Factors affecting rate hydroperoxides

Formation of hydroperoxide by Reaction 3 is known to occur (13, 72). It may be rate-controlling. Two opposing factors contribute to determine the rate of this reaction. As Reaction 3 is the same as Reaction 3, structural features affecting rate of hydrogen abstraction apply. However, as in many cases the R group is the same in RH and R02, ... 

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. 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

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