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

Abstract

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo�CR entities in a stoichiometric triple-bond metathesis event to furnish RC�CR and the corresponding dinuclear complex, 8, with a Mo�Mo core. In addition to the regular analytical techniques, 95 Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.

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

“…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. 114 As expected, the vicdibromoalkenes neither compromise the activity of the catalyst nor become damaged. This result adds another important entry to the list of functional groups that are compatible with these versatile molybdenum alkylidynes.…”

“…This exigent transformation worked again very nicely, independent of whether the in situ catalyst mixture ( 29 / 30 ) or the structurally defined canopy catalyst 31 − 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 . As expected, the vic -dibromoalkenes neither compromise the activity of the catalyst nor become damaged.…”

“…In the future, the significance of alkyne metathesis will also be defined by the upstream and/or downstream alkyne chemistry. Therefore, our laboratory is committed to explore opportunities along these lines in parallel to our ongoing work on fundamental and applied aspects of alkyne metathesis proper. ,,,, …”

“…Therefore, our laboratory is committed to explore opportunities along these lines 136 in parallel to our ongoing work on fundamental and applied aspects of alkyne metathesis proper. 35,36,114,137,138 ■ ASSOCIATED CONTENT * sı Supporting Information…”

Acceleration of alkyne metathesis in multicomponent catalytic systems by use of alternative Mo(0) sources under optimised conditions

“…Over the last decade, alkyne metathesis has emerged as a viable tool for organic synthesis. [1,2] Arguably, this fact can be attributed to almost explosive research [3] in alkylidyne complexes of molybdenum [4][5][6][7], tungsten [6], and, most recently, rhenium, [8] however, the original multicomponent catalytic system is the more accessible option that still can find uses. [9][10][11][12] Alkyne metathesis is, in principle, analogous to its better known relativeolefin metathesis.…”