| The influence that each ligand on the ruthenium phosphonium alkylidene framework [(L)(X)2Ru=C(H)PR3] [A] has on precatalyst initiation, longevity and efficiency performing olefin metathesis reactions, has been explored in detail.;Thermal decomposition studies were performed on complexes [(H 2IMes)(Cl)2Ru=C(H)PR3][ClB(C6F 5)3] (R =iPr, Cy) to probe the mechanism of precatalyst degradation. In addition, compounds with varying amounts of deuterium labeling at the o-methyl substituents on the N-aryl groups of the H2IMes NHC ligand (d6-H 2IMes, d11-H2IMes, d22-H2IMes) were synthesized and both intramolecular and intermolecular kinetic isotope effects were determined. Kinetic isotope effect values ranging between 4.4-6.9 provided excellent evidence that C-H activation at an o-methyl substituent on the N-aryl group of H2IMes is the rate-determining step in the thermal decomposition of these precatalysts.;Two complexes, [(H2IMes)(I)2Ru=C(H)P iPr3] [IB(C6F5)3] and [(H2IMes)(CF3COO)2Ru=C(H)P iPr3] [CF3COOB(C6F 5)3], containing anionic ligands other than chloride were synthesized, characterized, and thermally decomposed. The complex with iodide ligands was found to be more thermally stable, while the perfluoroacetate-containing complex was less stable relative to the chloride derivative. Complexes with different o-N-aryl substituents on the NHC ligand, namely H2IDEP (o-Et substituents) and H2ID iPP (o-iPr substituents), were also synthesized, characterized and thermally degraded. The complex containing the H2IDEP ligand was less thermally stable, while the complex containing the H2IDiPP ligand was more thermally stable than the derivative containing H2IMes. A series of standard metathesis reactions were performed to compare the various catalysts' activities, providing insight into the pertinent features necessary for efficient ruthenium phosphonium alkylidene precatalysts.;First, varying the steric bulk of the phosphorus substituents on the phosphonium alkylidene moiety in a series of precatalysts [(H2IMes)(Cl) 2Ru=C(H)PR3][ClB(C6F5)3] (R3 = Cy3, Cyp3, iPr 3, EtCy2, MeCy2, EtiPr 2, MeiPr2) demonstrated a clear structure-activity relationship in initiation reactions with o-isopropoxystyrene. Decreasing the steric bulk of the phosphonium group increased the rate of initiation as there was more room for the incoming olefin to bind to the ruthenium center. Further, evidence for two binding orientations of the monosubstituted olefinic substrates was found, suggesting two pathways of reactivity. The more sterically encumbered pathway produces the unsubstituted olefin ethene, which subsequently reacts with the precatalyst autocatalyzing the initiation reaction. |
