Intermolecular radical migration

Evidence of intermolecular radical migration of alkyl radicals... 

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). 

Reaction between iV,JV-dimethylaniline and chloranil in polar solvents yields a violet material with an absorption spectrum corresponding to that of the crystal violet cation. The diamagnetic complex (D A) and semiquinone radical were identified as intermediates. The overall course of the reaction is apparently as given above. The intermediate steps must involve an intermolecular methyl migration. 

The [1,2]-Wittig rearrangement is believed to proceed via a radical mechanism,which is illustrated in the example shown below. The reaction of 11 with MeLi provides organolithium intermediate 12, which imdergoes C-0 bond homolysis to form radicals 13 and 14. Radical 13 is converted to 15 via 1,2-lithium migration, and a subsequent intermolecular radical coupling of 15 with 14 yields alkoxide 16. The alcohol product 17 is obtained after an aqueous workup. The homolysis/recombination events are believed to occur inside a solvent cage. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

RADIATION-SENSITIVE GROUPS. Although the absorption of radiation energy is dependent only on the electron density of the substrate and therefore occurs spatially at random on a molecular scale, the subsequent chemical changes are not random. Some chemical bonds and groups are particularly sensitive to radiation-induced reactions. They include COOH, C-Hal, -SO2-, NHz, C=C. Spatial specificity of chemical reaction may result from intramolecular or intermolecular migration of energy or of reactive species -free radicals or ions. 

When the group R is chiral at the migrating carbon, the chirality is retained (excludes intermolecular processes via ionic or radical intermediates which would readily invert/racemize). 

Szocs [246] has studied the rate of decay at higher temperatures of samples subjected to excess pressure. At a pressure of 1 atm and a temperature of 100°C, the rate of decay is more than 100 times that at 100°C and 5000 atm. If the pressure is 5000 atm, free radicals can still be observed at 160°C. According to Szocs, the large influence of pressure indicates that the decay is due to intermolecular migration of free radicals. The radicals interact with neighbouring segments and are transferred and disappear by combination when then they meet other radicals. 

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

Thermolysis of the coenzyme Bi2-model complex, (10), led to an equilibrium mixture of the BnCo species and the Bn-migration macrocyclic complex, Scheme 7, without formation of bibenyl. " Surprisingly, the reaction proceeds intermolecularly via Co—C bond homolysis, leading to the free radicals Bn and Co . The latter, stable species is termed a persistent radical, thus allowing for the observed chemical selectivity of greater than 10 for the observed equilibrium species over bibenzyl. These observations represent a seminal example of Fischer s internal suppression of fast reactions, which requires that one of the radical intermediates formed be more persistent than the others, and the persistent and transient species must be formed at equal rates. Both criteria are met, as the Co complex is quite stable and Co—C homolysis ensures that both radicals are formed simultaneously. 

Besides allyl, benzyl, and methyl groups also migrate/In every case, the reaction is intermolecular with an induction period involving equilibration of the allylic termini (in that case) and is inhibited by thiophenol, and related chain-transfer agents clearly indicating free radical processes. 

Orthopedic UHMWPE has a molecular mass of 2 10" a.m.u. or higher. In this state, the polymer has a high viscosity, even in the molten state. Thus, macroradicals have very low mobility, either in the molten or in the solid state, while the H radical, which has a diameter smaller than 1 A, can migrate in the polymer mass, even in the crystalline phase, where distances between C atoms are in the order of 4 A. H radicals resulting from Reaction 3 are very mobile and they can extract other H atoms intermolecularly or intramolecularly producing hydrogen, following Scheme 2. 

The degradation reactions involved in PS include random scission (which reduces the molecular weight of the polymer), depolymerization (which yields monomer), intramolecular transfer reaction (which produces dimer, trimer, etc.), and intermolecular transfer reaction which reduces the molecular weight of the polymer. The initial degradation products from PVC are Cl radicals and HCl (Owen 1984 Ahmad and Mahmood 1996). The structure and composition of PVC and PS and the interaction of various products formed may give rise to some cross products formed from the radicals or molecules which migrate across these phase boundaries and can play an important role in the degradation of blends. 

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