Thesis advisor: Amir H. Hoveyda / We have synthesized Ru-carbene isocyanide complexes that promote both ring-opening metathesis polymerization of norbornene as well as cycloisomerization of diethyl diallylmalonate. We have also synthesized a N-heterocyclic carbene complex bearing a biphenylthiol moiety, and we installed this ligand on a Ru-carbene to produce a racemic chiral bidentate Ru-thiolate complex. Although the Ru-thiolate was found to initiate more slowly than the corresponding biphenoxide catalyst, both perform ring-opening/cross-metathesis (ROCM) with similar efficiency. Several other bidentate Ru-complexes were synthesized where the anionic ligand was varied (tosylate, pivalate, and phenylthiolate), as well as a new Re-alkylidene bis-pyrrolide. We have expanded the scope of Ru-catalyzed enantioselective ROCM of cyclopropenes utilizing a variety of ester, ketone, ether, and aliphatic olefin cross-partners. The utility of this method was demonstrated in the enantioselective total synthesis of the marine natural product (+)-sporochnol, which was synthesized in 8% overall yield across eleven linear steps. Additionally, we have developed an enantio- and Z-selective ROCM of enol ethers and oxabicycles; we propose the origin of Z-selectivity to arise from a lower barrier to ruthancyclobutane cleavage/formation for the cis-substituted ruthenacyclobutane vs the trans-substituted ruthenacyclobutane (which is favored for ROCM of oxabicycles and styrene). We also have found that stereogenic-at-Ru complexes are capable of undergoing non-metathesis isomerization through polytopal rearrangements. This observation may explain why cyclopropene ROCM suffers from low enantioselectivity for many substrates. We have developed a diasteroselective ROCM reaction, which utilizes commercially available ruthenium dichloride catalysts in the presence of chiral allylic alcohols and cyclopropenes. Our investigation revealed that the presence of a hydroxyl group dramatically accelerates the rate of ROCM vs the corresponding methyl ether and delivered products in high yield and diastereoselectivity. Furthermore, we found that the methyl ether delivered the opposite diastereomer vs the allylic alcohol; this led us to propose that intramolecular H-bonding between the hydroxyl proton and a chloride ligand controls the diastereoselectivity and enhances the rate of the ROCM. Protic additives have also been found to promote polytopal rearrangements in stereogenic-at-Ru complexes; H-bonding may facilitate olefin metathesis in a similar fashion to polytopal rearrangement by reducing the trans effect during the transition state to ruthenacyclobutane formation. A number of synthetically useful allylic alcohols and strained olefin substrates efficiently provide products in high diastereoselectivity and with good E:Z selectivity (89:11-97:3 dr, 4:1-11:1 E:Z). We have developed a Mo-catalyzed Z-selective cross-metathesis (CM) reaction. A wide range of olefin cross partners were found to be effective for both enol ether and allylic amide substrates (51-97% yield, 81 to 98% Z). We applied our Z-selective CM method to the synthesis of KRN7000, a potent immunostimulant (the Z-allylic amide was obtained in 85% yield and 96% Z). We also utilized Z-selective CM in the formal synthesis of an enol ether plasmalogen C18 (plasm)-16:0 (PC), a lipid membrane component found in mammalian brain tissue (the enol ether was obtained in >98:2 Z selectivity). Z-selective cross-metathesis is therefore a new tool for synthetic chemists to access important building blocks for the synthesis of biologically active molecules. We have developed a Z-selective cross-metathesis of vinyl and allyl boronates. Reactions of both substrate classes proceed to between 50-95% conv and deliver Z-vinylboronate and Z-crotylboronate products in 85-93% Z selectivity. Allylboronate CM provides Z-crotylboronates which can be used for diastereoselective crotylation. The utility of Z-selective vinylboronate CM was demonstrated in the synthesis of a dienyl boronate (obtained in 83% yield and >98% Z) that will be utilized in the total synthesis of the potent anti-cancer agent disorazole C1. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

O novo monômero-ligante 3-imidapiridina-exo-7-oxabiciclo[2.2.1]-hept-5-eno-2,3-dicarboxilato (oxaNBE(3imdpy)) foi sintetizado e caracterizado por análise elementar (CHN), FTIR, difração de Raios-X e RMN (1H e 13C). Trata-se de um ligante monodentado com uma unidade de piridina apta a se coordenar a um centro metálico por uma extremidade da molécula, enquanto que a outra extremidade possui uma olefina cíclica que pode ser polimerizada via metátese por abertura do anel. Foram realizados estudos adicionais com monômeros-ligantes similares apresentando duas unidades de piridina por composto (oxaNBE(3amdpy)2 e NBE(3amdpy)2). Foi também sintetizado o monômero oxaNBE(imdPh). O composto oxaNBE(3imdpy) reagiu com o complexo cis-[Ru(H2O)2(phen)2], obtido in situ a partir do cis-[RuCl2(phen)2] em uma mistura 1:1 EtOH/H2O, onde phen é 1,10-fenantrolina, formando o novo complexo cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. Esse complexo foi caracterizado por análise elementar (CHN), FTIR, RMN (1H; 13C; COSY; NOE), voltametria cíclica, espectrofotometria UV-vis, cálculos computacionais ab initio e investigou-se fotólise contínua e luminescência. Observou-se um único par redox por voltametria cíclica nos solventes MeOH (E1/2 = 0,63 V vs Ag/AgCl) e DMF (E1/2 = 0,72 V vs Ag/AgCl). Os espectros eletrônicos em MeOH, DMF, CH3CN, CH2Cl2 e CHCl3 apresentaram absorções típicas de complexos polipiridínicos de rutênio na regiâo de 440-500 nm que foram atribuídas a transferências de carga do metal para o ligante (MLCT) do Ru(4d) para a phen. A influência do meio sobre o comportamento em solução mediante fotólise foram avaliadas. Os espectros de absorção na região do UV-vis exibem perfis espectrais semelhantes em MeOH, CH2Cl2, CHCl3 e CH3CN, não obtendo-se correlações lineares com os parâmetros característicos dos solventes. Observou-se que os espectros eletrônicos na ausência ou presença de luz ambiente em MeOH, CH2Cl2 e CH3CN não apresentaram mudanças significativas, exceto para CHCl3. Fotólises a 450 nm indicaram mudanças espectrais nos solventes CHCl3 e CH2Cl2. Os espectros de emissão (excitação em 450 nm) do complexo não mostraram diferenças nos diferentes meios. O monômero-ligante oxaNBE(3imdpy) e o similar monômero oxaNBE(imdPh) formaram polímeros em presença de catalisadores de rutênio por 16 horas à temperatura ambiente, não ocorrendo o mesmo com o metalo-monômero cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. Os monômeros-ligantes oxaNBE(3amdpy)2 e NBE(3amdpy)2oxaNBE(3amdpy)2 não formaram polímeros, bem como seus respectivos complexos. / The novel monomer-ligand 3-imidepyridine-exo-7-oxabicycle[2.2.1]-hept-5-ene-2,3-dicarboxylate (oxaNBE(3imdpy)) was synthesized and characterized by elemental analysis (CHN), FTIR, X-ray diffraction and NMR (1H e 13C). It is a monodentate compound with a pyridine ligand able to be coordinated to a metal center via one side of the molecule, while the other side has a cyclic olefin that can undergo ring-opening metathesis polymerization. Additional studies were carried out using similar monomers featuring two pyridine units per molecule (oxaNBE(3amdpy)2 and NBE(3amdpy)2). It was also synthesized the monomer oxaNBE(imdPh). The compound oxaNBE(3imdpy) reacted with cis-[Ru(H2O)2(phen)2], obtained in situ from cis-[RuCl2(phen)2] in 1:1 EtOH/H2O mixture, where phen is 1,10-phenanthroline, given cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. This novel complex was characterized by elemental analysis (CHN), FTIR, NMR (1H; 13C; COSY; NOE), cyclic voltammetry, UV-vis spectrophotometry, quantum mechanical calculations, continuous photolysis and luminescence. The complex showed a single redox process in the cyclic voltammogram either in MeOH (E1/2 = 0.63 V) or DMF (E1/2 = 0.72 V vs Ag/AgCl) as solvent. The electronic spectra in MeOH, DMF, CH3CN, CH2Cl2 and CHCl3 showed typical absorptions of polypyridinic-ruthenium complexes in 440-500 nm range that were attributed to metal to ligand charge transfers (MLCT) from Ru(4d) to phen. The influence of the solvent on the complex behavior at room temperature as a function of time and under photolysis was studied. The UV-vis spectra showed similar spectral profiles in MeOH, CH2Cl2, CHCl3 and CH3CN, without linear clear correlations with characteristic parameters from the solvents. It was observed that the electronic spectra either in absence or presence of ambient light in MeOH, CH2Cl2 and CH3CN did not show significant spectral changes, except for CHCl3. The photolysis at 450 nm indicated changes in CHCl3 and CH2Cl2. The emission spectrum (excitation at 450 nm) of the complex did not show differences in different solvents. The monomer-ligand oxaNBE(3imdpy) and the similar monomer oxaNBE(imdPh) were polymerized in presence of ruthenium-based catalysts for 16 h at room temperature, contrary to the metal-monomer cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. The monomer-ligands oxaNBE(3amdpy)2 and NBE(3amdpy)2 did not form polymers, as well as the related complex, under similar conditions.

Thesis advisor: Amir H. Hoveyda / Chapter 1. Exploring Ligand Effects in Ruthenium Dithiolate Carbene Complexes. Ruthenium dithiolate metathesis catalysts discovered in the Hoveyda group have been a valuable addition to the field of olefin metathesis. While the catalyst shows unique selectivity and reactivity, quantifying and mapping key interactions in the catalyst framework to elucidate and explain causes is difficult. We, therefore, decided to use the neutral chelating or monodentate ligand, controlling initiation, as a structural probe. By altering its properties and observing changes in the catalyst, we sought to deepen our understanding of these complexes. We established a trans influence series with over 20 catalysts and correlated the impact on catalyst initiation. Further, we show that in the case of strongly σ-donating and π-accepting ligands such as phosphites and isonitriles, the complex exhibits fluxional behavior. The catalysts ground state is elevated to such a degree that thiolate Ruthenium bonds become labile and rapidly exchange. While Ruthenium dithiolate catalysts were readily applied to metathesis polymerization, their use in the synthesis of small molecules was initially less forthcoming. Specifically, reactions involving terminal olefins lead to rapid catalyst deactivation and only low conversion. We were able to determine that the potential energy stored in the trans-influence between the thiolate ligand and the NHC can be released in a sulfur shift to reactive Ruthenium methylidene species. Since methylidenes are formed by reaction with terminal olefins, use of an excess of internal olefins can prevent their formation. Chapter 2. Harnessing Catalyst Fluxionality in Olefin Metathesis. Depending on its use, material requirements can vary significantly. Materials that can easily be adapted to a given application, for example by varying tensile strength, melting point or solubility, are desirable. Controlling the polymers tacticity (the adjacent stereocenters in a polymer chain) is a straight forward way to achieve just that. Ru dithiolate catalysts should give highly syndiotactic polymers due to their single stereocenter undergoing inversion during every metathesis step. The fluxional nature of the catalyst allows for control of polymer tacticity from 50% (atactic) to ≥95% syndiotacticity by changing monomer concentration. We determined the factors which are responsible for fluxionality and synthesized complexes that give either high or low levels of tacticity over a broader range of monomer concentration. Chapter 3. Harnessing Catalyst Fluxionality in Olefin Metathesis. The importance of fluorine-containing molecules is hard to understate, keeping in mind the surge of new methodologies for their synthesis and the medical breakthroughs they enable. However, efficient and practical syntheses of stereodefined alkenyl fluorides are rare. In this context, we have developed enantioselective boryl allylic substitution of allylic fluorides, which yield enantioenriched γ-alkenyl fluoride substituted allyl boronate esters. The reaction is catalyzed by Cu-based catalysts that are prepared in-situ and delivered as products with high yield and enantioselectivity. Mechanistic inquiry shows the reaction is not a concerted allylic substitution. An intermediate Cu alkyl complex is formed after the Cu boron addition is made to the double bond, which only slowly undergoes β-fluorine elimination in the presence of a Lewis acid. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Amir H. Hoveyda / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

O novo monômero-ligante 3-imidapiridina-exo-7-oxabiciclo[2.2.1]-hept-5-eno-2,3-dicarboxilato (oxaNBE(3imdpy)) foi sintetizado e caracterizado por análise elementar (CHN), FTIR, difração de Raios-X e RMN (1H e 13C). Trata-se de um ligante monodentado com uma unidade de piridina apta a se coordenar a um centro metálico por uma extremidade da molécula, enquanto que a outra extremidade possui uma olefina cíclica que pode ser polimerizada via metátese por abertura do anel. Foram realizados estudos adicionais com monômeros-ligantes similares apresentando duas unidades de piridina por composto (oxaNBE(3amdpy)2 e NBE(3amdpy)2). Foi também sintetizado o monômero oxaNBE(imdPh). O composto oxaNBE(3imdpy) reagiu com o complexo cis-[Ru(H2O)2(phen)2], obtido in situ a partir do cis-[RuCl2(phen)2] em uma mistura 1:1 EtOH/H2O, onde phen é 1,10-fenantrolina, formando o novo complexo cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. Esse complexo foi caracterizado por análise elementar (CHN), FTIR, RMN (1H; 13C; COSY; NOE), voltametria cíclica, espectrofotometria UV-vis, cálculos computacionais ab initio e investigou-se fotólise contínua e luminescência. Observou-se um único par redox por voltametria cíclica nos solventes MeOH (E1/2 = 0,63 V vs Ag/AgCl) e DMF (E1/2 = 0,72 V vs Ag/AgCl). Os espectros eletrônicos em MeOH, DMF, CH3CN, CH2Cl2 e CHCl3 apresentaram absorções típicas de complexos polipiridínicos de rutênio na regiâo de 440-500 nm que foram atribuídas a transferências de carga do metal para o ligante (MLCT) do Ru(4d) para a phen. A influência do meio sobre o comportamento em solução mediante fotólise foram avaliadas. Os espectros de absorção na região do UV-vis exibem perfis espectrais semelhantes em MeOH, CH2Cl2, CHCl3 e CH3CN, não obtendo-se correlações lineares com os parâmetros característicos dos solventes. Observou-se que os espectros eletrônicos na ausência ou presença de luz ambiente em MeOH, CH2Cl2 e CH3CN não apresentaram mudanças significativas, exceto para CHCl3. Fotólises a 450 nm indicaram mudanças espectrais nos solventes CHCl3 e CH2Cl2. Os espectros de emissão (excitação em 450 nm) do complexo não mostraram diferenças nos diferentes meios. O monômero-ligante oxaNBE(3imdpy) e o similar monômero oxaNBE(imdPh) formaram polímeros em presença de catalisadores de rutênio por 16 horas à temperatura ambiente, não ocorrendo o mesmo com o metalo-monômero cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. Os monômeros-ligantes oxaNBE(3amdpy)2 e NBE(3amdpy)2oxaNBE(3amdpy)2 não formaram polímeros, bem como seus respectivos complexos. / The novel monomer-ligand 3-imidepyridine-exo-7-oxabicycle[2.2.1]-hept-5-ene-2,3-dicarboxylate (oxaNBE(3imdpy)) was synthesized and characterized by elemental analysis (CHN), FTIR, X-ray diffraction and NMR (1H e 13C). It is a monodentate compound with a pyridine ligand able to be coordinated to a metal center via one side of the molecule, while the other side has a cyclic olefin that can undergo ring-opening metathesis polymerization. Additional studies were carried out using similar monomers featuring two pyridine units per molecule (oxaNBE(3amdpy)2 and NBE(3amdpy)2). It was also synthesized the monomer oxaNBE(imdPh). The compound oxaNBE(3imdpy) reacted with cis-[Ru(H2O)2(phen)2], obtained in situ from cis-[RuCl2(phen)2] in 1:1 EtOH/H2O mixture, where phen is 1,10-phenanthroline, given cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. This novel complex was characterized by elemental analysis (CHN), FTIR, NMR (1H; 13C; COSY; NOE), cyclic voltammetry, UV-vis spectrophotometry, quantum mechanical calculations, continuous photolysis and luminescence. The complex showed a single redox process in the cyclic voltammogram either in MeOH (E1/2 = 0.63 V) or DMF (E1/2 = 0.72 V vs Ag/AgCl) as solvent. The electronic spectra in MeOH, DMF, CH3CN, CH2Cl2 and CHCl3 showed typical absorptions of polypyridinic-ruthenium complexes in 440-500 nm range that were attributed to metal to ligand charge transfers (MLCT) from Ru(4d) to phen. The influence of the solvent on the complex behavior at room temperature as a function of time and under photolysis was studied. The UV-vis spectra showed similar spectral profiles in MeOH, CH2Cl2, CHCl3 and CH3CN, without linear clear correlations with characteristic parameters from the solvents. It was observed that the electronic spectra either in absence or presence of ambient light in MeOH, CH2Cl2 and CH3CN did not show significant spectral changes, except for CHCl3. The photolysis at 450 nm indicated changes in CHCl3 and CH2Cl2. The emission spectrum (excitation at 450 nm) of the complex did not show differences in different solvents. The monomer-ligand oxaNBE(3imdpy) and the similar monomer oxaNBE(imdPh) were polymerized in presence of ruthenium-based catalysts for 16 h at room temperature, contrary to the metal-monomer cis-[Ru(phen)2(oxaNBE(3imdpy)2](PF6)2. The monomer-ligands oxaNBE(3amdpy)2 and NBE(3amdpy)2 did not form polymers, as well as the related complex, under similar conditions.

Rhodium complexes bearing multidentate nitrogen donor ligands were investigated for their ability to promote alkyne and olefin functionalization reactions. This thesis work is comprised of two projects in which rhodium-heteroatom reactivity is investigated: P-H bond activation reactions and olefin functionalizations via rhodaoxetane intermediates. [Tp*Rh(PPh3)2] [Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] and [Tp*Rh(cod)]2 (cod = cyclooctadiene) were evaluated for their activity in alkyne hydrophosphinylation in comparison to known catalysts for this reaction. [Tp*Rh(PPh3)2]and [Tp*Rh(cod)]2 were both shown to effect hydrophosphinylation of 1-octyne with diphenylphosphine oxide with high regioselectivity but moderate yields in comparison with Wilkinson's catalyst [C1Rh(PPh3)3]. [Tp*Rh(PPh3)2] was further shown to effect hydrophosphinylation of a range of aromatic and aliphatic alkynes with diphenylphosphine oxide, in each case exclusively providing the E-linear vinylphosphineoxide product. 1H and 31P NMR spectroscopy provided evidence that alkyne hydrophosphinylation in the presence of pyrazolylborate rhodium complexes follows an analogous mechanism to that proposed for this reaction catalyzed by [C1Rh(PPh3)3] or[C1Rh(cod)]2. The 2-rhodaoxetane [(TPA)Rhmec2_,-4u, 0-2-oxyethypr BPh4- (TPA = tris[(2-pyridal)methyl]amine) was investigated for its potential as an intermediate in proposed functionalization reactions of olefins. RTPA)Rh111(K2-C,0-2-oxyethyl)]+ BPh4- was prepared by two published methods with limited success. A third method involved the use of nitrous oxide to oxygenate [(12-ethene)(K4-TPA)Rh1]+ to RTPA)Rh1110(-2-C,0-2-oxyethyDr. Only a trace amount of [(TPA)Rhmoc2 -C,0-2-oxyethypr was observed in the 1I-1 NMR spectrum of this reaction mixture. Initial test reactions of [(TPA)Rhilioc2_C,0-2-oxyethypr combined with substrates (aniline, toluenesulfonamide, phenylboronic acid, or benzaldehyde) were inconclusive since the results were obscured by the impurity of the samples.

No description available.

Rhodium complexes bearing multidentate nitrogen donor ligands were investigated for their ability to promote alkyne and olefin functionalization reactions. This thesis work is comprised of two projects in which rhodium-heteroatom reactivity is investigated: P-H bond activation reactions and olefin functionalizations via rhodaoxetane intermediates. [Tp*Rh(PPh3)2] [Tp* = hydrotris(3,5-dimethylpyrazolyl)borate] and [Tp*Rh(cod)]2 (cod = cyclooctadiene) were evaluated for their activity in alkyne hydrophosphinylation in comparison to known catalysts for this reaction. [Tp*Rh(PPh3)2]and [Tp*Rh(cod)]2 were both shown to effect hydrophosphinylation of 1-octyne with diphenylphosphine oxide with high regioselectivity but moderate yields in comparison with Wilkinson's catalyst [C1Rh(PPh3)3]. [Tp*Rh(PPh3)2] was further shown to effect hydrophosphinylation of a range of aromatic and aliphatic alkynes with diphenylphosphine oxide, in each case exclusively providing the E-linear vinylphosphineoxide product. 1H and 31P NMR spectroscopy provided evidence that alkyne hydrophosphinylation in the presence of pyrazolylborate rhodium complexes follows an analogous mechanism to that proposed for this reaction catalyzed by [C1Rh(PPh3)3] or[C1Rh(cod)]2. The 2-rhodaoxetane [(TPA)Rhmec2_,-4u, 0-2-oxyethypr BPh4- (TPA = tris[(2-pyridal)methyl]amine) was investigated for its potential as an intermediate in proposed functionalization reactions of olefins. RTPA)Rh111(K2-C,0-2-oxyethyl)]+ BPh4- was prepared by two published methods with limited success. A third method involved the use of nitrous oxide to oxygenate [(12-ethene)(K4-TPA)Rh1]+ to RTPA)Rh1110(-2-C,0-2-oxyethyDr. Only a trace amount of [(TPA)Rhmoc2 -C,0-2-oxyethypr was observed in the 1I-1 NMR spectrum of this reaction mixture. Initial test reactions of [(TPA)Rhilioc2_C,0-2-oxyethypr combined with substrates (aniline, toluenesulfonamide, phenylboronic acid, or benzaldehyde) were inconclusive since the results were obscured by the impurity of the samples.

Olefin metathesis is an exceptionally versatile and general methodology for the catalytic assembly of carbon-carbon bonds. Ruthenium metathesis catalysts have been widely embraced in academia, and are starting to see industrial uptake. However, the challenges of reliability, catalyst productivity, and catalyst cost have limited implementation even in value-added technology areas such as pharmaceutical manufacturing. Key to the broader adoption of metathesis methodologies is improved understanding of catalyst decomposition. Many studies have focused on phenomenological relationships that relate catalyst activity to substrate structure, and on the synthesis of new catalysts that offer improved activity. Until recently, however, relatively little attention was paid to catalyst decomposition. The first part of this thesis explores a largely overlooked decomposition pathway for “second-generation” olefin metathesis catalysts bearing an N-heterocyclic carbenes (NHC) ligand, with a particular focus on identifying the Ru decomposition products. Efforts directed at the deliberate synthesis of these products led to the discovery of a succinct, high-yielding route to the second-generation catalysts. Multiple reports, including a series of detailed mechanistic studies from our group, have documented the negative impact of phosphine ligands in Ru-catalyzed olefin metathesis. Phosphine-free derivatives are now becoming widely adopted, particularly in pharma, as recognition of these limitations has grown. Decomposition of the phosphine-free catalysts, however, was little explored at the outset of this work. The only documented pathway for intrinsic decomposition (i.e. in the absence of an external agent) was -hydride elimination of the metallacyclobutane (MCB) ring as propene. An alternative mechanism, well established for group 3-7 and first-generation ruthenium metathesis catalysts, is bimolecular coupling (BMC) of the four-coordinate methylidene intermediate. However, this pathway was widely viewed as irrelevant to decomposition of second-generation Ru catalysts. This thesis work complements parallel studies from the Fogg group, which set out to examine the relevance and extent of BMC for this important class of catalysts. First, -hydride elimination was quantified, to assess the importance of the accepted pathway. Even at low catalyst concentrations (2 mM Ru), less than 50% decomposition was shown to arise from -hydride elimination. Parallel studies by Gwen Bailey demonstrated ca. 80% BMC for the fast-initiating catalyst RuCl2H2IMes(=CHPh)(py)2 GIII. Second, the ruthenium products of decomposition were isolated and characterized. Importantly, and in contrast to inferences drawn from the serendipitous isolation of crystalline byproducts (which commonly show a cyclometallated NHC ligand), these complexes show an intact H2IMes group. This rules out NHC activation as central to catalyst decomposition, suggesting that catalyst redesign should not focus on NHC cyclometallation as a core problem. Building on historical observations, precautions against bimolecular coupling are proposed to guide catalyst choice, redesign, and experimental setup. The second part of this thesis work focused on the need for more efficient routes to second-generation Ru metathesis catalysts, and indeed a general lack of convenient, well-behaved precursors to RuCl2(H2IMes). This challenge was met by building on early studies in which metathesis catalysts were generated in situ by thermal or photochemical activation of RuCl2(p-cymene)(PCy3) in the presence of diazoesters. Such piano-stool complexes (including the IMes analogue) have also been applied more broadly as catalysts, inorganic drugs, sensors, and supramolecular building blocks. However, RuCl2(p-cymene)(H2IMes), which should in principle offer access to the RuCl2(H2IMes) building block, has been described as too unstable for practical use. The basis of the instability of RuCl2(p-cymene)(H2IMes) toward loss of the p-cymene ring was examined. Key factors included control over reaction stoichiometry (i.e. limiting the proportion of the free NHC), limiting exposure to light, and maintaining low concentrations to inhibit bimolecular displacement of the p-cymene ring. A near-quantitative route to RuCl2(p-cymene)(H2IMes) was achieved using appropriate dilutions and rates of reagent addition, and taking precautions against photodecomposition. This approach was used to develop atom-economical syntheses of the Hoveyda catalyst, RuCl2(H2IMes)(=CHAr) (Ar = 2-isopropoxybenzylidene) and RuCl2(H2IMes)(PPh3)(=CHPh), a fast-initiating analogue of GII. Related p-cymene complexes bearing bulky, inflexible imidazolidene or other donors may likewise be accessible.

Olefin metathesis is a powerful tool for assembly of carbon-carbon bonds. Amines and related N-donors are problematic functional groups in Ru-catalyzed olefin metathesis - a well- documented, but poorly understood problem. The first part of this thesis focuses on amine-induced deactivation pathways; two of which are described in depth. Alkylidene abstraction, a previously unknown reaction for nitrogen nucleophiles, was observed for smaller and less Bronsted-basic amines. Deprotonation of the metallacyclobutane intermediate formed during catalysis is prominent for highly Bronsted basic or sterically bulky N-donors. Monosubstituted (and, by extension unsubstituted) metallacyclobutanes are particularly vulnerable to deprotonation. For each pathway, the fate of the alkylidene Ru=CHR functional group proved key in determining the nature of deactivation. Both pathways have been detected during catalysis, as evidenced by formation of diagnostic amine (RCH2NR2’) or substituted propene products. A combination of quantitative NMR and GC-MS analysis was used to identify these species on loss of the Ru-alkylidene functional group. The second part of this thesis focuses on incorporating amines into catalyst design – an under-utilized strategy in the context of Ru-catalyzed olefin metathesis. A modified Grubbs-type catalyst was developed featuring a bulky, relatively non-basic biaryldiamine ligand. Metathesis activity for this catalyst was comparable, and in some cases superior to the most widely-used homogeneous catalysts currently available. Several new, related Ru-benzylidenes were also prepared and fully characterized in conjunction with the mechanistic studies described above. Progress toward development of N-anion-containing metathesis catalysts is also discussed. Synthesis of Ru-hydride complexes originally intended for this purpose allowed for a fundamental study of the coordination chemistry and reductive elimination chemistry of the NPh2– anion.