Alkyne trimerisation

What is alkyne trimerisation?

The alkyne trimerisation, also known as Reppe alkyne cyclotrimerization, is a [2+2+2] cycloaddition reaction that involves the combination of three alkyne units (C≡C) resulting in the formation of a benzene ring. A metal catalyst is required to facilitate the reaction.

Alkyne trimerisation - general reaction scheme - Reppe alkyne cyclotrimerization
Alkyne trimerisation

In 1866, the first reported example of this reaction was the cyclization of acetylene to form benzene, which necessitates high temperatures and yields a combination of products. In 1948, Reppe discovered the first instance of a transition-metal catalyzed version of this reaction, which employed nickel and resulted in the formation of substituted benzenes.

Subsequently, catalysts based on transition metals, lanthanides, actinides, and aluminum have been developed for the cycloaddition of substituted alkynes to derivatives of benzene.

Alkyne trimerisation holds significance not just from a historical perspective but also in terms of its usefulness in organic synthesis. Due to its nature as a cycloaddition reaction, it has high atom economy. Several variations have been developed which include the cyclisation of mixtures of alkynes and alkenes, in addition to alkynes and nitriles.