Thioether tungsten

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]. 

If olehn metathesis is to be conducted in solution, solvents of low Lewis-basicity will generally give the best results (CH2CI2 > toluene > THF). As discussed above, metathesis is initiated by the formation of a jt-complex between the metal and the alkene. Hence, other nucleophiles will compete with the alkene for these coordination sites and in some systems even THF can lead to complete deactivation of the catalyst [786]. Tungsten-based catalysts which can even metathesize allyl thioethers have, however, been described [787]. 

The most active d metal peroxo complexes toward nucleophilic substrates, like amines, phosphines, thioethers, double bonds etc., are molybdenum, tungsten and rhenium derivatives vanadium and titanium catalysis is also important, in particular when... 

Cyclic thioethers, in tungsten complexes, 5, 643 Cyclic trigermane, halogenation, 3, 794 Cyclic voltammetry... 

In fluid-solution systems, the plain hydrogenation of thiophenes to thioether products has been found to be catalyzed by various transition metal complexes surprisingly, none of these catalysts contains either molybdenum or tungsten, which are essential components of heterogeneous HDS catalysts [1 b]. Unpromoted M0S2, on the other hand, is quite active for the HDS of thioethers with no need of assistance by a late transition metal promoter [lb]. 

This rare case of homogeneous C-S bond scission by a tungsten complex, shown in Fig. 4.12 has been extended to other nucleophiles and to other bridging thioethers e.g. (28). Its main relevance lies in the fact that it is based on one of the commonly used metals in practical HDS. Apparently, the acute W-S-W angle ca. 62.5°) arising from a very short W-W bond may actually contribute to the weakening of the C-S bonds. 

We have found that metathesis of olefmic sulfides can be successfully achieved by using, as homogeneous catalyst, the aryloxy(chloro)neopentylidene-tungsten complex 1. Thus, the self-metathesis of allyl methyl sulfide (4) and of 5-dkylthiocycloooctenes (5) (or their cometathesis with various acyclic or cyclic olefins without functional groups) can lead to a family of new olefins, dienes, or unsaturated polymers containing one or more thioether groups. 

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