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Electro-organic reactions are becoming increasingly important due to interests in energy efficient and green synthetic methodologies. Existing electro-synthetic methods largely focus on dimerization pathways performed on Pt or carbon electrodes. Catalytic reactions utilizing metal oxides are often done using nanoparticles. Traditionally, the oxide synthesis and subsequent reactivity screening occur separately in two independent experiments. Experiments in this dissertation were aimed at screening new electro-organic pathways, which requires the development of platforms capable of monitoring organic reactions quantitatively and qualitatively in real-time. The overarching objective was to use electrical energy (direct current/ DC) to form oxides in-situ on non-inert metal/alloy electrode surfaces via electro-oxidation while also screening catalytic activity of the as-formed nascent oxides simultaneously in a single step using Mass Spectrometry. We expected the presence of oxides to open new reaction pathways different from those available solely by electron-transfer-driven mechanisms. We harnessed the electrical energy inherent in electrospray ionization (ESI) to create the nascent oxides whose reactivities were studied in real-time using suitable chemical systems on the reactive nano-electrospray ionization mass spectrometry (nESI-MS) platform that was developed in this dissertation. The first chemical system studied involved the conversion of isosafrole to piperonal using positive-ion mode analysis (at 1.5 kV and MeOH/H2O (1:1, v/v). We tested several transition metals (Ir, Ru, Pt, Ag, etc.) as electrodes for nano-electrospray ionization and found Ir to be the most reactive electrode during the ionization process. Overall, we learned that the Ir electrode is active toward isosafrole oxidation only in the presence of applied DC potential. This observation supports the existence of a transient active species similar to those generated in the physisorbed active oxygen surface, MOx(•OH). Indeed, in relatively low pH medium, such as that existing in the nESI process, the physisorbed hydroxyl radicals are found to dissociate to yield charged surface sites that eventually facilitate the formation of hydrated species generally represented as MOx(OH)y. Extensive XPS analyses of used Ir electrode did not reveal the presence of permanent oxides. This can be explained by the unstable nature of MOx(OH)y and MOx(•OH). Therefore, it is presumable that nascent active oxides are formed during the application of the DC potential allowing reactions to be studied by mass spectrometry in real-time. With this established, we are studying a variety of reactions using this Ir-based nESI-MS platform. These include: (1) oxidation of aldehydes to the corresponding carboxylic acids (Chapter 2). The main objective was to understand the mechanism of this unique electro-catalytic reaction and employ it for the analytical purpose of detecting small, poorly ionizable analytes such as aldehydes. This resulted in an instantaneous oxidation of the aldehyde to the corresponding acid and its detection on the negative mode with an enhanced intensity without the need for any other derivatization steps that are traditionally employed. Preliminary experiments using various para-substituted derivatives of benzaldehyde have shown rapid conversion into the corresponding carboxylic acid. (2) Epoxidation reactions involving unsaturated fatty acids (Chapter 3). Since mass spectrometry is frequently transparent to constitutional isomers, we proposed to use our ability to oxidize C=C bonds as a means to differentiate fatty acid positional isomers. We expected direct cleavage of the C=C bond as observed for isosafrole. However, we observed epoxidation in the case of the larger unsaturated fatty acids such as oleic acid and vaccenic acid. This difference in the reactivity of the Ir-based nESI-MS platform was not surprising given that we utilized cathodic oxidation for fatty acid analysis. Isosafrole was analyzed using positive potential (anodic oxidation), which is a more efficient means to generate oxide films on a metallic substrate. Due to their high propensity to form deprotonated negative ions, we had to use negative potential for fatty acid analysis, which led to a less efficient cathodic oxidation. Further experiments indicated minimal involvement/production of reactive oxygen species such as H2O2 or O2–• after the application of negative potential. Through combination with tandem MS, we applied the Ir-based nESI-MS platform to localize the C=C bond in fatty acid isomers. Collisional activation of the fatty acid epoxide results in the formation of diagnostic product ion pairs that are indicative of the position of the double bond in the fatty acid. We have recently finished a manuscript where the fatty acid isomers, oleic acid and cis-vaccenic acid were successfully differentiated and quantified directly from serum at concentration levels as low as 1.2 μM. (3) A Facile CO2 insertion reaction across possible aryl-H bonds (Chapter 4). In the direction of exploring new reactions during anodic and cathodic cycles, we decided to start screening with derivatives of 1,2,3,4 tetrahydroquinoline- carbaldehyde and 1,2,3,4 tetrahydroquinoline-carboxylic acids as that can be detected in both positive and negative modes simultaneously. Interestingly, 1,2,3,4 tetrahydroquinoline-carboxylic acids add up CO2 molecule on our catalytic platform providing insights into newer reaction pathways. The nESI source fitted with reactive electrodes enabled anodic and cathodic electrochemical reactions to be explored for chemical synthesis via the oxidation system of 1,2,3,4 tetrahydroquinoline-carbaldehyde. |
