Vanadium catalysts olefin epoxidation

The mechanism of olefin epoxidation with iodosylbenzene in the presence of Cr(III) Schiff base complexes has been studied.The same reaction is catalysed by V0(acac)2 probably yij free radicals.Trans-stilbene is epoxidised by NalOi, in the presence of RuClg and substituted phenanthroline ligands. Vanadium(V) supported on a functionalised polystyrene resin is a good catalyst for the epoxidation of allylic alcohols by ButQOH a similar Mo(VI) catalyst is more suitable for cyclohexene epoxidation. ... 

Epoxidations. Vanadium based catalysts are largely used for olefin epoxidation by hydroperoxides (such as butyl hydroperoxide). These catalysts are very regioselective for epoxidation of double bonds of allylic alcohols [56]. 

Yamamoto has studied a variety of readily available, chiral hydroxamic-acid based ligands, which have proven effective in vanadium-catalyzed enantioselective epoxidations of allylic (Equation 14) [84, 85] and homoallylic alcohols (Equation 15) [86], as well as in molybdenumunfunctionalized olefins [87], For example, treatment of 74 and 77 with tert-BuOOH and 1 mol % vanadium catalyst 75 furnished epoxides 76 and 78 in 97 and 91 % ee, respectively [85]. 

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. 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

By analogy between the oxo forms of vanadium(V) and iron(IV), the latter being the active species in oxidations by cytochrome P-450, the system constituted by vanadium oxide as the catalyst, and t-butylhydro-peroxide, as the oxidant, gives good results in the conversion of olefins to the corresponding epoxides. With the supported "clayniac" catalyst, in the presence of i-butyraldehyde as a sacrificial reducer, olefins are epoxidized in good yields by compressed air at room temperature, in a convenient procedure. 

Typical procedure The mole ratio of alkene t-BuOOH catalyst was 10 10 0.25 and 40 mmol of the olefin serving as its own solvent. Thus, 10 mmol of TBHP (80% in di-tert-butyl peroxide) and 0.25 mmol of vanadium pentoxide were added to 50 mmol of the olefin and the reaction mixture was stirred at 60°C under a dinitrogen atmosphere. The products formed were analyzed by GC by comparison of their retention time with those of authentic samples. Good yields of epoxides were obtained only with an excess of olefin to TBHP of 5 1. That the olefin doubles up as the solvent makes for a more practical procedure. Typical results (34) are shown in Table 1 ... 

Oxovanadium(V) and oxomolybdenum(VI) were incorporated into crosslinked polystyrene resins functionalized with iminodiacetic acid or diethylenetriamine derivatives 921 The polymer complexes were used as catalysts in the oxidation of olefins with f-butylhydroperoxide. Vanadium(V) complexes promote the epoxidation of allylic alcohols in a highly regioselective manner, e.g., 2,3-epoxide was obtained in 98 % selectivity from e-geraniol at 80 °C. The catalytic activity of the vanadium(V) complexes is generally higher than that of the molybdenium(VI) complexes in the oxidation of allylic alcohols, whereas an opposed trend holds for the epoxidation of cyclohexene. 

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,... 

The most direct way to prepare VO(acac)2 is by the reaction of vanadyl sulfate with a source of the ligand. Vanadium(V), such as V205, can be reduced to vanadium(IV) by ethanol solvent in the presence of sulfuric acid. Reaction with acetylacetone in sodium carbonate yields the desired product. The synthesis we will use produces the complex in high yield directly in a system that can visually shed light on the active catalyst species in the epoxidation of olefins, Figure 9.4. 

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