Reducing Them Down To Charge Them Up: Low Temperature Catalyst Activation

The largest industrial application of olefin metathesis today is the synthesis of propylene from ethylene and butenes employing WO3 on SiO2, a relatively long-lived and regenerable catalyst that operates at 350–400 °C. It is widely proposed that high temperatures are required because the percentage of metal sites actually involved in the metathesis reaction is extremely low, or the reaction that generates alkylidenes is not a high yield reaction, or both. A recent paper by Copéret, Mashima, and co-workers tackles head-on the question concerning how in WO3/SiO2 catalysts the alkylidene is formed from an olefin alone. Hundreds of papers have attempted to answer this question, although one has to admit that there may not be a single answer for all supported oxide catalysts or all olefins. Copeŕet and Mashima employ Me4BTDP to reduce fourcoordinate (SurfO)2WO2 sites on silica in the absence of olefins to give 2,3,5,6-tetramethylpyrazine, hexamethyldisiloxane, and M(IV) sites (eq 1). Analogously, five-coordinate