Investigating applications of photocatalysis in synthesis and exploring original approaches to N-Heterocycles
Creators: name: Tang, Meiqiong
Publisher: name: University of Tasmania
Year: 2024

description: Photoredox catalysis has drawn considerable attention in recent times as this area of chemistry provides powerful tools for carbon–carbon and carbon–heteroatom bond formation in organic synthesis. This Thesis investigates the development and applications of transition-metal-based photoredox catalysts with the broad objective of exploring their scope and limitations in both organic chemistry and polymer chemistry. In Chapter 2, the application of photoactive copper complexes in photo-mediated RAFT polymerisation of methyl methacrylate (MMA) under visible light irradiation was explored. The RAFT polymerisation was shown to proceed through a “living” process under white light irradiation, yielding well-defined polymers with low dispersities. Temporal control was demonstrated via simple light “on/off” experiments in addition to successful chain extension reactions. However, significant background reactions (in the absence of photocatalyst) were observed in these experiments, attributed to the direct photo-excitation of the RAFT agent. In addition, the synthesis of hyperbranched polymers was demonstrated via a combination of PET-RAFT polymerization and thiol– alkynone chemistry (Figure 1). Poly(methyl acrylate) (PMA) was initially prepared by RAFT-polymerization, followed by end-group modification of the R group to introduce a protected terminal alkyne. Upon deprotection and reduction of the thiocarbonylthio end group, hyperbranched structures were formed through a thiol–alkyne click reaction at room temperature. The potential reversibility between the mono- and di-thiol adduct offers a potential route to hyperbranched polymers with tuneable composition.<br>[Image]<br><b>Figure 1</b>: Representative structure of hyperbranched polymers synthesised in Chapter 2<br>Chapter 3 discusses the study of photoactive metal carbonyl complexes and their evaluation as photoredox catalysts. Part One explored attempts to establish a solvent?controlled, divergent atom-transfer radical addition process employing a photolabile rhenium(I) catalyst and the transfer reagent triflyl chloride (Figure 2, left). Part Two investigated the synthesis and characterisation of novel photoactive metal carbonyl complexes bearing N-heterocyclic carbene ligands and the evaluation of these systems as photoredox catalysts (Figure 2, right). A number of structurally related chromium (<b>3.30</b>), molybdenum (<b>3.31</b>), tungsten (<b>3.32</b>) and manganese (<b>3.33</b>) complexes were synthesised and characterised. During the course of this study, a high background reaction was observed in Povarov-type reactions. Thus, Part Three explored the development of a catalyst-free, photochemical Povarov-type reaction that allowed for the preparation of tetrahydroquinolines in low to moderate yields. The reaction proceeded efficiently in DMF under blue light irradiation at room temperature in air.[Image]<br><b>Figure 2</b>: Solvent-controlled divergent ATRA reactions in MeCN and CH2Cl2 under investigation (left); the photoactive metal carbonyl complexes studied in Chapter 3 (right)<i>.</i><br>Chapter 4 discusses attempts to develop new routes to polycyclic alkaloids. Part One investigated an original approach to ent-stemaphylline via the coupling of lactone <b>4.35</b> and ketone <b>4.36</b> (Scheme 1). Advanced key bicyclic intermediate <b>4.36</b> was obtained via the cyclisation of epoxide <b>4.38</b> and oxidation of the ensuing product. However, attempts to unite fragment 4.36 with an appropriate derivative of lactone <b>4.35</b> via either Wittig or Julia olefinations were unsuccessful. Part Two explored an approach to access the core of the alkaloid natural product lycorine.[Image]<br><b>Scheme 1</b>: Retrosynthetic analysis of <i>ent</i>-stemaphylline explored in Chapter 4.