Hydrogen abstraction by radicals

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). 

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

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. 

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. 

In alkaline solution, the anion of acetone will be produced by proton transfer from the radical of 2-propanol to the hydroxide ion (R3). Not only electron attachment of the solvated electron (R7), but also electron transfer from the anion of acetone (R4) will be a significant process to form the anion of the halogenated carbons. The radical of 2-propanol will be reproduced due to hydrogen abstraction by radicals formed by the dissociation of an anion of the halogenated carbons (R6). This is a chain reaction and causes effective degradation. On the contrary, dissociative electron attachment is the only process to decompose halogenated hydrocarbons in the pure alcohol solution. 

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

Figure 1.8 Preferred site of attack in hydrogen abstraction by various radicals. 1.3.4 Stereoelectronic Factors...

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

Chain propogation is by hydrogen abstraction by rominc radicals. The weakest C-H bond is broken, that the allylic bond, to give the most stable radical (2)... 

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