Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by browse.metadata.advisor "Arnott, Gareth E."

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    Inherently chiral calixarenes; methodology and applications
    (Stellenbosch : Stellenbosch University, 2016-12) Castell, Dominic Christian; Arnott, Gareth E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The use of chiral directing groups has provided an efficient route to meta-functionalised inherently chiral calixarenes. Previously reported ortholithiation methods, incorporating chiral oxazolines have been reexamined, with the aim of revising the individual roles of the three major components of the reaction. The potential mechanistic ramifications of the solvent, additive and alkyllithium structure on the reaction outcomes were individually evaluated. The overarching complexity inherent in this chemistry, coupled with a wide scope of experimental results, point to a number of substrate, solvent and also reagent dependent reaction mechanisms. In addition to the oxazolines, the tert-butyl sulfoxide functional group has also been established as an effective chiral auxiliary for this ortholithiation strategy, yielding enriched diastereomeric mixtures of inherently chiral sulfoxide calixarenes. The absolute stereochemistry of these major and minor products were determined crystallographically. Despite restrictions in electrophile choice, an efficient desulfurization method afforded a route to a new class of meta-functionalised inherently chiral compounds. With the view to evaluating the potential of inherent chirality, oxazoline-directed ortholithiation methods were used to synthesize a new class of inherently chiral phosphine oxazoline ligands. The results from the application studies of these compounds, to both the Tsuji-Trost allylation and asymmetric Suzuki coupling reactions, suggested a definitive relationship between the configurations of the inherent chirality of the calixarene, and central chirality of the oxazoline auxiliary. A model of the -allyl palladium intermediate responsible for the enantioselection seen in the Tsuji-Trost reaction has been proposed for these calixarene ligands. The model was constructed using previously reported calixarene crystallographic data, in conjunction with numerous solid-state and computational studies of simpler Phox ligand systems. The experimental results obtained for this reaction were found to be in good correlation with the proposed model for the inherently chiral calixarene ligands. To confirm these observations, two additional inherently chiral Phox ligands, lacking a stereocenter on the oxazoline, were synthesized. Their preliminary application to the Tsuji-Trost allylation also yielded enantiomerically enriched product mixtures. These results point to a significant contribution made by the inherent chirality, toward the overall asymmetric induction observed in the reaction. These findings serve as a positive affirmation of the viability of inherently chiral calixarenes in the field of asymmetric catalysis, and create a platform for further development of new ligands containing this structural property.
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    An investigation into the functionalization of poly(styrene-co-maleic anhydride) with calixarenes and their potential applications
    (Stellenbosch : Stellenbosch University, 2019-12) Eghbali Dogaheh, Seyed Babak; Arnott, Gareth E.; Mallon, P. E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: This work describes the development of a new method for the synthesis of a calixarene grafted polymer. The method involves synthesizing a mono-alkylated calixarene that contains an amino-tether suitable for grafting onto polymers such as the polystyrene-maleic anhydride copolymer (PSMA). The amino-tether was synthesized through a Gabriel synthesis on the parent phthalimide-derivatized calix[4]arenes. Having successfully optimized the method, the next stage investigated the potential applications for this technology. Two divergent applications were investigated: 1) a colorimetric mercury sensor, and 2) supramolecular hydrogel formation. The first application was based on the known sensing ability of functionalized calixarenes for the detection of different guests (cations, anions, neutral species, etc.). The most interesting one has been the colorimetric approach. Of interest in this work was the reported distal bis- allyl bis-arylazo calix[4]arene (also known as Chung’s sensor), which is capable of detecting mercury cations with a rapid color change in the solution media (yellow to pink). Therefore, a polymer-supported version of the same calix[4]arene was targeted for synthesis to facilitate the formation of a sensing instrument for mercury. Despite all efforts (trying eight synthetic routes), the required functionalized calixarene could not be obtained. Several suggestions for future work have been offered to solve this challenge. The second application aimed at in this project was to attach water-soluble calix[4]arenes to PSMA in order to make the polymer water soluble and to form a potential hydrogel. The most well-studied compound in this category has been the p-sulfonated calix[4]arene, which has been shown to form supramolecular complexes with bis-quaternary amines in aqueous solution. The possibility to achieve hydrogelation through a p-sulfonated calix[4]arene-grafted PSMA was therefore investigated. The primary amine-tethered p-sulfonated calix[4]arene was successfully synthesized and grafted onto PSMA. Unfortunately, the resulting grafts were unable to form a hydrogel in the presence of multiple quaternary amine-based compounds in spite of varying a large number of experiment parameters (i.e. stoichiometry, pH, temperature, concentration and so forth). One problem was that the grafts were not as water soluble as expected. Thus a second option was to replace PSMA with another polymer with higher water solubility upon grafting with the calix[4]arene. The polymer investigated was a polyvinylpyrrolidone maleic anhydride copolymer (PVP-MA). The grafting reaction was successfully carried out and much higher water solubility for the graft was achieved, but once again a hydrogel could not be formed. It is believed that this failure most likely is due to the conformational issue of the calix[4]arene or the low degree of functionalization for the grafts. Several solutions to this these failures have been proposed.
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    Oxazoline directed lithiation of Calix[4]arene and Ferrocene
    (Stellenbosch : Stellenbosch University, 2011-12) Herbert, Simon Anthony; Arnott, Gareth E.; Stellenbosch University. Faculty of Sciences. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The use of chiral oxazoline directed lithiation provides a highly diastereoselective (up to >99% de) route to meta functionalised inherently chiral calixarenes. This methodology can be used on both the butylated and debutylated calixarene systems and is tolerant of a wide range of different electrophillic quenches allowing access to a structurally diverse range of inherently chiral metafunctionalised calixarenes. The oxazoline directing group can be removed via hydrolysis, generating a range of functionalised calixarene carboxylic acids in high ee. We also demonstrate that the use of derivative alkyllithiums such as cyclopentyl lithium can provide significantly enhanced diastereoselectivity over the conventional organolithiums such as sec-butyl lithium, when employed in ortholithiation reactions of this nature. The differences in diastereoselectivity associated with the different alkyllithiums can be tied, in certain cases, to the steric bulkiness associated with the individual reagents. In this regard we have found that the use of the so called Tolman angle or cone angle approach allows quantification of the relative steric bulk of the alkyllithium. We also detail that the oxazoline directing group provides a hitherto unknown ability to be diastereoselectively tuned through the choice of the ligand system in the ortholithiation reaction. In this regard the development of a series of diglyme based ligands have proved to provide a highly diastereoselective means of inverting the chirality from that which the use of the conventional TMEDA ligand is able to generate (up to –92% de). The use of diglyme ligands to invert the sense of chirality is also shown to occur on the ferrocenyloxazoline system and presents an apparently general and hitherto unknown facet of asymmetric oxazoline directed ortholithiation. This diglyme induced inversion has been shown to be controlled through a secondary nitrogen coordinated mechanism that is able to operate with chiral oxazolines.
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    Resorcin[4]arene based N-heterocyclic carbenes as catalysts for carbon-carbon formation
    (Stellenbosch : Stellenbosch University, 2016-12) Ngodwana, Lonwabo; Arnott, Gareth E.; Van Otterlo, Willem; Stellenbosch University. Faculty of Science. Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Resorcin[4]arenes are 3-dimensional cyclic tetramers belonging to a class of compounds called cyclophanes. Because of their properties, including several sites of functionalization and possession of a cavity, these compounds found their way into several applications. These include cation extraction, ion exchange mimics, molecular switches and catalysis. The latter is still at its infancy. This work was aimed at developing the first examples of resorcin[4]arene based Nheterocyclic carbene (NHC) complexes of palladium and evaluating their potential for catalysis of carbon-carbon bond formatting reactions. Using an ortholithiation technology that was developed in our group, a distally functionalized resorcin[4]arene di-ester was prepared. The reduction of the ester and bromination of the resulting di-ol led to a distal bromomethylresorcin[4]arene. The reactions of this new resorcin[4]arene with a variety of N-alkylimidazoles gave a small library of resorcin[4]arene imidazolium NHC precursor salts. Initially, the preparation of a bidentate palladium complex from the salts was attempted. The metal complex was isolated in 35% yield from an N-mesityl imidazolium salt. When the preparation of a dinuclear metal complex series, i.e. PEPPSI, was attempted, the compounds in this class could be isolated in yields over 90%. Although these compounds show the expected coordination modes (square-planer Pd center), they showed some variations in solid state conformations. Resorcin[4]arene NHC compounds, both pre-formed and prepared in situ, were evaluated for catalytic ability in the Suzuki-Miyauri and Tsuji-Trost reactions. These compounds could facilitate moderate to complete conversion of starting materials to products.
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    Studies in the synthesis of benzoxazole compounds
    (Stellenbosch : Stellenbosch University, 2015-12) Kleinhans, Dewald Johannes; Arnott, Gareth E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Benzoxazoles are an important class of π-electron-excessive, benzene-fused heterocyclic compounds found in natural products and display a wide range of pharmacological applications. It is therefore a widely used starting scaffold for drug and agrochemical discovery programs. Other applications include: chiral auxiliaries in asymmetric reactions, chiral receptors for the resolution of racemic mixtures, fluorescent whitening dyes, various photochromic materials and as ligands for a wide range of catalytic reactions. Due to our interests in resorcinarenes, we came across 4-hydroxybenzoxazoles, a structural motif that has not been explored as potential asymmetric ligands. In this thesis it was attempted to investigate the synthesis, functionalisation and coordination chemistry of these compound class and finally look at a method of synthesising chiral 4-hydroxybenzoxazoles from amino acids. A small library of achiral 4-hydroxybenzoxazoles were synthesised in good yields. These compounds were then reacted with various transition metals, of which only the Pd-salts proved to return any usable compounds. The first structural evidence of the bonding of 4- hydroxybenzoxazoles was recorded from single crystal X-ray diffraction analysis of the coordination compounds that formed. Different coordination modes were recorded, depending on the ligand and the Pd-salt used. The PdCl2 compounds were also tested for catalytic activity with a Heck reaction, showing good conversions for the reaction between iodobenzene and styrene to form stilbene. Further examination pointed to the ligands playing an insignificant role in the reaction and the products possibly due to only the PdCl2’s reactivity. During this period it was also attempted to functionalise the phenol group with P(III) groups and repeat the coordination and catalytic studies. Efforts to synthesise these compounds were not successful, with oxidation of the P(III) to P(V) groups or degradation of these compounds. Efforts to synthesise these via phosphorous protection, utilising BH3 or the in situ trapping of the compounds with transition metals, were also not successful. During the trapping experiments the phosphinite and Pd-salt formed a re-arranged product that is a known and useful catalyst on its own. Finally a small library of chiral benzoxazoles and 4-hydroxybenzoxazoles were synthesised, starting from amino acids and utilising a Mitsunobu reaction to perform the ring closing. Antimicrobial tests with these compounds did not return any appreciable results.