Hept-2-ynes, metathesis

Catalysts such as Mo(C())6/McCN also bring about the metathesis of hept-2-yne, but not of hept-l-yne instead oligomers are formed from the latter763. 

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. 

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