Tungsten Oxide Dispersed on Silica as Robust and Readily Available Oxo/Imido Heterometathesis Catalyst.

Continuing our investigation of catalytic oxo/imido heterometathesis as novel water-free method for C=N bond construction, we report here the application of classical transition metal oxides dispersed on silica (MO x /SiO 2 , M=V, Mo, W) as cheap, robust and readily available alternative to the catalysts prepared via Surface Organometallic Chemistry (SOMC). The oxide materials demonstrated activity in heterometathetical imidation of ketones, WO 3 /SiO 2 being the most efficient. We also describe a new well-defined supported W imido complex (≡SiO)W(=NMes) 2 (Me 2 Pyr) (Mes=2,4,6-Me 3 C 6 H 2 , Me 2 Pyr=2,5-dimethylpyrrolyl) and characterize it with SOMC protocols, which allowed us to identify the position of W on the oxo/imido heterometathesis activity scale (Mo<W<Ta) and showed that W in fact provides an optimal balance between activity and stability. Noteworthy, the performance of WO 3 /SiO 2 was only slightly inferior to that of the SOMC analog enabling the efficient synthesis of a broad range of ketimines hardly accessible by classical methods, in most cases W being a competitive and robust alternative to the most active albeit highly sensitive Ti imido systems. Combined with the availability of the catalyst, this facilitates a more widespread application of heterometathesis methodology in organic synthesis including potential larger scale applications.