In this thesis, I am not referring to my work, but to the efforts of friends, family, and community members who have helped me reach this point. Chris Daeffler and Jean Li (and the rest of the class above me) took me in when I got to Caltech that summer before most of the other members of my class did. Ian Stewart, Kevin Kuhn, Koji Endo, Renee Thomas, Myles Herbert, Alexey Fedorov, Pinky Patel, Vanessa Marx, and Lauren Rosebrugh worked very closely with me on several projects and essentially did most of the work described in this thesis.
I also want to thank all the people in the group I worked with for their time, encouragement, and general awesomeness. All crystal structures in this thesis were obtained by Larry Henling and Mike Day, for which we are especially grateful. Finally, I would like to thank my friends (especially Eric, Chethana, Ian, Rachel, Leslie, Ted, Andrew, Young In, Matt, Taylor, Justin, and many others) and family for.
Last, but not least, I would like to thank Chithra Krishnamurthy for her love and support over the years. If it weren't for her, I'd probably still be a country bumpkin (maybe I still am), and there's so much of the world I would've missed.
Introduction
Over the past 60 years, olefin metathesis has evolved from an unusual occurrence in petroleum distillation and cracking processes to the standard method for building new C-C double bonds. The application of olefin metathesis in these fields has been facilitated by the development of a wide variety of increasingly advanced and well-defined catalysts tailored to suit these applications. A variety of other metals in the transition metal block have shown metathesis activity, but they have not been explored to the same extent.4 Our group focuses on the preparation, development and study of Ru-based olefin metathesis catalysts.
The versatility of olefin metathesis is largely due to the variety of olefins that can react with or form from this reaction. The formation of kinetic products, such as Z-olefins, has continued to be a significant challenge for olefin metathesis since its discovery. Like many organometallic catalysts, the development of more efficient olefin metathesis catalysts has been facilitated by the preparation of new ligand frameworks.
A convincing argument could be made that further development of metathesis catalysts based on Ru and other olefins is no longer necessary. One example of a new application is Z-selective olefin metathesis, and new, selective Mo, W, and Ru-based olefin metathesis catalysts have only recently been reported.
Acid- and Photo-Activated Ruthenium Metathesis Catalysts
A closer inspection of the RCM reaction revealed that the conversion profile of 2.8 to 2.9 is highly dependent on the amount of acid added and its relative strength (pKa). HCl = hydrochloric acid, TFA = trifluoroacetic acid, PFP = pentafluorophenol .. and 2.7), trends in catalyst performance reflected the reactivity of the parent dichloride complexes (e.g., 2.1 for 2.2) (Figure 2.4).14 However, a interesting trend appeared when RCM of 2.8 was performed with catalysts 2.2-2.4. This result implies that the substitution of acac ligands is an essential step in catalyst activation.
This result indicates that at least one of the catalytically active species is the 14-electron complex 2.23. Mechanistic studies indicated that the identity of the exogenous acid and the electronics of the acac ligand play a critical role in catalyst activation. Unfortunately, UV irradiation of complex (2.28) did not result in dissociation of CO or indeed any change in the complex.
The contents of the tube were emptied and concentrated before purification of the product by column chromatography on silica gel. The contents of the NMR tube were emptied into a vial and the solvent removed under reduced pressure, after which the residue was dissolved in a minimal amount of THF and precipitated into cold MeOH (poly-(cyclooctene)) or cold 1:1 Et2O. /hexanes (polynorbornenes).
Preparation and Reactivity of Mesoionic Carbene (MIC)–Contain- ing Ruthenium Metathesis Catalysts and their Acid-Activated
Behavior
- In contrast to similar intermediates observed during the metalation of MICs 3.15–3.18, compound 3.29 was indefinitely stable and phosphine dissociation
The solid state structure of 3.31 (Figure 3.11) was consistent with previously reported bis-NHC complexes Figure 3.11. Benzylidene proton resonance 3.31 was monitored by 1H NMR spectroscopy after the addition of various amounts of TFA. The graph of the observed rate constant (kobs) against the TFA concentration in CD showed a second-order dependence on the TFA concentration (Figure Figure 3.13.
This behavior is consistent with protonation of 3.31 by an acid dimer instead of an acid monomer. To investigate this possibility and also to simplify the acid–base chemistry of the system, we decided to monitor the onset of 3.31 in CD3CN rather than in C6D6. For example, the reaction of 3.31 in C6D6 after the addition of excess HCl (> 15 equiv.) resulted in a decrease in the benzylidene proton signal Figure 3.16.
Regardless of the exact activation mechanism of 3.31 in C6D6 with HCl, the saturation behavior explains why, under some conditions, a weaker acid (TFA) can more efficiently activate 3.31 (e.g., Fig. Continuing our mechanistic studies, the growth of product 3.3. monitored after treatment of 3.31 with acid in the presence of varying amounts of olefin 3.33 as a scavenger A final question we wanted to answer was whether the behavior of 3.31 was due to the unique nature of the MIC ligand, or whether other conventional NHC ( in 3.3 ) would act in a similar way.
A portion (0.25 mL) of the above solution was transferred to an NMR tube and diluted with C6D6 (0.35 mL) so that the final concentration of 3.31 was ca. The exact temperature of the NMR probe was determined as described in the General Information. Reaction Mechanism Study by Mass Spectrometry: A 1 mM solution of 3.31 in C6H6 was prepared and TFA (5 mL) was added.
(Note: Over long periods of time (hours), 3.31 would decompose in the presence of CD3CN, therefore all samples for kinetic runs were prepared immediately before use.) The NMR tube was removed from the glove box and placed inside the spectrometer. Representative procedure and kinetic plots for reaction of 3.31 with TFA in CD3CN containing varying amounts of KTFA (Variable pH): A 1 mL volumetric flask was charged with KTFA (13.6 mg, 0.0907 mmol) and filled to the line with CD3CN. All other amounts of HCl showed good first-order behavior until the reaction was complete.
Degenerate (Nonproductive) Reactions with Ruthenium Metathesis Catalysts
For example, catalysts with N-aryl/N-aryl NHC ligands showed high selectivity for productive metathesis while those with N-aryl/N-alkyl NHC ligands showed selectivity for degenerate metathesis. Finally, the relationship between degenerate metathesis and selectivity for kinetic metathesis products is also discussed, together with the application of degenerate-selective catalysts for the ethenolysis of methyl oleate. In these studies, the rate of degenerate metathesis was found to exceed that of productive metathesis by approximately an order of magnitude.
For example, switching the aryl group of the NHC of Mes or ortho-tolyl (4.17) to the larger 2,6-diisopropylphenyl (DIPP, 4.18) resulted in a large increase in selectivity for degenerate metathesis (blue-green line in Figure 4.6). Similar selectivity for degenerate metathesis was measured when testing catalysts with N-aryl/N-alkyl NHCs. To investigate this, we performed the RCM of 4.5-d2 with catalyst 4.20, as this catalyst is relatively selective for degenerate metathesis but is also able to reach a very high TON.
Following our temperature studies, we next examined the effect of concentration on the selectivity of degenerate metathesis. To evaluate the effect of degenerate metathesis on a more challenging reaction, the RCM of 4.24 was tried. As before, productive metathesis was measured by GC while degenerate metathesis (at 4.24-d6 and 4.24-d2) was monitored by LCMS-TOF (Figure 4.11).
In other words, there are more possibilities for degenerate metathesis because the RCM of 4.24 is relatively slow. Due to the limitations described above, we turned to kinetic modeling to reproduce the selectivity curves in Figure 4.6 and Figure 4.11 and increase our understanding of the reactions that give rise to degenerate metathesis. During our research on degenerate metathesis, we noticed that catalysts had a higher selectivity for degenerate metathesis.
Recall that 4.20, as well as catalysts similar in structure to 4.31–4.39 showed increased selectivity for degenerate metathesis. We also investigated the consequences of degenerate metathesis selectivity in the ethenolysis of methyl oleate (4.26), a reaction with potential industrial applications. These results show that in some circumstances selectivity for degenerate metathesis can actually be beneficial.
Kinetics of Ruthenacyclobutanes Related to Degenerate Metathesis
Selective Ruthenium Metathesis Catalysts
- Similar results were achieved for the alcohol substrate 6.15 (Figure 6.16)
During our first attempts at the cross metathesis of 6.3 and 6.4, we observed a significant amount of the homodimer cross product 6.6 (Figure 6.4). Nevertheless, we reasoned that the reaction conditions could be optimized to provide good yields of 6.6 and good selectivity to the Z-isomer. We suspected that the poor performance of 6.2 under these conditions was a result of ethylene generated as a by-product of the contingency table.
This result is not surprising, since the onset of 6.2 should depend on olefin concentration. While the bidentate complexes exhibited initiation rates comparable to those of 6.A, complexes 6.2 and 6.24 initiated at significantly slower rates even at higher temperatures. For example, exposure of 6.18 to AgOOCCF3 led to immediate alkylene insertion and subsequent decomposition to the Crude olefin complex 6.32 (Figure 6.14).
Among the carboxylate-based catalysts, 6.19 was the least active, giving low conversion of 6.3 and poor selectivity for the desired product 6.6 . Monitoring the time course of reaction 6.7 with catalysts 6.24–6.27 revealed some subtle differences between the nitrate catalysts (Figure 6.15). After establishing the efficiency of 6.24 in several homodimerization reactions, we turned our attention to more complex reactions, incl. standard” cross-metathesis reaction between 6.3 and cis-1,4-diacetoxybutene Table 6.9.
In the extreme case, this was evidenced by the inactivity of monodentate ligands, but also by the lower activity of 6.20. For example, when a solution of 6.24 in C6D6 was exposed to air, the benzylidene resonance of 6.24 was still observed after 12 hours by 1 H NMR spectroscopy. This could be observed qualitatively because a solution of 6.24 and 6.34 remained purple (the color of 6.24) even after complete conversion of the monomer.
Similar to the ROMP of 6.38 (norbornene) and 6.39 (norbornadiene), increasing the temperature of the polymerization of 6.35 led to polymers with a lower cis content, although it never went below 80%. The use of 6.24 therefore resulted in a significant improvement in the cis content of poly-6.47, albeit to a lesser extent than expected. Nevertheless, we reasoned that a more significant increase in strain energy, due to the use of trans-cyclooctene (6.49), would provide access to the desired polymer.42 Indeed, reaction of 6.24 with 6.49 at RT in THF led to the immediate and high yield production of poly-6.49.
