Development of methodology and novel composite electrodes for the selective electrochemical fluorination of organic compounds for use in positron emission tomography

Abstract

The search for fast, safe and selective fluorination protocols has been the focus of much research in recent years as a result of the unique biological and functional properties afforded by fluorinated organic compounds, and the hazards associated with their production by traditional methods. The need for improved methodologies is especially great for the synthesis of fluorine-18 compounds, owing to the relatively short half-life of the isotope (109.7 min). One technique that could be utilised as a viable synthetic tool is electrochemical fluorination (ECF). Historically, ECF has been researched by a relatively small number of groups; consequently, the technique is not fully understood and is often overlooked by synthetic chemists. In order to make ECF a viable method for routine fluorination of organic compounds, more work is required to understand the full potential of the technique, and the mechanisms by which it occurs. In addition to this, the development of new technologies is essential to further improve control and selectivity of ECF reactions, and hence maximise the potential of the technique. This thesis is split into two main sections; the first details an investigation into the electrochemical fluorination (ECF) of organic compounds, including some aromatic substrates, with a particular focus on the ECF of cyclic carbonates, as these provide good models for electron-rich aliphatic molecules, the like of which are typically difficult to fluorinate by traditional nucleophilic methods. The effects of electrolyte and current density on the overall yield and distribution of fluorinated products has been investigated for the selective ECF of ethylene carbonate using the molten salt electrolytes Et4NF.3HF and Et4NF.4HF. The optimum conditions were then applied to the ECF of propylene carbonate, displaying the first synthesis of the mono-fluorinated product, 4-fluoro-4-methyl- 1,3-dioxolan-2-one (6) by a reaction that has not been reported previously. The main products of each reaction were characterised fully by NMR for the first time, highlighting the interesting AA`XX` spin system displayed by the di-fluorinated compounds (3) and (4). In-situ electrochemical electron pair resonance (EPR) spectroscopy was performed in an attempt to elucidate the mechanism by which ECF occurs. The results of these studies were inconclusive; however, a radical-cation mechanism is postulated, based on a combination of molecular modelling calculations and literature. The second section is concerned with the development of a completely new type of composite electrode which could potentially allow greater control of regioselectivity in ECF reactions as well as expanding the number of substrates to which the technique can be applied. The composite anodes are comprised of a TiO2 doped Si substrate upon which is deposited a metal grid. A battery connected between the metal grid and the back of the Si wafer allow a flow of electrons through the device (providing the bias circuit) in addition to the standard electrochemical circuit. The application of bias potential allows increased currents at lower electrochemical potentials. Fundamental investigation into the electrochemical behaviour of the anodes resulted in the postulation of a theoretical model detailing the mechanism by which the anodes work. The model is based on the theory of hole generation and transport through the device by means of Anderson localisation and Miller-Abrahams hopping. It was found that irradiation of the anode surface further increased bias current, hence affording greater control by presenting a three-dimensional surface on which to operate, as opposed to a one-dimensional axis, as is the case with conventional electrodes.