published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

No description available.

published_or_final_version / Chemistry / Master / Master of Philosophy

Polymer-supported catalysts and reagents have been widely used in organic chemistry because they could facilitate the purification procedures and usually be recycled. Much research has been directed to polymer-supported catalysts and reagents, mainly focusing on these aspects, such as new polymer support, new application in organic chemistry, different modifications and so on. Many polymer-supported phosphine reagents have been developed for Wittig reaction. However, most of them suffer from swelling issue or low loading. A new polyethlyeneimine-supported triphenylphosphine has been synthesized and used as a highly loaded bifunctional homogeneous reagent in a range of one-pot Wittig reactions. All the substrates afforded desired products in high yields after only simple purification procedures. Furthermore, it also served efficiently in reaction cascades involving a one-pot Wittig reaction followed by conjugate reduction of alkene products. In these transformations the phosphine oxide generated in Wittig reaction served as the catalyst for activating trichlorosilane in the subsequent reduction reaction. Triphenylphosphine oxide is always considered as a byproduct of Wittig and Mitsunobu reactions which complicates the purification procedures. One option to utilize it is its application in halogenation reaction with oxalyl halide. Heterogeneous polymer-supported triphenylphosphine oxides based on the rasta resin architecture have been synthesized, and applied as reagent precursors in a wide range of halogenation reactions. The rasta resin-triphenylphosphine oxides reacted with either oxalyl chloride or oxalyl bromide to form the corresponding halophosphonium salts, and these in turn reacted with alcohols, aldehydes, aziridines and epoxides to form halogenated products in high yields after simple purification. The polymer-supported triphenylphosphine oxides formed as a byproduct during these reactions could be recovered and reused numerous times with no appreciable decrease in reactivity. Another option is to use triphenylphosphine oxide as catalyst in organic synthesis. A highly regioselective 1,4-reduction of conjugated polyunsaturated ketones catalyzed by triphenylphosphine oxide is described. In the presence of triphenylphosphine oxide, conjugated di-, tri-, and tetraenones were selectively α,β-reduced using trichlorosilane without over reduction or isomerization, and all the substrates rendered desired products in high yields. Furthermore, 1,4-reduction products were successfully obtained in sequential one-pot Wittig/conjugate reduction reaction, triphenylphosphine oxide generated in Wittig reaction served as the catalyst for reduction reaction. In addition, natural moth pheromones and their analogues were synthesized in high yields using this method. Finally, the synthesis of γ-sanshool and hydroxy-γ-sanshool is depicted. The synthetic route started from simple and commercially available building blocks using an alkyne for E,E-2,4-diene group of the key synthetic intermediate 2E,4E,8Z,10E,12E-tetradecapentaenoic acid, which in turn was converted into both γ-sanshool and hydroxy-γ-sanshool by reaction with the appropriate amines. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / toc / Chemistry / Master / Master of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

The area of chemical research and synthesis increasingly recognises the need for improved technologies and methods, which involves chemical processes with less energy consumption, time savings, reduction and/or minimisation of waste, simple processes and an overall clean production. Microwave heating has been exploited in a variety of disciplines for many useful applications and organic synthesis is an area, which has benefited significantly over the past decade. The present study investigates organic reactions accelerated under microwave irradiations. In particular, the study involves use of recyclable Polymer and Inorganic Solid Supported Reagents for application in transfer hydrogenation. Reductions of electron deficient alkenes have been studied using polymer and inorganic solid supported formates. Microwave irradiations have been used to study transfer hydrogenations in presence of Wilkinson's catalyst [RhCl(PPh3)3]. The application of the Polymer Supported Reagents (PSR) has been investigated for studying transfer hydrogenation in N-benzyl deprotections. Microwave assisted synthesis of formamides from primary and secondary amines have been studied using supported formates. Microwave irradiations have also been applied in studying heterocycle synthesis by cycloaddition of nitrones with Pt (II) and Pd(II) bound organonitriles. The study broadly demonstrates a means of simplifying reaction procedures and purification along with reduction in waste of reagents and release of toxic residues. More importantly, use of microwave irradiations has been used to substantially improve the reaction yields and reduce reaction times, lower energy consumption and solvent volumes. The use of this methodology significantly benefits in the development of "Green Chemistry" and automated systems for chemical synthesis in many industrial sectors.

published_or_final_version / Chemistry / Master / Master of Philosophy

Polymer-supported reagents and catalysts, which allow for simple product separation and easy recycling, have been widely studied in the context of organic synthesis. The past decade has witnessed a number of new variations of polymeric materials, and among the most frequently immobilized functionalities are amines that possess versatile synthetic utilities. Polymers with new structures and improved properties for use in synthesis have been continuously developed since the support may impact the chemical reactions in which they are used in various ways. A new heterogeneous polystyrene-based amine, rasta resin-DMAP, has been synthesized and used in addition reactions of carbon dioxide to epoxides to afford cyclic carbonate products. This new material was found to be a more efficient catalyst than divinylbenzene cross-linked polystyrene supported DMAP, and was readily recycled without significant loss of catalytic activity. Compared to polymers bearing a single functionality, polymers possessing multiple different functional groups attached to a single polymer backbone would have greater potential utility, especially in reactions requiring multiple catalysts or reagents. As an example of this concept, a bifunctional polystyrene bearing both DMAP and piperazine groups has been prepared and applied as an organocatalyst for decarboxylative Doebner-Knoevenagel reactions of arylaldehydes and mono-ethyl malonate to produce (E)-,-unsaturated esters in high yields. Additionally, both non-cross-linked and cross-linked bifunctional polystyrenes featuring amine and thiourea groups have been developed, and their catalytic performance were evaluated in reactions of nitroalkenes with either nitroalkanes or sulfur ylides. Both polymers proved to be efficient catalysts in these reactions and the insoluble polymer demonstrated high recyclability. Control experiments using monofunctional polymers indicated that both catalytic groups of these bifunctional polymers are essential and they could work cooperatively to achieve efficient catalysis. Finally, a second generation bifunctional phosphine-amine polymer, rasta resin-PPh3-NBniPr2, was prepared and examined in tandem Wittig-reductive aldol reactions. In these reaction cascades, the phosphine oxide groups generated from the Wittig reaction served as the catalyst for the reductive aldol reaction, and moderate yields of structurally diverse -hydroxy ketones could be obtained from one-pot processes involving 5 sequential reactions. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2008. / Also available in print.