Vanadium epoxidation catalysts

Other transition metal-substituted molecular sieves have been synthesized and tested as epoxidation catalysts [21]. The stability of many of these materials towards leaching is, however, seriously in doubt [31]. Catalysis by vanadium-substituted molecular sieves has, in all cases studied, been shown to be homogeneous in nature, i.e. because of leached vanadium [31,55]. A priori, one would expect zirconium- [56] and tin- [57] substituted molecular sieves to be more stable but more rigorous proof is needed. 

Other metal-based epoxidation catalysts have been explored to overcome some of the hmitations of the Sharpless procedure. One drawback with the Sharpless asymmetric epoxidation is the slightly lower ees often obtained when using cis-olefin substrates. The group of Yamamoto have achieved highly enantioselective epoxidations of ds-alkenes using vanadium(V) oxytriisopropoxide in the presence of C2-symmetric bishydroxamic acid ligands such as (4.23). In contrast to the Sharpless procedure this process is not hampered by the presence of air or... 

Related polydentate ligands are the polymer-anchored bis(phosphonomethyl)amino and bls(2-hydroxyethyl) amino species, studied by Suzuki (13). These ligands have been attached to both microreticular and macroreticular polystyrene-co-divinyl benzene and coordinated to both oxo-vanadium (V) and oxo-molybdenum(VI). Using the catalytic epoxidation of (E)-geraniol as a model system (with t-BuOOH) it was found that the macroreticular oxo-vanadium(V) catalyst was the most reactive, particularly with the phosphorous ligand (Equation 3). 

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]. 

Sharpless advanced this notion through using a metal-based epoxidation catalyst (Scheme 4.18) [44]. This approach took advantage of the capacity of VO(acac)j to coordinate simultaneously to alcohols and peroxides and promote regioselective epoxidations of allylic and homoal-lylic alcohols with iBuOOH as the oxidant, as shown by the conversion of 87 to 88. This approach also provides stereoselectivity, as shown by the transformation of 89 to 90 in which the hydroxyl group directs a yn-oxidation of the alkene. This procedure is selective because alkyl peroxides generally do not epoxidize alkenes but can be activated by coordination to the alcohol-coordinated vanadium Lewis acid. 

SCHEME 34.15. Asymmetric epoxidation of allyl alcohols 54 leading to enantioenriched epoxides 56 mediated by vanadium(V) catalysts with a chiral hydroxamic acid ligand 55 and t-butyl hydroperoxide as a terminal oxidant. 

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). 

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. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

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. 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

SMPO [styrene monomer propylene oxide] A process for making propylene oxide by the catalytic epoxidation of propylene. The catalyst contains a compound of vanadium, tungsten, molybdenum, or titanium on a silica support. Developed by Shell and operated in The Netherlands since 1978. 

The reaction sequence is summarized below using isobucane as the hydrocarbon. The crucial and nearly incredible part of the process is in two parts itself but only shows as one. It is the second equation where the oxygen molecule transfers to the propylene molecule and the ring closes to form the epoxide. (Thats why they call it epoxidation). The magic that causes all that to happen is in the metal catalysts, molybdenum naphthenate or the soluble salts of titanium, vanadium, or tungsten. This molecular fancy-dance is but one of nniany examples in chemistry where catalysts can cause atoms to slide around molecules in unlikely ways. 

Aiming at easier workup conditions, immobilization of several transition metal catalysts, which show activity for the epoxidation of allylic alcohols, on polymer support has been investigated. For example, Suzuki and coworkers incorporated an oxo-vanadium ion into cross-linked polystyrene resins functionalized with iminodiacetic acid or diethylenetri-amine derivatives (Scheme 57), which afforded a heterogeneous catalyst that can promote... 

TABLE 17. Comparison of the epoxidation results obtained with different vanadium/hydroxamic acid catalysts and the Sharpless catalytic system (Ti/L-DET) (yields are given ee and abs. configuration of the epoxides obtained are given in parentheses)... 

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