No description available.

No description available.

No description available.

No abstract available. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2006.

The ability to construct C-N bonds is of great importance to organic chemists as exemplified by the vast number of natural products, pharmaceutical agents and fine chemicals that contain such linkages. An atom efficient C-N bond forming reaction namely hydroamination has attracted much interest to date due to its ability in forming amine, imine and enamine functionality. The scope of this project involved the attempted synthesis of a biologically active and nitrogen containing pyrrolizidine alkaloid isolated from cryptic thief ants and poison dart frogs namely 223H (xenovenine). The method of hydroamination was utilized as the pivotal ring forming step and was established as being a valuable synthetic tool towards the construction of 223H (xenovenine). The stereoselective synthesis resulted in the successful formation of ethyl (3R)-5-heptyl-3-methyl-2,3-dihydro-lH-pyrrolizine-7-carboxylate 74, a novel, and structurally analogous precursor to 223H (xenovenine) over 10 synthetic steps from (S)-pyroglutamic acid. The following research also resulted in the synthesis of two other novel compounds namely ethyl 3-[(2R)-2-methyl-5-thioxotetrahydro-lH-pyrrol-l-yl]propanoate 86 and ethyl 3-{(5R)-2-[(E)-2-ethoxy-2-oxoethylidene]-5-methyltetrahydro-lH-pyrrol-l-yl}propanoate 87. A catalytic hydroamination study on the conversion of C-propargyl vinylogous amides into pyrroles demonstrated that transition metal salts of groups 11 and 12 serve as effective hydroamination catalysts. The oxide, acetate, chloride and nitrate derivatives of group 11 and 12 metals namely Cu(II), Ag(I), Zn(II), Cd(II) and Hg(II) were employed as potential hydroamination catalysts in the oxidation states provided. The Zn(II) catalyst series with the exception ZnO provided the greatest hydroamination yields under mild reaction conditions owing to their high Lewis acidities however the Ag(I) and Hg(II) catalyst series also provided excellent yields of product under more forcing reaction conditions. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

There is enormous interest in the design and development of efficient chiral ligands for asymmetric catalysis, as a result, this field has become one of the most popular areas of research in organic chemistry. This project involved the investigation of the novel chiral pentacyclo-undecane (PCU) diol 54a, PCU bisimine 87 and PCU bis(oxazoline) 100 type ligands. The PCU diol ligand was synthesized, but proved to be difficult to obtain enantiomerically pure which hindered further investigation into this type of ligand. The PCU bisimine ligand 87 was synthesized. However due to its instability it was not further pursued. Synthesis of the PCU bis(oxazoline) ligand 100 was successful. This ligand was complexed to various metal salts and its efficiency as a chiral Lewis acid catalyst was evaluated on the asymmetric Diels-Alder reaction between 3-acryloyloxazolidin- 2-one 52 and cyclopentadiene 33. The anhydrous magnesium perchlorate ligand complex emerged as the best catalyst providing the endo-cycloadduct product 53 in 81 % enantiomeric excess at -40 oC. Optimizations of the possible conformations of the magnesium complex of ligand 100 with the substrate 52 were performed using Density Functional Theory (DFT) calculations. The more energetically favoured complex conformation was established. The Re-face of the dienophile which was less hindered produced the product consistent with the experimentally observed product 16. Based on the calculated bond lengths from the computational model binding of the ether oxygen on the PCU moiety to magnesium was observed. All the novel compounds were fully characterized using NMR, IR and mass spectroscopy as the main tools. / Thesis (M.Sc.) - University of KwaZulu-Natal, Westville, 2008.

Hydroxymethylation catalysis provides a valuable strategy for the high volume production of alcohols from α-alkenes. Generally this involves a hydroformylation-hydrogenation sequence, but the capacity to optimise selectivity for each transformation is limited. Condensation reactions between aldehyde products and alcohol products frustrate process economics. By an alternative scheme, all relevant bond-forming reactions occur in a single mechanism. This thesis describes several approaches to catalyst development and the application of derived systems for the hydroxymethylation of allyl alcohol. A review of auto-tandem hydroxymethylation and domino hydroxymethylation is presented in Chapter 1. In Chapter 2 the synthesis of bis-(diethylphosphine) ligands based on a modular series of chiral alicyclic scaffolds is described. High pressure NMR studies have shown that the catalytically active complex [RhH(CO)₂(L-L)] adopts preferentially ea geometry, with [Rh(CO)(L-L)(μ-CO)]₂ as the primary competing species. Catalyst performance can be correlated with the flexibility of the chelating ring; this favoured a high monomer/dimer ratio which enhances activity, but could not rigidify the configuration of the diethylphosphine groups which inhibits linear selectivity. Deuterium labelling studies were suggestive of a domino hydroxymethylation scheme. From the rhodium-hydroxyalkyl-hydride-carbonyl cation, a reductive elimination furnishes the diol derivatives and a β-hydride abstraction furnishes the hydroxyaldehyde derivatives. Up to 53 mol% selectivity to 1, 4-butanediol was attained. The catalysts could be recycled via biphasic separation, however poisoning by methacrolein caused a decline of activity upon reuse of the solution. An investigation of enhanced specific activity via the meta-effect is the subject of Chapter 3. The effect of systematic meta-substitution in triphenylphosphine upon physicochemical properties was investigated by IR spectroscopy and electrochemistry, both of which showed no significant structural impact on the uncoordinated triarylphosphine. Variable temperature ¹H NMR studies however revealed a change in the solution dynamics of the corresponding Vaska complex. The activation barrier to phosphorus-(ipso)carbon rotation increases as a function of meta-substitution, with rotation of substituted aryl rings past each other being more strained. This should create a well-defined coordination sphere around rhodium, and is proposed to account for the high linear selectivity observed in the hydroformylation of allylic alcohols with [RhH(CO){(3, 5-Me₂Ph)P}₃]. Linear-selectivity reached 96 mol%. Catalyst recycling was executed via biphasic separation, retaining on over twelve cycles an average of ~ 94 % efficiency. The kinetics of allyl alcohol hydroformylation with [RhH(CO){(3, 5-Me₂Ph)P}₃] was found to be well represented by Equation 11 (Section 3.6) A detailed analysis of how substrate-specific the influence of the meta-effect remains to be performed. In Chapter 4 domino hydroxymethylation by multi-component L-L/PEt3/Rh systems is described. The regioselective performance of a diphosphine rhodium catalyst in hydroformylation was translated for hydroxymethylation upon introduction of triethylphosphine at a L-L/PEt3 molar ratio ≥ 1. The highest observed selectivity to 1, 4-butanediol was 66 mol%. Competitive activity of triethylphosphine-modified rhodium species presumably accounts for the reduced linear selectivity observed when L-L/PEt3 molar ratio < 1. Despite aggravated catalyst decomposition at higher triethylphosphine concentrations, heterogeneous hydrogenation does not appear to take place. Deuterium labelling studies also discount a sequential homogeneous hydrogenation. There is evidence for the activation of a tris-phosphine-modified rhodium-acyl-carbonyl complex, but such a species could not be isolated from complexation reactions with a variety of precursors. It would be of interest to determine alternative promotors and to establish whether it is preferential to employ a high concentration of mildly acidic species or a low concentration of highly acidic species. The self-assembly of DNA base pair analogues 2-N-pivaloylaminopyridyl phosphine and isoquinolyl phosphine, each modified with diphenylphosphine, diethylphosphine, dicyclohexylphosphine and bis(3, 5-dimethylphenyl)phosphine, is described in Chapter 5. In the presence of a rhodium precursor, exclusive formation of the heteroleptic complex was observed. Although the intramolecular hydrogen-bonding network is sensitive to temperature and free hydroxyl functionalities, highly regioselective catalysts were generally afforded under the appropriate operating conditions. Only the catalyst based on the bis(dicyclohexylphosphine)-heterodimer performed poorly, presumably due to the formation of mono-phosphine complexes. High chemoselectivity was correlated with the heterodimer acidity constant, however this is rendered non-linear by a trans influence when electronic distinction between the platforms is high. Overall, complexes based on the assembly of a dicyclohexylphosphine platform and a bis(3, 5-dimethylphenyl)phosphine platform were found to be optimal; up to 73 mol% selectivity to 1, 4-butanediol was reached. It has been demonstrated in this thesis that in order to effect linear-selective domino hydroxymethylation of allyl alcohol, two distinct transition state structures must be optimised. High regioselectivity demands an asymmetric rhodium-hydride-dicarbonyl complex, which can be generated by an asymmetric chelate or by rigidifying the configuration of the substituents on phosphorus. Interestingly, chelation geometry in this transition state has little impact on this parameter. It has been shown that domino hydroxymethylation is activated by an electron-rich rhodium-acyl-dicarbonyl. The state of electron density on rhodium can be controlled by the substitution pattern on the phosphorus donors, but can also be changed by the inclusion of a suitable promoter. The chelation geometry in this transition state is more significant; placing the acyl functionality trans to a phosphorus donor concentrates the electronic effect in the rhodium-alkyldiol-hydride-carbonyl cation to such an extent as to impede hydride migration and reductive elimination of the diol, favouring β-hydride abstraction and reductive elimination of the hydroxyaldehyde.

Evidence is presented that apparent silene products obtained from the metalation of cyclopentadienyldimethyl - chlorosilane either with tert-butyl1ithium or with methylenetriphenylphosphorane actually arise from the metalated starting material, a silenoid, rather than from a silafulvene intermediate. Trimethylmethoxysi1ane is shown to be an effective trap for dimethylsilafulvene. A new dimethylsilafulvene precursor, bis(dimethylmethoxysi1yl) cyclopentadiene, which gives high yields of dimethyldimethoxysi1ane and the silafulvene at temperatures as low as 240°C is reported.

Call number: LD2668 .T4 1958 D63 / Master of Science

The aim of this study was to develop an optimized process for the manufacturing of hexazinone and to validate laboratory results on a larger scale, in this case a miniplant. Patent protection on hexazinone has expired and the objective of this study was to improve on the known commercial process to enable the cost-effective manufacturing of hexazinone. The generic market is highly competitive and hexazinone can be sourced from several suppliers at low cost. Ownership of improved technology for the production of hexazinone can give Dow AgroSciences a competitive advantage in the generic market. Various routes for the manufacture of hexazinone are reviewed and important aspects of scale-up is considered and summarized. The known commercial five-step synthesis was optimized on a laboratory scale, using experimental design where applicable. The optimized procedures were scaled to the mini-plant. Significant improvements over patent results were realized for the process efficiencies. This is in particular true for the synthesis of N-ethoxycarbonyl-N,N,Ntrimethylguanidine in step 3. Employing experimental design to optimize the reaction was instrumental in meeting the objective of the study. Higher overall yields lead to substantial cost savings because of more efficient raw material conversion to value adding products. Die doel van hierdie studie was om die vervaardigingsproses vir heksasinoon te optimizeer. Die geldigheid van die laboratorium resultate is bevestig op ‘n groter skaal, in hierdie geval, op ‘n loodsaanleg met ‘n vyftig liter kapasiteit. Patente op die bekende vervaardigingsproses van heksasinoon het verval. Die mark vir generiese produkte is hoogs kompeterend en heksasinoon kan teen lae koste aangekoop word van verskeie vervaardigers. ‘n Betekenisvolle verbetering op die bestaande tegnologie kan aan Dow AgroSciences ‘n kompeterende voordeel besorg in die generiese heksasinoon mark. Die studie gee ‘n oorsig oor verskeie roetes vir die vervaardiging van heksasinoon en belangrike aspekte van opskalering. Die bekende vyf-stap sintese van heksasinoon is suksesvol geoptimiseer in die laboratorium met behulp van eksperimentele ontwerp. Die optimum kondisies is bevestig op ‘n loodsaanleg. Die gebruik van eksperimentele ontwerp om die vervaardigingsproses te optimiseer, het gelei tot belangrike verbetering in die tegnologie. Die totale opbrengs van die vyf-stap sintese oorskry gedokumenteerde waardes. Die geoptimiseerde proses kan lei tot betekenisvolle besparings deur die effektiewe omskakeling van uitgangstowwe na ‘n waardevolle produk. / Prof. F. Van Heerden