Metal-hydroperoxide complexes, heterolytic

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Similar to homolytic mechanisms, the heterolytic reactions can be divided into three groups (i) reactions with hydroperoxides, (ii) activation of molecular oxygen, and (iii) direct reaction of metal complexes with substrate. 

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. 

In oxidative epoxidation reactions besides of oxidizing agent often various catalysts systems are used [14]. It was established that the rate of heterolytic decomposition of 0-0 bonds in tertiary-butyl peroxide in the presence of catalysts such as Mo(CO)g proceeds in result of complexation between metal and hydroperoxide. By the authors suggested the probable schemes transition condition, without discussion of valent state of metal (Scheme 2). 

The most important factor affecting the selectivity of the epoxidation reaction (226) is the choice of metal complex used as the catalyst [374-377]. Table 11 summarizes the results of several studies which indicate that in general, molybdenum complexes are superior catalysts for this reaction. The lower selectivity for several of the catalysts listed in Table 11 is due to competing metal catalyzed hydroperoxide decomposition via homolytic bond cleavage under reaction conditions. Sheldon and Van Doom have shown that half times for decomposition of tert-hnXyl hydroperoxide in benzene at 90 were in the order [Co(Oct)2] >[Cr(acac)3] >[VO(acac)2] > [Mo(CO)6] > [W(CO)6] > [Ti(OBu)4]. On the other hand, the relative rates of epoxide formation in reactions of ferf-butyl hydroperoxides with cyclohexene in benzene at 90°C were in the order [Mo(CO)6] > [VO(acac)2] > [Ti(OBu)4] > [W(CO)6j. Thus, the relative rates of homolytic decomposition pathways and heterolytic epoxidation for any given complex determine the epoxide selectivity. 

Disulfides of dithioic acids behave differently from the metal complexes in that heterolytic decomposition of hydroperoxides predominates at all molar ratios of the disulfide to In contrast to the metal complexes which show multistep hydroperoxide decomposition curves, disulfides show a single step which correspond to the third catalytic stage in the case of the metal complexes. Furthermore, comparison of the rate constant of this step with that of the final step of the metal complexes shows a close resemblance to the case of iron and nickel and is quite different from that of the zinc complex, confirming the intermediacy of the disulfide in the former cases but not in the latter. 

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