| Lizenz: Creative Commons Namensnennung 4.0 International (52MB) |
- URN zum Zitieren dieses Dokuments:
- urn:nbn:de:bvb:355-epub-365302
- DOI zum Zitieren dieses Dokuments:
- 10.5283/epub.36530
| Dokumentenart: | Hochschulschrift der Universität Regensburg (Dissertation) |
|---|---|
| Open Access Art: | Primärpublikation |
| Verlag: | University of Regensburg, Germany |
| Ort der Veröffentlichung: | Regensburg, Germany |
| Seitenanzahl: | 526 |
| Datum: | Januar 2018 |
| Begutachter (Erstgutachter): | Prof. Dr. Oliver Reiser und Prof. Dr. Burkhard König und Prof. Dr. Arno Pfitzner |
| Tag der Prüfung: | 8 Dezember 2017 |
| Institutionen: | Nicht ausgewählt |
| Sonstige Projekte: | Organic Chemistry, Visible-Light Photocatalysis |
| Stichwörter / Keywords: | Organic Synthesis, Visible-Light Photoredox Catalysis, Sustainable Chemistry |
| Dewey-Dezimal-Klassifikation: | 500 Naturwissenschaften und Mathematik > 540 Chemie |
| Status: | Veröffentlicht |
| Begutachtet: | Ja, diese Version wurde begutachtet |
| An der Universität Regensburg entstanden: | Ja |
| Dokumenten-ID: | 36530 |
Zusammenfassung (Englisch)
This thesis demonstrates the development of new and unprecedented activation methods in visiblelight mediated photoredox catalysis. The established transformations not only unlocked new avenues in organic synthesis but holds enormous potential for the future advancements and sustainable developments in this field. In chapter 1, we have outlined a short overview of visible-light-mediated ...
Zusammenfassung (Englisch)
This thesis demonstrates the development of new and unprecedented activation methods in visiblelight mediated photoredox catalysis. The established transformations not only unlocked new avenues in organic synthesis but holds enormous potential for the future advancements and sustainable developments in this field.
In chapter 1, we have outlined a short overview of visible-light-mediated processes that have been established over the years, starting with Ciamician’s idea of using sunlight as a clean and unlimited energy source for organic synthesis. Therein, we have also highlighted four principal activation modes associated with visible-light photocatalysis (i.e, oxidative quenching, reductive quenching, energy transfer, and a photocascade process). Accordingly, we stated the diversity of various synthetic methods relies upon the ability of photoredox catalysts such as [Ru(bpy)3]Cl2, [Ir{dF(CF3)ppy}2(dtbbpy)]PF6, [Ir(ppy)2(dtbbpy)]PF6, or fac-Ir(ppy)3 to undergo a number of productive quenching pathways from the excited state. The flexibility of these catalysts has allowed for the development of a series of transformations such as diradical formation by triplet photosensitization, cycloaddition reactions, vinyl radical formation by reductive radical dehalogenations, 1,6-HAT process or α-amino functionalization, sulfonylation as well as arylation reactions, etc. Notably, these photocatalysts certainly possess a series of advantages over the classical radical chemistry in light of sustainable developments.
In chapter 2, we have summarized the visible-light mediated activation of C-X bonds, generation of vinyl radical as a key intermediate, and its applications in organic synthesis that are developed in last few years, mostly from our group.
In chapter 3, we have presented the unprecedented activation of vinyl-bromides with molecular oxygen utilizing dual energy and electron transfer modes: Ortho-alkynylated α-bromocinnamates can be converted by a visible-light-mediated photocascade reaction with molecular oxygen into either indenones or dihydroindeno[1,2-c]chromenes. The one-step process features key photochemical steps; thus, the initial activation of vinyl bromides through energy transfer to give α-ketoradicals in a reaction with molecular oxygen, followed by α-oxidation of an arene moiety by 6-π electrocyclization, and subsequent hydroxylation by an electron-transfer process from the
Abstract
same photocatalyst leads to the dihydroindeno[1,2-c]chromenes. For this chapter, we have conducted a series of experiments in attempts of understanding the detailed reaction mechanism. While finding the mechanistic details, some of the attempts also ended up in disclosing the new reactions; for examples, visible-light mediated [2+2]-photocycloaddition of cinnamates with visible-light was unknown. As a consequence, we have subsequently developed the latter method and extended the scope of the stated transformation, which is separately described in chapter 4.
In chapter 4, an efficient method for the synthesis of substituted cyclobutanes from cinnamates, chalcones, and styrenes utilizing a visible-light triplet sensitization mode was described. This reaction provided a diverse range of substituted cyclobutanes in high yields under mild conditions without the need of external additives. Good regioselectivity was obtained due to strong π–πstacking of arene moieties, whereas diastereoselectivity relied on the electronic effects or orthosubstitution of the arene substrate. Furthermore, we have also shown the utility of this transformation by formal synthesis of the lignane natural product (±)-Tanegool.
In chapter 5, a tin-free method for the synthesis of substituted indolines was described generating vinyl radicals by visible-light-promoted photocatalysis in a reductive quenching cycle. This method offered a mild, robust, and high yielding pathway to a wide range of indolines containing diverse electronic substituents. The resulting 2,3-disubstituted indolines serve as valuable precursors for the synthesis of biologically active molecules, which was demonstrated with the formal synthesis of Tryptamines and Furoindolines. Notably, we have replaced the hazardous reagents and harsh reaction conditions with the mild reaction conditions.
In chapter 6, the unprecedented discovery of Triquinacenes was outlined: We have shown for the first time, a visible-light mediated cascade of C-C bond formations with vinyl chlorides and phenyl acetylenes. Noteworthy, this process involves four highly reactive vinyl radical intermediates, which was engaged in a productive reaction cascades with two phenyl acetylene molecules, leading to the complex molecular designs in a single photochemical step under the mild reaction conditions. This reaction could also be performed in the micro-flow reactors in the short reaction time. Arguably, the single step synthesis of these types of complex architectures is certainly impossible to accomplish by employing other methods at this stage.
Abstract
In chapter 7, we have highlighted the applications of sulfonyl chlorides. This has been found a cheap, biocompatible, readily available reagent for sulfonylation, trifluoromethylation or arylation reactions under visible-light conditions.
In chapter 8, the sequential [Cu] and [Ir] photocatalyzed method for the synthesis of βhydroxysulfones from sulfonyl chlorides and styrenes in the presence of water was demonstrated. We showed here the importance of each specific catalyst that is employed in the title transformation. In turn, this paper has already received a remarkable attention from the synthetic community.
Finally, in chapter 9, we further extended the scope of sulfonyl chlorides for arylation or sulfonylation of heterocycles at different temperature conditions. We showed for the first time a comparative study on the selective C-C and C-S bond formations just by adjusting the appropriate reaction temperature.
Overall, in my PhD work, I had a chance to work in the fascinating field of visible-light photoredox catalysis. As shown, I have come across several unique reaction pathways and closely witnessed these surprising discoveries. Undoubtedly, further discovery and development of new methods utilizing visible light-mediated photoredox catalysis will continue to grow in the next years, particularly due to the redox flexibility of photocatalysts to be employed with a range of reactions. However, a detailed study of how factors such as temperature, solvent choice, catalyst choice, or additives, etc…affect the reaction outcome. Such studies will be necessary to understand how to better improve existing methods and develop new methods. More importantly, these studies may also provide answers to many of the existing anomalous results that have been observed in this field. One example of an observation that could possibly be explained by more rigorous mechanistic studies involves the specific role of oxygen for the vinyl bromide activation (Chapter 3) or the high reactivity of vinyl chlorides over vinyl bromides in Triquinacene synthesis (Chapter 6), and so on. Another observation pertains to the unexpected C-S bond formation in the trifluoromethylsulfochlorination reaction in the presence of selected [Cu]-catalyst in contrast to the [Ir]- or [Ru]-photocatalysts (Chapter 8).
Abstract
Last, but not least, to understand the detailed reaction mechanism of new photoredox methods is still a challenging task. Preliminary results in our group especially for the activation of vinyl halides indicate that particular reactions are prone to a high level of selectivity and reactivity via different reaction pathways based on the specific substituents present on the starting materials or the photocatalyst employed. The design of special substrates or identification of key intermediates involved could open new chapters along with understanding the mode of action of a photocatalyst in the overall process.
Übersetzung der Zusammenfassung (Deutsch)
Diese Arbeit beschreibt die Entwicklung neuer Aktivierungsmethoden in der Photoredoxkatalyse mit sichtbarem Licht. Die hier beschriebenen Umwandlungen eröffnen nicht nur Wege in der organischen Synthese, sondern bergen auch enormes Potential für zukünftige Untersuchungen und nachhaltige Entwicklungen in organischer Chemie.
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