Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap. Issue 56 (26th August 2021)

• Record Type: Journal Article
• Authors: Hillenbrand, Julius; Korber, J. Nepomuk; Leutzsch, Markus; Nöthling, Nils; Fürstner, Alois
• Is Part Of: Chemistry. Volume 27:Issue 56(2021)
• Page Start: 14025
• Page End: 14033
• Publication Date: 2021-08-26
• Subjects: alkyne metathesis -- metal-metal bonding -- metal alkylidynes -- molybdenum -- tungsten; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202102080
• Languages: English
• ISSNs: 0947-6539

Abstract

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. Abstract : Vulnerable ? The activity and functional group tolerance of metal alkylidyne canopy catalysts for alkyne metathesis is excellent, but their stability leaves room for optimization. The search for the Achilles′ heels unveiled a new decomposition pathway through bimolecular coupling and led to the discovery of a facile cyclo‐tetramerization reaction by (reversible) opening of the tripodal ligand cage.