Mechanism and Stereoselectivity Control of Terminal Alkyne Dimerization Activated by a Zr/Co Heterobimetallic Complex: A DFT Study

Heterobimetallic complexes have recently
garnered considerable
attention in organic synthesis owing to their high activity and selectivity,
which surpass those of monometallic complexes. In this study, the
detailed mechanisms of terminal alkyne dimerization activated by the
heterobimetallic Zr/Co complex, as well as the different stereoselectivities
of Me3SiCCH and PhCCH dimerization, were
investigated and elucidated by using density functional theory calculations.
After excluding the three-molecule reaction and outer-sphere mechanisms,
the inner-sphere mechanism was determined as the most optimal process.
The inner-sphere mechanism involves four processes: THF dissociation
and coordination of the first alkyne; ligand migration and C–H
activation; N2 dissociation and insertion of the second
alkyne; and reductive elimination. The stereoselectivity between the E-/Z- and gem-isomers
is determined by the C–C coupling mode of the two alkynes and
that of the E- and Z-isomers is
determined by the sequence of the C–C coupling and hydrogen
migration in the reductive elimination process. Me3SiCCH
dimerization yields only an E-isomer owing to the
large differences in the distortion and interaction energies, whereas
PhCCH dimerization produces an E-, Z-, and gem-isomers owing to the reduced
interaction energy differences.