Olefin metathesis has had tremendous impact on synthetic approaches to the formation of new carbon-carbon bonds. Heterogeneous metathesis catalysts have been used in industry for decades, to effect redistribution of olefin chain lengths in petrochemicals processing. Only recently, however, has olefin metathesis emerged in pharmaceutical and specialty chemical manufacturing. The nearly 20-year gap between the discovery of easily-handled ruthenium catalysts and industrial implementation in these sectors is a result of many factors. One key contributor is the limited understanding of decomposition mechanisms that limit the reliability of olefin metathesis. Poor catalyst selectivities and yields remain a challenge for industrial uptake of olefin metathesis. Much academic effort focuses on designing methods and new catalysts for catalyst separation and reuse. Exploration of catalyst decomposition pathways has seen much less study in comparison. The limited recognition of decomposition as a problem in academia is undoubtedly due to the tendency to use high catalyst loadings, which mask catalyst decomposition problems. The catalyst loadings commonly reported in academia need to be decreased by a factor of 100–1000 for industrial viability in many processes. This thesis explores the decomposition of metathesis catalysts bearing a phosphine ligand. Such Grubbs-type catalysts, particularly second-generation versions containing an N-heterocyclic carbene ligand, are the most widely used metathesis catalysts in current use. This study follows up on an earlier discovery from the Fogg group, which showed that pyridine and amine donors drastically accelerate decomposition of the Grubbs catalysts. For the first-generation Grubbs catalyst, decomposition of the resting-state methylidene complex RuCl2(PCy3)2(=CH2) Ru-6 was shown to proceed via nucleophilic attack of PCy3 on the Ru=CH2 bond, forming a σ-alkyl complex that was intercepted and characterized crystallographically. Further reaction led to liberation of the methyl phosphonium salt [MePCy3]Cl 2. Under the same conditions, the second-generation methylidene complex Ru-4a decomposes rapidly without any detectable σ-alkyl intermediate. In this thesis, decomposition of the important second-generation catalysts is shown to proceed via the methylidene-abstraction pathway. These studies centered on the methylidene complex RuCl2(NHC)(PCy3)(=CH2), where the NHC ligand is H2IMes (Ru- 4a) or IMes (Ru-4b). The short lifetime of the σ-alkyl complex was tentatively attributed to the ease of activation of a C–H bond on the NHC ligand (specifically, those on the mesityl o-methyl group). This intermediate could not be observed for Ru-4a, but could be observed for the IMes system Ru-4b. This is suggested to reflect the slower rotation of the H2IMes ligand about the Ru–NHC bond, which promotes C–H activation. Also examined is the ability of other Lewis donors to trigger this methylidene abstraction pathway during catalysis. Lewis donors are shown to greatly accelerate decomposition of a wide range of phosphine-stabilized metathesis catalysts. Remarkably, even weak donors such as water, ethers, alcohols, and nitriles (i.e. functionalities that are widespread among contaminants, functional groups, and “green solvents”) are shown to promote this pathway. Phosphine-stabilized catalysts are generally regarded as more robust towards such donor functionalities, and the detrimental impact of such weak donors has gone widely unrecognized. These findings have profound implications for catalyst choice and use. Finally, a deuterium-labelling study was undertaken, in which the mesityl substituents on the NHC ligand were fully deuterated, to confirm that the proton in the phosphonium salt 2 indeed originates in the mesityl methyl group. Of note, the energies of the PCy3 dissociation and C–H activation steps were found to be closely similar. This was unexpected, given prior evidence that PCy3 loss is rate-determining in the Ru-2 precatalysts, and the much stronger Ru–PCy3 bonding known to be present in the methylidene complexes. This finding highlights the need to impede C-H activation for phosphine-stabilized metathesis catalysts. More broadly, it underscores the risks inherent in the use of phosphine-stabilized catalysts, or indeed of catalysts that are stabilized by a donor group that can function as both a Lewis base, and a nucleophile.

Olefin metathesis is increasingly popular for the construction of carbon-carbon double bonds. In the past two decades, ruthenium metathesis catalysts have seen extensive development. Two marvelous early developments were the introduction of N-heterocyclic carbene (NHC) ligands, which greatly improved catalyst activity, and replacement of a nucleophilic stabilizing ligand (the alkylphosphine PCy3) by a chelated benzylidene-ether. A more recent breakthrough is the introduction of cyclic alkyl amino carbene (CAAC) ligands as alternatives to the NHCs. Over the past 5 years, the CAAC catalysts have drawn much attention for their breakthrough productivity in challenging reactions, including ethenolysis and RCM macrocyclization. Nevertheless, important challenges remain. As discussed in the first part of this thesis, these include the synthesis of key ligands (in particular, the styrenyl ether ligands H2C=13CH-C6H4-2-OiPr and H2C=13CH-C6H3-2-OiPr-5-NO2), which represent the source of the chelated benzylidene-ether noted above) and key catalysts (e.g., Hoveyda- and Grela-class catalysts bearing an H2IMes or CAAC carbene ligand). A further challenge lies in understanding the behaviour of the styrenyl ether ligands – for example, how they contribute to catalyst partitioning between active, resting-state, and decomposed species, and the impact of the carbene ligand on that partitioning. An initial objective was synthesis of labelled styrenyl ethers that would enable synthesis of Hoveyda and Grela catalysts with a 13C-label at the alkylidene carbon. While H2C=13CH-C6H4-2-OiPr could be accessed, its nitro analogue could not, probably owing to attack on the NO2 substituent. The remainder of Chapter 2 assessed reported routes to HII/nG and HII-C1/nG-C1, and reports on challenges and reproducibility issues. Difficulties in synthesis of the CAAC catalysts are attributed to the need for in situ-generated CAACs, and catalyst decomposition by the base required for deprotonation (i.e KHMDS or LiHMDS). The second part of this thesis explores the impact of the NHC or CAAC ligands on initiation and “boomerang” recapture of the styrenyl ether ligands for Hoveyda- and Grela-class catalysts. Examination of the kinetics of initiation with bulky t-butyl vinyl ether (tBuVE) revealed a linear dependence of kobs on [tBuVE], and faster reaction by the p-NO2-substituted Grela catalyst. These data suggest an associative or interchange-associative (IA) mechanism. A systematic comparison of initiation rate constants revealed the trend HII > nG-C1 > HII-C1 in chlorinated and aromatic solvents. Recapture of added styrenyl ether ligand was examined in macrocyclization (mRCM). Rate plots indicated inhibition by this ligand, even at the high dilutions required for mRCM, implying that boomerang re-uptake of the styrenyl ether is indeed operative for both Hoveyda- and Grela-class catalysts. However, inhibition was found to be more profound for HII than HII-C1 or nG-C1. That is, the NHC catalysts are much more susceptible to partitioning into the off-cycle (precatalyst) form than are the CAAC catalysts. This higher commitment to the active cycle may be an important contributor to the impressive productivity of the CAAC catalysts. In addition, the slow initiation of these catalysts indicated above may be an asset, rather than a limitation, as it inhibits their susceptibility to bimolecular decomposition.

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Reactive distillation is a combination of a traditional multi-stage distillation column with a chemical reaction. The primary benefits of a reactive distillation process are reduced capital costs for equipment and energy in addition to enhanced conversion for equilibrium-limited reactions. One such equilibrium-limited reaction is an olefin metathesis. Olefin metathesis is a catalyzed reaction that breaks the double bond in olefins and rearranges the alkene fragments into new olefinic products. A comprehensive investigation of a reactive distillation based olefin metathesis and supporting experimentation is documented here. A small pilot plant study was performed for pilot scale performance comparison. Bench reactor experimentation was conducted for the purposes of learning detailed information on specific metathesis reactions. Lastly, a process simulation study was completed for comparison in performance with the small pilot plant process. The small pilot plant study involved the design, construction, testing, operation, and optimization of a reactive distillation column. Continuous operation campaigns at two different hydraulic capacities within the reactive zone were performed and their performances were compared. A higher hydraulic capacity proved to be more efficient and more selective for the conversion of medium molecular weight olefins into both lighter and heavier olefinic products. Bench reactor experiments were designed with the intent of investigating specific alpha olefin metathesis reactions and obtaining conversions, selectivities, and yield structures for future simulation work. However, under conditions similar to that within the small pilot plant process, there existed a high frequency of secondary double bond isomerization (possibly due to an isomerization activity for alumina). There was also an observed dependence on temperature for both the primary metathesis and secondary isomerization reactions. A process simulation representative of the small pilot plant process was constructed in AspenPlus. Using a simplified reaction network based on assumptions and analysis of the reactive zone, its performance was compared with that of the small pilot plant process. The simulation performance tended to underpredict overhead compositions, but accurately simulated the bottoms product composition. Because reactive distillation has not been used with a heavy olefin metathesis reaction, this dissertation demonstrates the uniqueness and effectiveness of a reactive distillation based heavy olefin metathesis. / text

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This thesis details the silver(I) coordination chemistry of thirty four alkene-containing ligands. The synthesis of thirty two of these ligands is described of which fifteen are unreported compounds. The ligands were designed to fully explore the potential of the silver(I)-alkene synthon in metallosupramolecular chemistry. Five series of ligand were designed each exploring a different facet of ligand design. Three series explored different ligand cores which included benzene, naphthalene and single atoms such as carbon, oxygen and nitrogen. Another series explored ligands of higher denticity including tri-, tetra- and hexa-substituted benzenes. The last series investigated ligands with functional groups in addition to olefins, in particular, heterocyclic nitrogens. A metal-centred ligand was created from a bifunctional ligand subunit. The silver(I)-alkene synthon has been used to create a range of assemblies. Polymeric structures were favoured with a variety of one-dimensional polymers with linear, ladder, helical and necklace type structures. Two-dimensional networks were formed, with some showing porosity. Three-dimensional metallopolymers were formed, including an interpenetrated three-dimensional network. Discrete complexes are commonly of the type Ag2L2 but with the occasional formation of Ag2L. It is shown that silver(I)-alkene interactions can coexists with other stronger interactions such as silver(I)-nitrogen. The deliberate use of bifunctional ligands allowed the formation of many interesting assemblies including an Ag3L2 heterotopic helicate. A Cu(I) complex with copper(I)-alkene interactions was identified. Techniques used to characterise the ligands and complexes include NMR, mass spectrometry, elemental analysis and X-ray crystallography. The crystal structures of seven organic compounds and forty six complexes are discussed.

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