D.Sc. (Chemistry) / The aim of this study was to develop new methodology for the transformation of unsaturated carbohydrates utilising organometallic compounds. The first half of the study was directed toward developing a general synthesis of complex allyltins and in determining their application to carbon-carbon bond formation. It was decided to utilise carbohydrate substrates in this regard to develop a novel method of producing glycosides...

M.Sc. (Chemistry) / Please refer to full text to view abstract

Previous investigation of the kinetics and mechanism of the substitution reaction of bidentate substituted transition metal carbonyl complexes with Lewis bases (L') to yield the cis distributed bidentate substitution product, where M=Cr, Mo, or W and S-S = DTO (2,2,7,7-tetra-methyl-3, 6-dithiaoctane) or DTN (2,2,8,8-tetramethyl-3,7-dithianonane) have suggested that this reaction proceeds via a mechanism involving initial reversible dissociation of one end of the bidentate sulfur ligand followed by rapid attack of L' on the resulting five-coordinate intermediate.

The chemistry of homogeneous transition metal systems offer parallels to the reactions on the surfaces of industrial hydrodesulphurization catalysts. The reactions of several ruthenium complexes with sulphur-containing reagents are described, with an emphasis on the kinetics and mechanisms thereof. The complex Ru(CO)₂(PPh₃)₃ (2), for example, reacts quickly with thiols and disulphides, producing cct-RuH(SR)(CO)₂(PPh₃)₂ (9) and cct-Ru(SR)₂(CO)₂(PPh₃)₂ (14), respectively, although 2 fails to react with unstrained thioethers. Reactions of the related complex Ru(CO)₂(PPh₃)(dpm) (dpm=Ph₂PCH₂PPh₂) are complicated by the lability of all of the three different ligands. The two dihydrides cct-RuH₂(CO)₂(PPh₃)₂ (3) and RuH₂(dpm)₂, as a cis/trans mixture (7), react with thiols to produce the hydrido-thiolato complexes 9 and RuH(SR)(dpm)₂ (13). respectively. The mechanisms appear to depend on the basicity of the hydride ligands; the more basic dihydride, 7, is probably protonated by the thiol, giving an unobserved molecular hydrogen intermediate, while 3 reacts by slow reductive elimination of H₂. The same rate constant, rate law, and activation parameters are found for the reaction of 3 with thiols, CO or PPh₃. The reaction of 3 with RSSR produces mostly 9, with small amounts of 14. The complete characterization of several members of the series 9 and 14 is described, including the crystal structure of the p-thiocresolate example of each. The reactions of 9 with other thiols, P(C₆H₄pCH₃)₃, CO, RSSR, HCl, PPh₃, and H₂, are also reported. The first three of these reactions share the same rate law and rate constant, the common rate determining step probably being initial loss of PPh₃. Some equilibrium constants for the exchange reactions of 9d (R=CH₂CH₃) with other thiols were tetermined, the Keq values increasing with the acidity of the incoming thiol. The mercapto hydrogens of 9a and 14a (R=H) exchange with the acidic deuterons of added CD₃OD. The hydridic and ortho-phenyl hydrogens exchange more slowly, presumably by intramolecular processes. Complex 14b (R=C₆H₄pCH₃) is unstable in the presence of light, exchanges phosphines rapidly with added P(C₆H₄pCH₃)₃, exchanges thiolate groups with added thiols, and is converted by high pressures of H₂ to a mixture of 9b and 3. Intermediates proposed for the mechanism of the thiol exchange reactions of 9 and 14 contain two or three thiolate groups sharing a proton. A related complex, [Ru(CO)₂(PPh₃)(μSEt)₂(μ₃SEt)Na(THF)]₂, which contains three thiolate groups on a ruthenium centre sharing a sodium cation, was isolated from the reaction of cct-RuCl₂(CO)₂(PPh₃)₂ with sodium ethanethiolate. In acetone, 9b and 14b can be formed cleanly from cct-RuHCl(CO)₂(PPh₃)₂ and cct-RuCl₂(CO)₂(PPh₃)₂, respectively, by reaction with p-thiocresolate. Complex 3 or cheaper analogues could be used as catalysts for the reduction of disulphides by H₂, or as recyclable reagents for the non-oxidative extraction of thiols from thiol-containing mixtures such as oil fractions. The chemistry described above will help to guide future researchers to systems that more closely parallel the processes occurring on the surfaces of industrial hydrodesulphurization catalysts. / Science, Faculty of / Chemistry, Department of / Graduate

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The purpose of this investigation is threefold: (1) to examine in detail the cycloaddition of halogenated ketenes and carbonyl compounds, (2) to study the decarboxylation of the resulting halogenated 2-oxetanones,and (3) to investigate the effect of halogens in the halogenated 2-oxetanones on the nucleophilic addition reaction.

<p> The catalytic asymmetric transformation of carbonyl compounds via iminium and enamine intermediates using chiral amines as organocatalyst has grown remarkably over last decade. Various "metal-free" reactions including aldol, Mannich, Michael, alkylation and Knoevenagel types have now been reported efficiently using simple amino acids as the sole organocatalysts. Amongst these, proline has proven to be particularly useful as a general catalyst in such catalytic asymmetric syntheses due to its unique properties. Nonetheless, proline suffers inherent fundamental disadvantages as an organocatalyst in terms of its solubility in organic media, limiting reactions to polar aprotic solvents such as DMF or DMSO. We have been interested in the synthesis of derivatives of proline incorporating functional groups that would render the chiral secondary amine fully soluble in organic solvents such as THF or dichloromethane and others that could be employed in aqueous media. The synthesis of these catalysts and their application towards the asymmetric synthesis will be presented.</p> <p> The thesis also describes isolation and identification of natural products and synthesis of their semi-stabilised derivatives. Sakuranetin and dihydrowagonine were isolated from Prunus avium plant parts, menisadurilide was isolated from Dicentra spectabilis extracts while narciclasine was isolated from Narcissus pseudonarcissus plant parts. Semi-synthetic derivative, trans-dihydronarciclasine was prepared from natural product narciclasine. Both the derivatives were subjected to antifungal and human cytochrome inhibition activity and their correlation is explained. The Amaryllidaceae family alkaloids and their semi-synthetic derivatives were subjected to biological testing and an important structural property is correlated to their activities.</p> / Thesis / Master of Science (MSc)

<p> It has been shown that the hydroxy compounds la-4a possess triplet states which are substantially shorter-lived than those of their methoxy analogs (lb-4b). In each case the efficient quenching of the excited carbonyl triplet state has been attributed to intramolecular phenolic hydrogen abstraction. Evidence for this is found in the detection of biradical species generated from triplet decay, primary hydrogen-deuterium kinetic isotope effects and thermodynamic parameters which support both atom transfer (enthalpy) and high degrees of order (entropy) in the transition state necessary for the proposed abstraction process. The difference in triplet lifetimes (16 ns to 1135 ns) is due almost exclusively to varying degrees of difficulty in obtaining a transition state geometry in which the phenoxy hydrogen is properly lined up for intramolecular hydrogen abstraction. </p> / Thesis / Master of Science (MSc)