The mechanism of the trimerization of alkynes in the presence of an Ir complex
bearing a hydrotris(pyrazolyl)borate (Tp) ligand has been studied using density functional
theory calculations at the B3LYP and M06 levels. In this reaction, the initial oxidative coupling
of two alkyne molecules yields an iridacyclopentadiene intermediate, which reacts with a third
alkyne molecule to give a benzene TpIr complex. There are two possible mechanisms for the
formation of the benzene complex in this reaction, including the intramolecular [4+2]
cycloaddition and Schore mechanisms. The formation of unsubstituted benzene was initially
investigated using acetylene molecules, and then the oxidative coupling reaction of 1,4-dimethyl-
2-butyne-1,4-dioate (CH3OCOCCCOOCH3) followed by the formation of the substituted
benzene complex with 2-butyne (H3CCCCH3) was studied. It has been possible to clarify the
favorable reaction pathway and the effects of different substituents on the reaction mechanism.
In the unsubstituted reaction of acetylene the [4+2] cycloaddition is more favorable than the
Schore mechanism, whereas the reaction could proceed only via the Schore mechanism in the
reactions involving substituted alkynes because of the effect of the substituents. Notably, the
effects of additional water molecules on the stability of the reaction intermediates were also evaluated because the water
complexes of several intermediates have been experimentally isolated and identified.