Ruthenium-Based Olefin Metathesis Catalysts: Synthesis, Mechanism, and Activity

Citation

Dias, Eric Lee Kuiokalani (1998) Ruthenium-Based Olefin Metathesis Catalysts: Synthesis, Mechanism, and Activity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/30mh-mq42. https://resolver.caltech.edu/CaltechTHESIS:11012023-164245860

Abstract

Several ruthenium-based olefin metathesis catalysts of the formula (PR₃)₂X₂Ru=CHCHCPh₂ have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined: X = I < Br < Cl, and PR₃ = PPh₃ << PⁱPr₂Ph < PCy₂Ph < PⁱPr₃ < PCy₃. Additional studies were conducted with the catalyst (PCy₃)₂Cl₂Ru=CH₂ to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways: the dominant one in which a phosphine dissociates from the ruthenium center, and a minor one in which both phosphines remain bound. Higher catalyst activities could be achieved by the addition of CuCl to the reaction.

The carbenes (PCy₃)₂Cl₂Ru=CHR (R = CHCPh₂, Ph) react with the bridged-chloride dimers [(p-cymene)RuCl₂]₂, [(p-cymene)OsCl₂]₂, and [(ᵗBu₂Cp)RhCl₂]₂ to quantitatively form bimetallic, bridged-chloride ruthenium carbenes and and one equivalent of each corresponding piano-stool complex. In the ring-opening metathesis polymerization (ROMP) of 1,5-cyclooctadiene, catalyst activity was found to increase in the order M = Ru < Os < Rh for the ancillary metal centers, with all of the bimetallic catalysts having higher activities. The kinetics of ROMP were studied, and the data support an associative mechanism of olefin metathesis.

Reaction of excess pyridine with (PCy₃)₂(Cl)₂Ru=CHPh produces the stable, bis(pyridine) adduct (PCy₃)(pyr)₂(Cl)₂Ru(CHPh). In solution, an equilibrium is established with the mono(pyridine) adduct (PCy₃)(pyr)(Cl)₂Ru(CHPh). Reaction of thallium salts of ,B-diketonates with (PCy₃)₂ChRu=CHR (R = CHCPh₂, Ph) produces the complexes (PCy₃)(L)₂Ru(CHR) (L = acac, ᵗBu₂acac; R = CHCPh₂, Ph). While the pyridine complexes are stable and completely initiate RCM, the propagating methylidene is very unstable, decomposing as fast as, or faster than, it is formed. The β-diketonate complexes initiate ROMP in the presence of HCl and RCM in the presence of CuCl.

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