Molybdenum complexes olefin epoxidation

Thus, vanadium complexes preferentially epoxidize small amounts of allylic oxygen species formed in situ to give epoxy alcohols whereas molybdenum complexes catalyze epoxidation of the excess of unreacted olefin to give epoxides. The mechanism of vanadium catalyzed epoxidation of allylic alcohols has been discussed in an earlier section. 

The addition of an oxygen atom to an olefin to generate an epoxide is often catalyzed by soluble molybdenum complexes. The use of alkyl hydroperoxides such as tert-huty hydroperoxide leads to the efficient production of propylene oxide (qv) from propylene in the so-called Oxirane (Halcon or ARCO) process (79). 

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. 

The selective oxidation is catalyzed by silver, which is the only good catalyst. Other olefins are not converted selectively to the epoxides in the presence of silver. However, propylene epoxidation is appHed commercially the catalysts are either molybdenum complexes in solution or soHd Ti02—Si02 (see... 

Molybdenum hexacarbonyl [Mo(CO)6] has been vised in combination with TBHP for the epoxidation of terminal olefins [44]. Good yields and selectivity for the epoxide products were obtained when reactions were performed under anhydrous conditions in hydrocarbon solvents such as benzene. The inexpensive and considerably less toxic Mo02(acac)2 is a robust alternative to Mo(CO)6 [2]. A number of different substrates ranging from simple ot-olefms to more complex terpenes have been oxidized with very low catalytic loadings of this particular molybdenum complex (Scheme 6.2). The epoxidations were carried out with use of dry TBHP (-70%) in toluene. 

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] ... 

The formation of molybdenum complexes with diols (formed by olefin oxidation) was proved for the use of the molybdenum catalysts. Therefore, the participation of these complexes in the developed epoxidation reaction was assumed [242]. 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

Diperoxo(oxo)molybdenum(IV) complex bearing (S)-lactic acid piperidineamide as a chiral ligand has been used for the epoxidation of E-2-butene (Scheme 6B.8) and moderate enantiose-lectivity (49%) is achieved wherein the reaction is stoichiometric [16]. Two possible mechanisms have been proposed for this reaction. One mechanism includes coordination of an olefin prior to epoxidation, which makes the olefin electrophilic and facilitates the nucleophilic attack of the proximal oxygen atom of the peroxide on the olefin. The other one is that an olefin nucleophilically attacks the peroxo group of the molybdenum complex. 

Molybdenum complexes are the most effective catalysts known for the selective epoxidation of olefins with alkyl hydroperoxides (210-212). Commonly known is the Arco or Halcon process for the large-scale manufacture of propylene oxide from propylene. This process uses t-BuOOH or ethyl benzene hydroperoxide (EBHP) as an oxidant and Mo(CO)6, for example, as a source of Mo. The Mo(CO)6 acts as a catalyst precursor, which is converted into a soluble active form by complexation with diols (3). Chemists have designed several supported versions of the catalysts for this epoxidation chemistry. A clear classification can be made on the basis of the nature of the support. 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

Sharpless et a . have conhmied this mechanism in part by labeling experiments which demonstrated that the epoxide oxygen is derived exclusively from the peroxo ligands of the complex and not from the oxo oxygen. However, the reactivity of the molybdenum complex toward olefins closely parallels that of peracids, for which a three-membered cyclic transition stale is favored. ... 

Epoxidation of olefins (2, 287). The procedure for epoxidation of olefins with t-butyl hydroperoxide catalyzed by molybdenum hexacarbonyl has been published. Kinetic data have been obtained from the reaction. The mechanism is believed to involve I) reversible complex formation between the catalyst and the hydroperoxide, 2) reversible inhibition by the coproduct alcohol, and 3) reaction of the hydroperoxide-molybdenum complex with the olefin to form the epoxide and by-product alcohol. 

D. V. Deubel, J. Sundermeyer, G. Frenking, Mechanism of the olefin epoxidation catalyzed by molybdenum diperoxo complexes Quantum-chemical calculations give an answer to a longstanding question, J. Am. Chem. Soc. 122 (2000) 10101. 

Enantiomerically pure manganese complexes using ligands other than the salen structure have been reported, but so far with lower enantioselectiv-ities. Better results have been achieved using molybdenum complexes bearing hydroxamic acid ligands and TBHP or cumylhydroperoxide as oxidant. This system has been used to effect the epoxidation of a range of olefins with up to 96% ee. 

Catalytic olefin epoxidation with T -cyclopentadienyl molybdenum complexes 12COC16. 

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