Homolytic oxidants

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

Heterolytic oxidations require releasable coordination sites on the metal, involve strained metallocyclic intermediates and are highly selective. By contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate association and are less selective. 

The oxidation of hydrocarbons by cobalt(lll) acetate has been thoroughly investigated, due to its relevance to industrial homolytic oxidation processes.56 361 547 Radical intermediates are produced from one-electron oxidation of hydrocarbons according to an electron transfer or an electrophilic substitution mechanism previously described in equations (200)-(203). These oxidations are dramatically accelerated by the presence of strong acids or halide salts. 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

Saturated hydrocarbons are homolytically oxidized by complexes (205) into alcohols, ketones and t-butyl peroxide products. The hydroxylation reaction occurs at the more nucleophilic C—H... 

In this context, homolytical oxidation reactions (autooxidation) proceeding by the free-radical mechanism seem to be the most suitable model. In autooxidation reactions ROOH dissociation catalysis (in the most reduced case, H202) plays the key role, but does not eliminate selectivity. This is the most urgent problem of homolytical oxidation. 

In the autooxidation system, the initial stage only (dissociation) is catalyzed. Other stages are usually insensitive or less sensitive to the catalyst action. This situation strictly limits the abilities of catalyst selection for selective homolytical oxidation. 

For high-temperature homolytical oxidation of hydrocarbons, it should be taken into account that the difference in reactivities of different C—H-bonds in relation to radical attack becomes insignificant, which provides for formation of many different reaction products. 

We have seen in the first section how the concepts of electron and ligand transfer via 1-electron changes provides a basis for the understanding of homolytic oxidation mechanisms. Similarly, the concepts of substrate activation by coordination380 to metal complexes and by oxidative addition381 386 provide a basis for discussing heterolytic mechanisms. Examples of the former are the activation of hydroperoxides (Section III.B.2) and olefins (Section III.D) to nucleophilic attack by coordination to metal centers. 

Since copper(II) is a known homolytic oxidant and since nitrobenzene gives an essentially identical reaction constant (p+ of -0.45 versus -0.48), the logical implication is that nitrobenzene also acts as a homolytic oxidant under these conditions. 

Oxidation of 1-(4-methoxyphenyl)-2-( -substituted phenyl) et hands, 6, by Cerium(IV). Dehydration prevented the oxidative-cleavage study of l-(4-hydroxyphenyl)-2-(4,-substituted phenyl)ethanols, 4, (8). As an alternative study, the oxidation of these phenolic compounds using the homolytic oxidant ceric ammonium nitrate (CAN) in an acidic environment was initiated. However, preliminary oxidations of these compounds were unsuccessful due to the apparent formation of complexes of cerium(IV) with the phenolic hydroxyl groups. 

The homolytic oxidative reagent, nitrobenzene, either abstracts an electron from a hydroxide ion as reported by Ashby and coworkers (21,22) (Scheme 1 of Figure 3) or from the benzylic hydroxyl group (Scheme 2 of Figure 3). 

Homolytic oxidations involve free radical intermediates, are catalyzed by first row transition metals and characterized by one-electron redox steps. Some examples are shown in Figure 2. These metals vary considerably in their redox properties. However, the redox potentials change substantially with changes in solvent and changes in the ligand bound to the metal ion. 

Their behaviour in oxidation is largely dependent on the type of metal used as catalyst. With one-electron redox systems homolytic oxidation prevails and hydroperoxides are simply decomposed in the catalytic system to generate radical species. Figure 7, also known as the Flaber-Weiss mechanism, shows the intermediates formed with for example a Co(II)/Co(III) redox couple that may eventually lead to decompositon of the hydroperoxide with formation of dioxygen and alcohol (or water). 

With Ti-substituted Keggin polyoxometalates, for example, Na5 H PTiWn04o, the oxidation of cyclohexene with HP in an acetonitrile solvent yields traws-1,2-cydohexandiol as the main reaction product, via a heterolytic oxygen-transfer mechanism, when n> 2 in the compound. If the polyoxometalate contains only one proton, the main products are those of allylic oxidation, namely 2-cyclohexene-l-ol and 2-cydohexene-l-ol, produced via a homolytic oxidation mechanism [36tj. 

Conversion of eyclooctene to epoxycyclooctene was studied with different catalysts. As can be seen products of homolytic oxidation are much less prominent compared with cyclohexene. Heterolytic epoxidation of eyclooctene compared to cyclohexene is a faster reaction, and in Table 3 it is shown that whereas VO(salen) did not give significant epoxide yields with cyclohexene, with eyclooctene 78% epoxide selectivity was found. The eyclooctene oxidation is therefore less sensitive to ligand effects, and the reaction was mainly used to study catalyst stability. Experiments to test for leaching were performed in two ways. In the first method the reaction mixture was filtered after 1 h, and the filtrate allowed to react further (Table 3, note g). It is important to filter the solution at the reaction temperature because readsorption can take place on cooling. In the second method the zeolite is incubated... 

Waters, W. A Homolytic oxidation processes. In Progress in organic chemistry, Volume 5. London Butterworths 1961, pp. 1-45... 

This scheme indicates that while the initiation steps involve hetero-lytic cleavage of H2, during the actual hydrogenation and propagation steps the major reactions involve a reductive elimination [H2m (olefin) m s)n alkane] and a subsequent (homolytic) oxidative addition of H2 to one metal atom. The latter is, of course, the more usual process which has been established for [Rh(PPh3)3Cl] catalyzed hydrogenation (14,15,16),... 

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