Alkyne trimerisation
An alkyne trimerisation reaction is a 2+2+2 cyclization reaction in which three alkyne molecules react to form an aromatic compound. The reaction is 'pseudo' pericyclic since it has not been observed to occur without the assistance of metal catalysis; and the metal catalyst assembles the ring stepwise via intermediates which are not directly in between (in a geometric sense) the starting material and products.
Berthelot reported the first example of cyclotrimerization leading to aromatic products in 1866, the cyclization of acetylene to benzene. The reaction required very high temperatures to proceed (400 °C) and produced a complex mixture of products, and research in the field remain dormant until the late 1940s. In 1948, Reppe discovered that nickel could catalyze the formation of substituted benzenes from acetylenic compounds; since his initial discovery, cyclotrimerizations to produce substituted benzenes have been catalyzed by no less than seventeen transition metals, including:
Control of the substitution pattern of the aromatic product is good in many cases, and cyclotrimerization can be used in cases when functionalization of pre-formed aromatic materials (through electrophilic aromatic substitution, for instance) is not selective. The reaction is highly chemoselective for triple bonds and can tolerate a wide variety of functional groups on the starting materials.
For cotrimerizations involving two or three different acetylenes, a variety of regioisomers may form. Operationally, these reactions usually require elevated temperatures (>60 °C) and sometimes require irradiation to facilitate the dissociation of strongly binding carbon monoxide ligands. Catalyst deactivation can occur through the formation of stable, 18-electron η4-complexes incorporating cyclobutadiene,cyclohexadiene, and arene ligands. The most problematic side products of the reaction are due to cyclotetramerization (leading to cyclooctatetraenes) and alkyne dimerization (leading to enynes).
...
Wikipedia