Metal-hydroperoxide complexes, heterolytic reactions

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. 

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. 

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. 

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