Direct hydrogen abstraction

The value of CM has been determined by a number of groups as 6x 10 5 (Table 6.14.1. " However, the mechanism of transfer has not been firmly established. A mechanism involving direct hydrogen abstraction seems unlikely given the high strength of vinylic and aromatic C-M bonds. The observed value of Cy is only slightly lower than Ctr for ethylbenzene ( 7x 10"5). w... 

When the reactant is cyclohexene, in the first step of Scheme 26, the direct hydrogen abstraction for the allylic oxidation (path 1) competes with the electron transfer (from the alkene to the M-oxo complex) for the epoxidation (path 2). Because the manganese complex is more readily reduced than the chromium... 

Mechanism II. Amines may be able to stabilize free radicals formed during the reaction between the fuel and oxygen. This could be done either by forming an adduct between a fuel radical, R , and the amine, followed by reaction to form stable products (28), or by direct hydrogen abstraction (25, 28, 29) to form a radical, T, from the inhibitor, which does not take any part in the fuel-oxygen chain reaction ... 

For example, no deuterium isotope effect is observed on the rate constant when CH2OD is substituted for CH2OH (Grotheer et al., 1988 Pagsberg et al., 1989), as would be expected if a direct hydrogen abstraction were occurring. 

In competition with the formation and subsequent reaction of the OH-acid complex, there is also a direct hydrogen abstraction channel ... 

The oxidation of hydrazine derivatives with diethyl azodicarboxylate is of particular interest because it involves direct hydrogen abstraction. The oxidation of keto hydrazones with lead tetraacetate leads to azoacetates, presumably by a free radical mechanism. 

Carbonyl halides containing a hydrogen atom are likely to undergo reaction in the troposphere with hydroxyl radicals, which dominate the day-time chemistry. Reaction 37 of carbonyl halides CHXO (where X = F or Cl) may occur by two mechanisms (i) direct hydrogen abstraction or (ii) radical addition to carbonyl. 

The best evidence for a CT process rather than direct hydrogen abstraction involves the values of kT s-butyl- and ferf-butylamine display much the same value 156> triethylamine and ferf-butyldimethylamine are equally reactive and some 50 times more so than primary amines 155>. Thus the rate constant for reaction is independent not only of the type of C—H bond a to the nitrogen but also of the presence or absence of a-hydrogens. Such evidence demands that abstraction of an a-hydrogen not be involved in the rate-determining quenching reaction. Moreover, the relative reactivity of amines (tertiary > secondary > primary) is proportional to the ease with which they are oxidized. 

A final possibility for the radical anion is fragmentation to yield the relatively stable aryl radical and an anion. The end result is either reduction (whether via direct hydrogen abstraction or via a second electron transfer - the radical is usually easier to reduce than the original substrate - and protonation of the resulting anion) or substitution. 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

For these systems, direct hydrogen abstraction by benzophenone triplets was observed in benzene whereas in a polar solvent electron transfer and hydrogen-atom abstraction were observed. Electron transfer followed by an intramolecular proton transfer was observed in these systems although such proton-transfer reactions are not observed in unlinked systems of primary and secondary amines. The observed differences between the linked and unlinked systems have been attributed to the dependence of electron transfer, proton transfer, and hydrogen transfer on mutual distance and orientation. In the unlinked systems, rotational and translational motion of two reacting molecules are usually much faster than those in linked systems. 

Stereoselective radical addition on the exo-face of levoglucosenone 250 leads to the C-linked dimer 251 in a modest 26% yield. The expected competing reaction is the direct hydrogen abstraction by the initial radical species. Stereoselective hemiketal and ketone reductions afford the 1,6-anhydro derivative 252, which is deprotected and opened to give the final C-ana-log of a /3-(1 4) disaccharide 253. 

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. 

Equation (15) does not require an unusual reversibility of oxygen addition, direct hydrogen abstraction by oxygen, or generation and decomposition of a hydroperoxide-containing radical. It is also consistent with the observed differences between alcohols and ethers. The reported rapid unimolecular decomposition of hydroxyalkylperoxy radicals to HOO radicals and a carbonyl compound in aqueous solution under mild conditions appears to lend strong support to this proposal [25]. The production of HOO radicals has been proposed to explain inhibition in a variety of hydrocarbon oxidations [10, 27]. 

Owing to their relatively low ionization energies (IE) of ca 8.0-8.5 eV, phenols are also good electron donor solutes. Recent experimental studies of phenols in non-protic solvents showed that ionized solvent molecules react with phenol to yield not only phenol radical cations by electron transfer, but also phenoxy radicals by hydrogen transfer. An obvious question is whether, under these conditions, the latter radicals were formed from ionized phenols rather than by direct hydrogen abstraction, because proton transfer reactions could be facilitated upon ionization. This also raises a question about the influence of solvent properties, both by specific and non-specific interactions, on the mechanism and kinetics of deprotonation processes ... 

Benzophenones and thioxanthones can also work through a direct hydrogen abstraction reaction in the presence of H donor such as alcohols or THF (10.49). 

By contrast, 0-demethylation is considered to begin with direct hydrogen abstraction because of a relatively large kinetic isotope effect (kHlko 5-14) (135-137). The isotope effect for the deprotonation of an ether cation radical to... 

This latter product may arise via direct hydrogen abstraction or via a single-electron oxidation of the radical to the cation followed by elimination of a proton. Results of CYP17A catalyzed oxidation of substrates bearing deuterium labels at the Cl6, Cl7 and the methyl group a to the ketone are in agreement with this mechanism . The 17-0-acetyl-testosterone is probably best explained as... 

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