Towards Highly Active and Robust Alkyne Metathesis Catalysts: Recent Developments in Catalyst Design

Abstract

alkylidynes · alkynes · homogeneous catalysis · metathesis reactions · molybdenumThough alkyne metathesis has crossed several barriers after the initial discovery of the heterogeneous catalyst system based on tungsten oxide and silica, it is still in its infancy as a synthetic method.[1] It is highly desired to develop a catalyst that can operate under ambient conditions and tolerate air and moisture without compromising the metathesis activity and functional-group compatibility. [1,2] This Highlight is intended to showcase some recent developments in the catalyst design and spotlight the immense potential of this unique multi-bond-exchange reaction.Subsequent to the discovery of the Mortreux system consisting of [Mo(CO) 6 ] and phenol additives, [1, 3] Schrock and co-workers pioneered the development of metal alkylidyne complexes, such as the prototype neopentylidyne complex [Me 3 CC W(OCCMe 3 ) 3 ]. [2,4] Since then, a significant number of reports have appeared in the literature regarding the synthesis and applications of several molybdenum-and tungsten-based alkylidyne complexes, including solid-supported catalysts.[2, 4-9] The Mo III trisamido species [Mo-(NArR) 3 ] (Ar = 3,5-dimethylphenyl, R = tert-butyl), initially developed by Cummins and co-workers, [5] has received particular attention, as it can serve as the precursor to metathesis-active catalysts, either by in situ activation using dichloromethane, [6] leading to the formation of mixed species such as [MoCl(NArR) 3 ] and [MoCH(NArR) 3 ], or by a reductive recycling approach [7] in the presence of excess of magnesium and R'CHCl 2 (R' = H, Me, Et) to generate the corresponding alkylidyne [Mo(CR')(NArR) 3 ]. The alkylidyne [Mo(CR')(NArR) 3 ] reacts with substituted phenols and generates highly active catalysts in situ and has been successfully employed in the synthesis of conjugated polymers and shape-persistent macrocycles with high efficiency. [2d, 7, 8] The alkylidyne was also grafted onto silica and polyhedral oligomeric silsesquioxane (POSS), a homogeneous mimic of silica, where the silanols replace the aniline ligands and the resulting catalyst system is reported to have high stability, presumably owing to the greatly minimized bimolecular decomposition pathway of the catalyst. [9] In addition, the polymerization of small alkyne substrates (Scheme 1) such as 2-butyne, the common metathesis by-product of propynyl substrates, can also be inhibited, as the bulky POSS spatially blocks concurrent substrate binding at the two open binding sites of the hexavalent molybdenum and tungsten centers. However, lack of structural tunability represents a potential drawback of this silica-based heterogeneous catalyst system. Over the past two decades, even though the alkylidynebased alkyne metathesis catalysts have shown great promise, many serious issues remained unsolved, such as their high sensitivity to air and moisture, incompatibility with terminal alkyne substrates, low or no reactivity towards heterocyclic substrates, and the undesired polymerizatio...

Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes

“…With both mechanistic scenarios, the selectivity towards diyne metathesis is explained satisfactorily, whereas diyne disproportionation can be rationalized only by deviation from the α,α‐mechnism, e.g., with triyne formation via an α,β‐dialkynyl‐MCBD. To further validate these assumptions, we would like to present in this contribution a combined experimental and theoretical study that will help to further unravel the mechanism of the 1,3‐diyne metathesis reaction, which we regard as an important expansion of the increasingly important field of alkyne metathesis …”

Highly Active Multidentate Ligand‐Based Alkyne Metathesis Catalysts

“…[17][18][19] Numerous molybdenum(VI) and tungsten(VI)a lkylidyne complexes have been developed. [20][21][22][23][24][25][26][27][28][29][30] Most well-defined catalysts are Mo/W-based alkylidyne catalysts, which can be formulatedas X 3 MCÀR. The ligands (X) in the complex play ac rucial role in catalystp erformance.…”

Synthesis of a conjugated porous Co(ii) porphyrinylene–ethynylene framework through alkyne metathesis and its catalytic activity study

“…38 Recently, we demonstrated that poly(aryleneethynylene) networks constructed through dynamic alkyne metathesis [39][40][41][42] shows signicantly improved specic surface areas and thermal stability than the frameworks constructed through commonly-practiced irreversible crosscoupling reactions. 38 Recently, we demonstrated that poly(aryleneethynylene) networks constructed through dynamic alkyne metathesis [39][40][41][42] shows signicantly improved specic surface areas and thermal stability than the frameworks constructed through commonly-practiced irreversible crosscoupling reactions.…”