Browsing by Author "Wilbers, Derik"

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    Hydrocarbon oxidations using metal-containing polypropylene-imine dendrimers
    (Stellenbosch : Stellenbosch University, 2019-12) Wilbers, Derik; Mapolie, Selwyn Frank; Luckay, Robert C.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The production of oxygenates from hydrocarbon feedstocks is an important industrial process. However, this process suffers from various weaknesses such as poor catalytic activity, chemoselectivity, and often requires the use of harsh reaction conditions. In this thesis attempts to address these shortcomings are described. The synthesis and characterization of a number of transition metal complexes, many of which are novel, as well as their application in the oxidation of hydrocarbons (both unsaturated and saturated) are described. The complexes include both iron metallodendrimers as well as iron mononuclear complexes (model complexes) based on pyridine-imine ligands. In addition to the iron complexes, a number of cobalt salicylaldimine complexes (metallodendrimers and model complexes) as well as cobalt thioether-amine complexes were synthesized. The complexes were then employed as catalyst precursors in the catalytic oxidation of cyclohexene. Both the cobalt model complexes C1 and C2 as well as their dendritic counterparts (MD1 and MD2) performed poorly as catalyst precursors in the oxidation of cyclohexene when using H2O2 as oxidant. The poor performance of the model complexes was attributed to the disproportionation of the oxidant, a process catalyzed by the model cobalt complexes C1 and C2. When H2O2 was replaced by TBHP as oxidant, virtually no disproportionation of this oxidant was observed and an improvement in catalytic activity was obtained, however, turnover numbers (20 mol product/ mol catalyst) remain relatively low. The cobalt complexes exhibited poor activity in the oxidation of n-octane obtaining product yields of around 2% under the optimized conditions (2 mmol cyclohexene, 1 mol% catalyst, 4 mmol TBHP, 40°C, MeCN). The iron complexes were more active for catalytic cyclohexene oxidation than the cobalt complexes. C4 and MD5 (iron pyridine-imine complexes) gave cyclohexene conversions of around 50% with turnover numbers around 60. In all cases high selectivity towards the allylic oxidation product 2-cyclohexen-1-one was obtained. The iron complexes were also more active than the cobalt complexes in n-octane oxidation achieving product yields of around 7% (2 mmol octane, 0.5 mol% iron, 4 mmol TBHP, 60°C, MeCN). The kinetics of the reaction of selected complexes with peroxide-based oxidants were investigated in an attempt to rationalize the observed differences in catalytic activity for these systems. It was found that the cobalt salicylaldimine complex MD1 has a significantly higher activation energy than the iron complex MD5 which might serve as a possible explanation for the observed differences in catalytic activity of the two catalyst systems. Further investigations attempted to ascertain the most likely mechanism by which hydrocarbon oxidation, catalyzed by these complexes, is achieved. It was found that these complexes are likely to operate via a free radical-centered mechanism.
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    Preparation, characterization and applications of macrocycle-dendrimer conjugates
    (Stellenbosch : Stellenbosch University, 2013-12) Wilbers, Derik; Mapolie, S. F.; Luckay, Robert C.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: In this thesis we describe various attempts at incorporating macrocycles into dendritic architectures to form macrocycle-dendrimer conjugates with the aim of preparing materials that would exhibit properties that are more than the sum of the constituent parts, in this case macrocycles and dendrimers. A further aim was the synthesis and characterization of metallodendrimers based on such scaffolds and to test these as catalyst precursors in the catalytic oxidation of alcohols. The synthesis of two different types of conjugate systems was attempted; viz. dendrimers functionalized with macrocycles on the peripheries and dendrimers with macrocyclic cores. The synthesis of conjugate systems based on cyclam as the macrocycle was attempted. This required the mono functionalization of cyclam with a linker molecule capable of further reaction with the functional groups at the periphery of commercially available N,N,N,N-tetrakis(3-aminopropyl)-1,4-butanediamine dendrimer. Several approaches were taken in trying to make such conjugate systems but they were not entirely successful. One of the major issues was the final deprotection step, of the Boc-protected cyclam units which proved difficult in our hands. Another approach to prepare the target conjugates involved the use of click chemistry in order to synthesize a dendrimer with an aromatic core and cyclam peripheries. A dendrimer with Boc-protected cyclam peripheries that are bonded through triazole groups to the aromatic core was synthesized. However, subsequent attempts at de-protection of the cyclam functionalities of this conjugate failed to yield the pure de-protected dendrimer. Greater success was achieved with the preparation of a dendrimer with a macrocyclic core. A cyclam cored dendrimer with salicylaldimine peripheries was successfully synthesized and characterized. This dendritic ligand was complexed to Cu(II), Ni(II) and Zn(II) metal ions respectively to form a series of new metallodendrimers. These metallodendrimers were fully characterized using a range of analytical techniques including FT-IR spectroscopy, mass spectrometry, elemental analysis, thermogravimetric analysis, magnetic susceptibility measurements and NMR spectroscopy where appropriate. The Cu(II) and Ni(II) metallodendrimers were tested as catalyst precursors in the catalytic oxidation of benzyl alcohol to benzaldehyde. The catalytic system consisted of the appropriate metallodendrimer, the free radical, 2,2,6,6-tetramethylpiperidinyl- 1-oxyl (TEMPO) and O2 as the oxidant. The reaction parameters, namely the nature of the solvent, catalyst loading, substrate concentration and reaction temperature were sequentially optimized to achieve the best catalytic efficiency. The Cu(II) catalyst precursor exhibited relatively high catalytic activity and achieved TOF’s between 40 and 30 when operating under the optimized conditions, while the Ni(II) catalytic system showed very poor catalytic activity.