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No description available.

This thesis describes advances in the chemistry of alkenes with group 6 transition metals, with a focus on improving activation methods, ligands and mechanistic understanding for selective alkene trimerisation catalysis. Chapter 1 describes the current state of the field for this catalytic process, including the most important developments in ligand design, catalyst activation and substrate scope.

Six new R3 TACCrCl3 complexes, displaying branching on the second or third carbon of the ligand R group, were prepared for catalytic studies for ethylene oligomerisation and polymerisation. Five of the complexes made were activated with MAO and their reactivity towards ethylene, at ambient pressure and in a pressurised autoclave tested under various conditions. A number of the complexes were found to give good activities and selectivity for ethylene oligomerisation under optimum conditions. Chapter 2 discusses the design and synthesis of new R3 TACCrCl3 complexes displaying branching on the second or third carbon of the ligand R group. The complexes synthesised were characterised by paramagnetic NMR, magnetic moments, mass spectrometry, X-ray crystallography and elemental analysis. Chapter 3 explores the catalysis of an active selective ethylene oligomerisation R3TACCrCb complex which at optimal conditions shows higher activity for oligomerisation (trimerisation to l-hexene and higher oligomers) than the best system of its type from literature (tested in tandem). Experiments were carried out at various conditions discovering raised pressure and lowered temperature increased the selectivity for the competing polymerisation processes. The selectivity for oligomerisation was found to improve when the solvent was changed from toluene to heptane. Effort was made to identify some of the higher oligomers produced. Chapter 4 investigates the effect of new R3 TACCrCb complexes containing halogenation of the ligand on the ethylene trimerisation reaction, giving evidence that chlorination and bromination of the ligand have a positive effect on the selectivity for ethylene trimerisation. The catalysis of the most active system, bearing chlorination of the ligand, was explored at various conditions. Chapter 5 discusses a new R3TACCrCh system branched at the third carbon of the ligand R group which shows good activity for ethylene polymerisation. The catalytic reaction was explored at different catalyst Joadings and temperatures, finding lower temperature and an optimum catalyst loading gave high activity.

The metal-catalysed allylic oxidation of alkenes has emerged as a powerful method for the functionalisation of Sp3 C-H bonds. This transformation has allowed for the expedient preparation of synthetically useful materials from hydrocarbon building blocks. With the development of air stable and environmentally benign copper(I)-NHC catalysts from readily available materials, it has been shown that these catalysts can participate in the allylic oxidation of alkenes in an effective manner. We have developed a powerful protocol for the functionalisation of alkenes into allylic alcohols and enones, by the use of different terminal oxidants in a divergent fashion. The highly regio- and chemoselective copper(I)-NHC-catalysed allylic oxidation has been examined in the syntheses of functionalised cyclopentenones and cyclohexenones, which has provided mechanistic insights into the oxidation. The system displays excellent tolerance of a plethora of sensitive functional groups and provides a general approach with high efficiency. It has been shown that high regioselectivity is not necessary straightforward and can depend on many factors, including stereoelectronic interactions. The studies towards the enantioselective variant via the desyrnrnetrisation of the proposed prochiral intermediate utilising a range of chiral copper(I)-NHC catalysts was unsuccessful. The synthetic utility of this transformation has been validated by the total synthesis of (±)-untenone A in the shortest and most efficient approach to date. Studies towards the total synthesis of cephalimysin A are currently ongoing, which would employ the late stage copper(I)-NHC-catalysed allylic oxidation on a densely functionalised intermediate.

This thesis describes our investigations into the development of a general method for the catalytic, asymmetric bromination of alkenes. The bromination catalysts employed in the research are ortho-substituted iodobenzenes, which are hypothesised to deliver Br+ to the alkene substrate via a hypervalent I(III)-Br bond. Initially, endeavours to achieve a large scale preparation of our asymmetric bromination catalyst, 2,6-di-[(4R,5R)-4,5-diphenyl-4,5-dihydro-1H-imidazol-2-yl]iodobenzene, or R-IBAM, are detailed. In order to facilitate this, a large quantity of enantiopure 1,2-diphenylethylene diamine was required to form the chiral amidine moieties of our R-IBAM catalyst. Thus, the development of two novel methods for the synthesis and resolution of 1,2-diphenylethylene diamine are described and the subsequent application of each route to a large scale preparation of the enantiopure diamine. The subsequent novel and optimised preparation of our catalyst to produce 25 g of R-IBAM is detailed. The following studies into the catalytic asymmetric bromination of alkenes include the screening of the various reaction conditions, stoichiometric addition of N-bromosuccinimide to the catalysts and the synthesis and screening of a range of R-IBAM derivatives and analogues. An improved understanding of the catalytic cycle and the possible mechanisms of loss of enantioexcess in our brominated product is detailed. The final section of the thesis describes research into the exchange of Br+ between enantiopure bromonium ions and alkenes. The generation of an enantiopure bromonium ion in the absence of alkene was achieved via the rearrangement of enantiopure bromohydrin, (2S)-1-bromo-1-phenylpropan-2-ol. The intermediate bromonium ion was trapped by chloride to produce the enantiopure bromochlorinated product. This, to the best of our knowledge, represents the first example of the generation and trapping of an enantiopure bromonium ion. Our subsequent investigations into Br+ transfer from the bromonium ion to added alkene are described.

No description available.

The cobalt-catalysed oxidative cyclisation of 5-hydroxy alkenes has been demonstrated to be a powerful synthetic tool for the formation of trans-THFs with excellent diastereoselectivity. This thesis describes the utilisation of this methodology in the synthesis of the FG fragment of pectenotoxin-4, allowing the scope of the reaction to be further explored. Introduction: This section introduces the pectenotoxins, a family of structurally complex closed-chain polyether macrolides with promising biological activities. The isolation, structural elucidation, and biological properties of the pectenotoxins are reviewed, along with a summary of previous syntheses towards the FG fragment of pectenotoxin-4. In addition, the cobalt-catalysed oxidative cyclisation of 5-hydroxy alkenes and its application in synthesis is discussed. Results and Discussion: An outline of the synthetic strategy employed in this project and details of the novel retrosynthesis of the C31-C40 fragment of pectenotoxin-4 is described. The synthetic studies carried out towards the synthesis of the FG fragment of pectenotoxin-4 are discussed in detail, with the exploitation of a cobalt-catalysed oxidative cyclisation as the key step to form the trans-THF F-ring. Overall, the FG fragment, which contains six stereogenic centres, was achieved in 18 total synthetic steps (13 longest linear sequence).