Alkyne Metathesis with Silica-Supported and Molecular Catalysts at Parts-per-Million Loadings

Improvement of the activity, stability, and chemoselectivity of alkyne‐metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBuF6)3] (1, Mes=2,4,6‐trimethylphenyl, OtBuF6=hexafluoro‐tert‐butoxy) onto partially dehydroxylated silica gave a well‐defined silica‐supported active alkyne‐metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBuF6)2] (1/SiO2‐700). Both 1 and 1/SiO2‐700 showed very high activity, selectivity, and stability in the self‐metathesis of a variety of carefully purified alkynes, even at parts‐per‐million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO2‐700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO2‐700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface. Up to speed: Extremely low catalyst loadings and turnover numbers as high as 54 000 were possible with the new heterogeneous alkyne‐metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBuF6)2], as well as turnover numbers up to 185 000 with its molecular precursor [MesC≡Mo(OtBuF6)3] (see picture; OtBuF6=hexafluoro‐tert‐butoxy). High reactivity, selectivity, and stability of the catalysts were observed with carefully purified alkynes.