Molybdenum and Tungsten-Catalyzed Epoxidations

Epoxidations of Aikenes Cataiyzed by Eariy Transition Metais 

High-valent early transition metals such as titanium(lV) and vanadium(V) have been shown to efficiently catalyze the epoxidation of aikenes. The preferred oxidants using these catalysts are various alkyl hydroperoxides, typically tert-butyUiydroperox-ide (TBHP) or ethylbenzene hydroperoxide (EBHP). One of the routes for the industrial production of propylene oxide is based on a heterogeneous Ii 7Si02 catalyst, which employs EBHP as the terminal oxidant [6], 

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Molybdenum and tungsten compounds have long been known to catalyze the transformations of alkenes into epoxides and diols by hydrogen peroxide.171"173 This reaction was found to be suitable for the epoxidation of water-soluble alkenes such as allylic alcohols (equation 30)174,175 or unsaturated carboxylic acids (equation 31).171 Tungsten catalysts were found to be more active and selective in aqueous solution than molybdenum complexes. 

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. 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

The selectivity pattern of d° transition metal catalyzed epoxidations is much less readily understood. In the molybdenum-catalyzed epoxidation of (S)-limonene (Table 2, entries 1 and 2) the cis selectivity could perhaps be explained by a directing effect due to -coordinating of the second double bond to molybdenum. Such a selectivity is completely missing in the analogous tungsten-catalyzed reaction of (S)-limonene (Table 2, entry 4) in the absence of a second double bond as, for example, in 3/(-acetoxy-5-cholestene (Table 4) reactions with both metals afford similar diastereomeric ratios. 

Many hydroxylated linalools [including compounds 105, 106, 108, and 110, both (Z)- and ( )-isomers], as well as the epoxides of both furanoid (109) and pyranoid (see section on pyrans) linalyl oxides, have been identified in papaya fruit (Carica papaya). At the same time, the first reported occurrence of die two linalool epoxides (112) in nature was made. These epoxides are well known to be unstable and easily cyclized (see Vol. 2, p. 165) and have been made by careful peracid oxidation of linalool. An interesting new method has now been described. While the vanadium- or titanium-catalyzed epoxidation of geraniol (25) gave the 2,3-epoxide (see above), as does molybdenum-catalyzed epoxidation with hydrogen peroxide, the epoxidation of linalool (28) with molybdenum or tungsten peroxo complexes and hydrogen peroxide led, by reaction on the 6,7-double bond, to 112. ... 

Selective photochemical epoxidation of olefins cyclohexene, 2-hexene) with dioxygen (irradiation with a 500 W tungsten lamp) is catalyzed by oxoethoxo(tetra-p-tolylporphinato)molybdenum(V) and niobium(V) [73,74]. 

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