Abstraction by radicals

The hydrogen atoms of the -CH2- group located between the two double bonds of linoleic ester (Lin-H) are especially susceptible to abstraction by radicals. 

In many cases both Kolbe and non-Kolbe products are isolated from a reaction. Carboxylic acids with an a-alkyl substituent show a pronounced dual behaviour. In these cases, an increase in the acid concentration improves the yield of the Kolbe product. An example of the effect of increased substrate concentration is given in Kolbe s classical paper [47] where 2-methylbutyric acid in high concentration affords mostly a dimethylbexane whereas more recent workers [64], using more dilute solutions, obtained both this hydrocarbon and butan-2-ol. Some quantitative data is available (Table 9.2) for the products from oxidation of cyclohexanecar-boxylic acids to show the extent of Kolbe versus non-Kolbe reactions. The range of products is here increased through hydrogen atom abstraction by radical intermediates in the Kolbe reaction, which leads to some of the monomer hydrocarbon... 

The alkylation of quinoline by decanoyl peroxide in acetic acid has been studied kineti-cally, and a radical chain mechanism has been proposed (Scheme 207) (72T2415). Decomposition of decanoyl peroxide yields a nonyl radical (and carbon dioxide) that attacks the quinolinium ion. Quinolinium is activated (compared with quinoline) towards attack by the nonyl radical, which has nucleophilic character. Conversely, the protonated centre has an unfavorable effect upon the propagation step, but this might be reduced by the equilibrium shown in equation (167). A kinetic study revealed that the reaction is subject to crosstermination (equation 168). The increase in the rate of decomposition of benzoyl peroxide in the phenylation of the quinolinium ion compared with quinoline is much less than for alkylation. This observation is consistent with the phenyl having less nucleophilic character than the nonyl radical, and so it is less selective. Rearomatization of the cr-complex formed by radicals generated from sources other than peroxides may take place by oxidation by metals, disproportionation, induced decomposition or hydrogen abstraction by radical intermediates. When oxidation is difficult, dimerization can take place (equation 169). 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

Zavitsas AA, Chatgilialoglu C (1995) Energies of activation. The paradigm of hydrogen abstraction by radicals. J Am Chem Soc 117 10645-10654... 

The rates of hydrogen atom abstractions by radicals are subject to the same factors that control rates of alkene additions [130]. Both enthalpic and polar... 

Further applications of the radical-induced reactions in the Kolbe decarboxylation reactions involve hydrogen abstraction and coupling with in situ generated radical species from additives. For example, hydrogen abstraction by radical intermediates derived from paraconic acids (XLVIII) occurs selectively in MeOH-MeONa-Fe powder—(Pt) and MeOH—MeONa—(C) systems [Eq. (22)] [20]. The effect of Fe powder is significant since butenolides (L) are obtained exclusively in an MeOH-MeONa-(Pt) system. 

Applying M /halogenid catalysts, two different mechanisms are responsible for the start of the oxidation reactions [14, 19] the reaction of the substrates to radicals and the direct hydrogen abstraction by radicals X formed from XCo "(OAc)2. 

Variations Halogens are also commonly abstracted by radicals, and the reaction favors the weakest C-X bond, I > Br > Cl > F. Sulfur and selenium are also capable of being abstracted by radicals, with a transition state reminiscent of an Sn2 substitution. 

Most organic compounds have more than one type of C-H bond, and hydrogen abstraction by radicals usually proceeds in an unselective manner, giving complex mixtures of products. An exception is when the abstraction is intramolecular, when for example a long-chain alkoxyl... 

The hydrogen atoms situated in the a position against the etheric oxygen atoms of the polyetheric chain are very susceptible to radical attack (hydrogen abstraction by radical mechanism), giving transfer reactions [1, 12, 18]. 

Hydrogen chain transfer reactions, which may occur as intermolecular or intramolecular processes, as depicted in Scheme 4.1. This hydrogen abstraction by radicals leads to the formation of olefinic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

Selective hydrogen atom abstraction by radicals. Giesc and Curran2 note that radicals such as 4 abstract hydrogen atoms in a ratio that is remarkably similar to that observed by Cram for reduction of ketones by lithium aluminum hydride. 

A typical example for stereoselective reactions of acyclic radicals occurs during the reaction of alkene 3 with BuHgCl/NaBH4. The reactions proceed by addition of a tert-hvXy radical to 3 and subsequent stereoselective H atom abstraction by radical 4 that leads to 5 as main product (Scheme 1). 

Figure 4. Transition state energies of the H-abstraction by radical 7...

The reaction paths describing the formation and subsequent reactions of the active radical (phenyl) via abstraction by radical species (e.g. OH or Cl, H, or O) are important to include in mechanisms because sufficient thermal energy is available to overcome the endothermicity which is around 10 kcal mof ... 

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