Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts

The
intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2
+ 2] cycloaddition-based mechanism is a well-established paradigm
in alkyne metathesis with alternative species viewed as off-cycle
decomposition products that interfere with efficient product formation.
Recent work has shown that the exclusive intermediate isolated from
a siloxide podand-supported molybdenum-based catalyst was not the
expected metallacyclobutadiene but instead a dynamic metallatetrahedrane.
Despite their paucity in the chemical literature, theoretical work
has shown these species to be thermodynamically more stable as well
as having modest barriers for cycloaddition. Consequentially, we report
the synthesis of a library of group VI alkylidynes as well as the
roles metal identity, ligand flexibility, secondary coordination sphere,
and substrate identity all have on isolable intermediates. Furthermore,
we report the disparities in catalyst competency as a function of
ligand sterics and metal choice. Dispersion-corrected DFT calculations
are used to shed light on the mechanism and role of ligand and metal
on the intermediacy of metallacyclobutadiene and metallatetrahedrane
as well as their implications to alkyne metathesis.