A-Hydrogen atom abstraction

The oxidation of di-2-chloroethyl ether is first-order with respect to ether, but is autocatalytic and chloride ion is liberated. A hydrogen atom abstraction process, similar to that above, probably takes place, viz. 

Nature has designed the active sites in the enzymes GO and glyoxal oxidase in order to perform a hydrogen-atom abstraction reaction from the a-carbon atom of an O-bound alcoholate (or aldehyde) in the rds. The essence of this chemistry is depicted in Fig. 8. 

The O-dealkylation of ethers, while not as frequently encountered as N-dealkylation in drug metabolism studies, is still a common metabolic pathway. Mechanistically it is less controversial than N-dealkylation in that it is generally believed to proceed by the HAT pathway, i.e., a-hydrogen atom abstraction followed by hydroxyl radical rebound, and not a SET pathway (Fig. 4.58). The product of the reaction is unstable, being a hemiacetal or hemiketal depending on the number of hydrogens on the a-carbon, which dissociates to generate an alcohol and an aldehyde or ketone. 

The rate constants of a-hydrogen atom abstraction from four hydrocarbons by cumylperoxy, tetralylperoxy, and 9,10-dihdroanthracyl-9-peroxy radicals and by the normal chain-carrying peroxy radicals are compared in Table VIII. The results show that the reactivities of peroxy radicals are affected by the nature of the organic group. The relatively low propagation constant for the oxidation of pure cumene may be caused by the low reactivity of the cumylperoxy radical. 

Somewhat analogous reactions would be expected for the reaction of ethylene with 02 ions but the observed reaction rate is lower than for propene, suggesting that the reaction pathway may be controlled by the C—H bond energies. For reactions of propane and 1-butene with 02, oxygenated compounds of the same carbon number as the reactants were produced. The initial step is thought to involve a hydrogen atom abstraction from a secondary carbon atom. 

Figure 10.9. Orbitals for a hydrogen atom abstraction reaction. The middle is the orbital diagram for the transition state for H transfer.

In Saveant s terminology this process is called chemical catalysis with electrochemical regeneration while Shono uses the expression heteromediatory system . In the following equations the principle is exemplified in the simple case, in which the redox step is connected with a hydrogen atom abstraction (oxidation) ... 

The A-hydroxyphthalimide is anodically oxidized to the phthaliraide iV-oxyl radical which subsequently performs a hydrogen atom abstraction from the substrate. Regeneration of the active radical takes place at the anode in the presence of a base like pyridine or 2,6-lutidine (mechanism B, Sect. 2.3) (Eq. (113)). The turnover... 

With alkyl radicals higher than methyl, the rate constant for addition to NO is pressure independent for all the conditions studied. However, a hydrogen-atom abstraction is possible and the reactions are... 

Benzene has been observed as a product of both the OH- and NQz-initiated oxidation of cyclohexa-1,3-diene, indicating a hydrogen atom abstraction in both reactions. In the presence of NO and molecular oxygen, the N02-initiated reaction leads to removal of cyclohexa-1,3-diene by reaction with both NO2 and OH. Formic acid was detected as a product in this system, providing evidence for significant formation of stabilized C6o -hydroxyperoxy radicals from the OH-initiated chemistry, and their subsequent reaction with NO. Mechanisms consistent with the observations have been proposed.80... 

An alternative possible route for the formation of carbamoyl radicals may involve a hydrogen atom abstraction from formamide by the excited acetone molecule in its triplet, state (45, 57). Ketonic compounds at this state of excitation mainly if the excitation is of an n - -7r transition, are known to be hydrogen atom abstraction agents, due to the electron deficient oxygen in the excited state. Thus, the formation of carbamoyl radicals through the second route may be summarized by the following scheme ... 

The light-induced addition of 2-pyrrolidone to olefins when induced directly by light leads to traces only of the desired 1 1 adducts. On the other hand, when acetone is used as a photosensitizer for this reaction it results in relatively high yields of the 1 1 adducts and may be exploited for synthetic purposes in some cases. As in the case of y-butyrolactone the initiation step can be regarded involving a hydrogen atom abstraction process from the lactam by the excited acetone molecule,... 

The idea of radicals selectively abstracting hydrogen atoms from C—H bonds weakened by this n-a interaction was also used by Deslongchamps et al. (1972a) in their initial treatment of the ozonolysis of [1] and [2], Although the balance of evidence now points to this reaction being one of hydride abstraction, at the time a hydrogen atom abstraction mechanism was entirely reasonable. 

With other Ta complexes, alkylidene transfer does not occur. Instead, a-hydrogen atom abstraction gives carbyne compounds ... 

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