Switching on the Metathesis Activity of Re Oxo Alkylidene Surface Sites through a Tailor-Made Silica-Alumina Support

Re oxo alkylidene surface species are putative active sites in classical heterogeneous Re‐based alkene‐metathesis catalysts. However, the lack of evidence for such species questions their existence and/or relevance as reaction intermediates. Using Re(O)(=CH‐CH=CPh2)(OtBuF6)3(THF), the corresponding well‐defined Re oxo alkylidene surface species can be generated on both silica and silica–alumina supports. While inactive on the silica support, it displays very good activity, even for functionalized olefins, on the silica–alumina support. Al‐ctivation: Supporting a well‐defined Re oxo alkylidene on a silica–alumina support gives a highly active metathesis catalyst, while the corresponding species on silica is inactive. This work shows the importance of Lewis acid sites in activating these catalysts.