Browsing by Author "Ngodwana, Lonwabo"

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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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    Selective distal functionalization of resorcinarenes via an ortholithiation approach
    (Stellenbosch : Stellenbosch University, 2012-12) Ngodwana, Lonwabo; Arnott, Gareth E.; Van Otterlo, Willem; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Resorcinarenes are tetramers, belonging to a class of [1]4metacyclophanes, easily formed by the acid- or base-catalysed condensation of resorcinol and aldehydes. Properties include, amongst others; bowl-like shape, adhesion to hydrophilic surfaces and formation of hexameric capsules. Their uses are, to mention some: components in liquid crystals, photoresistors, selective membranes, surface reforming agents, HPLC stationary phases, ion-channel mimics, metal-ion extraction, molecular switches and ligands for metal catalysts. Selective functionalization of resorcinarenes has been explored and achieved via relatively inefficient methodologies which limit studies, structural architecture and new applications. In this work, synthesis of C4v symmetric resorcinarene ethers was performed which were used as templates in undertaking studies towards selective derivatisation of resorcinarenes via an ortholithiation approach. Conditions for the efficient synthesis of distally substituted resorcinarenes using ortholithiation were optimized and tested with a range of electrophiles, lower rim lengths, scale, base equivalents, reaction times and solvent effects. Ortholithiation gave distally substituted resorcinarenes in reasonable yields (>80%). Ortholithiation and its ability to form distal-resorcinarene esters could possibly be used as a way to synthesize distal-chloromethyl resorcinarene precursors whose further functionalization would furnish a range of distal-resorcinarene imidazolium salts, a class of distal bidentate carbene ligand staring materials for transition metal coordination.