School of Chemistry - Theses
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ItemThe Carbonylation of Organic Compounds by Visible Light Photoredox Catalysis( 2020)Palladium-catalysed alkoxy- and aminocarbonylation of aryl (pseudo)halides provides efficient access to aromatic esters and amides. The broad application of this approach has been restricted by functional group tolerance, high reaction temperatures and moderate catalyst efficiency. Free-radical carbonylation is a complementary approach not confined by the same inherent limitations of palladium-catalysed carbonylative cross-coupling methodology. The development of free-radical carbonylation has been hindered by the ability to selectively generate the carbon-centred radical species and the high pressures of carbon monoxide required to drive the carbonylation step. This thesis describes the development of visible light photoredox-catalysed alkoxy- and aminocarbonylation of aryl (pseudo)halides. Visible light photoredox-catalysis is a potent method to generate carbon-centred radicals selectively under mild reaction conditions. Aryl radicals can be trapped by carbon monoxide to afford carbonyl compounds. Continuous flow chemistry is utilised throughout, employing tube-in-tube semipermeable membrane reactor technology, to enable precise control over reactions conditions and safe use of carbon monoxide. Chapter 1 introduces carbonylation and elaborates on carbonylative cross-coupling of aryl (pseudo)halides. It further introduces continuous flow processing in synthetic chemistry (flow chemistry) and details the application of flow chemistry to carbonylative cross-coupling and photochemical reactions. Chapter 2 established a continuous flow platform for high pressure gas-liquid photochemistry. The flow system consisted of a pumping module, a reagent delivery module, a Teflon AF-2400 tube-in-tube reactor for saturation of the reaction stream with carbon monoxide, a photoreactor and pressure regulation devices. The photoredox-catalysed alkoxycarbonylation of aryl diazonium salts was selected to evaluate the performance of the flow system. It was determined that excellent yields of the benzoate ester could be achieved at significantly lower partial pressures of carbon monoxide and processing time than in batch. Chapter 3 details the development of a free-radical annulative addition/alkoxycarbonylation cascade reaction. The developed methodology was applied to the synthesis of a diverse library of novel 3-acetate functionalized 2,3-dihydrobenzofurans from widely accessible allyl aryl diazonium ethers. Application of the previously established continuous flow system enabled dilute reaction conditions to effectively control the propagation of competitive intermolecular radical addition side reactions without compromising on reaction throughput or space-time yield. Chapter 4 describes the development of photoredox-catalysed aminocarbonylation of aryl halides. The developed methodology was applied to the synthesis of both electron rich and electron deficient benzamides at room temperature. Spectroscopic and theoretical computational studies were conducted to elucidate the reaction mechanism. A novel tandem photoredox catalytic manifold was proposed that features the transformation of Ir(dtbbpy)(ppy)2]PF6 in the presence of DIPEA to generate a distinct highly reducing Ir-complex capable of engaging energy demanding aryl halides. Chapter 5 provides a summary of the work described in this thesis. Supplementary data is included in the appendix.
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ItemCoinage metal hydrides: reactive intermediates in catalysis and significance to nanoparticle synthesis( 2019)The coinage metal hydrides of copper, silver and gold have applications in catalysis and nanoparticle synthesis. Coinage metal hydrides are key intermediates in the chemical transformations of a range of substrates including fine chemical syntheses and chemical storage of hydrogen. Ranging from mononuclear coinage metal hydrides to clusters and nanoparticles, a fundamental understanding of their atomic and molecular interactions is invaluable in developing innovative solutions to practical problems. The reactive sites can be identified using a range of spectroscopic methods allowing the “tuning” and/or “reshaping” of the reactive site by ligands to control the reactivity. Mass spectrometry provides a means to identify coinage metal hydrides in solution and further allows isolation of discrete coinage metal hydrides that can be: (i) characterised, for example by spectroscopic methods, (ii) reacted with neutral substrates, or (iii) fragmented to generate reactive intermediates in the gas phase. The use of borohydride in nanoparticle synthesis is well-known. Chapter 2 describes a mass spectrometry directed synthesis to afford the first isolable silver hydride borohydride cluster, [Ag3(μ3-H)(μ3-BH4)L3]BF4 (L =bis(diphenylphosphino)methane), structurally characterised by X-ray crystallography. Gas-phase experiments and DFT calculations reveal ligand (L) loss from [Ag3(H)(BH4)L3]+ results in the loss of BH3 and a geometry change of the cluster to yield [Ag3(H)(BH4)Ln]+ (n = 1 or 2). This work reveals links between silver hydride/borohydride and silver hydride nanoclusters adding to our understanding of silver nanoparticle synthesis using borohydride salts. Chapter 3 examines that the reactivity of CO2 with the binuclear silver hydride cation core, [Ag2H]+, can be controlled by design. Reshaping the geometry and reaction environment of [Ag2H]+ using a range of phosphine ligands (bis(diphenylphosphino)methane, 1,2- bis(diphenylphosphino)benzene and bis(diphenylphosphino)ethane) allows “tuning” of the active site’s reactivity toward formic acid to produce H2. Gas-phase ion-molecule reactions, collision-induced dissociation, infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. The gas-phase studies were then translated to solution-phase studies using NMR to show that H2 could be produced from solutions comprising well-defined ratios of ligand, AgBF4, NaO2CH and HO2CH at near ambient temperature. Chapter 4 further developed the concept of altering the reactive site by changing the binuclear metal centres of the [LAg2H]+ core to compare all six possible combinations of copper silver and gold i.e. [LAg2H]+, [LCu2H]+, [LAu2H]+, [LCuAgH]+, [LCuAuH]+ and [LAgAuH]+ in the gas phase. DFT calculations, gas-phase ion-molecule reactions and gas-phase energy-resolved collision-induced dissociation showed both metal centres play a role in the reaction with formic acid. One metal site functions as an “anchor” for an oxygen of formic acid or formate while the other facilitates the dehydrogenation step resulting in the formation of H2. It was found that the copper homobinuclear species performed best overall. Attempts to isolate the reactive intermediate [LAg2(O2CH)]+ by using a range of bisphosphine ligands resulted in the isolation of an unusual co-crystal in the case of L = dcpm as described in Chapter 5. Single crystal X-ray diffraction of crystals suitable for crystallographic analysis revealed two discrete tetranuclear silver clusters [(μ2-dcpm)Ag2(μ2-O2CH)(η2-NO3)]2·[(μ2- dcpm)2Ag4(μ2-NO3)4]. The solution-phase studies, tracked by NMR, show that H2 could be produced from solutions comprising well-defined ratios of ligand, AgBF4, NaO2CH and HO2CH at 65⁰C. Gas-phase studies indicate that while the tetranuclear cluster [L2Ag4(O2CH)3]+ undergoes sequential decarboxylation reactions, none of the resultant hydrides react with formic acid. These results highlight important role of the binuclear hydride [LAg2(H)]+ in the catalytic decarboxylation of formic acid. Hydrido cuprate [CuH2]- has been explored for its applications in hydrogen storage. Chapter 6 indicates two chemically induced routes for the liberation of hydrogen when [CuH2]- is reacted with various chemical substrates. One path occurs via homocoupling of both hydride ligands giving the substrate-coordinated copper, the other by protonation with acids.