Photocatalyzed metathesis

Fig.5 shows the dependence of the catalytic activity of Nb20s/PVG photoactivated in the presence of ethene on the niobium content in the catalyst. The activity increases monotonously with increase in the Nb content. On the other hand, Anpo et al-have shown that tetrahedrally coordinated Mo or V ion with double-bonded oxygen ions(see Reactions[1] and [2]), which take part in photochemical process of their oxides, is predominantly formed at a rather low content in porous Vycor glass (16,17). Furthermore, it has been found that photocatalyzed metathesis (13) and photooxidation of carbon monoxide observed for Nb205/PVG show a maximum activity at 1 mg g-PyG" of the Nb content. This difference in the dependence of the activities on the Nb content... 

Finally, one may note the curious behaviour of alkynes. If Mo(CO)6/non-4-yne is irradiated at room temperature and an excess of 3-chlorophenol then added, there is rapid metathesis to give oct-4-yne and dec-5-yne with nearly 100% selectivity (Mortreux 1977). In contrast, W(CO)6/CCl4//iv (350 ran) induces polymerization of hept-2-yne while causing metathesis of pent-2-ene present in the same reaction mixture (Stockel 1978). The difference in behaviour of the two systems presumably lies in the ability of Mo(CO)6/non-4-yne// v to generate a metal carbyne on addition of the phenol, whereas W(CO)6/CCl4/Av gives only a metal carbene. Photoassisted polymerization of terminal alkynes takes place with WCV/iv in hydrocarbon solutions (Landon 1985) photocatalyzed polymerization of substituted alkynes is induced by W(CO)6/SnCl4// v (Tamura 1994). For related systems, see Ch. 10. 

Others have reported analogous photocatalyzed polymerizations when W(CO)6 is irradiated in halocarbon solvents in the presence of alkynes and halocarbene complexes have been suggested as the key intermdiates [110]. Halocarbene intermediates have also been implicated in the photocatalysis of olefin metathesis reactions by the group VI carbonyls M(CO)6 in carbon tetrachloride [111,112]. 

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