Tungsten-catalysed

Scheme 1. Tungsten catalysed cross-metathesis of allyl methyl sulphide with pent-2-ene and but-2-ene...

Tungsten-catalysed oxidation of alcohols by hydrogen peroxide is achieved in high yield in the presence of tetra-n-butylammonium hydrogen sulphate [20-22]. Secondary alcohols are converted into ketones (>90%) [e.g. 21], but primary alcohols generally are oxidized completely to the carboxylic acids [21], Aldehydes are also oxidized to the carboxylic acids [e.g. 21]. In contrast, using procedure 10.7.8.B, which is adaptable to scale up, benzyl alcohols are converted into the aldehydes benzoic acids are only formed with an excess of hydrogen peroxide [22],... 

A method for the synthesis of eight-membered carbocycles based on a new tandem tungsten-catalysed cycloisomerization-cyclopropanation reaction has been reported (Scheme 99).143 This is a catalytic reaction in which a heteroatom-stabilized Fischer... 

Tungsten-catalysed BV oxidation with hydrogen peroxide has been studied by DFr.471... 

Kitatani et al. found that tungsten(VI) chloride would catalyse the formation of a range of oxazoles from benzoyl(phenyl)diazomethane and nitriles (Scheme 17).<74TL1531, 77BCJ1647> The reaction with acetonitrile was studied with a range of other metal chlorides, but all proved less satisfactory than WCle. They attributed the catalytic nature of tungsten(Vl) chloride to both its Lewis acidity and the affinity of tungsten for carbenes. 

Cyclisation of l-aryl-6-hydroxyhex-2-ynyl carbonates to the 2-arylidene-5,6-dihydropyran catalysed by Pd(0) is steieospeciBc, giving only the (Z)-isomer <96SL553>, whilst alk-l-yn-5-ols form dihydtopyranylidene carbenes under the influence of the tungsten pentacarbonyl-THF complex. The carbenes are a source of 2-stannyldihydropyrans <96TL4675>. 

White H, G Strobl, R Feicht, H Simon (1989) Carboxylic acid reductase a new tungsten enzyme catalyses the reduction of non-activated carboxylic acids to aldehydes. Eur J Biochem 184 89-96. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

The successful cross-metathesis of allyl methyl sulphide with ris-pent-2-ene and czs-but-2-ene, catalysed by the tungsten alkylidene 5, was reported by Basset and co-workers in 1993 [20] (Scheme 1). 

The heat of formation of [MolCOlg] has been determined as -960 + 12 kJ mol by measuring its heat of decomposition. The Mossbauer parameters for the 100 keV transition of in [W(CO)g] and some tungsten(vi) complexes have been measured and discussed in terms of known bonding and structure. Secondary ions [M (CO) ] (M = Mo, m = I or 2 M = W, m = 1—4 n = 0—14) formed by ion-molecule reactions have been observed in the mass spectra of the hexacarbonyls. A mixt u re of [Cr(CO) ] and [MolCO) ] vapours affords [CrMo(CO) ] ( = 5—7). [MofCOl ] and [WICO) ] catalyse the condensation of isocyanates with aldehydes to give imines in high yields. ... 

Catalytic activities of [Ru PW (H30)(0)3, ] -/03/CH3CN/80°C and of [Ru PMOjj(H30)(0)3, ] "/03/CH3CN were compared. The tungsten complex did not catalyse the aerobic oxidation of cumene and 1-octene to cumyl alcohol and 1-octene oxide while the Mo analogue did so the tungsten complex underwent structural change with to an inert form, while its molybdenum analogue did not... 

Most of the studies to date have employed either palladium [222—229] or platinum [220,224,226,228—235], commonly as Adams reduced platinum oxide, although nickel [228,236,237], rhodium [238,239], ruthenium [239], iridium [239], iron [237] and tungsten [237] have also been used. Many of these studies have been concerned with the stereochemistry of the hydrogenation of disubstituted cycloalkenes. Table 32 shows some typical results for the platinum- and palladium-catalysed hydrogenation of disubstituted cyclohexenes. Table 33 shows comparative results for the hydrogenation of 1,4-dialkylcyclohexenes over palladium, platinum and rhodium catalysts. 

Enyne intramolecular metathesis reactions, of the type shown in equation 61, can be very useful in organic synthesis. A number of such reactions, catalysed by tungsten or chromium carbene complexes, have been reported634,635,737 - 740. The ruthenium carbene catalysts 18-20 (Table 2) are likely to be increasingly used for this purpose because of their stability, ease of handling and good yields, as in the synthesis of various 5-, 6- and 7-membered heterocycles, e.g. equation 67741. 

Molybdenum(iv) and Tungsten(iv).—A wide variety of molybdenum(iv) complexes dissolved in non-aqueous media have been shown to catalyse the reduction of acetylene by sodium borohydride. The optimum yield was 2.8 moles of C2H2 reduced per g. atom of MoIV, with both C2H4 and C2H6 being formed.265... 

It is possible to speed up aliphatic tertiary amine oxidation by adding tungstate or molybdate catalysts.334 However, for oxidation of aromatic and particularly heterocyclic tertiary nitrogen, a stronger system than hydrogen peroxide alone is required. iV-Oxidation of heterocycles is of pivotal importance in industrial chemical synthesis.335 Catalysed systems have been applied and these are dominated by metal peroxo systems based on molybdenum or tungsten. For example, quinoxaline and pyrazine may be oxidized to mono- or... 

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