Dioxygen hydrogen-atom abstraction

The bimolecular reaction with hydrogen atom abstraction by dioxygen from the weakest C—H bond. Ester as a polar medium accelerates this reaction due to solvation of the polar TS. 

The reduction of o- and />-nitrotoluene and /7-ethylnitrobenzene in DMF in the presence of dioxygen to o- and />nitrobenzoic acid and / -nitroacetophenone plus - p-nitrophenyl)ethanol, respectively, has been suggested [128] to proceed via a proton or hydrogen atom abstraction from the side chain followed by reaction of this species with superoxide ion and/or dioxygen to the isolated products. 

Scheme 12. A mechanism for dioxygen evolution by the OEC as proposed by Babcock that employs the hydrogen atom abstraction concept. [Reproduced with permission from (182). Copyright 1998, the American Chemical Society.]...

Scheme 13. A mechanism for dioxygen evolution by the OEC involving hydrogen atom abstraction as proposed by Pecoraro.

N-alkylidene-2-hydroxyanilines (65) can be oxidized by dioxygen in the presence of CuCl in pyridine to 2 substituted benzoxazoles (66) in good yield [80]. The phenoxy radical formed via hydrogen atom abstraction attacks the adjacent double bond to bring about ring closure rather than dimerization ... 

In accord with this mechanism, free peroxyl radical of the reaction product hydroperoxide activates the inactive ferrous form of enzyme (Reaction (1)). Then, active ferric enzyme oxidizes substrate to form a bound substrate radical, which reacts with dioxygen (Reaction (4)). The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

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