Tungsten stereochemistry

Table 23.2 Oxidation states and stereochemistries of compounds of chromium, molybdenum and tungsten...

Tungsten, tetrakis(diethyldithiocarbamato)-stereochemistry, 1,94 Tungsten, tetrakis(ethyltrithiocarbonato)-stereochemistry, 1, 94... 

Another class of octahedral structures whose stereochemistry was established by NMR is tungsten hexafluoride complexes with donor molecules, e.g., WF6-(C6H6)3P (87a). For these, the F19 spectra consist of a low-field doublet, a medium-field quintet and a high-field singlet of relative intensities 4 1 1. The doublet and quintet resonances establish the geometry... 

Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 

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. 

A couple of immediate targets for applying the general methodology proposed in this book can be indicated, based on an analysis of the literature. The first candidate might be the valence bond approach to the analysis of stereochemistry of compounds of heavy transition metals (e.g. tungsten) proposed in a series of works [2-4]. It... 

Table 18-C-i Oxidation States and Stereochemistry of Molybdenum and Tungsten... 

The anti addition of amines to the double bond of cationic (alkene)(cyclopentadienyl)di-carbonyliron complexes and to the analogous molybdenum and tungsten complexes has been reported31 33. The adducts underwent carbonyl insertion-cyclization to give chelate complexes, which were then oxidized to /8-lactams. For example, from the Fp complexes of ( )- and (Z)-2-butene the corresponding /8-lactams were obtained diastereoselectively in 10-15% yield by the direct oxidation of the benzylamine adducts with chlorine at low temperature33. The stereochemistry was determined by H-NMR spectroscopy. 

The [2 + 2] cycloaddition of the propynyl tungsten complex (12b) has been found to be stereospecific with the cis and trans isomers of eAyl prc nyl ether." ITie reaction with c/s-propenyl ethyl ether gave rise to the cyclobutenyl complex (18), in addition to a minor amount of the ring-opened dienyl carbene complex (19). The stereochemistry about the double bonds in the dienyl complex (19) revealed that it was derived from a thermal electrocyclic conrotatory ring opening of the c/s-cyclobutenyl carbene complex (18) and thus the stereochemistry is completely retained in this cycloadditirxi. In support of this analysis it was found that cyclobutenyl complex (18) could be thermally opened to the dienyl complex by heating in an inert solvent at 50 C. In the cycloaddition of the propynyl complex (12) with trans-... 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

F n.m.r. has been used to probe the stereochemistry of tungsten(vi) with mixed uni- and bi-dentate ligands and F chemical shifts measured for the compounds ROWFj (R = alkyl or alkyl halide) and interpreted in terms of ct- and it-bonding effects. ... 

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