Epoxidation With Molybdenum Complexes

A number of kinetic studies have been made on the epoxidation of olefins in the presence of molybdenum complexes. It has been reported that the Mo(CO)6-catalyzed reactions of 1-octene [375], 2-octene [375], styrene [378], and cyclo- 

Free energy relationships for substituted styrenes showed considerable scatter but a po plot gave p = — 1.4 0.6 (95% confidence levels). Thus, it appears that the rate determining step involves electrophilic attack upon the alkene [378]. 

With styrene as solvent the reaction is first order in hydroperoxide but displays a complex dependence on molybdenum concentration. The apparent order in molybdenum complex is 3/2 at low concentrations and 1/2 order at high Mo concentrations. 

More detailed studies of the oxidation of propylene [380, 381] and 1-octene [382] have shown that the actual rate equation is of the Michaelis-Menten type with the co-product alcohol being the competitive inhibitor. The fact that alcohols inhibit the molybdenum catalyzed epoxidation of olefins is well known [377, 378, 383]. In addition, it has been noted that induction periods are observed [378, 382] during which time the active catalyst, a molybdenum(VI) species [384, 379] is formed. 

The kinetics of the [Mo02(acac)2]-catalyzed epoxidation of cyclohexene by tert-hutyl hydroperoxide in cyclohexane have also been examined [385]. Reactions are nearly first order in catalyst but of nonintegral order in olefin with rates more nearly proportional to [olefin] at low concentrations than at high. A similar behavior was noted for hydroperoxide. This kinetic behavior was ascribed to the formation of molybdenum hydroperoxide and molybdenum olefin complexes. Kinetic evidence for the formation of a wide variety of metal-olefin and metal-hydroperoxide complexes has been reported during the molybdenum versatate-catalyzed epoxidation of cyclohexene with ethyl benzene hydroperoxide, and equilibrium constants for their formation were calculated [386]. 

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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 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. 

Liquid-Phase Epoxidation with Hydroperoxides. Molybdenum, vanadium, and tungsten have been proposed as Hquid-phase catalysts for the oxidation of the ethylene by hydroperoxides to ethylene oxide (205). tert- uty hydroperoxide is the preferred oxidant. The process is similar to the arsenic-catalyzed route, and iacludes the use of organometaUic complexes. 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

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. 

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]. 

The transformations discussed in Sects. 2.2-2.3 highlight several important features of the RCM process. Firstly, the compatibility of the ruthenium initiator 3 with a wide range of functional groups including epoxides, vinyl iodides, thia-zoles and alcohols is demonstrated. The versatility of 3 is further illustrated in Sect. 2.3, where it is used to effect RCM of polymer-bound substrates. Previously, the molybdenum complex 1 has been reported to be more sensitive than 3 [19]. Experiments reported here are consistent with this view (Sect. 2.2.3) [14b]. 

The molybdenum-mediated arylamine cyclization was also applied to the total synthesis of pyrano[3,2-a]carbazole alkaloids (Scheme 26). Reaction of the 5-aminochromene 71 with the complex salt 62 affords the complex 72, which on oxidative cyclization provides girinimbine 73, a key compound for the transformation into further pyrano[3,2-a] carbazole alkaloids. Oxidation of 73 with DDQ leads to murrayacine 74, while epoxidation of 73 using meta-chloro-perbenzoic acid (MCPBA) followed by hydrolysis provides dihydroxygirinim-bine75 [113]. 

The chiral ligand (44) was prepared starting from the cyclic a-amino acid (S)-proline80). Recently, similar chiral catalysts and related molybdenum complexes involving optically active N-alkyl-P-aminoalcohols as stable chiral ligands and acetylacetone as a replaceable bidentate ligand, were designed for the epoxidation of allylic alcohols with alkyl hydroperoxides which could be catalyzed by such metal complexes 8,). 

Of other related systems, molybdenum(V) porphyrin exhibits very high stereoselectivity with tert-BuOOH as the oxygen source (97% m-2-hexene oxide and 99% trans-2-hexene oxide from cis- and trans-2-hexene, respectively).329 Nonporphyrin complexes of iron were found to be stereoselective in the epoxidation of stil-bene isomers. Iron cyclam, a nonporphyrin iron complex, gives the corresponding cis and trans epoxides in epoxidation with H202.330 Fe(acac)3, in contrast, yields the trans epoxide from both stilbene isomers.331... 

To probe hydroperoxide reactivity in these systems we studied the reaction of tert-butyl hydroperoxide in the presence of [C5H5V(CO)4]. In contrast to the rhodium(I) and molybdenum complexes, [C5H5V-(CO)4] catalyzed the rapid decomposition of tert-butyl hydroperoxide to oxygen and tert-butyl alcohol in both toluene and TME (Table II). When reaction was done by adding the hydroperoxide rapidly to the vanadium complex in TME, no epoxide (I) was produced. However, when the TME solution of [C5H5V(CO)4] was treated with a small amount (2-3 times the molar quantity of vanadium complex) of tert-butyl hydroperoxide at room temperature, a species was formed in situ which could catalyze the epoxidation of TME. Subsequent addition of tert-butyl hydroperoxide gave I in 13% yield (Table II). This vanadium complex also could catalyze the epoxidation of the allylic alcohol (II) to give tert-butyl alcohol and IV (Reaction 14). Reaction 14 was nearly quantitative, and the reaction rate was considerably faster than with TME. 

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