Improving the Efficiency of Ruthenium-Catalyzed Olefin Metathesis with Solid-Supported Catalysts, Microfluidic Reactors, and Novel X-Type Ligands

Citation

Van Wingerden, Matthew Martin (2013) Improving the Efficiency of Ruthenium-Catalyzed Olefin Metathesis with Solid-Supported Catalysts, Microfluidic Reactors, and Novel X-Type Ligands. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2A9B-HY16. https://resolver.caltech.edu/CaltechTHESIS:01062013-162840550

Abstract

Olefin metathesis has become an important tool in modern organic chemistry. Key to the development of olefin metathesis as a methodology has been the discovery of the highly active, selective, and tolerant ruthenium-based Grubbs catalysts. The overall efficiency and utility of these catalysts are determined by a complex set of parameters including catalyst design, reaction conditions, reactor design, and purification strategy. These parameters can be varied to achieve higher catalyst turnovers, better product selectivity and reduced product contamination. This research seeks to improve the efficiency and utility of olefin metathesis using three strategies; the covalent attachment of catalysts to silica supports, the development of biphasic microfluidic reactors, and the synthesis of novel catalyst architectures.

Solid-supported catalysts present an effective strategy to eliminate metal contamination in metathesis products. These catalysts, however, are generally ill defined and their active species and decomposition pathways are poorly understood. In order to further study both the activity and decomposition of silica-supported catalysts, both a brominated alkylidene ligand and a cleavable linker were prepared. The brominated ligand was designed to bind only active catalyst, but was found to indiscriminately bind all ruthenium species. The cleavable linker was synthesized with an ortho-benzyl nitro ether moiety, rendering it cleavable by UV light. Future studies will use this UV-triggered lability to study the solid-supported catalysts with solution phase techniques.

Biphasic microfluidic reactors were developed to address the generation or consumption of ethylene gas in metathesis. By using either alternating flow or parallel flow gas-liquid reactors, the mass transfer of ethylene was facilitated. The enhanced mass transfer gave higher yields and catalyst turnovers in ethenolysis and ring-closing metathesis reactions.

Novel catalyst architectures were designed and synthesized to increase catalyst activity. While chloride-based catalysts have generally been used because of their higher activity, the activity of fluoride and hydroxide catalysts remains under explored, due mainly to practical challenges associated with their synthesis. A set of fluoride catalysts based on the Piers-type catalyst and a hydroxide catalyst based on the recently developed Z-selective catalysts were synthesized and characterized. The hydroxide catalyst showed promising activity while the fluoride catalyst was found to be inactive under all but the most forcing of conditions.

In general, the utility of ruthenium-based catalysts has caused rapid growth in the field of olefin metathesis. The work presented herein covers a variety of strategies to improve the overall utility and efficiency of these catalysts, including insights into decomposition pathways, controlling phase interactions, and synthesizing novel catalysts. Further pursuits of these strategies will prove valuable to the advancement of olefin metathesis.

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