Hydrogen abstraction generation

The decomposition by the iron(II) salt of hydroxylamine-O-sulfonic acid in the presence of formamide, alkylformamides, or A -alkylacetamides provides the amino radical cation +NH3 which, by hydrogen abstraction, generates carbamoyl and a-Wamidoalkyl radicals. Both kinds of radicals selectively attack protonated quinoxaline. 

One additional H abstraction reaction must be mentioned. Internal 1,5 (Reaction 54) or 1,6 (Reaction 55) hydrogen abstraction generates an alcohol and a radical (21) in a position that may or may not be normal for autoxidation. Intramolecular H abstraction involving a six-membered transition state (Reaction 55) has been identified in saturated alkyls with long side chains (304). Occurrence of the corresponding reaction in unsaturated fatty acids would produce oxidation at sites previously attributed to HO attack (314). 

This proposal, however, has been criticized on the basis of transition state theory (74). Hydroperoxy radicals produced in reaction 23 or 24 readily participate in chain-terminating reactions (eq. 17) and are only weak hydrogen abstractors. When they succeed in abstracting hydrogen, they generate hydrogen peroxide ... 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

If selenide additions are carried out in the presence of tri- -butylstannane, the radical generated by addition can be reduced by hydrogen abstraction. The chain is then continued by selenide abstraction by the stannyl radical. This leads to nonselenated addition and cyclization products. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

It was found that the sulfate radical anion S04 produced photochemically in Scheme (46) is responsible for generating the cellulose derivative macroradicals by hydrogen abstraction, which added the vinyl monomer to produce the grafted copolymer. The main disadvantage of this method is the production of large quantities of undesirable homopolymers in addition to the grafted copolymers. 

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

Free radicals can be generated on the cellulose chain by hydrogen abstraction, oxidation, the ceric ion method, diazotization, introduction of unsaturated groups, or by y-irradiation. 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

Phosphinyl radicals (e.g. 103-107) arc generated by photodecomposition of acyl phosphinates or acyl phosphine oxides (see 3.3.4.LI)282,466 474,473 or by hydrogen abstraction from the appropriate phosphine oxide.467... 

To summarize under favorable conditions the acidity of a-hydrogens facilitates the generation of a-sulfoxy and a-sulfonyl carbanions in thiirane and thiirene oxides and dioxides as in acyclic sulfoxides and sulfones. However, the particular structural constraints of three-membered ring systems may lead not only to different chemical consequences following the formation of the carbanions, but may also provide alternative pathways not available in the case of the acyclic counterparts for hydrogen abstraction in the reaction of bases. 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

Entries 18 to 19 pertain to cyclizations of electrophilic radicals generated by oxidations. Entry 18 is the prototype for cyclization of a number of more highly substituted systems. The reaction outcome is consistent with oxidation of the less-substituted enolic position followed by a 6-endo cyclization. The cyclized radical is then oxidized and deprotonated. In Entry 19, the vinyl radical formed by cyclization is reduced by hydrogen abstraction from the solvent ethanol. 

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