin et al. 2003). 

Compound 18, prepared by a modification of this method, underwent free-radical dehalogenation as shown in Equation (4) to give the product of radical migration 20 as well as the expected product 19 <1995TL3867>. 

Although the radiation chemistry of Polyox would not be expected to be exactly the same (because of the influence of crystal structure on radical migration, deactivation of excited species, etc.), it is still reasonable to use their ratios to estimate the initial radical production in our system. The corresponding values for total bond cleavage in Polyox are as follows (calculated for a molecule containing 8 C—H bonds, 4 C—O bonds, and 2 C—C bonds) ... 

In the irradiation of polyethylene Dole et al.71 have found that the initially present vinylidene unsaturation decreases markedly with dose and have suggested a free-radical migration mechanism. Charge transfer could equally well explain these results this would be a direct extension of the results from solid n-hexane to the polyethylene system. The close correspondence of these two systems in their response to high-energy radiation will be pointed out below. 

Subsequent one-electron transfer and intramolecular hydrogen migration lead to radical 102 followed by reaction with 02 to yield hydroperoxide radical 103. Radical 103 is further oxidized to a dihydroperoxide (104), which decomposes to anthra-quinone. Alternatively, 103 may be transformed to a diradical that eventually gives anthracene as a byproduct. The ratio of the two products strongly depends on the solvent used. The highest yield of anthraquinone (85% at 100% conversion) was achieved in 95% aqueous pyridine. 

It is believed that the reaction starts with homolytic cleavage of the cobalt-carbon bond (at a cost of perhaps 100 kJ mol-1)8 to yield a Co(ll) atom and a 5 -decxy-adenosyl radical. This radical then abstracts a hydrogen atom (in Eq. 19.35 from the methyl group). Migration of the —GO)SR group takes place, followed by return of the hydrogen atom from 5 -deoxyadenosine to the substrate. This regenerates the 5 -deoxyadenosyl radical, which can recombine with the Co( I) atom to form the coenzyme. 

Note also the radical migration of the benzoyl group from C-2 to C-3, as in the anionic version (Scheme 29). 

Redox and homolytic substitution reactions almost never directly form C—C, C—N and C—O bonds. Such bonds are generated in radical addition reactions (Scheme 14). Intermolecular addition reactions are presented in this chapter. Cyclization reactions have important similarities with, and differences from, bimolecular additions, and they are presented in Chapter 4.2 of this volume. Falling under the umbrella of addition reactions are radical eliminations (the reverse of addition) and radical migrations (which are usually, but not always, comprised of an addition and an elimination). 

The ionization of a molecule and the rupture of a chemical bond by ionizing radiation necessarily result in the pairwise formation of radical species. The pairwise correlation of radical species will be more or less retained in solid polymers where the radical migration is restricted. This heterogeneity of spatial distribution of radical species affects the radiation chemistry of polymers. Another source of spatial heterogeneity is the heterogeneous deposition of radiation energy [6, 7]. Low LET radiations such as y-rays produce an ensemble of isolated spurs. Each spur is composed of a few ion-pairs and/or radical... 

In principle, Stevenson s rule still applies. By this rule, the cleavage of the adjacent bond could involve radical migration and charge retention if the ionization energy of YR7 is less... 

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