Productive Alkyne Metathesis with “Canopy Catalysts” Mandates Pseudorotation

Molybdenum alkylidyne
complexes of the “canopy catalyst”
series define new standards in the field of alkyne metathesis. The
tripodal ligand framework lowers the symmetry of the metallacyclobutadiene
complex formed by [2 + 2] cycloaddition with the substrate and imposes
constraints onto the productive [2 + 2] cycloreversion; pseudorotation
corrects this handicap and makes catalytic turnover possible. A combined
spectroscopic, crystallographic, and computational study provides
insights into this unorthodox mechanism and uncovers the role that
metallatetrahedrane complexes play in certain cases.