Productive Alkyne Metathesis with “Canopy Catalysts” Mandates Pseudorotation

Abstract

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.

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap

“…[ 17 , 22 , 23 ] Combined spectroscopic, crystallographic and computational studies unveiled the unorthodox mechanism by which such “canopy catalysts” operate as a consequence of the geometric constraints imposed onto the reactive intermediates by the tripodal silanolate ligand sphere. [ 24 , 25 ]…”

“…The Lewis acidity of the W(+6) center in 5 is upregulated to the extent that the catalytic turn‐over ceases. [ 24 , 25 , 26 ] Therefore, a fundamentally different ligand design had to be pursued: [26] complex 6 with an expanded tris‐alkoxide (rather than siloxide) chelate ligand framework constitutes a promising new lead in that it outperformed a classical Schrock catalyst of type 7 (R=2,6‐dimethylphenyl),[ 27 , 28 , 29 , 30 ] although it still does not rival the best molybdenum catalysts known to date.…”

“…While the basic features of the new catalyst families are fairly well understood,[ 16 , 17 , 18 , 24 , 25 , 26 ] it is mandatory to refine the picture in order to lay a solid ground for further improvements. To this end, the following important aspects need to be addressed, which had only briefly been touched upon in our original disclosures: how stable are the podand ligand frameworks of 1 and 6 ?…”

A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I

“…This exigent transformation worked again very nicely, independent of whether the in situ catalyst mixture ( 29 / 30 ) 56 or the structurally defined canopy catalyst 31 ( 33 − 35 ) was used. The need for fairly high loadings is again tentatively ascribed to the forcing conditions necessary to override the incipient ring strain of the polycyclic product 70 , which certainly also accelerate catalyst decomposition (for reaction optimization, see the SI ); this notion is supported by recent results from our laboratory.…”

“…Gratifyingly, we found that the newly developed and molecularly well-defined molybdenum alkylidyne complex 31 distinguished by a tripodal silanolate ligand sphere is at least equipotent. 33 − 35 Although a fairly high loading and rather forcing conditions proved necessary to overcome the strain of the incipient tetracyclic scaffold, cycloalkyne 28 was obtained in 83% yield on a 1.3 g scale (for reaction optimization, see the Supporting Information (SI)); its constitution and stereostructure were unambiguously established by 2D NMR spectroscopy and X-ray diffraction ( Figure 1 ).…”