Tailored Ru-NHC heterogeneous catalysts for alkene metathesis

The preparation of highly active and stable Ru-NHC (ruthenium-N-heterocyclic carbene ligands) alkene metathesis catalysts through surface organometallic chemistry on hybrid mesostructured materials was reported. Two types of mesoporous hybrid materials were prepared, displaying the same texture, porous network structure and concentration of organic functionalities. The only difference was in the nature of the spacers, propylmesityl imidazolium iodide (M-ImPr) versus benzylmesityl imidazolium chloride (M-ImBn). M-ImPr and M-ImBn were converted into their corresponding Ru-NHC derivatives, M-RuPr and M-RuBn, by reaction with potassium hexamethyldisilylazide (KHMDS). The catalytic activity of these materials was tested in the metathesis of ethyl oleate. The M-RuBn displays comparable performances with a slightly lower TOF of 30 min^-1. The stereoselectivity at low conversions was then used as a tool to characterize the active sites in metathesis. These materials display an initial E/Z ratio of about 2, which shows that they behave like their NHC homogeneous homologues.