Tungsten acetylene

Hydration and/or dehydration reactions are frequently catalyzed by metallopro-teins. Examples are proteins containing nickel (urease), zinc (e.g., peptidases), molybdenum (the hydratase partial reaction of formate oxidoreductase), tungsten (acetylene hydratase). An obvious difference between Ni, Zn, on the one hand, and Fe, Mo, W, on the other, is that the first are directly coordinated to the protein whereas the latter are also part of a cofactor. With reference to the Fe/S cluster in aconitase it has been suggested that cofactor coordination may provide an added flexibility to the active site, in particular to the substrate binding domain [15],... 

The molybdenum and tungsten acetylene complexes [M(RCCR)3 (CO)] and [W(RCCR)3 (MeCN)] are very interesting. All diphenylacetylene molecules in... 

In attempting to extend this reaction to derivatives of chromium and molybdenum, completely different results were obtained under comparable reaction conditions. Reaction of 3-hexyne with (CH CN) Mo(CO) did not appear to give a compound corresponding to the tungsten acetylenic complex, Hexaethylbenzene isolated from the reaction mixture indicates that the molybdenum complex causes cyclotrimerization of the acetylenic molecule. 

Titanium tetraiodide can be prepared by direct combination of the elements at 150—200°C it can be made by reaction of gaseous hydrogen iodide with a solution of titanium tetrachloride in a suitable solvent and it can be purified by vacuum sublimation at 200°C. In the van Arkel method for the preparation of pure titanium metal, the sublimed tetraiodide is decomposed on a tungsten or titanium filament held at ca 1300°C (152). There are frequent hterature references to its use as a catalyst, eg, for the production of ethylene glycol from acetylene (153). 

In the polymerization of phenyl acetylene [27] by tungsten and mo]ybdenum hexcarbonyls, high-polymer yields were obtained in CCI4 solvent. The following reaction scheme was proposed, which is different from that reported by Bamford and coworkers [17-20] ... 

Acetylene hydratase from the anaerobe Pelobacter acetylenicus is a tungsten-iron-sulfur enzyme that resembles molybdopterin with W replacing Mo (Meckenstock et al. 1999), and catalyzes the addition of the elements of water to acetylene (Figure 3.30a). 

Meckenstock RU, R Krieger, S Ensign, PMH Kroneck, B Schink (1999) Acetylene hydratase of Pelobacter acetylenicus. Molecular and spectroscopic properties of the tungsten iron-sulfur enzyme. Eur J Biochem 264 176-182. 

Rosner BM, B Schink (1995) Purification and characterization of acetylene hydratase of Pelobacter acetyle-nicus, a tungsten iron-sulfur protein. J Bacterial 177 5767-5772. 

On the basis of the fact that tungsten(VI) alkylidene complexes will metathesize olefins one might predict that acetylenes should be metathesized by tungsten(VI) alkylidyne complexes (29). Acetylene metathesis is not unknown, but the catalysts are inefficient and poorly understood (30, 31). 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

Carbonyl insertions into metallocarbenes have previously been observed for several different metals, including iron48 (see Section VI,C) and manganese.49 Indeed, carbonyl insertions into chrominum and tungsten diphenyl-carbenes have been shown to be viable processes.50 Most importantly, Wulff has isolated51 an 774-vinylketenecobalt (I) complex from the reaction between a cobalt carbene and an acetylene, a transformation that necessitates such a carbonyl insertion (see Section V,B). 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

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