Radical reactions hydrogen atom abstraction

The addition of thiols to olefins (thiolene reaction), to form thioethers, is a well-known reaction. The process can occur by either free-radical or ionic mechanisms. The free-radical reaction can be initiated thermally via a peroxide or by UV irradiation with benzophenone. The initiation step involves the formation of a thiyl radical by hydrogen atom abstraction. Both of these species are capable of starting polymer chains (Table 2.30). 

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (commonly /-butyl peracetate or /-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hydrogen atom abstraction and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

Recently, Kim has reported a modified Hofmann-Loffler-Freytag reaction under reducing conditions [70], Treatment of alkyl azides with Bu3SnH afforded N-tributylstannyl radicals. These radicals proved to be more reactive than ordinary aminyl radicals towards hydrogen atom abstraction presumably because of their higher nucleophilicity. Two examples are shown in Scheme 14. In the first one (Eq. 

Additional proof for the difference between Gif- and radical bromination was obtained from bromination of cyclohexyl bromide. Radical induced hydrogen atom abstraction occurrs at the p-position of bromoalkanes and in accordance with "Skell-Walling effect" results in the formation of the /ra 5-l,2-dibromide60. This was confirmed qualitatively for the radical bromination of cyclohexyl bromide. In contrast, this was not the case in the GoAggll bromination reaction. The ra 5-1,2-dibromide was found to be only a minor product, while trans-lA- and cw-l,3-dibromocyclohexanes were the major products. Thus all this data illustrates the different nature of Gif-reactions and radical reactions. Here it is worth mentioning that tertiary C-H bonds appeared to be the least reactive in the bromination process, as is found in the Gif- oxidation reactions. 

The greater selectivity of bromination can be illustrated by comparing the reaction coordinate diagrams for the formation of primary and tertiary free radicals by hydrogen atom abstraction from 2-methylpropane with bromine versus chlorine radicals (Figure 4.20). 

A radical-type hydrogen atom abstraction/oxygen rebound reaction occurs, the net effect of which is hydroxylation and re-formation of the iron(III) form of the enzyme. 

While phenyliodine(ni) diacetate and TMSN3 are known to form PhI(N3)2 in solution and homolyticaUy cleave to the azide radical (see this section, below), the combination is sensitive to decomposition at elevated tanperatures. A more stable azide source was developed by Zhdankin and cowoikers. Azidoiodinane 11 is thermally stable and in fact needs a radical initiator to affect the radical reaction. Following radical initiation, hydrogen-atom abstraction leads to a carbon-centered radical... 

Although phosphino radicals (R2P ) have been generated by photolysis of tertiary phosphines such as PPh3 in a biological system reactions of PR3 compounds via phosphino radicals are likely to be more important for PH3 and primary and secondary phosphines. For these, phosphino radicals can be generated by radical-initiated hydrogen atom abstraction ... 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

The most direct evidence that stereoelectronic effects are also important in these reactions follows from the specificity observed in hydrogen atom abstraction from conformationally constrained compounds,18 60 C-H bonds adjacent to oxygen113"118 or nitrogen110 and which subtend a small dihedral angle with a lone pair orbital (<30°) are considerably activated in relation to those where the dihedral angle is or approaches 90°. Thus, the equatorial H in 20 is reported to be 12 times more reactive towards /-butoxy radicals than the axial 11 in 21.115... 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

Radicals can react with bases via hydrogen atom abstraction or, more commonly, by addition to the pi bonds in the heterocyclic nucleobases (Scheme 8.1). These reactions have been extensively studied in the context of hydroxyl radical (HO ), which is generated by y-radiolysis of water. When DNA is exposed to the hydroxyl radical, approximately 80% of the reactions occur at the bases. Many base damage products arising from the reaction hydroxyl radical with DNA have been characterized (Fig. 8.2). ... 

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