Thesis (Ph. D.)--University of Hong Kong, 2005. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references. Available also in a digital version from Dissertation Abstracts.

The reaction of chlorodimethylvinylsilane with tertbutyllithium was investigated in the presence of several conjugated dienes. In all cases except with 2,5-dimethylfuran, [2+4] cycloadducts of a silene intermediate are obtained in hydrocarbon solvents. The presence of THF in the reaction mixture suppresses the formation of cycloadducts in favor of 1,3-disilacyclobutanes. In the reaction of dimethylethoxyvinylsilane or dimethylmethoxyvinylsilane with tert-butyllithium the main product is the 1,1-dimethyl2-neopentyl-4-(dimethylalkoxysilyl)silacyclobutane. It is concluded that lithium chloride elimination to give silene intermediates occurs in hydrocarbon solvents. In the presence of strong Lewis bases or when the leaving group on silicon is an alkoxy group, the addition reaction giving a-lithiosilanes occurs and products arising from their coupling reactions are obtained.

The novel complexes [special characters omitted] and [special characters omitted] were prepared and characterized. Photoredox quantum yields for the formation of [special characters omitted] from the above compounds were measured on irradiation at 360 nm to be 0.065, 0.082, 0.0088 and 0.0040 respectively. With added thiocyanate a significant increase in ΦCo2+ occurred. This can be modeled in two ways; (i) scavenging of thiocyanate radical from an initial caged radical pair giving 6-25 ps estimates for the lifetime of the latter species; (ii) photolysis of a thiocyanate/complex ion pair, giving formation constants of 0.19, 0.09, 0.08 and 0.05 for the complexes [special characters omitted] and [special characters omitted] respectively. Subnanosecond laser flash photolysis studies showed evidence for the formation of [special characters omitted]. The effects of added electrolytes and of viscosity on the formation and decay of [special characters omitted] were also investigated. To help to distinguish between the above two mechanisms, the zero-charged novel complex [special characters omitted] (tacn = 1,4,7-triazacyclononane) was synthesized and characterized. It is thermally stable in aqueous/DMSO solution, but on irradiation at 360 nm undergoes parallel photosubstitution to form DMSO and aqua-substituted products with an overall quantum yield of 0.012. The product yields increase linearly with added thiocyanate. For a 1 M thiocyanate solution, the quantum yield for disappearance of the starting complex rose to 0.022 and a small redox yield of 0.0008 was found. Under these same conditions, ns laser flash photolysis at 355 nm revealed a transient absorption owing to [special characters omitted], which was produced with a quantum yield of 0.036. These results are interpreted in terms of scavenging of radical pair species by thiocyanate ion followed by back electron transfer to give a photosubstituted product, and a radical pair quantum yield of 0.29 and lifetime of 12 ps was derived. The emission of [special characters omitted] (where pop = μ-pyrophosphite-P,P’) can be quenched by the complexes [special characters omitted] (where X = [special characters omitted]) only in the presence of electrolytes. The salt effects have been studied using the salts MCl, M'Cl2, or [special characters omitted] (where M, M’ and R represent alkali, alkaline earth metals, and alkyl respectively, with n = 0-3), and [special characters omitted]. For 0.5 M cation concentration, second-order quenching rate constants kq lie in the range [special characters omitted]. For the different quencher complexes used, kq decreases in the order [special characters omitted]. The oxidative quenching products [special characters omitted] are observed, and their quantum yields are 0.083 and 0.027 respectively for the reaction of [special characters omitted] with [special characters omitted] and [special characters omitted] in 0.5 M KCl / pH2 solution. The quenching occurred by atom transfer (dominant) and electron transfer (minor) for quencher [special characters omitted] or [special characters omitted], while only electron transfer was observed for [special characters omitted] and [special characters omitted] quenchers. The quenching efficiency of the cobalt complexes increases with electrolyte concentration and specific cation effects are observed in the kq with the following trends Li+ < Na+ < K+ < Cs+: Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4+ < MeNH3+ < Me2NH2+ < Me3NH+: Et3NH+ < Et2NH2+ < EtNH3+: n-PrNH3+ < EtNH3+ < MeNH3+. / Graduate

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M.Sc. / Porphyrazines, having similar structures to porphyrins, exhibit interesting characteristics as well as applications. These mainly stem from the highly conjugated nature of these compounds, making them useful in fields ranging from functional dyes to molecular devices to biomedical applications. Of these a highlight must be the photodynamic treatment of cancer. The usefulness of porphyrazines in these fields is very closely linked to the structure of the macrocycles. Therefore it is necessary to find methods to customise these compounds in the hopes of improving the current applications as well as broaden the horizons. There exists a gap in porphyrazine synthesis in the sense that it has not been possible before to ensure the formation of cis-porphyrazines. Therefore, the main effort in this project was directed towards addressing this void. A focus of the research was to investigate introduction of a linker between two monomer dinitrile units, used during the macrocyclisation step, to force the cis geometry into the porphyrazine. Attention was also given to finding a way to functionalise the macrocycles in such a way that the functionality could be removed at a later stage to be able to differently functionalise the porphyrazines to yield other potentially interesting structures. By manipulating existing methods to prepare porphyrazines, we were able to synthesise, with a great deal of selectivity, a cis-porphyrazine from monomers that were generally viewed as trans-directors. This represented a breakthrough in porphyrazine research. It was also possible to prepare a macrocycle with removable functionality (ally! groups) on the peripheral oxygens, thereby opening up the way towards further functionalisation. Building on this success, in which two new porphyrazines (one of which was cis functionalised) had been synthesised, it was attempted to introduce the linker unit as previously described, such that cis geometry only was possible in the macrocycle. Various reaction conditions and reaction types, ranging from Mitsunobu reactions to imine formation and reduction, were utilised in various synthetic approaches to these moieties. This effort allowed the successful synthesis of a variety of linked dinitrile dimers, which were set up to undergo Linstead macrocyclisation. However due to time constraints, these compounds have not yet been subjected to macrocyclisation but represents a challenge for the future.

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

Increased interest in porphyrazines is due to the role they play as photosensitisers in tumour therapy. A continuing problem in photodynamic therapy is selective uptake of the photosensitiser into cancerous tissue versus healthy tissue. A potential solution would be to functionalise the photosensitiser with biological molecules to assist in its selective uptake and for enhanced water solubility. The main objective of the project was the synthesis, deprotection, metalation and characterisation of peripherally carbohydrate substituted porphyrazines. A succesful synthetic pathway for the incorporation of carbohydrate moeities into porphyrazine systems in the form of 1:3, where 1 represents the carbohydrate-substituent and 3 represents the propyl-substituted pyrrole unit, has been demonstrated by this study. To this end, a range of selectively protected monosaccharide precursors was coupled to a small variety of phthalonitriles in nucleophilic aromatic substitution reactions. The products were co-macrocyclised to form hybrid porphyrazines via Linstead macrocyclisation reactions to form the desired 1:3 cyclisation products. Similarly, two protected disaccharides were synthesised de novo, coupled with the phthalonitriles and macrocyclised into their porphyrazine derivatives. Several problems were encountered in the latter set of reactions, but various methods were devised to overcome these issues. Demetalation of the porphyrazine products mentioned above was effected with relative ease using glacial acetic acid to afford the free-base pigments, which were subsequently converted into their zinc or nickel complexes. One of the problems encountered with some of the metalated products was poor solubility in spectroscopic solvents used for their characterisation. The UV-visible spectra revealed some features related to those of free-base porphyrazines or phthalocyanines but demonstrated diagnostic unsplit Q-bands confirming the success of each metalation reaction. In the second part of this investigation, some of the porphyrazines were deprotected of their isopropylidene groups of the carbohydrates to increase their hydrophilicity. This was done by using a catalytic amount of p-TsOH acid in the presence of a nucleophilic solvent. This extraction coefficients were determined as measured between 2-octanol and PBS solution. Comparison of the partition coefficients of the carbohydrate-substituted porphyrazines and their deprotected counterparts to those of non-carbohydrate substituted pigments showed that the structural alteration offers a way to significantly increase the hydrophilicity of substituted porphyrazines. Finally, photophysical studies on the porphyrazine products were performed, and these include singlet oxygen generation, triplet lifetimes and -yields, etc. The results of this part of study showed that the products are good candidates for PDT and that the nature of the carbohydrate played an insignificant role in the activity of the pigments. Rather, the position of attachment and the presence/absence of a metal in the core cavity were found to be more important. / Prof. D.B.G. Williams

D.Sc. / Please refer to full text to view abstract