Study of Photoredox Catalysis for the Preparation of Organofluorine Compounds and Its Applications for Surface Functionalization

光催化氧化還原在制備含氟有機化合物及表面功能化的應用研究

Student thesis: Doctoral Thesis

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Award date20 Dec 2019

Abstract

Organofluorine species are well-known to possess unique chemical, physical and biological properties. These characteristics have inspired intensive investigations with the aim of developing new medicinal and agrochemical compounds, as well as new materials through the incorporation of fluoroalkyl substituents. However, the uncommon chemical reactivity of the organofluorine substituents also creates synthetic challenges for chemists, because many conventional organic synthesis methods fail or are unproductive for compounds with highly fluorinated substituents. Recently, the high versatility of photoredox catalysis in organic transformations has been reported and has attracted particular attentions. The successful generation of perfluoroalkyl radicals (Rf) using photoredox catalysis has also been documented. In this context, it is hypothesized that these perfluoroalkyl radicals Rf could be useful in the preparation of a wide variety of organofluorine species, which are well-known to have important applications in the fields of drug discovery, biomedical and materials science.

In this thesis, new photocatalytic reactions for the preparation of perfluoroalkyl amides from amines and esters from alcohols have been reported. The optimization of the photocatalysis reaction conditions has been studied, including the effects of excited-state redox potentials and the properties of the photocatalysts. Detailed mechanistic studies based on excited-state quenching experiments and the use of radical trap for the characterization of radical intermediates have been performed to elucidate the catalytic cycle as well as the role of the different reagents.

On the basis of the results obtained from photoredox catalysis for amide formation, which demonstrate the successful generation of a wide variety of Rf from the corresponding perfluoroalkyl halides, photoredox catalysis for cross-coupling reactions between alkyne and perfluoroalkyl iodides have been developed. Detailed investigation during the process of reaction optimization revealed that the addition reaction between alkyne and perfluoroalkyl iodides occurred prior to the formation of the cross-coupling reaction product. Reaction conditions for the preparations of the addition products and cross-coupling products have been optimized. These reactions provide moderate to good yields. The emission quenching study showed that the quenching of the excited state of the photocatalysts with perfluoroalkyl iodide is highly efficient with diffusion-controlled quenching rate constants; while the quenching rate constant is slower with the addition products (iodo-perfluoroalkyl substituted alkene). As a result, the addition product is the major reaction product when the photocatalysis is performed in an air-saturated solution with a relatively shorter irradiation time. In contrast, when the photoreaction is performed in an anaerobic condition with a prolonged irradiation time, the cross-coupling product becomes the major product.

Apart from the organic transformations, applications of the above-mentioned photocatalytic amidation for surface functionalization have also been studied. To apply amidation for surface chemical modifications, the hydroxyl groups on the solid surfaces, including glass and cellulose-based materials, were silanized with low-cost 3-(trimethoxysilyl) propylamine. The surface modification of the amines functionalized surface by photocatalytic amidation with perfluorooctyl iodide was then studied. The surface properties, including wettability and hydrophobicity, were evaluated. For the perfluorooctamide-functionalized filter paper, its wetting selectivity and application for the separation of aqueous/organic mixtures have also been investigated.