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

Abstract

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.