The synthesis of high molecular weight copolymer of carbon disulphide (CS2) and propylene oxide (PO) has not reported in literature. In the present work, zinc isopropyl xanthate (Zn(Xt)2) was used as catalyst for the copolymerisation of PO and CS2 into high copolymer. However, the product can be fractionated into high and low molecular weight components. High molecular weight copolymer was rubbery products, but low molecular weight copolymers were oily products containing cyclic dithiocarbonates. Copolymers were characterized by elemental, end group analysis, DSC, TGA, GPC, Light Scattering, UV, IR, NMR spectroscopy, polarized microscopy and refractometry. Copolymerization process was zeroth order with respect to monomers, and its non-terminated but suffered from several types of transfer reactions. As a result of transfer reactions S-(C=S)-S, O-(C=S)-O, O-(C=O)-O groups in the backbone of copolymer and SH groups at the chain terminals and cyclic dithiocarbonates are formed. Apart from SH groups, OH and double bonds were found and their amounts were determined at the chain terminals. Copolymers with high mole fractions of PO units (F1) in the copolymer are crystallized in the shape of Malta&amp / #8217 / s Cross. Melting points of products were obtained from DSC. The F1 values are calculated from elemental analysis as well as zeroth order rate constants and from melting point of the crystals. All three results were in close agreement and changed between 0.9 &amp / #8211 / 0.7. However, these ratios depend on reaction conditions (temperature, catalyst and monomer concentrations, time and dielectric constant of reaction medium). A mechanism for coordination-copolymerization on the basis of above observation was proposed.

Extensive research has been performed on the interaction of dithiocarbonates (xanthate) with a wide variety of substrates. This study the focuses on the interaction of trithiocarbonates (TTC) with pyrite and copper. The mechanism of adsorption of the xanthate is compared to that of the TTC. For the xanthate to adsorb it is necessary for an oxidant to be present, since xanthate adsorbs via charge transfer processes (electrochemical processes). It was found by the use of cyclic voltammetry and contact angle measurements that collector adsorption of the TTC can occur in both oxidising and reducing (thus the absence of an oxidant) conditions. Neither the TTC monomer nor the dimer could be detected on the surface by the use of Raman spectroscopy. The collector species on the surface was the TTC decomposition products namely the thiol or thiolate. Electrochemical impedance spectroscopy (EIS) confirmed that the TTC can interact under oxidising and reducing conditions. EIS showed that the rate of adsorption of the collector species for anodic currents increases relatively to the rate of adsorption for cathodic currents. Different adsorption mechanisms are realised for the different polarisation conditions. It is postulated that the TTC species serves as an intermediate for the adsorption of the thiol or thiolate on the surface, ultimately rendering the surface hydrophobic. Decomposition tests, performed by employing UV/Vis spectroscopy, indicated that the TTC is very unstable between a pH of 4 and 11. The thiol or thiolate however does not readily adsorb onto the substrates (indicated by the EIS measurements). Microflotation tests confirmed the thiolate’s inability to render pyrite hydrophobic. The microflotation tests also indicated that the TTC became less effective in recovering pyrite after it was left to decompose for a couple of hours. / Dissertation (MSc (Metallurgy))--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / unrestricted

This project investigates the optimisation of platinum-group metals recovery at the Mimosa Mine in Zimbabwe. The first part of this research investigates how collector performance can be improved by using collector combinations (mixtures) of the standard sodium iso-butyl xanthate with a secondary collector. The synergistic effect of the sodium-iso-butyl-xanthate (SIBX) combined with trithiocarbonates (TTCs) or dithiophosphates (DTP) was investigated. The short chain and the long chain TTCs were investigated. These collector combinations were studied at various molar percent ratios. The SIBX:DTP combination was studied at a 1:1 mass ratio or molar ratio of 1:1.3. Amongst all the collector combinations, it was established that the SIBX/DTP combination improved sulphur recoveries by 10% and subsequently the PGE+Au recovery by 10% compared to when the SIBX was used on its own. The C4-TTC showed poor performance in the sulphur recovery; however an improvement in PGE + Au recovery was noticeable. There was generally no significant metallurgical improvement with respect to final grades and recoveries with C10 and C12TTC mixtures with SIBX. However, the 6.25 molar per cent substitution of SIBX by C12TTC appeared to show some improvement on sulphur but not on PGM+Au recovery and grade. It was therefore concluded that the SIBX/DTP combination at 1:1 mass ratio (total moles of 0.64) showed a potential to improve the grades and recoveries of the Mimosa Mine. The Mimosa ore is characterised by an unusually large amount of talc, which often causes problems during the flotation of the PGM minerals due to the fact that talc consumes high amount of reagents. Therefore the purpose of the second part of this thesis was to investigate the possibility of removing talc prior to the flotation process by de-sliming. Two cyclones (36.9 mm and 76.0 mm) were used for de-sliming the flotation feed. De-sliming the ore was able to reduce the depressant dosage up to 300 g/t as opposed to 500 g/t that is used at Mimosa Mine. / Dissertation (MSc)--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / unrestricted

Cette thèse se divise en trois parties. La première concerne le développement d'une nouvelle voie d'accès à des hétérocycles azotés. Cette synthèse est basée sur deux étapes: l'addition radicalaire d'un xanthate, portant une fonction 1,2-dione, sur diverses oléfines suivie de sa condensation sur une diamine. Cette étude a permis de préparer des composés présentant un intérêt biologique certain tels que des pyrazines. De plus, une nouvelle méthodologie permettant l'accès à des structures imidazoles a été mise en évidence. La seconde partie a été consacrée à la préparation d'un support soluble à l'aide du procédé Madix. La dernière étude porte sur le développement de nouveaux agents modérateurs de la polymérisation radicalaire. Cette étude nous a amenés tout d'abord à synthétiser une large gamme de composés originaux de type dihydro-pyrazolone. Par la suite, la mise en place d'une étude modèle nous a permis de mesurer l'aptitude à se dissocier de ces composés puis de sélectionner les meilleurs candidats. Enfin, une étude poussée en polymérisation des agents sélectionnés a permis de mieux comprendre leur mécanisme d'action. Nous sommes en présence d'un système de type initer original. Ce système de polymérisation n'est cependant pas vivant du fait de l'irréversibilité de l'addition du contre radical dihydro-pyrazolone sur la chaîne polymère.

The thesis entitled ‘Chemistry of molybdenum xanthate (MoO2[Et2NCS2]2): Applications in organic synthesis’ is presented in 4 chapters. Molybdenum (IV and VI) oxo-complexes are the subject of significant interest due to their functional and structural similarities with several molybdo-enzymes.1 Literature survey suggests that, molybdenum (VI as well as IV) xanthate2 1 resembles the active sites of various molybdo-enzymes. Therefore, in the present thesis, we are presenting our attempts directed towards exploiting molybdenum xanthate 1 in developing various useful methodologies. Figure 1: Molybdenum xanthate Chapter 1 discloses the utility of molybdenum xanthate (1) in catalytic, aerobic oxidation of organic azides and alcohols as presented in part A and B. Part A: A mild molybdenum xanthate catalyzed, chemoselective oxidation of benzylic azides to the corresponding aldehydes3 under aerobic condition is described. This oxidation turned out to be a general method and a variety of benzylic azides were oxidized to the corresponding aldehydes. This oxidation protocol tolerates a variety of functional groups including alcohols, esters, ketones, halides and olefins. More importantly, the oxidation of azides stops at corresponding aldehyde stage without further oxidation to the corresponding carboxylic acids. A few examples are presented in scheme 1. Part B: As our attempts to oxidize alcohols with molybdenum xanthate 1 were unsuccessful (Chapter 1, Part A), we have attempted supporting the reagent 1 and investigated its utility in the oxidation of alcohols. As a consequence, polyaniline supported molybdenum xanthate (MoO2[Et2NCS2]2) is designed and used in an aerobic and mild chemoselective oxidation of alcohols4 to the corresponding aldehydes and ketones. The scheme to use polyaniline as the support for molybdenum xanthate was derived from the fact that polyaniline is known to increase the redox activity of various metal complexes by coordinating to the metal centre.5 The present oxidation strategy tolerates a variety of functional groups such as olefin, ketones, sulfides, tertiary amines, propargyl group etc. This oxidation strategy also works very well for the oxidation of secondary benzylic alcohols. Interestingly, the supported catalyst can be filtered after the reaction and reused for further oxidation without loss of its activity. Some representative examples are presented in Scheme 2. Chapter 2 describes the chemoselective and efficient reduction of azides to the corresponding amines. In this chapter, we have shown that a catalytic amount of molybdenum xanthate (1, MoO2[S2CNEt2]2) with PhSiH3 is an effective catalyst for the reduction of azides to the corresponding amines.6 This reduction of azides by 1, was inspired by the reductive silylation of aldehydes through the activation of silanes.7 This reduction tolerates a variety of reducible functional groups such as olefin, aldehydes, ketones, esters, amides and ethers, acetals etc. This strategy was also extended to various aliphatic azides to synthesize amine and their N-Boc derivatives in good yields. Scheme 3 illustrates few examples. Chapter 3 discloses convenient methods for the synthesis of substituted thiourea derivatives as presented in part A and B. Part A: A convenient method for the synthesis of tri-substituted thiourea derivatives by the reaction of primary amines with molybdenum dialkyl dithiocarbamates is presented in Part A.8 Primary amines on reaction with molybdenum xanthate produce corresponding thioureas in moderate to good yields. Similar reactions with propargylamine and 2-aminoethanol produce cyclic thiaoxazolidine and oxazolidine derivatives respectively. This methodology has been successfully adopted for the synthesis of amino acids derived chiral thioureas. Some examples are presented in Scheme 4. Scheme 4: Molybdenum xanthate mediated synthesis of thioureas Part B: An efficient method for the synthesis of symmetrical and unsymmetrical substituted thiourea9 derivatives by simple condensation of amine and carbon disulfide in aqueous medium is extensively studied. Present method describes the involvement of amino dithiol moiety as an intermediate. Though this method is not successful with secondary amines and aryl amines, it works smoothly with aliphatic primary amines to afford various di- and tri-substituted thiourea derivatives. The present method is also useful in synthesizing various substituted 2-mercapto imidazole heterocycles in moderate yields. A few examples are seen in Scheme 5. Scheme 5: Synthesis of thiourea derivatives in aqueous medium Chapter 4 describes a chemoselective deprotection10 of terminal acetonides (isopropylidines) by using aqueous TBHP (70%). A variety of acetonide derivatives on reaction with aq. TBHP in water:t-BuOH (1:1) as solvent mixtures furnish the corresponding acetonide deprotected diol products in good yields. This unprecedented deprotection strategy, tolerates a variety of acid sensitive functional groups such as silyl ether, trityl, olefin, propargyl, methoxymethyl ether, N-Boc, lactones, esters etc. A few examples are documented in Scheme 6. Scheme 6: Chemoselective deprotection of acetonides (For structural formula pl see the pdf file)

Ces travaux de thèse ont consisté à développer de nouvelles réactions radicalaires dans le domaine de la glycochimie. Deux cascades radicalaires, n'utilisant aucun dérivé stannylé et impliquant un transfert d'hydrogène intramoléculaire, ont été étudiées. La première permet de synthétiser des motifs spirocétaliques [6.5] nonanomériques et la deuxième consiste à débenzyler régiosélectivement un éther de benzyle par proximité. Les spirocétals [6.5] nonanomériques sont des motifs présents dans de nombreuses structures de produits naturels. Pour obtenir ce squelette, dont la synthèse est généralement difficile, nous avons développé une cascade radicalaire en chaîne impliquant des précurseurs homopropargyliques et des dérivés phosphorés non toxiques. Plusieurs étapes se succèdent dont l'addition du radical phosphoré sur la triple liaison, un transfert 1,5 d'hydrogène permettant de générer un radical anomère de O-glycoside, à l'origine de la diastéréosélectivité du centre spiranique, et une cyclisation 5-exo-trig. Cette stratégie s'est révélée particulièrement efficace puisque de bons rendements et une excellente diastéréosélectivité ont été obtenus notamment en série glucose et glucosamine. La nouvelle réaction de O-débenzylation par proximité, développée dans la deuxième partie, permet de déprotéger sélectivement un éther de benzyle en α d'un groupement hydroxyle préalablement fonctionnalisé sous forme d'éther de silyle xanthate. Cette réaction se déroule en deux étapes successives dans le même ballon. La première est une cascade radicalaire constituée, entre autres, d'un transfert 1,7 d'hydrogène et de l'addition du radical benzylique, ainsi formé, sur le peroxyde de dilauroyle. L'acétal mixte intermédiaire obtenu est alors hydrolysé lors de la deuxième étape. Cette méthodologie a été appliquée avec succès à divers mono- et disaccharides polybenzylés et s'est révélée efficace en présence de nombreuses autres fonctionnalités chimiques (acétal de benzylidène, azido..).

Cette thèse traite essentiellement de différents aspects de la chimie radicalaire du groupement xanthate. Le premier chapitre est une introduction à la chimie radicalaire générale puis dresse un aperçu de la chimie radicalaire des xanthates. Le chapitre suivant présente une nouvelle méthodologie de synthèse de composés poly-aromatiques à partir d'époxy-oléfines tendues par voie radicalaire. Le troisième chapitre décrit la synthèse totale de la structure supposée du flossonol, puis d'un isomère de position de ce dernier et conclut par la résolution de la structure du produit naturel. La partie suivante rapporte l'étude de différentes stratégies pour la synthèse du pseudoptéroxazole. Lors de ce travail, une méthode d'isomérisation contra-thermodynamique de doubles liaisons carbone-carbone trisubstituées endocycliques a été mise au point et fait l'objet du cinquième chapitre. La sixième partie du manuscrit traite d'une tentative de synthèse du squelette des eudesmanes et enfin, le septième chapitre présente une méthodologie de synthèse de dérivés benzéniques par transfert 1-5 d'un groupement acyle sur un dérivé de 3-méthylène-1,4-cyclohexadiènes.

Extensive research has shown that electrochemistry is one of the factors that govern the flotation of sulfide minerals. Flotation is often adversely affected by uncontrolled oxidation, which is also an electrochemical process. The interest in pyrrhotite recovery arose after observing that there is a substantial loss of PGM due to the depression of pyrrhotite and the subsequent loss of any PGMs associated with it. The first part of this study focuses on the influence of chemical composition and crystal structure on the electrochemical behaviour of pyrrhotite in a 0.05 M Na2B4O7 solution. Rest potential and polarisation resistance measurements, as well as anodic polarisation diagrams, showed that the magnetic 4C type pyrrhotite is anodically more reactive than the non-magnetic 6C type pyrrhotite. It was also shown in cathodic polarisation diagrams that the non-magnetic 6C type pyrrhotite is a better substrate for oxygen reduction and is less susceptible to oxidation. ToF-SIMS showed the formation of an oxide layer on the pyrrhotite surface after oxidation. In the second part of this work, the influence of galvanic interactions on the electrochemical behaviour of pyrrhotite in contact with pentlandite was investigated. It was observed that, under oxygen-saturated conditions, as the amount of pentlandite increases, the reactivity towards oxidation of the mixed mineral system is reduced. Impedance measurements showed a decrease in capacitance values, indicating the formation of a continuous oxide layer on the surface and an increase in oxide layer thickness with decreasing pentlandite content. Anodic polarisation diagrams showed that under oxygen-deficient conditions and in the low potential region, pentlandite behaves as an inert material and does not have an influence on the oxidation behaviour of pyrrhotite. Hence, the anodic activities of the different magnetic 4C type pyrrhotites from Sudbury Gertrude, Phoenix and Russia were compared. It was shown that the oxidation reactivity decreased in the following order: Sudbury Gertrude magnetic 4C pyrrhotite > Phoenix magnetic 4C pyrrhotite > Russian magnetic 4C pyrrhotite; it also varied according to location. In the transpassive region, higher anodic currents were observed on the mixed samples because both pentlandite and pyrrhotite reacts. The reactivity increased in the order: pure pyrrhotite (Russia) < medium-pentlandite (Sudbury Gertrude) < high-pentlandite (Phoenix). In the presence of potassium ethyl xanthate, there was no change in the initial anodic reactivities of the different pyrrhotites. The anodic polarisation diagrams of the pure and mixed samples showed a reduction in the maximum anodic peak current, suggesting the presence of xanthate on the surface, which hinders oxidation of the mineral surface. In addition, the influence of cleaning of oxidised pyrrhotite with gaseous carbon dioxide was studied, using electrochemical and microflotation measurements. Electrochemical measurements indicated that CO2 treatment resulted in depassivation of the oxidised surfaces; this was supported by ToF-SIMS measurements that demonstrated a reduction in the oxide layer thickness after CO2 treatment. Anodic polarisation diagrams showed a higher anodic peak current, indicating that the surface is more reactive. Gaseous carbon dioxide conditioning of oxidised pyrrhotite resulted in improved flotation response of pyrrhotite with the aid of copper activation and higher air flow rate. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

Cette thèse est divisée en huit chapitres. Les trois premiers chapitres bibliographiques constituent respectivement un rappel des grands principes de la chimie radicalaire, une présentation des possibilités offertes par la chimie radicalaire des xanthates et une revue des méthodes d'allylation radicalaire, principale thématique de nos travaux. Les trois chapitres suivants sont consacrés à des applications d'une méthode d'allylation radicalaire de simples alcools allyliques, récemment découverte au laboratoire. Ces applications nous ont permis de définir des nouvelles voies de synthèse d'éthers d'énols régio-définis, de cétones et dicétones-1,4, de sulfures vinyliques, et enfin d'oléfines fluorées et trifluorométhylées. Les dicétones-1,4 et les sulfures vinyliques obtenus ont en particulier été mis à profit pour la préparation de carbo- et d'hétérocycles fonctionnalisés. Un effet de polarité remarquable a par la suite été découvert pour le processus de fragmentation radicalaire lors de la synthèse d'alcènes trifluorométhylés. Ce dernier a permis le développement d'une nouvelle méthode de synthèse de cétones et de dicétones-1,5 dans laquelle la nouvelle cétone formée provient de la rupture homolytique inusuelle d'une liaison carbone-carbone. Les résultats obtenus sont présenté dans un septième chapitre. Enfin, une nouvelle voie d'accès au alpha-céto carbinols vinyliques, composés à fort potentiel synthétique, est décrite dans un huitième chapitre. Cette méthode exploite le réarrangement sigmatropique de Mislow-Braverman-Evans de sulfoxydes allyliques. Elle constitue l'approche la plus directe au alpha-céto carbinols vinyliques et présente l'avantage de partir de simples cétones.