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• W3208365903 abstract "This thesis presents different methods for the photocatalytic activation of carboxylic acid and diol derivatives. Visible light-mediated cross-couplings or cleavage of these biomass-derived substrates have been achieved and mechanistic investigations are described.As carboxylic acids are abundant, non-toxic and inexpensive compounds with high structural diversity, they are ideal starting materials for modern, sustainable organic synthesis. Extrusion of CO₂ from different classes of carboxylic acids gives radical intermediates that react in various cross-coupling reactions. Chapter 1 summarizes recent developments in the field of photocatalytic decarboxylative reactions and compares different methods regarding scope, product yields, atom economy, reaction time and toxicity. Chapter 2 and Chapter 3 present the decarboxylative cross-coupling of natural carboxylic acids with a green light-mediated, environmentally friendly method. In both cases, amino acids, α-oxy acids and fatty acids from renewable resources are activated by esterification to N-(acyloxy)phthalimides. The decarboxylation takes place upon reductive quenching of the excited organic dye eosin Y to its radical anion by DIPEA and subsequent reduction of the N-(acyloxy)phthalimide. An alkyl radical is formed which can undergo C–C bond formation reactions.Chapter 2 describes the trapping of these alkyl radicals with electron-deficient alkenes. After cross-coupling, alkylated products without double-bonds are obtained. Although especially substrates with α-hetero atoms give good yields, even simple fatty acids are suitable, albeit giving lower yields.In Chapter 3, alkyl radicals which are formed upon decarboxylation react with acetylenic sulfones under elimination of a sulfonyl radical. Starting from easily available natural carboxylic acid derivatives, this method enables the synthesis of building blocks with an alkyne moiety which are important for organic synthesis. With this method, even complex starting materials like protected sugar acids, gemfibrozil or cholic acid can be alkynylated in good yields. Another important and abundant class of natural compounds are diols. Chapter 4 presents a photocatalytic method for the C–C bond cleavage of vicinal diols to aldehydes and ketones. This reaction proceeds via ligand-to-metal-charge-transfer (LMCT) with a cerium-catalyst and under irradiation with blue light. The scope includes aryl as well as alkyl substituted diols and the protocol for the valorization of diols is operationally simple as it works under air and at room temperature.The selective, visible light-induced fragmentation of 1,2-diol derivatives with one protected and one unprotected hydroxyl group is described in Chapter 5. The most abundant biomass-derived representatives of this class of compounds are lignin β-O-4 model compounds, a type of aromatic backbone diol derivative. Redox neutral cleavage of the β C(sp³)–O bond within one step is achieved with two different homogeneous photocatalysts, in the presence of a thiol co-catalyst and blue light. Different leaving groups are compatible with the system. With the metal-free catalyst 4CzIPN, the reaction proceeds under more robust conditions but is limited to benzylic diol derivatives. With [Ir(ppy)₂dtbpy]PF₆, the photoreactions have to be carried out under nitrogen atmosphere, but on the other side, even C–O cleavage of aliphatic diol derivatives is possible. A mechanistic pathway including reductive quenching of the excited photocatalyst and HAT of the α-hydrogen atom of the benzylic alcohol to the generated thiyl radical is postulated." @default.
• W3208365903 created "2021-11-08" @default.
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• W3208365903 date "2020-09-01" @default.
• W3208365903 modified "2023-09-27" @default.
• W3208365903 title "Photocatalytic Activation of Carboxylic Acids and Diol Derivatives" @default.
• W3208365903 hasPublicationYear "2020" @default.
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