Trimerization of Alkynes in the Presence of a Hydrotris(pyrazolyl)borate Iridium Catalyst and the Effect of Substituent Groups on the Reaction Mechanism: A Computational Study

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.