Olefin Metathesis Catalyst Supported by a Hemilabile NHC Ligand Bearing Polyether Arms: Structure, Activity, and Decomposition.

An olefin metathesis catalyst (8) bearing a hemilabile N-heterocyclic carbene (NHC) ligand with four methoxyethoxy arms was synthesized and analyzed by X-ray crystallography and NMR spectroscopy. The solid-state structure of the catalyst reveals that one of the aromatic rings is rotated toward the plane of the NHC heterocycle, which enables one ortho-O→Ru coordination. Ring-closing metathesis (RCM) activity was tested at low catalyst loadings. Catalyst 8 was found to display a significant rate enhancement relative to the commercial catalyst 3 but offered no improvement in turnover numbers (TON). The solid-state structure of the catalyst's major degradation product (9) in the presence of 1-hexene reveals a dinuclear compound containing a bridging methylidene (Ru-CH2-Ru), indicating that the methoxyethoxy arms may not be strong enough to stabilize the (NHC)RuCl2(=CH2) catalyst resting state.