Catalysts tungsten

The success of tungstate as catalyst has stimulated a wealth of research in this area. Recently it was shown that a silicotungstate compound, synthesized 

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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],... 

Homogeneous Systems Using Molybdenum and Tungsten Catalysts and Alkyl Hydroperoxides or Hydrogen Peroxide as the Terminal Oxidant... 

A particularly interesting system for the epoxidation of propylene to propylene oxide, working under pseudo-heterogeneous conditions, was reported by Zuwei and coworkers [61]. The catalyst, which was based on the Venturello anion combined with long-chained alkylpyridinium cations, showed unique solubility properties. I11 the presence of hydrogen peroxide the catalyst was fully soluble in the solvent, a 4 3 mixture of toluene and tributyl phosphate, but when no more oxidant was left, the tungsten catalyst precipitated and could simply be removed from the... 

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

Sato et al. (1997) have shown that 2-octanol can be converted to 2-octanone, with 95% selectivity, with 3-30% aqueous H2O2 in the presence of a tungsten catalyst and a PTC like trioctylmethylhydrogen sulphate. A similar strategy works for converting A-tert butylcyclohexanol to A-tert butylcyclohexanone. 

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Although the application of tungsten catalyst 5 to the cross-metathesis reaction of other alkenes has not been reported, Basset has demonstrated that to-un-saturated esters [18] and glycosides [21], as well as allyl phosphines [22], are tolerated as self-metathesis substrates. 

The report by Basset and co-workers on the metathesis of sulphur-containing alkenes using a tungsten alkylidene complex, mentioned previously for the acyclic cross-metathesis reaction (see Sect. 2.2), also contained early examples of ring-opening cross-metathesis of functionalised alkenes [20]. Allyl methyl sulphide was reacted with norbornene in the presence of the tungsten catalyst 5, to yield the desired ring-opened diene 35 (Eq. 29). 

Tungsten catalysts are also known, and are of considerable interest because they are mimics for tungsten-based oxotransferase enzymes found in thermochemical bacteria. Studies on such compounds have been reported (16,17). 

The molybdenum analogue of this tungsten catalyst is known as Schrock s catalyst [19]. It is less active than its tungsten counterpart, but it is much more resistant to polar groups in the substrate. 

The rate of decomposition of ammonia on a tungsten catalyst is independent of the concentration of ammonia, since [NHs] = 1. 

Experimental Procedure 3.2.7. RCM with a Tungsten Catalyst in Homogeneous Phase Diethyl 3-Cyclopentene-l,l-dicarboxylate... 

A complementary route to carbohydrate-based oxepines was developed by the McDonald group.67 It is based on the endo-selective cycloisomerization of alkynyl alcohols in the presence of molybdenum or tungsten catalysts to give the cyclic enol... 

SCHEME 23. c m/o-Selective cycloisomerization of alkynyl alcohols in the presence of molybdenum or tungsten catalysts. 

FIGURE 17.7 Catalytic cycle for SCR reaction over vanadium/tungsten catalyst. (With permission from Haldor-Topsoe, Inc.)... 

In 2002, Musaev and coworkers performed the first theoretical investigation of the mechanism with the aid of density functional theory calculations [26]. They first studied the mechanism of cycloisomerization in the absence of a tungsten catalyst, as shown in Scheme 4.14. The D FT calculations showed that the exo-cycloisomerization of 4-pentyn-l-ol via a concerted transition state leading to a five-membered-ring exo product had a high barrier (52.0 kcalmol ) (path a of Scheme 4.14). The pathways leading to a six-membered-ring endo product have also been calculated (paths b and c... 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Group 16 metal carbonyls are also effective in the PKR. Hoye prepared a pre-activated tungsten catalyst (W(GO)sTHF) by replacing one of the COs on tungsten with THF photochemically, and successfully applied it to PKR. This semicatalytic system constitutes one of the early examples useful even for the substrates bearing electron-withdrawing groups. 

SCHEME 84. Epoxidation of cyclohexene with tungsten catalysts immobilized on organophosphoryl macroligands... 

Mark Overhand of Leiden University recently reported (Tetrahedron Lett. 2004,45,4379) an example of alkyne-alkyne metathesis, the cyclization of 17 to 19. For this reaction, a tungsten catalyst was used. 

Whereas only limited stereoselectivity is characteristic of the metathesis of acyclic olefins, ring-opening metathesis polymerization of cycloalkenes may be highly stereoselective provided the proper catalysts and reaction conditions are selected. Cyclopentene, for instance, is transformed to either all-cis [Eq. (12.25)] or all-frans polypentenamers [Eq. (12.26)] in the presence of tungsten catalysts 21 92... 

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