Doctoral Degrees (Chemistry)

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Structural and synthetic studies of sesquiterpenoids and flavonoids isolated from Helichrysum species
(2008) Lourens, Anna C. U.
The genus Helichrysum (Asteraceae) consists of approximately 500 species worldwide, with 245 indigenous to South Africa. As a result of the large number of species, the chemistry and biological activity of several species have not yet been investigated. The aim of this project was to investigate the phytochemistry of three species and propose a synthetic route to one of the antibacterial compounds isolated. An extensive literature review regarding the widespread traditional uses, biological activity and phytochemistry of the South African Helichrysum species is provided. From Helichrysum splendidum, a plant used traditionally to treat rheumatism, two monomeric guaianolides and a dimeric guaianolide, helisplendidilactone, were isolated. The stereochemistry of these known compounds was confirmed and the NMR assignments for certain peaks of helisplendidilactone were corrected. An X-ray structure for helisplendidilactone was obtained for the first time. The phytochemistry of Helichrysum montanum was investigated for the first time and new diastereoisomers of known guaianolides were isolated. The phytochemistry of H. splendidum and H. montanum is remarkably similar and supports their morphological classification in the same taxonomic group. The chloroform:methanol extract of H. montanum yielded a new dimeric guaianolide, 13’-epihelisplendidilactone, which is related to helisplendidilactone, as well as three monomeric guaianolides (of which one is a new diastereomer of a known compound). The extract also yielded spathulenol (a sesquiterpene), umbelliferone (a coumarin) and 4’,5,7-trihydroxy-3,3’,8-trimethoxyflavone (a flavonoid). Thirty-five Helichrysum species were screened for antimicrobial activity against six microorganisms and a preliminary cytotoxic assay, which included the use of “normal” and cancer cell lines, was performed. H. excisum was selected for further study based on the fact that it exhibited promising antimicrobial activity and relative low toxicity. Furthermore, with the exception of the essential oil, the phytochemistry of this species has not been investigated. From the aerial parts of H. excisum, five flavonoids, identified as pinocembrin, gnaphaliin, lepidissipyrone, 5-hydroxy-7,8-dimethoxyflavone and isoscutellarein 7-O-b-glucoside were isolated. Four of these flavonoids have an unsubstituted B-ring, a phenomenon often observed in flavonoids isolated from Helichrysum species. The active antimicrobial component of H. excisum has been identified as lepidissipyrone. Owing to the interesting biological activities reported for phloroglucinol a-pyrones and the synthetic challenges associated with these molecules, lepidissipyrone was selected for a synthetic study. Both the flavanone and pyrone moieties present in lepidissipyrone have been successfully synthesised. A successful strategy towards the CH2 linker between the two units has been illustrated. The strategy could be used to synthesise similar phloroglucinol-derived pyrones.
Approaches to the total synthesis of a novel diarylheptanoid
(2008) Vela, Nomandla Magnificent
The total synthesis of a novel diarylheptanoid isolated from a South African medicinal plant, Siphonochilus aethiopicus, was investigated. S. aethiopicus (Indungulu in Zulu) is the only South African species of the Zingiberaceae plant family and is widely used in traditional medicine. One of the compounds isolated from this plant is a novel diarylheptanoid. Diarylheptanoids constitute a distinct group of natural plant metabolites characterized by two aromatic rings linked by a linear seven-carbon aliphatic chain, with varying functional groups on the aryl and the aliphatic chain. The target molecule for our synthesis contains two highly oxygenated aryl rings linked by an aliphatic chain with two stereogenic centres and a trans-alkene. In this study we present our investigation of different strategies to a viable synthetic method that could provide material to supplement the relatively small quantity of product that can be isolated from the plant extract. The major challenges of this synthesis were to develop procedures for the preparation of the homobenzylic trans-alkene, the stereogenic centres and to attach the electron-rich aromatic rings to the aliphatic chain. In this thesis the following aspects are described: • Various types of olefination reactions (including Wittig, Julia and organometalic-mediated type of olefination reactions) • Various types of alkylation reactions (including Grignard, Friedel-Crafts and organometalic-mediated type of alkylation reactions) • Incorporation of the stereogenic centres (including asymmetric hydroxylation and use of chiral starting materials) The synthesis will not only give a viable synthetic route to the target compound but is also versatile enough to allow the preparation of analogues.
Sedimentation and chemical processes on the Lower Mkuze floodplain : implications for wetland structure and function
(2008) Humphries, Marc
The Mkuze Wetland System, situated in northern KwaZulu-Natal, is South Africa’s largest freshwater wetland area. The system plays a vital role in the functioning of the local landscape and has been identified as an important site for the retention of a number of solutes. The mechanisms through which this retention occurs were investigated through analysis of sediment, groundwater and porewater samples collected from the lower floodplain. Sample analysis was achieved through the use of several techniques, including Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), X-ray Fluorescence (XRF), electron microscopy and sequential extraction.
Supramolecular resorcin [4] arene-capped porphyrins : ligands towards homogeneous catalysis
(2008) McKay, Michael.
The synthesis of cavitand-capped porphyrin ligands, with a view towards their potential as ligands in homogeneous catalysis, is described. The ligand apertures, one of which is outlined in the figure below, are focal with the aim of synthesising a ligand which can control access to the active site of the porphyrin via these apertures Synthesis of the target ligand (where R' = CH2 in the figure presented) was attempted via two pathways. Synthesis commenced by using an in situ protocol, which used successive functionalisation of the cavitand structure towards the required aldehyde precursor for porphyrin formation. It was found that subsequent in situ cyclisation and porphyrin formation was hindered by steric factors, arising directly from the short -CH2O- bridges used to link the cavitand to the porphyrin. Ligand synthesis was thus unsuccessful. In a second approach, the porphyrin was synthesised in isolation before being coupled with the cavitand in a direct capping protocol, which gave more promising results. In the case of R = C11H23 (in the figure above), preliminary UV-Vis analysis indicated a successful synthesis. Subsequent analysis of the reaction product by NMR techniques and mass spectrometry could not conclusively confirm the synthesis of the target ligand. The synthesis could therefore not be deemed a success; conceivably the short bridge length being the decisive factor once more. Computational chemistry was used to investigate synthetic results, and therefore the viability of using the -CH2O- bridges to afford limited access to the porphyrin active site. By using molecular mechanics, -CH2O- bridges were found to be too short, giving an aperture of insufficient size to enable only the terminus of a linear paraffin to gain access to the inner cavity of the ligand. Further investigation using molecular dynamics indicated that a ligand bearing bridges four or five atoms in length would afford an aperture of the desired size to accommodate the terminus of a paraffin exclusively. Consequently, synthesis was redesigned towards the preparation of two new ligands, bearing - O(CH2)2O- (four atom, R' = O(CH2)2 in the figure above) and -O(CH2)3O- (five atom, R' = O(CH2)3 in the figure above) bridges. Using 2-phenylethyl feet (R = CH2CH2C6H5 in the figure presented) and adopting the in situ synthetic protocol, both ligands were successfully synthesised. Characterisation using UV-Vis and NMR spectroscopic techniques, as well as mass spectrometry confirmed that both ligands had been obtained pure. Additionally, the in situ cyclisation (in both ligands) was performed via the use of microwave heating, a technique hitherto unreported. A viable synthetic route was thus established for the preparation of two new cavitand-capped porphyrin ligands towards their use in size-selective catalysis. In addition, a number of crystal structures of synthetic intermediates are described, five of which are newly reported. These illustrated notable structural features regarding resorcin[4]arene cavitands and their abilities as host molecules. In particular, the structure of the aldehyde precursor to capped porphyrin formation following the (initial) in situ synthetic protocol was significant in illustrating the reason as to why in situ cyclisation was unsuccessful for the synthesis involving -CH2O- bridges.
Thermal transformation of organoboranes : applicability of ¹¹B NMR spectroscopy and supporting molecular modelling.
(2008) Mzinyati, Andile Bulelani.; Jaganyi, Deogratius.
The high temperature transformations of trialkylboranes were investigated in the range: 50- 200 ºC. The extent of dealkylation was found to be linked to temperature with ca. 10% octene liberation from tri-n-octylborane at 150 ºC in the absence of bulk solvent. Analysis of the oxidised samples from the dealkylation investigation shows that, whereas the control experiment shows no back-isomerisation of tri-n-octylborane at 150 ºC, the addition of 10 mol% of DMF, DMSO, HMPA and trimethoxyphosphate results in back-isomerisation of the alkyl chain. In general, the addition of Lewis base catalyst was found to enhance the extent of dealkylation. In a supporting 11B NMR spectroscopy study to understand the interaction of trialkylboranes and Lewis bases, the interactions of a series of oxygen and phosphorous donor Lewis bases with tri-n-butylborane were found to be favourable, as indicated by large negative binding enthalpy ( HBIND) and entropy ( SBIND) values. Only the trialkylamine Lewis bases were found to have unfavourable interactions with tri-nbutylborane, as indicated by positive HBIND and SBIND values. The results also show that the chemical shift of the adduct at infinite dilution ( 11 B = ) is not as reliable a measure of the interaction between the two species and that correlation of binding constant (logKBIND) at 25.0 ºC to GBIND defines a linear trend that orders the Lewis bases according to spontaneity of the interaction with the strength of the dative bond formed. The applicability of 11B NMR spectroscopy to the study of the reactions of boranes and alkylboranes was extended to the investigation of the reduction of nitriles by BH3.SMe2 in dichloromethane (15-30 ºC). Results from the kinetic study indicate that the overall reduction with BH3.SMe2 is associative ( Sactivation = -71 ± 10 J K-1 mol-1), with the dependence of kobs data on SMe2 concentration highlighting the importance of the dissociation of the SMe2 from BH3 to the reduction process. The lack of reaction with propionitrile and benzonitrile at 25 ºC can be attributed to lack of stability of their adducts with BH3 as demonstrated by the small equilibrium constants for the formation of their adducts with borane; as determined by 1H NMR spectroscopy and further illustrated by computational calculation of their energies at the B3LYP/6-31G* level of theory.
Rhodium-catalyzed and uncatalyzed synthesis of boronate esters and their subsequent utility in the Suzuki realm.
(2008) Hadebe, Siphamandla Wiseman.
This study has shown that alkythioboranes 1,3,2-dithiaborolane and 1,3,2-dithiaborinane, disproportionated significantly during their synthesis. Their interaction with 1-octene has been investigated, and the rate constants, enthalpies and entropies of the hydroboration process, have been determined. The thermodynamic and kinetic parameters obtained have shown that the hydroboration reaction is sluggish and proceeded via an associative mechanism.
Scope of microporous and mesoporous molecular sieves in the oxidation of higher alkanes with ozone
(2008) Pullabhotla, V. S. R. Rajasekhar.
Alkane-rich fractions including n-paraffins are the most important chemical feedstock used in the
The Brix-Free water capacity and sorption behaviour of fibre components of sugar cane (Saccharum officinarum).
(2008) Hoi, Yin Lun Wong Sak.; Martincigh, Bice Susan.; Lionnet, Georges Raoul Edouard.; Autrey, Louis Jean Claude.
Milling data from sugar factories in Mauritius were examined from 1960 to 2004 to assess the trend in the quality of cane received at mills and the change in factory performance. A deterioration in overall quality was apparent due to the increased level of extraneous matter delivered in the cane supply. Comparison was made with available data from other countries in the world, notably those of South Africa and Australia. Controlled addition of extraneous matter to clean cane was effected under laboratory conditions to determine the relative impact of dry leaves, green leaves and cane tops on the quality of cane and the resulting juice, and to predict through derived equations, their impact on cane processing. The addition of dry leaves was found to have the most adverse effect followed by green leaves and cane tops. In the case of dry leaf addition to cane the detrimental effects were found to be masked by an increase in the concentration of solutes in the juice extracted. This phenomenon was thought to be due to the selective sorption of water (so-called Brix-free water) by dry leaves. To test this assertion, the sugar cane stalks of four different cane varieties aged 52, 44 and 36 weeks were separated into their component parts by means of a method devised in this work. There were nine component parts: stalk fibre, stalk pith, rind fibre, rind fines, top fibre, dry leaf fibre, dry leaf fines, green leaf fibre and green leaf fines which, on characterisation by Fourier transform infrared spectroscopy and scanning electron microscopy, were very similar except that stalk pith was more flaky and had a higher surface area than the others. Various analytical techniques were tested for the determination of Brix-free water. The most convenient method proved to be a refractometric method which was improved so as to be applicable to the wide range of cane components fibres studied. Statistical analysis of the Brix-free water content of the separated samples showed that when the combined effect of fibre and pith in the cane stalk of three ages was considered, the four cane varieties were not different. This was not the case for dry leaf, green leaf, top and rind. Of the nine cane components, stalk pith exhibited the highest Brix-free water value of about 20 g/100 g fibre, whereas all the other components exhibited values of about 15 g/100 g fibre, which are much lower than the traditionally accepted value of 25% for cane. The latter was found to be the fibre saturation point of bound water determined at 20 oC, which is the sum of dissolved and hydrated waters, and which is normally greater than the Brix-free water value as determined in this work. The water sorption characteristics of the various cane component parts were further investigated by making measurements to determine the equilibrium moisture contents at various water activity values. These data were used to construct adsorption isotherms. These were fitted to 17 existing isotherm models, of which two, namely, the Hailwood-Horrobin and Guggenheim-Anderson-de Boer models, gave the best fit. The sorbed water was subsequently characterised in terms of various parameters, namely, the monolayer moisture content, the number of adsorbed monolayers, the percentage of bound water, the total surface area for hydrophilic binding, the heats of sorption of the monolayer and multilayers, the net and total isosteric heats of sorption and the entropy of sorption. From the monolayer moisture content and the amount of “hydrated water” as calculated from the Hailwood-Horrobin model, it is clear that at EMC values between 0 and 5% (aw = 0 – 0.3), the non-freezable water is tightly bound to the surface of the fibre. The second region starts at EMC values from 5% to 10 – 15% (aw = 0.3 to 0.6 – 0.8) depending on the cane components, and the bound water in this region is termed the freezable water. The third type of water is essentially free water, it exists after the second region and ends at EMC values of about 25%. From this study, it is apparent that the Brix-free water as measured in this work measures the amount of water bound in the first two regions.
A study of chiral pentacyclo-undecane derived macrocycles and ligands.
(2004) Govender, Thavendran.
Several chiral macrocyclic crown ethers and related analogues have shown to be capable of
A phytochemical investigation of members of the hyacinthaceae family and biological screening of homoisoflavanones and structurally related compounds.
(2004) Du Toit, Karen.; Drewes, Siegfried Ernst.; Mulholland, Dulcie Aca.
The Hyacinthaceae family is richly represented in southern Africa. Of the five subfamilies, three are found in southern Africa. These are the Urgineoideae (URG), Ornithogaloideae (ORN) and the Hyacinthoideae (HYA). The overview of Pfosser and Speta (1999), revealed chemotaxonomic trends at a subfamily level for the Hyacinthaceae family of the Flora of southern Africa region. Homoisoflavanones were found to define the Hyacinthoideae subfamily whilst the Ornithogaloideae subfamily and the Urgineoideae subfamily are defined by steroidal compounds namely, cholestane glycosides and bufadienolide glycosides respectively. Representatives of all three subfamilies were investigated phytochemically. From Eucomis comosa (HYA), five homoisoflavanones were isolated. Omithogalum tenuifolium (ORN) contained a spirostanol saponin of which the crystals were amenable to X-ray analysis. Evidence of a novel stereoisomer was obtained. Extraction of the bulbs of Galtonia princeps (ORN) led to the isolation of two cholestane glycosides, one known and one novel, and a homoisoflavanone. Two novel bufadienolides were isolated from Urginea Iydenburgensis (URG). Structures were elucidated on the basis of spectroscopic data and chemical evidences. Homoisoflavanones and related compounds were then screened for antibacterial and anti-inflammatory activity. Several compounds showed antibacterial activity against Staphylococcus aureus, a gram-positive bacteria. Inhibition of the inflammatory process in microsomal cells was first evaluated, followed by screening of specific inhibition of cyclooxygenase enzymes. These are membrane-associated enzymes occurring in different isoforms. High levels of anti-inflammatory activity were detected especially in microsomal cells. This biological information made it possible to rationalize the ethnomedicinal use of some of the plants from which the compounds were isolated. 15 Biological screening was followed by a computer-based quantitative structureactivity relationship (QSAR) study. This study produced five equations with significant prediction value of anti-inflammatory and antibacterial activity for homoisoflavanones and related compounds. The derived models also provided valuable parameter guidelines of those properties influencing the antiinflammatory and antimicrobial activity of the studied compounds.
Extractives from samadera madagascariensis and toddaliopsis bremekampii.
(2004) Naidoo, Dashnie.; Mulholland, Dulcie Aca.
This work involves the isolation, structural elucidation and the biological screening of compounds from the Madagascan Samadera madagascariensis and Toddaliopsis bremekampii from South Africa. Samadera madagascariensis Jussieu (locally known as "fatriana") belongs to the Simaroubaceae family and is endemic to Madagascar. The leaves of Samadera madagascariensis are used in Madagascar for the treatment of stomach aches and dysentry, and the juice of the fresh leaves is used to treat wounds and burns. Samadera madagascariensis leaves were investigated in this work for the presence of quassinoids and triterpenoids. Eighteen compounds, including seven quassinoids: samaderine A (i), the novel 5~,6-dihydrosamaderine A (ii), the novel 2chlorosamaderine A (iii), the novel samaderine DN (iv), samaderine B (v), cedronin (vi) and the novel 3,4~-dihydrosamaderine C (vii), and eleven triterpenoids: the novel protosamaderine A (viii), 1,2-dihydrobruceajavanin A (ix), chisocheton compound A (x), the novel protosamaderine B (xi), the novel protosamaderine C (xii), the novel protosamaderine D (xiii), the novel protosamaderine E (xiv), the novel protosamaderine F (xv), the novel protosamaderine G (xvi), the novel protosamaderine H (xvii) and the novel protosamaderine I (xviii) were isolated from the Samadera madagascariensis leaves. The quassinoids isolated in this work are of the C18 and C19 classes, and only five of the C18 type have been published previously. Quassinoids exhibit a range of biological activities such as antileukemic, antiviral, antimalarial, anti-inflammatory, antifeedant and amoebicidal. 2-Chlorosamaderine A (iii), cedronin (vi) and 3,4~-dihydrosamaderine C (vii) were found to have anticarcinogenic properties. The triterpenoids isolated in this work are of the protolimonoid type and protolimonoids are known to exhibit anticarcinogenic and antifungal properties. Toddaliopsis bremekampii I. Verd. belongs to the Rutaceae family. Members of the Toddaliopsis genus are found in the warmer regions of southern Africa. The phytochemistry of this species and genus has not been investigated previously, however, other members of the Rutaceae have yielded a wide range of alkaloids and other compounds. The leaves and branches of Toddaliopsis bremekampii were investigated in this work and yielded four novel acridone alkaloids: 1,2,3trimethoxyacridone (i), 1,2,3-trimethoxy-1 O-acetoxymethylacridone (ii), 1-hydroxy-2,3-dimethoxy10- acetoxymethylacridone (iii) and 1,2,3-trimethoxy-1 O-methoxy methylacridone (iv). Acridone alkaloids are known to have antineoplastic and antitumour properties. These alkaloids were active in the chemiluminescence assay, which showed the suppression of the reactive oxygen species generated by polymorphonuclear leukocytes (PMNs) in humans. The PMNs play a key role in the host's defence. However, the reactive oxygen species generated as a result of this defence mechanism cause tissue damage.
Silver-catalysed hydroamination : synthesis of functionalised pyrroles.
(2004) Dovey, Martin Charles.; Gravestock, David.; Robinson, Ross Stuart.
The aim of this project was a broad one, namely to develop general methods for the preparation of nitrogen-containing heterocycles. This topic also encompasses the preparation of precursors to heterocycles with the goal of obtaining compounds that can undergo various transformations to give different cyclic products (general synthetic precursors). Workers in our group have previously reported the preparation of indolizinones from N-propargyl enaminones. Immediate goals were to elucidate the mechanism of this transformation, optimise its yields and explore its generality. Initial work revealed a possible reaction intermediate in the form of isomeric N-allenyl enaminones. These compounds can be easily prepared from N-propargyl enaminones using potassium tertbutoxide. Although parallels between the two reactions were noted, the presence of N-allenyls as reaction intermediates was not substantiated. In spite of this, N-allenyl enaminones were considered to be potentially valuable compounds in their own right and their chemistry was briefly explored. It was shown that they undergo hydroboration smoothly yielding only one hydroboration product. This was observed using dynamic iiB NMR spectroscopy. Subsequent oxidation gave interesting results and these are discussed further. At this stage it was considered important to prepare piperidine and acyclic analogues of the N-allenyl enaminones already synthesised. Both preparations proved problematic and were ultimately abandoned. Although these attempts were not fruitful, a number of interesting observations were made en route and contribute to the discussion. Whilst attempting to prepare acyclic analogues it was found that when a mixture of a secondary enaminone and propargyl bromide was treated with silver nitrate functionalised pyrroles were formed. This method of pyrrole formation, although low yielding (~25%), was both novel and intriguing. As pyrroles are important heterocycles and common synthetic targets, further work was carried out to elucidate the mechanism in operation during this conversion. A thorough investigation led to a proposed mechanism involving silver-mediated hydro amination followed by an intramolecular nucleophilic substitution. This proposed mechanism is discussed further. Carrying out the reaction over two steps was found to improve the procedure. Carbon-carbon bond formation prior to an intramolecular silver-catalysed hydroamination provides superior overall yields. Intramolecular hydroamination can be carried out at room temperature or using microwave irradiation. Employing microwave irradiation reduces reaction times from sixteen hours to sixty seconds without any decrease in yields. This process was extended to bicyclic systems, namely N-bridgehead pyrroles. The preparation of the necessary cyclic precursors via a protection-deprotection strategy is described. N-Bridgehead pyrroles provide the framework for pyrrolizidine, indolizidine and lehmizidine alkaloids and general synthetic protocols toward the synthesis of such alkaloids is proposed.
Synthesis of novel benzimidazole derivatives and their platinum (II) complexes.
(2010) Madlala, Bongiwe Pridesworth.; Soares, Anna Mampe.
Benzimidazole and its derivatives have attracted many organic chemists due to their interesting biological activities. These include activities against viruses such as, HIV, RNA, herpes (HSV-1), influenza, and cytomegalovirus (HCMV); antimicrobial and antitumor activities. Even though a lot of research has been conducted on the synthesis of benzimidazoles, factors such as, drug resistance present a need for synthesis of more structural analogues of these compounds. In chapter three, the synthesis of 2-aryl-1Hbenzimidazoles (46a-c) and 2-aryl-1-arylmethyl-1H-benzimidazoles (49a-d) is described. The yields for these products ranged from 44-79 % and 62-72 %, respectively. The synthesis of novel bisbenzimidazole derivatives is described in chapter four. Direct condensation of 3,3'-diaminobenzidine (1 mmol) with 2-thiophenecarboxyaldehyde (2 mmol) afforded 2, 2’-di-2-thienyl-5,5-Bi-1H-benzimidazole (52) in 65 % yield. Except in the case of 2-furancarboxyaldehyde, the acid catalyzed condensation of 3,3'- diaminobenzidine (1 equivalent) and heteroaromatic aldehydes (4 equivalents) gave novel bisbenzimidazoles where the aldehyde added three times to 3,3'-diaminobenzidine. The four times addition product, 1,2-di-2-furanylmethyl-2,2-di-2-furanyl benzimidazole (53) was obtained in 53 % yield. On the other hand, the three times addition product, 1,2-di-2- pyrrolylmethyl-2,2-di-2-pyrrolyl (54); 1,2-di-2-thienylmethyl-2,2-di-2-thienyl (55); and 1,2-di-2-pyridylmethyl-2,2-di-2-pyridyl benzimidazoles (56) were obtained in 85, 12 and 10 %, respectively. Full characterization of bisbenzimidazoles (54-56) was achieved by 1H, 13C NMR and LCMS spectra. Although benzimidazoles have been proven to be active against various cancers, their use as ligands for platinum (II) has been reported to enhance this activity. Three new benzimidazole Pt (II) complexes were synthesized. N, N, N-bound Pt (II) complexes of 2- quinolyl-1-quinolylmethyl-1H-benzimidazole (60) and 2-pyridyl-1-pyridylmethyl-1Hbenzimidazole (63) were obtained in excellent yields of 82 and 72 %, respectively. S, Nbound Pt (II) complex of 2-thienyl-1-thienylmethyl-1H-benzimidazole (64) was isolated in 63 % yield. From 195Pt NMR spectra analysis, it was concluded that the method reported by Morgan and Burstall is more efficient for the synthesis of these complexes. In addition to 195Pt NMR, platination was also confirmed using 1H and 13C NMR spectra.
Dinuclear complexes of CU(I), PD(I), PT(I) and RU(I) : catalysts for the electrochemical reduction of CO2.
(2001) Cripps, Garth Hereward.; Haines, Raymond John.; Field, John Stainer.
No abstract available.
Influence of bridging groups on the reactivity of dinuclear platinum (II) complexes with bis(2-pyridylmethyl)amine chelate headgroups.
(2009) Mambanda, Allen.; Jaganyi, Deogratius.
The influence on the reactivity of both the length as well as the structural nature of diamine bridges linking dinuclear Pt(II) complexes with homotopic bis(2-pyridylmethyl)amine headgroups has been investigated. For this purpose, three sets of square-planar Pt(II) complexes sharing a common non-labile bis(2-pyridylmethyl)amine chelate were synthesized and characterized by various spectroscopic methods. The substitution of the coordinated aqua ligands by three thiourea nucleophiles of different steric demands was studied in acidic aqueous medium under pseudo first-order conditions. The reactions were studied as a function of concentration, temperature and in some cases under an applied pressure using the standard stopped-flow technique and UV-visible spectrophotometry. Their thermodynamic properties were investigated by studying the acid-base equilibria of the coordinated aqua ligands using a spectrophotometric titration method. DFT Quantum mechanical calculations were also performed to determine their geometry-optimized structures and energies of the frontier molecular orbitals. The first set of Pt(II) complexes comprise dinuclears, all bridged by a flexible α,ω-alkyldiamines. The second set of complexes is Pt(II) amphiphilic mononuclear analogues of the former set, formed intuitively by excising off one of the Pt(II) chelate headgroups. The last set of complexes comprises Pt(II dinuclear complexes which are structurally related to the first set, but are linked by relatively rigid linkers, which are made up of either phenyldiamine or diaminocyclohexane fragments. In two of the complexes, a single methylene spacer (CH2 In general, the substitution reactions of the coordinated aqua ligands of all the Pt(II) complexes by the three sulfur donor nucleophiles (Nu) proceed via a two-step reaction pathway. The first step, whose rate constant is denoted in subsequent text as k) group is incorporated between the rigid moieties of the diamine bridge so as to elongate the average distances separating their Pt(II) atoms as well as to modulate the rigidity of the complexes. For comparison purposes, two monomeric analogues bearing the phenyl and cyclohexyl appended groups, respectively, were studied and reported together with these complexes. 2(1st), involves the substitution of the aqua ligands. The second step, induced by the coordination of the strong labilizing thiourea nucleophiles and whose rate constant is denoted in the text as k2(2nd), is ascribed to the dechelation of the one of the cis-coordinated pyridyl units. Thus, the substitution of the aqua ligands and the subsequent dechelation of the pyridyl units, can be expressed as kobs(1st/2nd) = k2(1st/2nd)[Nu] for all the reactions. Negative entropy of activation, negative volume of activation (in cases where measurements were carried out) and second-order kinetics for the substitution reactions all support an associative mode of activation. The substitution reactivity of all the dinuclear complexes is influenced to a greater extent by the steric influences conferred by the bridge as well as a weak electronic effect. The steric influences are mutual, axially exerted and seemingly unique to the square-planar terdentate chelate headgroups. The steric influences depend strictly on length of the diamine (i.e., the average distances separating the Pt centres of the dinuclears) as well as molecular symmetries and shapes of the complexes. The molecular symmetries and hence the shapes of the complexes depend on the parity of the connecting bonds in the diamine (whether even or not). If the connecting bonds of the bridges are even, C2h structures and hence slip-up molecular geometry are preferred. Their overlap geometries cause mutual and axial steric influences on the Pt(II) square-planar chelates which retard substitutional reactivity when the bridge is short. When odd, bowl-shaped complexes of the C2v point group symmetry are preferred in which the axial steric influences are absent at their Pt(II) chelates. In addition their bowl geometry causes an entrapment of the incoming nucleophiles, causing unusually high reactivity when compared to their even-bridged counterpart. For both molecular symmetries (C2h or C2v), the reactivity of the dinuclear complex depends on the average distances separating the Pt(II) centres of the dinuclears. In the former type of complexes, when the average distances separating their Pt(II) centres are long, the axial steric influences at each Pt(II) chelate due to their C2h The chain length as well as the structural make-up of the linker also determines the amount of electron density donated inductively from the linker to the Pt ions as well as the effective nuclear charge at each Pt(II) centre due to charge addition. These are two opposing overlap geometry is weakened, leading to enhanced reactivity as the chain length is increased. In the latter type of complexes, this weakens the ‘entrapment’ effect of their bowl-shaped geometry, resulting in a steady decrease in reactivity when the chain length of the linker is increased. In addition rigidity and planarity within the backbone of the diamine bridge has been found to distort the bowl cavity causing weakening of the ‘entrapment effect’ resulting in the lower rates than expected. iv factors which also influence the rate of substitution in these complexes to some extent. The inductive effect as well as the presence of a domineering steric influence in the C2h overlap geometry was verified by studying the reactivity of the analogous amphiphilic Pt(II) complexes.
Synthesis and emission studies of polypyridyl complexes of platinum (II).
(2002) Gertenbach, Jan-Andre.; Haines, Raymond John.
Chapter One serves as an introduction to the photophysical properties of square-planar 2,2':6',2/1-terpyridyl and 6-phenyl-2,2'-bipyridylligand complexes of platinum(II). A brief description is provided of the absorption and emission spectra of each complex, the latter generally both in solution and the solid state. The assignments made to the absorption and emission bands are reported, together with the lifetimes ofthe emitting states where these have been measured. The relationship between the molecular and/or crystal structure of the complex and its photophysical properties is discussed. Given this background, the overall aims of the work in this dissertation are presented. In Chapter Two the synthesis and characterisation of nine novelligands are described. These are 4'-(O-naphthyl)-2,2':6',2/1-terpyridine (4'-fJNp-terpy), 4'-(meta-biphenyl)-2,2':6',2/1-terpyridine (4'-mBiph-terpy), 4'-(para-biphenyl)-2,2':6',2"-terpyridine (4'-pBiph-terpy), 4-(fJnaphthyl)- 6-pheny1-2,2'-bipyridine (H4-fJNp-phbipy), 4-(para-biphenyl)-6-pheny1-2,2'bipyridine (H4-pBiph-phbipy), 4-phenyl-6-(2'-pyrazinyl)-2,2'-bipyridine (4-Ph-pzbipy), 4(ortho-CH3-phenyl)-6-(2'-pyrazinyl)-2,2'-bipyridine (4-oMePh-pzbipy), 4-(ortho-CF3-phenyl)6-pyrazinyl-2,2'-bipyridine (4-oCF3Ph-pzbipy) and 4-phenyl-2,6-bis(2'-pyrazinyl)-pyridine (4-Ph-pybipz). In Chapter Three the synthesis, characterisation and photophysical properties of platinum(II) complexes ofthe ligands 4'-jlNp-terpy, 4'-mBiph-terpy and 4'-pBiph-terpy are described. The complexes are salts prepared of the following composition: [Pt(4'-jlNp-terpy)CI]SbF6 (1), [Pt(4'-jlNp-terpy)Cl]BF4 (~), [Pt(4'-jlNp-terpy)Cl]CF3S03 (J), [Pt(4'-mBiph-terpy)Cl]SbF6 (1), [Pt(4'-mBiph-terpy)Cl]BF4 (~), [Pt(4'-mBiph-terpy)Cl]CF3S03(~), [Pt(4'-pBiph-terpy)CI]SbF6 (1), [Pt(4'-pBiph-terpy)Cl]BF4 (~) and [Pt(4'-pBiph-terpy)Cl]CF3S03 (2). Each ligand is characterised by a substituent in the 4'-position that has been designed to permit free rotation of that substituent about the interannular bond to the terpyridyl fragment. The rotational freedom allows the 4'-substituent to adopt an orientation nearly coplanar with that of the terpyridyl fragment. As a result ofthe extensive n-conjugation, there is a strong ligand-centred influence in their excited state emission spectra. Evidence is also presented for 3MLCT (metal 111 to ligand charge transfer) character in the emitting state, in particular that the emission is quenched in coordinating solvents. In addition, an 3ILCT (intraligand charge transfer) contribution has been proposed to account for the exceptionally long lifetimes measured in fluid solution for [Pt(4'-j3Np-terpy)C!t (12 jJs), [Pt(4'-pBiph-terpy)Cq+ (4 jJs) and [Pt(4'-aNpterpy) C!t (17 jJs). We conclude that large easily ionisable substituents in the 4'-position of the terpyridyl moiety ensure long lifetimes in fluid solution for platinum(II) complexes of these ligands. The same assignments apply to the solid state spectra of the yellow or pale orange salts that exhibit monomeric emission in the solid state, these being: [Pt(4'-j3Np-terpy)CI]SbFu (l), [Pt(4'-j3Np-terpy)CI]BF4 (I), [Pt(4'-j3Np-terpy)CI]CF3S03(J.), [Pt(4'-pBiph-terpy)CI]SbFu (1) and [Pt(4'-pBiph-terpy)CI]BF4 (~). In the solid state two additional types of solid state emission have been identified in cases where intermolecular interactions are present. The first type is typical of red or dark orange salts viz., [Pt(4'-mBiph-terpy)Cl]SbFu(1), [Pt(4'-pBiphterpy) Cl]BF4 (red form) (~) and [Pt(4'-pBiph-terpy)Cl]CF3S03 (.2). All three salts exhibit MMLCT (metal-metal to ligand charge transfer) emission brought about by close intermolecular Pt···Pt interactions in the solid. Finally, the three orange compounds [Pt(4'mBiph-terpy)Cl]SbFu.CH3CN (4a), [Pt(4'-mBiph-terpy)Cl]BF4 (~) and [Pt(4'-pBiphterpy) Cl]CF3S03(.2) exhibit simultaneous emission from IL (intraligand) and MMLCT states that are similar in energy. In the case of [Pt(4'-mBiph-terpy)Cl]SbF6.CH3CN (4a) the assigmnent ofMMLCT emission is supported by a X-ray crystal structure determination that shows the cations interacting in dimeric pairs which have a platinum-platinum separation of 3.356(2) A. In Chapter Four the synthesis, characterisation and photophysical properties of the neutral complexes [Pt(4-j3Np-phbipy)Cl] (10) and [Pt(4-pBiph-phbipy)Cl] (11) are described. The ligands, 4-j3Np-phbipy and 4-pBiph-phbipy are closely related to 4'-j3Np-terpy and 4'-pBiphterpy, the only difference being that the terpyridyl fragment is replaced by a phenyl-bipyridyl moiety that binds to the platinum via a deprotonated carbon atom in the ortho-position of the phenyl ring. The stronger a-donor strength of the anionic carbon atom is shown to result in: (i) a marked shift to the red in the MLCT absorption bands for the two complexes and (ii) a significant lowering of the energy ofthe emitting state, as compared to that observed for the cationic terpyridylligand derivatives. The assignment of the emission as 3ILPMLCTin origin remains the same, however, both in solution and in the solid state. In Chapter Five we report the synthesis, characterisation and photophysical properties of a series of platinum(II) complexes of the ligands 4-Ph-pzbipy, 4-oMePh-pzbipy, 4-oCFJPhpzbipy and 4-Ph-pybipz viz., [Pt(4-Ph-pzbipy)CI]BF4 (12), [Pt(4-oMePh-pzbipy)CI]SbF6 (14), [Pt(4-oMePh-pzbipy)CI]BF4 (15), [Pt(4-oMePh-pzbipy)CI]CFJSOJ (16), [Pt(4-oCFJPhpzbipy) CI]SbF6 (17), [Pt(4-oCFJPh-pzbipy)CI]BF4 (18) and [Pt(4-oCFJPh-pzbipy)CI]CFJSOJ (19). These ligands coordinate to platinum via a binding domain analogous to the terpyridyl fragment except that one (or both in the case of 4-Ph-pybipz) of the outer pyridine rings is replaced with a pyrazine ring. The ligands 4-oMePh-pzbipy and 4-oCFJPh-pzbipy are distinguished from 4-Ph-pzbipy by the presence of bulky methyl and trifluoromethyl substituents, respectively, in the ortho position of the peripheral phenyl ring. Monomer emission measured in room temperature fluid solution in a 77 K rigid glass for the three luminophores viz. [Pt(4-Ph-pzbipy)CIt, [Pt(4-oMePh-pzbipy)CIt and [Pt(4-oCFJPhpzbipy) Clr derives from an excited state of JILPMLCT mixed orbital parentage. Their emission energies are stabilised compared to their terpyridyl analogues but emission lifetimes are essentially the same. Solid state emission spectra have been recorded for salts ofthe [Pt(4oMePh- pzbipy)CW and [Pt(4-oCFJPh-pzbipy)CW cations. In every case emission is derived from an excited state brought about by intermolecular interactions, either in the form of excimeric or of MMLCT emission. In general orange compounds ([Pt(4-oCFJPhpzbipy) CI]SbF6 (orangeform ) (17) and [Pt(4-oCFJPh-pzbipy)CI]BF4 (18)} displayexcimeric emission and dark red compounds {[Pt(4-oMePh-pzbipy)CI]SbF6 (14), [Pt(4-oMePhpzbipy) CI]BF4 (15), [Pt(4-oMePh-pzbipy)CI]CFJSOJ(16), [Pt(4-oCFJPh-pzbipy)CI]SbF6 (red form) (17) and [Pt(4-oCFJPh-pzbipy)CI]CFJSOJ (19)} display emission from a MMLCT excited state. The [Pt(4-oCFJPh-pzbipy)CI]SbF6 (17) salt displays polymorphism, existing in two formS, one red and the other orange. In all cases the lower energy of the n-acceptor orbitals of the pyrazinyl-bipyridyl moiety causes emission to be at lower energies compared to that recorded for closely related terpyridyl analogues. Finally, attempts to coordinate the bis-pyrazinylligand, 4-Ph-pybipz, to platinum to form [Pt(4-Ph-pybipz)CI]SbF6 (13) were unsuccessful.
Extractives from the ptaeroxylaceae and the mesembryanthemaceae.
(2001) Koorbanally, Neil Anthony.; Mulholland, Dulcie Aca.
This work is an account of the investigations into the chemistry of two Cedrelopsis species from the Ptaeroxylaceae, Cedrelopsis grevei and Cedrelopsis microfoliata and a species from the Mesembryanthemaceae, Khadia alticola, as well as investigations into the synthesis of hydroxylated and prenylated chalcones. Cedrelopsis grevei, commonly called Katrafay, is amongst the many medicinal plants of Madagascar, being used to relieve muscle fatigue when the bark is soaked in hot water. Previous investigations of the wood and stem bark of this plant, have yielded chromones and coumarins and a recent investigation of the stem bark of a specimen collected in the north of Madagascar has yielded two novel limonoids of unusual structure, cedmilinol and cedmiline. The fruit and seed of Cedrelopsis grevei have not been studied previously and a phytochemical investigation of these plant parts was undertaken in this work. The dichloromethane extract of the fruit and seeds yielded, after column chromatography, a dihydrochalcone, uvangoletin, a flavanone, 5,7dimethoxypinocembrin, two simple chalcones, cardamonin and flavokawin Band three prenylated chalcones, 2’-methoxyhelikrausichalcone, cedreprenone and cedrediprenone. Three of these compounds, 2’-methoxyhelikrausichalcone, cedreprenone and cedrediprenone have not been isolated previously. Cedrelopsis microfoliata is another medicinal plant used in Madagascar. The leaves of this plant are used to prepare a decoction for woman to drink after childbirth. This is the first phytochemical investigation of Cedrelopsis microfoliata. The hexane extract of the dried stem bark yielded three compounds after column chromatography, a chalcone, microfolian and two flavanones, microfolione and (+)-agrandol. The dicholoromethane extract of this compound yielded four compounds after column chromatography, three coumarins, cedrecoumarin A, obliquin and microfolicoumarin and a sesquiterpenoid, sesquichamaenol. Four of the compounds isolated from Cedrelopsis microfoliata, microfolidione, microfolione, (+)-agrandol and microfolicoumarin have not been isolated previously. Khadia alticola is one of the species added to "Khadi", a Tswana/South Sotho name for beer brewed traditionally using the fleshy roots of a variety of taxa. Khadia is also reported to be used medicinally by the Manyika people of the Umtali district of Zimbabwe. The phytochemical investigation of the roots of Khadia alticola, which have not been studied previously, was undertaken to determine whether mesembrine type alkaloids were present in this species and thus contributing to the "potency" of the beer brewed traditionally. No mesembrine alkaloids were isolated in this work, however, a common sterol, sitosterol was isolated from the acidic chloroform fraction of the roots of this species and a flavonoid, 3,4',5,7-tetrahydroxyflavan was isolated from the basic chloroform fraction. Two chalcones, 3’,5’-dihydroxychalcone and 2’-hydroxychalcone were synthesised using the Claisen condensation. An isoprenylated acetophenone intermediate and an isoprenylated chalcone were also synthesised.
Synthesis and characterisation of novel hienylphosphines and their gold(I) complexes.
(2000) Lakoba, Elena.; Field, John Stainer.; Haines, Raymond John.
This study comprises the preparation and characterisation of various novel tertiary monoand polyphosphines as well as their gold(I) complexes. The possible modes of coordination of mono-, bi- and tridentate phosphine ligands to gold(I) are summarised in Chapter One, thus serving as an introduction to coordination behaviour of the new ligands to gold(I). The synthesis and characterisation of twelve novel tertiary mono- and polyphosphines as well as the three previously known thienylphosphines, diphenyl(2-thienyl)phosphine (PS), phenyldi(2-thienyl)phosphine (PDS) and tris(2-thienyl)phosphine (PTS), are reported in Chapter Two. There are four phosphines derived from a monothiophene unit i.e., diphenyl(5-bromo-2-thienyl )phosphine (PSBr), 2,5-bis(diphenylphosphino)thiophene (PSP), phenylbis[(5-diphenylphosphino)-2-thienyl]phosphine (PDSP) and tris[(5-diphenylphosphino)- 2-thienyl]phosphine (PTSP); two from a 2,2'-bithiophene unit i.e.. diphenyl[5( 2,2'-bithienyl)]phosphine (PSS) and 5,5'-bis(diphenylphosphino )-2,2'-bithiophene (PSSP); h hi fr ') ')'. two P osp mes om a _,_ .)-,,_')"-terthiiophene um.t t..e., diipheny1[)--('_)._')'..)-,._')"-terthiieny1)]- phosphine (PSSS) and 5,5"-bis(diphenylphosphino)-2,2':5',2"-terthiophene (PSSSP); three phosphines from a 2-(2'-thienyl)pyridine unit i.e., diphenyl[5-(2'-pyridyl)-2-thienyl]phosphine (PSN), diphenyl[6-(2'-thienyl)-2-pyridyl]phosphine (PNS) and diphenyl[5-(2'{ 6'-diphenylphosphino}pyridyl)-2-thienyl]phosphine (PSNP); and one phosphine from a 2,6-bis(2'-thienyl)pyridine unit i.e., 2,6-bis[(5'-diphenylphosphino )-2'-thienyl]pyridine (PSNSP). A combination of IH, l3C and 31p NMR spectroscopy as well as mass spectrometry and microanalysis is employed to establish the structures for all the compounds, while in three cases (for the ligands PSSP. PSNP and PSNSP) the structural assignment is confirmed by X-ray crystallography. The synthetic strategies used for the preparation of the phosphine ligands vary widely from a traditional reaction between a metallated thiophene and a chlorophosphine on one hand, to a Ni-catalysed coupling reaction involving phosphine-substituted heterocycles on the other. the choice depending not only on the parent heterocyclic molecule. but also on the number of phosphorus atoms present. A new synthetic methodology leading to a potentially water-soluble diphosphine ligand is also explored. Although the target compound itself is not isolated, the preparation of its precursor, 5.5'-bis(phenylphosphino )- Xl 2,2'-bithiophene (HPSSPH), represents the first example of the synthesis of a secondary phosphine containing a thiophene moiety. In Chapter Three the reactions of the mono- and diphosphine ligands PS, PDS, PTS, PSBr, PSS, PSSS.- PSN.. PNS.- PSP.' PSSP.. PSNP.. PSSSP and PSNSP with the. gold(I) precursors (such as HAuCI4, reduced in situ with thiodiglycoL and [Au{MeCNh]SbF6) are studied. The structures of the products are elucidated by means of 'n and 31 p NMR and IR spectroscopy as well as X-ray crystallography in the case of [CIAuPSS], [CIAuPNS], [CIAu(PDS)2], [CIAu(PS)3] and [Au2(~-PSNh](SbF 6)2. Neutral complexes containing twocoordinate gold(I) are obtained for all the ligands, having general formulae [ClAuP] and [CIAu(~-PP)AuCI] , where P and PP represent a mono- and a diphosphine respectively. Isolation of neutral complexes with three-coordinate gold(I) proved more difficult and is only achieved for the complexes [CIAuP2] (P = PS, PDS, PTS and PSN) and [CIAu(~PP)] n (PP = PSP and PSSSP). A 31 p NMR study of the behaviour of complexes in solution shows the complexes with three-coordinate gold to be very labile with respect to dissociation of the third ligand. The complex [CIAu(PS)3], where gold(I) exhibits a coordination number intermediate between 3 and 4 in the solid state (as shown by the Xray crystallographic analysis), is serendipitously obtained from a reaction of [CIAu(PPh3)] and PS in a I :1 molar ratio. Both mono- and dinuclear complexes with two-coordinate gold(I) are formed when the monophosphines PSN and PNS are reacted with suitable precursors (viz. [CIAuPNS] and [Au{MeCNh]SbF6). The mononuclear complexes are of formula [Au(r{Ph]SbF6, as confirmed by IH and 3lp NMR and IR spectroscopy. In the case of the dinuclear complexes. [AU2(~-PhJX2 (P = PSN. X = SbF6: P = PNS. X = PF6), bridging is achieved via the phosphorus atom and nitrogen atom of the pyridine ring 1n both ligands. This coordination mode is consistent with the IH and 31 p NMR data for both complexes. and it is also confirmed by an X-ray structure determination of the former complex. The diphosphine ligands PSSP, PSNP and PSNSP tend to produce insoluble polymeric products when reacted with cationic gold(I) precursors. while the diphosphines PSP and PSSSP allow discrete species (e.g. [Au{r!'-PSSSP}z]PF6 and [Au2 {~-PSP hHPF6 }z ) to be isolated under similar conditions. None of the gold(I) complexes synthesised in this work displays any evidence of the thiophene ring being one way or the other coordinated to the metal, XII Chapter Three concludes with a short description of the luminescence properties of a selection of gold(I)-phosphine complexes both in solution and in the solid state. Our results confirm the previously established trend that complexes containing three-coordinate gold(I) tend to luminesce. They also show, for the first time, that a dinuclear complex with no metal-metal interaction, and where each gold atom is coordinated to one phosphorus and one nitrogen donor atom i.e., [Au2(Il-PSN)2](SbF6L exhibits luminescent properties. The results of the cytotoxic testing of a number of mono- and diphosphines (PS, PDS, PTS, PSS, PSSS, PSSP, PSSSP, PSNSP and dppe) as well as of some heterocyclic compounds and the gold(I ) complexes [CIAuPSS] and (CIAu(Il-PP)AuCI] (PP = PSSP and dppe) against two cell lines are presented in Chapter Four. The complex [CIAuPSS] shows great potency against the HepG2 (liver cancer) cell line, far exceeding that of the known cytotoxic agent, [Cl.Autu-dppej.Auf'l]. For the other cell line. A549 (lung cancer), the cytotoxicity measurements are compared for 'in the dark' and under DVA light conditions. The results show that application of DVA light generally increases the cytotoxic properties of the compounds tested. Although there appears to be a link between the phototoxic properties of a compound and its photosensitising ability (a photophysical property of thienylphosphine ligands and their gold(I ) complexes, reported at the beginning of Chapter Four) , it is not yet possible to establish a quantitative correlation between the two.
Artificial neural networks for the classification of Meliaceae extractives.
(1998) Fraser, Leigh-Anne.; Mulholland, Dulcie Aca.
The goal of this project was the development of a computer-based system using artificial intelligence to classify the limonoids, protolimonoids and triterpenoids isolated from the family Meliaceae by the Natural Products Research Group of the University of Natal, Durban. A database of samples was obtained between 1991 and 1996, part of which time the author was a member of the group and isolated compounds from Turraea obtusifolia and Turraea floribunda. Over and above the problem of complexity and similarity in structures of the above mentioned natural products, are other difficulties. These include very small amounts of sample being isolated producing very weak peak signals in the C-13 NMR spectra, extraneous peaks in the NMR spectra due to different impurities and instrument noise, non-reproducible spectra due to the pulsed Fourier transform intervals and the nuclear Overhauser effect, impure samples often isolated as stereoisomeric mixtures or as mixed esters and superposition of peak signals in the NMR spectra due to carbons in the same environment within the same compound. These factors make identification by traditional computational and expert systems impossible. As a result of these shortcomings, the author has developed a novel approach using artificial neural network techniques. The artificial neural network system developed used real data from the 300 MHz NMR spectrometer in the Department of Chemistry, Durban. The system was trained to discriminate between limonoids, triterpenoids and flavonoids/coumarins from the C-13 NMR spectra of pure, impure and unseen compounds with an accuracy of better than 90%. Further differentiation of the glabretals from the rest of the protolimonoids as well as from the rest of the triterpenoids showed similarly significant results. Finally, individual limonoid discrimination within the limonoid dataset was extremely successful. Apart from its application to the extractives from Meliaceae, the methodology and techniques developed by the author can be applied to other sets of extractives to provide a robust method for the spectral classification of pre-identified natural products.
Experimental and theoretical studies of molecular complexes.
(1999) Govender, Maganthran Ganesan.; Ford, Thomas Anthony.
The binary complexes comprising the molecules hydrogen fluoride, hydrogen chloride, water, hydrogen sulphide, ammonia, phosphine, methane and silane have been studied by ab initio molecular orbital theory thus generating a matrix of eight by eight molecular complexes. The structures, energetics and vibrational spectra of these molecular complexes, both homodimers and heterodimers, have been studied at the second order level of Møller-Plesset perturbation theory using the 6-31G** basis set, by means of the Gaussian 92 computer program. The self consistent field interaction energies have been further dissected into their various components according to the Morokuma scheme, using the Monstergauss computer program. An analysis of the computed interaction energies of the complexes has also been carried out. A complete normal mode analysis, identifying the normal modes of Vibration, has been carried out using the Vibra program. The infrared spectra of some of the associated species have also been determined experimentally by means of the matrix isolation technique using argon and nitrogen as matrix gases. The combined use of the theoretical and experimental approaches has enabled reliable vibrational assignments to be made, which have been used in determining the molecular structures of the aggregates formed on complexation.

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