Hydrogen abstraction by carbon-centred radicals

Hydrogen Abstraction by Carbon-centred Radicals. . Hydrogen Abstraction by Heteroatom-centred Radicals... 

The kinetics of hydrogen abstraction by the CF3COC(C2F5)2 radical have been smdied by ESR with the degree of delocalization calculated using the MNDO/PM3 method. All aspects of the solution structure, stability, and chemistry of carbon-centred fluorine-containing free radicals have been discussed. ... 

Oxidation of aliphatic ketones in trifluoroacetic acid leads to hydrogen abstraction by the carbonyl oxygen radical-cation fonning a carbon radical, then further oxidation of the radical to the carbocation and migration of this centre along the carbon chain by a series of hydride transfer steps. Long chain ketones yield a mixture of alcohol trifluoToacetates by reaction of the carbocation centres with the solvent [3]. 

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). 

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

The initial propagation step in the oxidation of fuel is it s conversion from unreactive hydrocarbon to reactive carbon centred radical by hydrogen abstraction. The most important abstracting species are OH, H and O radicals... 

As depicted in Figure 33.28, A" VPA is not formed by dehydration of 4- or 5-hydroxy valproic acids, which are, with the glucuronide conjugate, the major metabolites of valproic acid. The mechanism is proposed to involve an initial hydrogen abstraction to generate a transient free radical intermediate. It has been demonstrated that the carbon-centred radical was localized at the C4 position. The radical undergoes both recombination (which yields 4-hydroxy valproic acid) and elimination (which produces... 

Several methods have been employed to generate the required carbon-centred radical, many depending on the initial formation of oxy or methyl radicals, which then abstract hydrogen or iodine from suitable substrates, as illustrated below. The re-aromatisation of the intermediate radical-cation is usually brought about by its reaction with excess of the oxidant used to form the initial radical. 

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