Radical reactions atom abstraction

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

The intramolecular hydrogen atom transfer occurs with lower activation energies in comparison with the intermolecular transfer (see the values of Ee for both types of reactions in Table 6.11). The values of the activation energies of intramolecular radical H-atom abstraction calculated by the IPM method are given in Table 6.15. 

REACTION WITH FREE RADICALS HYDROGEN ATOM ABSTRACTION AND ONE- OR THREE-ELECTRON BONDING... 

In contrast to eq. 2.29, eq. 2.30 shows the oxidative conversion of aldehydes (62) to amides (63) via acyl bromides with NBS/AIBN/R2NH under refluxing conditions in CC14 [74]. The reaction comprises of the abstraction of the formyl hydrogen atom by the succinimidyl radical, bromine atom abstraction from NBS by the acyl radical, and lastly,... 

Reaction (6-14a) is the initiation step, while reactions (6-14b) and (6-l4c) are atom abstraction propagation reactions. Atom abstraction reactions in free-radical polymerizations are called chain transfer reactions. They are discussed in some detail in Section 6.8. 

Unlike radical addition and fragmentation reactions, atom abstraction has no common counterpart in carbocation chemistry. 

The most important reactions of radicals are atom abstraction and addition to multiple bonds. Atom abstraction is almost always hydrogen or halogen. 

Organic compounds can generate the initiators of free radical sequences through the primary photochemical processes homolytic dissociation into radicals, hydrogen-atom abstraction, photoionization, and electron transfer reactions. The homolytic dissociation reactions are limited to compounds containing relatively weak bonds (<98 kcal), such as sulfides, peroxides, and some halides and ethers. Representatives of all of these classes of compounds are certainly present in seawater, but the limited information on the qualitative and quantitative aspects of their occurrence does not allow for an estimate of their importance in the promotion of free radical reactions. The same is true for electron transfer reactions, which may be an important photochemical process for organic transition metal complexes. 

A procedure widely applied for effecting O-atom insertions (olefin epoxidation, hydrocarbon hydroxylation or ketonization) with cobalt(II) catalyts is the addition of sacrificial 2-methylpropanal. It is converted via H-atom abstraction to an acyl radical, which upon reaction with O2 produces an acylperoxyl radical. H-atom abstraction by the latter leads to a hydroperoxide, which is capable of effecting O-atom insertions via free-radical chain reactions. The sacrificial aldehyde is lost via oxidation to an acid or an ester. 

The mechanisms and enantioselectivities of the Rh2L4 (L = formate, Al-methyl form-amide, S-nap)-catalysed intramolecular C-H aminations of 3-phenylpropylsulfamate ester have been studied with BPW91 DFT computations. The Rh2(II,II)-catalysed reactions start with the oxidation of the Rh2(n,n) dimer to a triplet mixed-valent Rh2(II,ni)-nitrene radical, which facilitates radical H-atom abstraction. However, a direct C-H bond insertion is postulated for the Rh2(HCOO)4-catalysed reaction. The Rh2(Al-methylformamide)4-catalysed reaction is a two-step process and so is the mechanism of the Rh2(5-nap)4 (41)-catalysed reaction of 3-phenylpropylsulfamate ester. The mechanistic proposal is supported by the calculated 94.2% ee which is in good agreement with the observed 92% ee ... 

These questions may be pertinent to every chapter in this book. For this chapter specifically, some additional questions arise What does radical-mediated mean Can a radical hydrogen-atom abstraction meet the requirements of C—H activation Does it depend on the particular mechanism of the reaction, the C—H BDE, or the timing of the radical abstraction step How can a researcher be sure that absolutely no transition metals are present and active during the reaction What if a transition metal is an additive, but only serves as an electron shuttle and does not specifically cleave to or react with the C—H bond ... 

The Fan group and Nicholas group independently propose the radical mechanism in the amination reaction they developed. While the source of the iodine-centered radical differs, the mechanistic concept is the same. An N-iodo species can homolytically cleave to a nitrogen-centered radical. Hydrogen atom abstraction from the benzylic C—H bond and iodine atom abstraction from the A-iodo species form a benzylic iodide. Substitution of the iodide with the amine yields the product. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

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