Metal molybdenum complex

Table 1. Metal—Metal Bonding in Molybdenum Complexes and Clusters...

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

From the energetics point of view, the epoxidation act should occur more easily (with a lower activation energy) in the coordination sphere of the metal when the cleavage of one bond is simultaneously compensated by the formation of another bond. For example, Gould proposed the following (schematic) mechanism for olefin epoxidation on molybdenum complexes [240] ... 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Alkenes can be transformed into epoxides by hydroperoxides and a catalyst, which often is a high-valent titanium or molybdenum complex acting as a Lewis acid. The mechanism is not clear in great detail in Figure 2.34 a suggested mechanism is given. Coordination of the alkene to the metal prior to attack of the electrophilic oxygen is not considered as a necessary step. 

Better control of the reaction is achieved by using stable, isolable metal-carbene complexes, such as the Schrock initiators based on molybdenum and tungsten (LXIV, Mt = W, Mo) and the Grubbs initiators based on mthenium (LXV). The Schrock initiators... 

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