Saudi Cultural Missions Theses & Dissertations
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Item Restricted Iridium-Catalysed Hydrosilylation of Carbonyl Derivatives: Design and Applications(University of Oxford, 2028-06-15) Almehmadi, Yaseen A; Dixon, Darren JChapter 1: the tertiary amide is a ubiquitous functional group and plays an irreplaceable role in medicinal chemistry. Its robust nature has meant – in the past – that selective manipulation of this motif remained elusive. The reductive activation through hydrosilylation of tertiary amides – using Vaska’s complex (IrCl(CO)(PPh3)2) – has emerged as a powerful strategy for the chemoselective transformation of amides into reactive enamines and iminium ions. Furthermore, these synthetically valuable species can be accessed in the presence of traditionally more reactive functional groups. This approach to amide reductive activation via hydrosilylation has been exploited in a range of downstream C–C bond forming processes, and has seen significant applications in total synthesis, enabling streamlined routes for the synthesis of complex natural product architectures. This chapter covers the development of this synthetic strategy, from initial hydrosilylation studies, to its flourishing use in the reductive functionalization of amide-containing molecules, both simple and complex. Chapter 2: a new reductive strategy for the stereo- and regioselective synthesis of functionalized isoquinuclidines has been developed. Pivoting on the chemoselective iridium (I) catalyzed reductive activation of β,γ-unsaturated δ-lactams, the efficiently produced reactive dienamine intermediates readily undergo [4+2] cycloaddition reactions with a wide range of dienophiles, resulting in the formation of bridged bicyclic amine products. This new synthetic approach was extended to aliphatic starting materials, resulting in the efficient formation of cyclohexenamine products, and readily applied as the key step in the shortest (5-step) total synthesis of vinca alkaloid catharanthine to date, proceeding via its elusive biosynthetic precursor, dehydrosecodine. Chapter 3: three-dimensional nitrogen-rich bridged ring systems are of great interest in drug discovery owing to their distinctive physicochemical and structural properties. However, synthetic approaches towards N–N bond containing bridged heterocycles are often inefficient and/or require tedious synthetic strategies. Herein, we delineate an iridium-catalyzed reductive approach to such architectures from C,N,N-cyclic hydrazide substrates using IrCl(CO)[P(OPh)3]2 and tetramethyldisiloxane (TMDS) which provided efficient first time access to the unstabilized and highly reactive C,N,N-cyclic azomethine imine dipoles. These were stable and isolable in their dimeric form, but, upon dissociation in solution, reacted with a broad range of dipolarophiles in [3+2] cycloaddition reactions with high yields and good diastereoselectivities, enabling the direct synthesis of nitrogen-rich sp3 pyrazoline polycyclic ring systems. Density functional theory (DFT) calculations were performed to elucidate the origin of diastereoselectivity of the cycloaddition reaction, and principal moment of inertia (PMI) analysis was conducted to enable visualization of the topological information of the dipolar cycloadducts. Chapter 4: The synthesis of sterically hindered alpha- or beta-tertiary ethers has long been constrained by the limitations of traditional SN2 and related SN1 approaches owing to poor reactivity arising from steric hindrance or competitive elimination / rearrangement pathways. Herein, we describe a general solution to the hindered ether synthesis problem via an iridium catalyzed reductive deoxygenation of readily prepared ester starting materials. Employing the IrCl(CO)(P[OCH(CF3)2]3)2 complex at 1 mol% and 4 equivalents of tetramethyldisiloxane (TMDS) as the terminal reductant, this alternative synthetic approach to hindered and non-hindered alkyl, aryl and benzyl ethers features mild reaction conditions in a single vessel using low catalyst loadings and with readily available starting materials to access both acyclic and cyclic product ethers in good to excellent yield. Control experiments demonstrated that the IrCl(CO)(P[OCH(CF3)2]3)2/TMDS catalyst system could not only rapidly hydrosilylate esters to mixed silyl/alkyl hemiacetal intermediates but also catalyze reduction of acetals directly to ether functionality, revealing the necessary Lewis acidic and hydridic properties required for this deoxygenative transformation.16 0Item Restricted Polymeric Ionic Liquid Supported Catalysts Incorporating Polyoxometalate and/or Metal Nanoparticles(Newcastle University, 2024-04-18) Alrubayyi, Aeshah; Errington, JohnMetal nanoparticles have a significant interest because of their applications in catalysis and nanoscience, and their synthesis requires stabilisers to prevent nanoparticle aggregation. Metal nanoparticles can be stabilised by ionic liquids. Polymer ionic liquids have recently attracted attention. Doherty and Knight group has developed and utilized them as support to immobilize polyoxometalates, which were Keggin-type polyoxometalates [XM12O40]n- and Lindqvist type- polyoxometalates [M6O19]n- that Errington group designed and developed, and nanoparticles, and explored their applications. The stabilisation of metal nanoparticles such as Pd, Pt, Ag, Au, Ir, Rh, and Ru by polyoxometalates has been investigated by Weinstock and Papaconstintinou. This represents that polyoxometalate provides redox and Brønsted acid functionally that stabilised Metal nanoparticles, which are applied in chemical synthesis electrochemistry and photocatalysis. This project aimed to investigate the interface between the metal nanoparticles and polyoxometalates. A convenient reduction method by hydrazine was used to synthesize polyoxometalate-stabilised Auᵒ nanoparticles by forming electron-rich polyoxometalates by adding multiple electrons to the fully oxidized polyoxometalates, which increases the electron density. Reduced polyoxometalates can act as stabilisers and reducing agents to synthesize polyoxometalates-stabilised Au° nanoparticles. Chapter 1 briefly discusses the history of the development of polyoxometalates, the structure of polyoxometalates, and their application. Developing ionic liquids and utilizing polymer ionic liquids as support are introduced based on previous research and discussed. The second chapter describes the synthesis and characterization of polyionic liquid immobilised Au° nanoparticles and a hybrid catalyst incorporating Au° nanoparticles and polyoxometalates. The polystyrene-based immobilised ionic liquid was prepared via free radical polymerization and used to support Au° nanoparticles (PIIL@AuNP) and incorporate phosphotungstates H3PW12O40 (PIIL@AuNP-PW12). Various techniques, including thermogravimetric analysis (TGA), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), have used to characterize all the prepared catalysts. The third chapter presents the results of the systematic evaluation of the efficacy of the newly prepared PIIL@AuNP and PIIL@AuNP-PW12 systems as catalysts for the selective reduction of nitrobenzene. PIIL@AuNP efficient catalysts in a 1:1 mixture of water and ethanol as the solvent for reduction of nitrobenzene to N-arylhydroxylamine, azoxybenzene, or aniline and incorporation of H3PW12O40 into PIIL@AuNP-PW12 switches the selectivity from N-arylhydroxylamine to aniline. This might be ascribed to the synergic creation of Brønsted acid sites in the presence of polyoxometalates. Chapter 4 describes the multi-electron reduction of phosphomolybdate (H3PMo12O40 and (TBA)3[PMo12O40]) and phosphotungstates (H3PW12O40 and (TBA)3[PW12O40]) and the interactions between the reduced anions and metal nanoparticles in an aqueous, non-aqueous and solid state. Transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FT-IR), and 31P NMR spectroscopy were used to investigate the initial reduction of polyoxometalates with hydrazine and the synthesis of polyoxometalates-stabilised Au° nanoparticles (POM@AuNP) from reduced polyoxometalates/oxidized polyoxometalates, and metal precursors. In a solid-state system, polyoxometalates-stabilised metal nanoparticles (POM@MNP) (M=Au, Ag) are also prepared by using polyoxometalates as stabilisers in the presence of sodium borohydride using a ball mill. Chapter 5 describes the synthesis and characterization of a range of polyionic liquid immobilized Lindqvist-type polyoxometalates ([MW5]@PIIL), where [MW5]= (TBA)3[(MeO)4TiW5O18], (TBA)6[{MnW5O18H}2], (TBA)6[{FeW5O18H}2], and (TBA)6[{CoW5O18H}2]. [(MeO)TiW5O18]3-@PIIL (TiW5@PIIL), [{MnW5O18}2]3-@PIIL (MnW5@PIIL), [{FeW5O18}2]3-@PIIL (FeW5@PIIL), [{CoW5O18}2]3-@PIIL (CoW5@PIIL) were prepared in methanol and acetonitrile solvent at room temperature. Various techniques, including solid-state NMR spectroscopy, thermogravimetric analysis (TGA), energy dispersive X-ray (EDX), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) have used to characterize all the prepared compounds.15 0Item Restricted On-site hydrogen peroxide electrosynthesis via partial oxygen reduction reaction enabled by advanced carbon-based electrocatalysts and electrochemical flow reactors(2023-05-13) Rawah, Basil; Li, WenzhenPursuing sustainable alternatives to fossil fuels for our energy and chemical demands is crucial for mitigating climate change and environmental pressures. Electrochemical processes have become an attractive route for chemical synthesis, owing to their inherent sustainability, as they enable the direct use of renewable energy sources such as solar or wind. Along with operating at mild temperature and pressure, electrochemical processes are also highly efficient, resulting in less by-product wastes than conventional energy-intensive thermochemical processes. Since electrochemical transformations and processes are driven by the electrode potential, optimizing the cell potential can significantly influence reaction kinetics and pathways. Thus, rational design of electrocatalysts and optimization of electrochemical processes are expected to play an essential role in our future energy and chemical production. Hydrogen peroxide (H2O2) synthesis via electrocatalytic partial oxygen reduction has attracted growing interest in recent years. This electrochemical process offers an efficient and sustainable route for on-site generation of H2O2 under ambient conditions. Achieving remarkable performance of H2O2 synthesis is often associated with the activity of the electrocatalyst. A promising electrocatalyst needs to be affordable, highly selective and, active, while also exhibiting high stability and durability in long-term operation. The goal of my Ph.D. research is primarily focused on designing carbon-based electrocatalysts for the cathodic hydrogen peroxide electrosynthesis via partial oxygen reduction with the aim of making electrochemical approaches feasible alternatives. Carbon compounds are attractive catalysts for two-electron ORR (2e- ORR) because of their abundance, moderate selectivity, and desirable stability under reaction conditions. More importantly, the carbon's surface and morphology are highly tunable, which can enhance the electrochemical properties and hence the 2e- ORR selectivity. It is worth noting that the electrolyte pH has a substantial impact on the activity and selectivity toward H2O2 generation, even for the same electrocatalyst. Thus, diverse 2e- ORR electrocatalyst mechanisms in different pH media motivated me to rationally design and finely tune the suitable active sites for H2O2 synthesis in different pH electrolytes. In my Ph.D. study, I explored carbon-based electrocatalysts' ORR activity in acidic media as they often exhibit high ORR overpotential. To address this issue, we hypothesized that by developing cobalt-nitrogen (Co-Nx) activity sites on the carbon catalyst framework, the ORR overpotential might be reduced. However, incorporating (Co-Nx) activity sites requires complicated syntheses and high-cost precursors. In my work, using a facile and cost-effective method, I successfully synthesized nitrogen-doped carbon featuring catalytically active cobaltnitrogen (Co-Nx) sites. Electrochemical measurements in an acidic media demonstrated that the present material significantly enhanced the ORR current density, accompanied by a positive shift in the onset potential and durable performance. Additionally, the present catalyst has shown approximately 90% selectivity toward H2O2 over a broad potential range. In the second project, my goal was to synthesize H2O2 in an alkaline media using a practical flow cell. It is noteworthy that ORR is kinetically facile in alkaline media; therefore, the inclusion of metals is unnecessary. Creating a metal-free carbon catalyst that is both highly active and durable became a major challenge for this project. In order to achieve optimal catalytic performance, I proposed a carbon catalyst featuring an improved structure with tuned nitrogen dopants. I used a simple, solvent-free technique to synthesize metal-free nitrogen-doped ordered mesoporous carbon (N-OMC) by in situ transforming glycine (carbon and nitrogen precursors) in a mesoporous SiO2 template (KIT-6), followed by thermal treatment at various temperatures. The improved structural properties with the optimal N-pyrrolic/N-graphitic, P/G carbon ratio provided remarkable electrocatalytic activity boosting H2O2 with high selectivity and generation rate in alkaline media. Furthermore, its practical capability was demonstrated in our self-designed flow cell, where it produced 9.43 mol gcat-1 h-1 H2O2 at 0.35 VRHE and nearly 100% FE at a cathode potential of 0.6 VRHE for 12 hours without degradation. The final phase of my research involved advancing H2O2 electrosynthesis technology toward more practical applications and validating its use at industrially relevant production rates. I focused primarily on synthesizing H2O2 in a neutral pH, particularly in its pure aqueous form collected in deionized water (DI). Pure aqueous H2O2 electrosynthesis is the most desirable approach, as it is ready-to-use and pH-adjustable. Recently an innovative standard solid-electrolyte flow cell with dual membranes (SE-FCAEM/CEM) was reported, in which the anode and cathode "sandwiched" the cation-exchange-membrane (CEM) and anion-exchange-membrane (AEM) layers, separated by a solid-electrolyte, thus allowing H+ and HO2– ions to recombine to form pure H2O2 in DI water stream. One key research needs to effectively deploy this flow cell is to address the stability and engineering difficulties of using an AEM, which creates significant drawbacks in cell performance and lifespan. In this study, I report a modified SE-FC without involving AEM (SE-FCAEM-FREE) to achieve better performance of H2O2 electrosynthesis. To validate SE-FCAEMFREE for industrial-relevant production rates. First, I further enhanced the performance of our nitrogen-doped carbon catalyst in the new settings by combining glycine, the nitrogen precursor, with affordable and highly conductive commercial carbon black in various nitrogen-to-carbon ratios. Among all samples, the catalyst N-C(2:3) contains high carbon and a proper nitrogen precursor that boosted its activity, resulting in excellent half-cell performance with faradaic efficiency (FE) above 90% at different pH-electrolytes. Secondly, we optimized the catalyst microenvironment by applying a PTFE layer. The Layered-PTFE (5wt.%) arrangement suppressed hydrogen evolution reaction (HER) and exhibited high 2e-ORR activity with a high current density of 380 mAcm-2 (about 6.53 mmol cm-2 h-1) at 90% FEH2O2 with no degradation for a 50-hour durability test.14 0