Development of Ruthenium Catalysts for Ethenolysis

Abstract

Olefin metathesis catalysts are now broadly used to construct carbon−carbon double bonds in the synthesis of pharmaceuticals, petrochemicals, fine chemicals, and polymers. The thesis is to develop novel ruthenium-based metathesis catalysts for ethenolysis (cross-metathesis with ethylene). The activity, selectivity, and stability of ruthenium metathesis complexes depend on the steric and electronic properties of ligands on the metal complexes. Notably, the sterically unsymmetrical substitution on ligands turned out to be crucial for the highly α-olefin selectivity. The use of highly σ-donating ligands is believed to stabilize the key intermediates for the enhanced productivity in the metathesis. Futhermore, the use of additives can improve catalyst activity and stability in metathesis reactions. This thesis describes the development of ruthenium metathesis catalysts and their structure−activity relationship studies directed at understanding the factors. Chapter 2 describes the preparation of ruthenium metathesis catalysts containing fluorinated imidazo[1,5-a]pyridin-3-ylidene carbene ligands (F-ImPy–Ru). This fluorine substitution appears to be pivotal to have stable ImPy–Ru catalysts via Ru−F interaction. Effects of ligand structure variations on catalyst activity, selectivity, and thermal stability were studied. The ruthenium catalysts exhibited high selectivity in ethenolysis and thermal robustness under an ethylene atmosphere. Chapter 3 explores the bimetallic system containing fluorinated-imidazo[1,5-a]pyridine abnormal carbene ruthenim complexes (F-aImPy–Ru) and tricyclohexylphosphine copper chloride (Cy3P–CuCl) for ethenolysis/propenolysis. These catalysts showed the remarable efficiencies (turnover number of 100,000 for ethenolysis and 200,000 for propenolysis) and excellent α-olefins selectivity. The computational study indicated that ruthenium and copper metal centers could be connected through bridging chloride. The coordination of Cy3P–CuCl to the ruthenium metal center could enhance the stability of the ruthenium methylidene species by blocking decomposition pathways. Chapter 4 details the preparation and their properties of acyclic aminooxycarbene ruthenium catalysts for ethenolysis. The superior activities (turnover numbers of 100,000 for methyl oleate and 89,000 for cis-cyclooctene) of ruthenium catalysts was correlated to their ambiphilic properties as strong σ-donating and π-accepting.