published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

An atomically thin film of semiconducting transition metal dichalcogenides (TMDCs) is emerging as a class of key materials in chemistry and physics due to their remarkable chemical and electronic properties. The TMDCs are layered materials with weak out-of-plane van der Waals (vdW) interaction and strong in-plane covalent bonding enabling scalable exfoliation into two-dimensional (2D) layers of atomic thickness. The growth techniques to prepare these 2D TMDC materials in high yield and large scale with high crystallinity have attracted intensive attention recently because of the new properties and potentials in nano-elctronic, optoelectronic, spintronic and valleytronic applications. In this thesis, I develop methods for the chemical synthesis of 2D TMDCs films. The relevant growth mechanism and material characteristics of these films are also investigated. Molybdenum disulfide (MoS2) is synthesized by using molybdenum trioxide (MoO3) and sulfur (S) powder as the precursor. The films are formed on substrate pre-treated with reduced graphene oxide as the catalyst. However, this method cannot be extended to other TMDC materials such as molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) because reduced graphene oxide (rGO) reacts with selenium to form alloy materials rather than TMDC films. At the same time, the conversion of MoO3 to MoSe2 or that of tungsten trioxide (WO3) to WSe2 without the assistance of hydrogen in the chemical reaction is not thermodynamically feasible because the oxygen in the metal oxide cannot be replaced by selenium due to lower reactivity of the latter. On the other hand, I demonstrate that MoSe2 film can be synthesized directly by using MoSe2 and Se powder. Furthermore, the method of sulfurization or selenization of pre-deposited metal film can be promising due to precise thickness/size controls. Finally, some perspectives on the engineering challenges and fabrication methods of this family of materials will be given. / published_or_final_version / Physics / Master / Master of Philosophy

Two new synthetic methods for the construction of the C-D moiety of streptonigrin have been developed. The first is the cyclization of beta, gamma unsaturated ketals to cyanopyridines. These ketals were prepared from akylidenemalononitriles. The second method utilized is the ortho-directed metalation of benzamide or oxazoline derivative to give keto compounds. However, attempts to transform these keto compounds to akylidenemalononitriles by Knoevenagel condensations were unsuccessful. With the ease of the reaction and ready availability of starting materials, the beta, gamma unsaturated ketals offer versatile synthons for pyridine C ring synthesis.

Alkoxy alkyl groups migrate to benzylic carbon when alkyl alkylphenyl ethers are treated with n-butyllithium and potassium t-butoxide. For alkyl 2,6-dialkylphenyl ethers, yields of the rearrangement products range from 45-70%. Rearrangement products are obtained in 10-30% yield from other dimethylanisoles and methylanisoles. The reactions appear to proceed by homolytic cleavage of the alkoxy alkyl group of alkoxybenzyl anions followed by recombination of the resulting radical pair in a different way. The reaction is useful for preparing 2,6-dialkylphenols and their corresponding ethers. The rearrangement can be avoided by using methyl ethers and working at or below room temperature. This was shown by reacting the dianion from methyl 2,6-dimathylanisole with dialkyl sulfates to give methyl 2,6-dialkylanisoles, with a,ω-dihalides to give methoxy[n]metacyclophanes (n = 8-15), dimethoxy[n.n]metacyclophanes (n = 5-10) and trimethoxy[S.S.S]metacyclophane, and with oxidizing agents to yield dimethoxy[2.2]metacyclophane.

This work involved synthetic approaches to the anti-tumor agent deoxybouvardin. Numerous reactions were explored in an attempt to build an amino acid from an aromatic aldehyde under mild conditions. From these reactions new and useful synthetic methods have been discovered for making α-acylamino alcohols and unsymmetrical imides. Some dehydroamino acids were successfully prepared from aromatic aldehydes and an N-acylphosphorylglycine ester. Progress towards a synthesis of 5- η-thiodeoxybouvardin will also be discussed.

Amino acids are important building blocks for the synthesis of a large number of biologically active compounds and drugs. Amino acids with the pentacyclo-undecane (1) and trishomocubane (2) frameworks fall into the class of conformationally constrained non-natural amino acids. Conformationally constrained amino acids are found in many naturally occurring, biologically active compounds. It was found that incorporating cage structures into drugs induces a range of positive effects: promotes transport across the cell membrane, drugs can be designed to target the central nervous system, increased receptor site specificity, and retards metabolic degradation. In the light of this, it was decided to investigate the incorporation of cage amino acids into peptides. A synthetic route has been established for the efficient synthesis of amino acids 1 and 2, and for their incorporation into peptides. Several chiral macrocyclic crown ethers and related analogues have been shown to be capable of forming complexes enantioselectively with chiral organic ammonium salts. The design and synthesis of host chiral macrocycles which are able to distinguish between the enantiomers of guest organic ammonium salts is of interest in the areas that include synthesis of enzymes, electrodes for specific ions or molecules, drugs targeted for specific sites, and enantiomer separation. A synthetic procedure has been established for the synthesis of cage annulated chiral crown ethers derived from amino acids. The advantage of using cage compounds in crown ethers is due to increased rigidity, increased solubility in non-polar solvents and increased chirality. Various techniques for the determination of enantiomeric recognition have been studied and include NMR spectroscopy, fluorescence emission spectroscopy and computational methods. The cage crown ether 3 represents a typical example of these new cage annulated, chiral crown ethers. / Thesis (M.Sc.)-University of Natal, Durban, 2001.

The increasing emergence of antibiotic resistant strains of bacteria and fungi is driving the need to increase the production of current antibiotics and produce novel antimicrobial compounds. This study worked to increase the production of cycloheximide and streptomycin antibiotics by co-culturing Streptomyces griseus with other industrially important microbes. 1-3 industrial challenge microbes at a time were added to a culture of S. griseus and allowed to grow for one week in shake flask cultures before harvesting and quantifying antibiotic production. Fifteen different industrial challenge microbes placed in 35 different combinations were used in the study and 17 of these combinations were found to significantly increase antibiotic production after analysis with ANOVA. Antibiotic production was confirmed using bioautograms. Three of the successful different co-cultures were then subjected to a study to see when industrial challenge microbe addition was optimal. Results suggest that the optimal time to add the challenge microbes was 1-3 days following the original S. griseus inoculation. Dead challenge microbes were also added to a culture of S. griseus and it was found that these significantly increased cycloheximide as much as the live co-cultures did. / Department of Biology