Tungsten-catalysed reactions metathesis

The tungsten alkylidyne complex W(=CBu )(OBu )3, which, if monomeric, is only 12-electron, catalyses the metathesis of 3-heptyne at a spectacular rate. Note that the presence of strongly Ti-donating alkoxy groups destabilizes some of the metal d-orbitals (p. 1 51) so that the 18-electron rule is no longer expected to hold. In this case the reactions are too fast to obtain evidence for tungstenacy-clobutadiene intermediates. Another route to the catalysts, however, is provided by the remarkable reaction between the triply bonded tungsten alkoxides W IOR) and alkynes. 

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. 

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. 

A number of authors have reported successful alkyne metathesis reactions catalysed by either tungsten " molybdenum" complexes. 

The possibilities of 1-methylcyclobutene as an isoprene equivalent have again attracted attention. Reaction of methylcyclobutene with -3-hexene catalysed by tungsten hexachloride and tetramethyltin gave the metathesis product (319) in ca. 25 % yield as a mixture of isomers. In a similar reaction with geraniol acetate, small amounts of farnesol acetate were detected. 

This review summarises the most interesting approaches in the polymerisation of alkynes and the ring opening metathesis polymerisation of cyclic olefins catalysed by seven-coordinate tungsten(II) and molybdenum(II) compounds. Special attention is given to the catalytically active intermediate compound formed in the reaction of the metal complex and the organic substrate (alkyne or cyclic olefin) and to the reaction mechanism. 

J. Chem. Soc. Chem. Commun. 311 (1979) J. L. Harrison and Y. Chauvin, Catalyse de transformation des olefines par les complexes du tungstene. II. Telemerisation de olefines en presence d olefines acycliques, Makromol. Chem. 141 161 (1970) R. H. Grubbs, The olefin metathesis reaction. Prof. Inorg. Chem. 24 ... 

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