Oxovanadium -catalyst

Several other asymmetric Mannich-type processes have been described. Propargyl alcohols (11) undergo an addition to imines (12), to give 2-acylallylic carbamates (13), using an oxovanadium catalyst.28 The reaction always gave the (Z)-enone, but a trial with a chiral propargyl alcohol showed virtually no enantioselectivity. 

Oxidative polymerization of bis(l-naphthoxy) monomers is known as the Shell Reaction (eq. 11) (228). 1,4-Dialkoxybenzenes have been pol5mierized nsing FeCls (229,230) and oxovanadium catalyst with dioxygen (231) to give poly(2,5-dialkoxy-l,4-phenylene)s (eq. 12). Poly(4,6-di-n-butyl-l,3-phenylene) was also obtained (232). 

TABLE 1 Exploration of Biaryl Bond Formation with Bimetallic Oxovanadium Catalysts... 

Dynamic kinetic resolution (DKR) of a racemic alcohol with lipase and metal catalysts is often conducted in organic solvents. Since the maximum yield of kinetic resolution of racemic alcohol by lipase is only 50% with 100% ee, metal catalysts to racemize the unreactive enantiomer substrate is necessary to achieve 100% yield and 100% ee [22]. For example, mthenium catalysts have been widely used for this process as shown in Figure 3.12a [22b]. Vanadium has also been used, and to improve both catalytic activity and compatibility of the oxovanadium catalysts with the lipases, a novel oxovanadium catalyst (V-MPS) immobilized inside mesoporous silica (MPS) with pores of approximately 3 nm in diameter was prepared. With this immobilization preparation, a complete division of the racemization site and the enzymatic reaction site was achieved. 

The Akai group S5nithesized (R)-imperanene by employing the DKR of allylic alcohol as the key step. In this synthesis, racemic alcohol intermediate rac-17 was converted to its enantiomeric acetate (S)-28 via the DKR, which was performed using lipase PS-IM and oxovanadium catalyst immobilized inside mesoporous silica (V-MPS). The target molecule of >99% ee was then obtained via four steps (Scheme 5.44) [66]. 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

The same authors later on presented their results of the epoxidation of various cyclic and acyclic olefins employing a heterogeneous catalyst with an oxovanadium(IV) ion incorporated on a sulfonic acid ion-exchange resin and TBHP as oxidant . Selectivities... 

The surface oxovanadium species are the active sites in the V20g-Ti02 (anatase) catalysts for the selective oxidation of o-... 

One should mention another, concurrent and near-contemporary methodology, also based on catalysis by an oxovanadium species. Mukaiyama and coworkers used oxovanadium (IV) complexes with the general formula OVL2 to perform epoxidations (40) A catalytic amount (4 mol%) of the complex is used. Molecular oxygen is the oxidant, under 3 atm of pressure. An alcohol in stoichiometric amount (1.5 equivalent) is introduced as a sacrificial reducer. The efficiency of the catalyst increases with a... 

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 Schiff base-oxovanadium(IV) complex formulated as 19 was found to catalyze the asymmetric oxidation of sulfides with cumene hydroperoxide (Scheme 6C.8) [70]. Various aryl methyl sulfides were used for this process (room temperature in dichloromethane and 0,1 mol equiv. of the catalyst). Chemical yields were excellent, but enantioselectivities were not higher than 40% for the resulting methyl phenyl sulfoxide, Complex 16a, where [Ti] was replaced by VO, was also examined in the oxidation of sulfides, but the reactions gave only racemic sulfoxides [68],... 

Lewis-acid complexes 6 are formed upon addition of the aluminium alkyl to oxovanadium(v) species and are mildly active catalysts for homogeneous ethylene polymerization (16). System 6 appeared to be a promising candidate for an... 

There was already a considerable body of knowledge on catalysts of this type [29]. For those used for selective oxidations, there was much evidence to show that the active phase was a monolayer of oxovanadium species chemically bonded to the TiC>2 surface such a material would have about 1 wt% V2O5 for a TiC>2 area of 10m2g l, but technical catalysts usually contained substantially larger amounts. The excess appeared to be in the form of V2O5 microcrystals which neither helped nor hindered in selective oxidation it seemed to serve as a reserve supply to replenish the monolayer, should it become depleted. There was also evidence that uncovered TiC>2 surface was harmful, in that it could cause deep oxidation to carbon oxides. In these applications, the anatase form of TiC>2 was generally used, and unless the contrary is stated the formula TiC>2 will imply this form. 

In the same year, Fujita s group63 reported the asymmetric oxidation of aryl methyl sulfide by hydroperoxides (TBHP, CHP) and an optically active catalyst formed by a Schiff base-oxovanadium(IV) complex 32, giving (S)-sulfoxides in low ee (up to 40%) (Fig. 4). Later, they developed64 a more promising approach using 33, a binuclear Schiff base-titanium(IV) complex (4 mol% equiv) to catalyze the asymmetric oxidation of methyl phenyl sulfide by trityl hydroperoxide in methanol at 0 °C. The (ft)-methyl phenyl sulfoxide was obtained with 60% ee. 

Fortunately, the equilibrium of the active catalyst (HPS)VO(OH) and another species (HPS)VO(OR) permits a direct probe of the active species. The addition of ethanol or methanol significantly retards the rate of tribromide formation when the catalyst is derived from (HPS)VO(OEt)(EtOH), although the rate is unchanged (within experimental error) when the catalyst is derived from sodium or ammonium vanadate. This disparity is readily rationalized by considering the coordination environment in the two catalysts. In (HPS)VO(OH), only one site is available to bind peroxide, the equatorial site occupied by hydroxide. The presence of ROH in solution results in a competition between peroxide and alkoxide for the sole available site. The equilibrium between (HPS)VO(OR) and (HPS)VO(02) favors the peroxo species, but addition of 0.13 M methanol (20-fold excess over [H202]) results in a greater than 2-fold decrease in rate. By contrast, oxovanadium(V) has several available coordination sites, as well as the ability to bind two peroxide moieties. The addition of 0.5 M methanol does not cause any change in the rate of tribromide formation. 

Furthermore, the catalytic activity in -butane oxidation over the VPO catalysts was lost when nitrous oxide, a well-known oxotransfer agent, was used instead of molecular oxygen [75]. This suggests that the q -superoxo or rf-peroxo species is indeed involved in the oxidation. In the case of the trans-oxovanadium(IV) dimer site proposed as active, the activation of dioxygen... 

Vanadium. A residual oil was desulphurized (673 K, 115 atm) with a non-stoicheiometric vanadium sulphide (S/V, 0.8-1.8) formed in situ from VS4 Vanadium sulphide catalysts have been prepared by in situ sulphiding of vanadium complexes, e.g., bis(acetylacetonato)oxovanadium(IV), dissolved in crude petroleum.Vanadium compounds occurring in heavy oils have been activated as desulphurization and demetallization catalysts by treatment with triethylaluminium. Catalysts consisting of vanadium promoted by nickel can be prepared in situ by deposition of the metals from heavy crude oils. Ni-V hds and hdm catalysts on silica or carbon have been claimed. 

Several oxovanadium(IV) complexes (141) with ON-donor ligands were prepared from [YOCl2(thf)2] (thf = tetrahydrofuran) and /// /////-substituted phenols to study the electronic effects of para substituents in order to develop better [VCl2(OR)2]-type olefin polymerization catalysts.676 The hyperfine coupling constants, the HOMO-LUMO transitions, and the oxidation potentials were all found to be linearly related to the Hammett a constant of the substituent on the monoanionic aryloxy ring.676... 

SbF6)2 [61], and an oxovanadium(IV) complex [62] are effective catalysts. Matsuda et al. recenfly reported that a cationic Ir complex generated in situ from [Ir(COD) (PPh3)2]OTf and H2 catalyzes aldol and aldol-type reactions of aldehydes and acetals with silyl enolates in CH2CI2 [63]. 

Chiral (salen)oxovanadium complexes for sulfide oxidation were first investigated by Fujita [29,43]. In the presence of 4 mol % of the catalyst optically active sulfoxides were obtained in good yields, however, the enantiomeric excesses remained only moderate (up to 40%). 

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