3- Hexyne cocycloaddition

Chemoselective cocycloaddition of two molecules of a terminal alkyne together with one molecule of an internal alkyne into a benzene product is possible due to the relative unreactivity of phosphine nickel carbonyls towards simple trimerization of the internal alkyne itself (equation 28). ° " Careful control of reaction conditions has also permitted selective intermolecular cocycloaddition in the presence of cobalt catalysts as well for example, diphenylacetylene and 3-hexyne give rise to a 57% yield of 1,2,3,4-tetra-pheny 1-5,6-diethylbenzene. ... 

The cocycloaddition of a,d)-cyanoalkynes with alkynes is another efficient entry to bicyclic pyridines. In this process an intermediate metallacyclopentadiene is formed intermolecularly from the cyanoalkyne and the alkyne. The larger alkyne substitutent ends up nearer the metal with very high regioselectivity except in the case of 2-hexyne, where selectivity is reduced to about 3 2 (equation 43). ... 

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