Cyclotrimerization
An alkyne trimerisation reaction is a 2+2+2 cyclization reaction in which three molecules of alkyne react to form an arene. The reaction requires metal catalysts. The process is of historic interest as well as being applicable to organic synthesis. Many variations have been developed including cyclization of mixtures of alkynes and alkenes as well as alkynes and nitriles.
In 1948, Reppe discovered that nickel compounds catalyze the formation of substituted benzenes from acetylenes:
Both 1,3,5- and 1,2,4-C6R3H3 isomers are observed.
Since Reppe's discovery, many other cyclotrimerizations have been reported.
The reaction does not occur without the assistance of metal catalysis. The reactions begin with the formation of metal-alkyne complexes. The combination of two alkynes within the coordination sphere affords a metallacyclopentadiene. Starting from the metallacyclopentadiene intermediate, two pathways are usually discussed:
For cyclotrimerization reactions of unsymmetrically substituted acetylenes, the substitution pattern about the product arene is determined in two steps: formation of the metallocyclopentadiene intermediate and incorporation of the third equivalent of alkyne. Although both head-to-head and tail-to-tail metallocyclopentadienes lead to 1, a number of acetylenic substrates selectively form regioisomers of type 2. Steric bulk on the alkyne coupling partners and catalyst have been invoked as the controlling elements of regioselectivity.
Chiral catalysts have been employed in combination with arynes to produce non-racemic atropisomeric products.
Catalysts for cyclotrimerization are highly selective for triple bonds, which gives the reaction a fairly wide substrate scope; alcohols, ethers, amines, and carbonyl compounds (ketones, esters, amides, and carboxylic acids) are all well tolerated.Nitriles can react with alkynes to give pyridines.
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