Oxidative addition atom abstraction

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

The isolation, separation, and chemistry of dithio- and perthioaryl-carboxylate complexes of Ni(II), Pd(II), and Pt(II) were reported in two complementary reports (381, 415). The perthiocarboxylate complexes have also been obtained by oxidative addition of sulfur to the dithiocar-boxylic acid complexes. The abstraction of the sulfur atom adjacent to carbon by PPha was again observed, and rationalized as follows. 

The initiator-derived radical products generate a-tocopheroxyl radicals (2) from a-tocopherol (1). The radicals 2 are further oxidized to ort/io-quinone methide 3 in a formal H-atom abstraction, thereby converting benzoyloxy radicals to benzoic acid and phenyl radicals to benzene. The generated o-QM 3 adds benzoic acid in a [ 1,4] -addition process, whereas it cannot add benzene in such a fashion. This pathway accounts for the observed occurrence of benzoate 11 and simultaneous absence of a 5 a-phenyl derivative and readily explains the observed products without having to involve the hypothetical C-centered radical 10. 

The peroxyl radical of a hydrocarbon can attack the C—H bond of another hydrocarbon. In addition to this bimolecular abstraction, the reaction of intramolecular hydrogen atom abstraction is known when peroxyl radical attacks its own C—H bond to form as final product dihydroperoxide. This effect of intramolecular chain propagation was first observed by Rust in the 2,4-dimethylpentane oxidation experiments [130] ... 

The coordination of dioxane and subsequent oxidative addition to the catalytic species (step (a) in Scheme 20.16) probably proceeds after the oxygen atom coordinates to the rhodium (47), followed by abstraction of a hydrogen atom. The cationic species (48) then rearranges to a complex in which the dioxane is bound to the rhodium via the carbon atom (40) (Scheme 20.17) [60]. 

At night, when the sun s radiation is minimal, the dominant VOC oxidant is nitrate radical (NO3 ). The chemistry initiated by NO3 differs from that initiated by HO radical in that NO3 prefers to react with unsaturated compounds via addition to one of the carbons of the 7t-system, rather than by hydrogen atom abstraction ... 

Once the initial benzene ring has cyclized, it can undergo sequences of H-atom abstraction followed by acetylene addition, to yield PAHs. This is known as the H-abstraction-C2H2-addition (HACA) process, proposed by Frenklach and Wang. As an aromatic species aggregates to a size over 500 amu, it adopts a particulate form and can coalesce with other PAHs to further increase in size. When many of these particles agglomerate, they form soot. Efforts to minimize soot production are widespread. Notably, decreasing the carbon content relative to oxidizer concentration in a fuel/oxidizer mixture decreases the amount of soot formed. 

The methoxy radical may subsequently react to form formaldehyde (H atom abstraction) or methanol (H atom addition). The sequence of reactions (R15) through (R17) is strongly chain branching and serves to build up a radical pool. Once this radical pool is established, another chain-branching oxidation route becomes dominating. Methane consumption now occurs mainly by the reactions [254]... 

Hydride abstraction from alkylamines forms the corresponding imi-nium ions, whose coordination to transition metals gives either a ir-complex or cr-bonded three-membered ring (Scheme 15) (26). Ligation of the cationic dehydro amines to Rh is aided by substantial electron donation from the metal to the electron-deficient carbon atom to produce the Rh(IH) complex with a covalent C—Rh bond and an N—Rh dative bond, consistent with the long C—N bond (1.467 A) and the small H— C( 1)-—N—C(2) dihedral angle (124.6°) as well as the noncoplanarity of the CH2—CH bond and a possible CH—NH2 plane seen in the allylam-ine oxidative addition product (24). 

Major destruction routes of OH radicals are the addition to olefins, the 11 atom abstraction from olefins and aldehydes, and the reaction with C( > Another radical, hydroperoxyl (H02), has been considered as a major oxidizing agent for NO and to a lesser extent for hydrocarbons. The HO, radicals are probably formed by the photolysis of formaldehyde [see Section VI1-4, p. 277]... 

The oxidation rates for bromoform were slower than the oxidation rates of unsaturated chlorinated aliphatic compounds, including the TCE. Because the hydroxylation rate constant of TCE is 109 Mr1 s 1 and the hydrogen abstraction of bromoform is 1.1 x 108 M 1 s aromatics and alkenes react more rapidly by hydroxyl addition to double bonds than does the more kinetically difficult hydrogen atom abstraction. No oxidative destruction of chloroform by Fenton s reagent was experimentally observed an explanation for this is that both H202 and Fe2+ have rate constants about one magnitude higher with respect to hydroxyl radicals than chloroform. 

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