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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Asymmetric allylation of carbonyl compounds : kinetic resolution of sec-allylboronates and total synthesis of natural products

Incerti-Pradillos, Celia A. January 2014 (has links)
Asymmetric allylation of aldehydes with stoichiometric allylmetal reagents has evolved into an efficient and well established methodology for the synthesis of enantiomerically enriched homoallylic alcohols. The use of enantioenriched secondary allylboronates gives rise to a mixture of E/Z homoallylic alcohols with the opposite configuration at the stereogenic centre (reaction 1). The first part of this thesis presents a novel and conceptually different solution to attain high stereoselectivity in the allylation of aldehydes with secondary allylboronates. The method revolves around an efficient kinetic resolution of chiral racemic allylboronates (reaction 2). Catalysis by a chiral Br??nsted acid ensures a face- and Z-selective allylation of aldehydes. This asymmetric allylation has proved successful over a wide range of aldehydes with different electronic and steric properties. The methodology provides a shortcut to enantio- and diastereomerically enriched homoallylic alcohols finding a wide use in pharmaceutical and fine chemicals development. The second part of the present thesis describes the asymmetric total synthesis of two bioactive metabolites of the Pseudopterogorgiane elisabethae family, (???)-elisabethadione and (???)-erogorgiane. Three key reactions steps to introduce the stereogenic centres in the natural product scaffold include asymmetric allylation, oxy-Cope rearrangement and cationic cyclisation.
12

A computational modelling study of the catalytically active perovskite LaCoO₃

Khan, Saira January 2007 (has links)
Atomistic and quantum mechanical computational modelling of the bulk and surface structures of the catalytically active perovskite LaCoC>3 have been undertaken to develop a better understanding of the processes involved during catalytic oxidation. Two catalytically important effects have been studied firstly the creation of oxygen vacancies resulting from Sr2+ doping at the La3+ site or reduction of Co3+ and secondly substitution by Ce4+at the La3+ site. Bulk atomistic calculations were carried out using the GULP code to compare the Mott Littleton and supercell models. Results for the former, which represent the dilute defect case, suggested that, generally, for the various defect reactions studied, interactions between nearest neighbours determines structure stability as anticipated from simple electrostatic considerations. In contrast, the supercell calculations, which represent a higher concentration of defects, showed that interaction between defect clusters is also important. In addition, calculations for Ce doping confirmed that Ce has only limited solubility in bulk LaCoC>3. However, since catalysis is a surface process the above defect calculations were repeated at the surface of LaCoC>3 using the MARVIN code. For the oxygen vacancy creation reactions, there is a general tendency that the smaller the cation defect- vacancy separation, the lower the energy of the cluster. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. For cerium doping, computational modelling has shown that Ce is more soluble at the surface of LaCoC>3 compared to the bulk. This result has been discussed in terms of its impact both on low temperature, suprafacial and high temperature, intrafacial oxidation catalysis. Quantum mechanical supercell simulations were carried out on bulk stoichiometric and oxygen vacancy containing LaCoC>3 using DMol3 to study electronic effects. These bulk calculations showed that the low spin is favoured compared to the high spin. For the defect calculations the high spin ferromagnetic state was the most favoured. These simulations have also shown that, in the presence of oxygen vacancies, electrons are localised on cobalt.
13

Sythesis and characterisation of ferrocenyl monolayers on silicon surfaces

Abdualla, Mufida January 2014 (has links)
A series of ferrocenyl monolayers on hydrogen-terminated Si(111) single crystal and porous silicon have been prepared and characterised by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Hydrosilylation modification of hydrogen-terminated Si(111) single crystal and porous silicon with vinylferrocene and ethynylferrocene was shown to produce three different silicon–carbon tethered ferrocenyl surfaces, Si-CH₂-CH₂-Fc, Si-CHCH-Fc, and Si-CC-Fc. Three different grafting procedures were used, a thermally activated method at 110 C in toluene using vinylferrocene produced the Si-CH₂-CH₂-Fc monolayer, EFc. Hydrosilylation at room temperature using ethynylferrocene in the minimum amount of dichloromethane produced the Si-CHCH-Fc surface, VFc. Finally, the reaction of n-butyllithium with ethynylferrocene via a nucleophilic substitution reaction (acetylide anion) at the silicon hydride electrode produced the Si-CC-Fc surface, EnFc. Transmission FTIR analysis confirmed that three different linkages were formed on porous silicon. Data recorded using cyclic voltammetry showed that ferrocenyl silicon monolayers were more stable in acetonitrile than aqueous electrolyte. The maximum surface coverage measured in acetonitrile was 1.3×10-¹º mol cm-₂ for the EFc surface, 2.7×10-11 mol cm-₂ for the VFc surface, and 5.4×10-¹¹ mol cm-₂ for the EnFc surface. However, reproducibility of the EFc, VFc and EnFc surfaces was not easy to control. In order to find a way to improve reproducibility, a series of 50:50 mixtures of vinylferrocene and simple alkenes were investigated. The alkenes used for this purpose were hexene (EFc/C₆), octene (EFc/C₈), decene (EFc/C₁₀), and undecene (EFc/C₁₁). AFM, XPS and CV were used to analyse these surfaces. Different surface coverages for 50:50 mol:mol in bulk solution of EFc/C₆, EFc/C₈, EFc/C₁₀, and EFc/C₁₁ were obtained from CV in aqueous and acetonitrile electrolytes. Higher coverage was observed in acetonitrile than water for all 50:50 mixed monolayers, and the EFc/C₈ surface was the more reproducible surface with a surface coverage of 4.0×10-¹¹mol cm-². Therefore, a range of more dilute EFc/C₈ surfaces, 20:80, 10:90 and 1:99 were prepared via reaction with a vinylferrocene/octene mixture and characterised using AFM, XPS and CV. The cyclic voltammograms of dilute mixed monolayers EFc/C₈ gave different surface coverages in acetonitrile electrolyte. The average observed were 3.03×10-¹¹ mol cm-², 1.84×10-¹² mol cm-² and 7.95×10-¹³ mol cm-² for 20:80, 10:90 and 1:99 EFc/C₈ surfaces respectively. The coverage of mixed EFc/C₈ monolayers determined via CV was found to depend on the mole fraction of vinylferrocene in the initial alkylation solution, and it is possible to control the separation of ferrocene units on the silicon surface by controlling the mole fraction. In addition, longer alkyl chain linkages between the ferrocene and silicon surface were investigated by two different approaches. In the first approach a ferrocenyl group with an extended tether was first synthesised, 1-(but-1-en-3-yne)ferrocene, and was then attached as a complete unit to the silicon surface using a one-step reaction to give the BuFc surface. In the second approach, the ferrocene monolayer was built up from the silicon surface in a two-step reaction procedure. In the first step, the silicon hydride surface was modified with 5-iodo-3-ethyl-2-ethynylthiophene to give the Tp surface. In the second step, the Tp surface was coupled to ethynylferrocene via the Sonogashira reaction to give the TpFc surface. In XPS a new Fe 2p peak and the concurrent disappearance of the I 3d peak was observed for the TpFc surface, which confirmed the replacement of the iodine by ethynylferrocene. The surface coverage obtained from CV for BuFc, (6.25×10-¹¹ mol cm-²), was higher than for TpFc surfaces, (7.2×10-1¹² mol cm-²). This is could be due to a lower overall reaction yield being associated with each reaction in the two-step process. In order to investigate the utility of ferrocene modified silicon surfaces in applications such as charge storage and memory devices, the EFc surface was studied using electrochemical scanning tunnelling spectroscopy which showed enhanced tunnelling current response that is directly related to the presence of the redox active ferrocene unit. The rate of electron transfer from ferrocenyl-monolayers to n-Si(111) was measured for both pure and mixed monolayers using light step photoelectrochemical chronoamperometry (PC). The rate constant associated with ferrocene oxidation is weakly dependent on applied potential, and is independent of the type of linkage, and the length of the n-alkyl chains in the mixed monolayers. The photoelectrochemical experiment provides a simple means to detect the extent to which the monolayer discharges in the dark and therefore can be used to assess the feasibility of charge storage in these monolayers.
14

Ambiphilic C-H activation routes to heterocycles

Khamker, Qudsia January 2014 (has links)
This thesis describes investigations of Cp*Rh- and (p-Cy)Ru-catalysed C-H functionalisation reactions of various substrates with alkynes and alkenes for the formation of several heterocycles and carbocycles. Mechanistic studies and DFT calculations are also presented. Chapter One includes a discussion of different mechanisms of C-H activation namely oxidative addition, σ-bond metathesis, 1,2-addition, electrophilic activation and AMLA/CMD. The applications of these different mechanisms of C-H activation in catalysis are also discussed with a particular emphasis on the use of AMLA/CMD in direct arylation reactions. Chapter Two gives an overview of stoichiometric and catalytic studies of AMLA C-H activation and subsequent reactivity with alkynes at Ir, Rh, and Ru. The results of Cp*Rh- and some (p-Cy)Ru-catalysed reactions of C-phenylpyrazoles with alkynes are presented. N-H and C-H activation occurs, leading to heterocycles. Mechanistic studies and DFT calculations show that C-H activation is reversible and rate limiting in the cases examined. Chapter Three is similar to Chapter Two but focusses on reactions with alkenes. The Cp*Rh-catalysed reactions of C-phenylpyrazoles with alkenes lead to mono or divinyl products which may undergo further aza-Michael cyclisations if the alkene is a good Michael acceptor. Mechanistic studies and DFT calculations are also discussed. Chapter Four deals with Cp*Rh-catalysed coupling reactions of other directing groups, including imidazole, imidazoline, pyrazolidinone, hydrazine, carboxylic acid and oxime with alkynes. Again, there is discussion on the different factors affecting product selectivity. Chapter Five gives a summary of all the conclusions on the work presented in this thesis. Throughout the thesis, all new compounds are characterised spectroscopically and several compounds have been characterised by X-ray crystallography.
15

Characterizing electroactive polymer films : from fundamentals to fingerprints

Sapstead, Rachel Marie January 2014 (has links)
Visualisation of latent fingerprints present on metallic surfaces has been demonstrated by means of spatially selective deposition of conducting copolymers. This novel technique utilises the inhibition of electrochemical processes on areas that have been masked by the fingerprint. This results in electropolymerisation between the ridges, generating a negative image of the fingerprint. The efficiency of recovery in forensically challenging scenarios of the copolymers was compared with the corresponding homopolymers. An extension of this electrochromic enhancement to include fluorescence has been developed using a novel synthetic pathway aiming to create more free volume to aid the inclusion of bulky fluorophore moieties. The unique selectivity of neutron reflectivity (NR), with isotopic contrast variation, permitted the diagnosis of chemical and structural changes within the depth profile of the polymer during the incorporation of the fluorophore moieties. The properties of conducting polymer films are determined by film composition and structure which, this thesis will show, can lead to different routes to electroneutrality maintenance during electrochemically controlled redox switching (doping/undoping). NR was used to quantify the diverse permeation characteristics of conventional solvents and ionic liquids into an electroactive copolymer. It revealed how the availabilities of these mechanisms are dictated by anion and cation sources and sinks in the film and liquid phases where molecular solvent is/is not present. In multi-layered systems, the nature of the polymer/polymer interface is central to the rectifying (segregated) or capacitive (interdiffused) characteristics of the films, such that the spatial distribution of the different polymer components determines the optical and electronic properties of the films. These properties are important for the potential applications of these multi-component systems in energy storage. NR was used to probe the extent of segregation and solvation properties of multi-layer films and revealed the order the components are deposited has a great effect on the final film properties.
16

The photo-oxidation and stabilization of poly(butadiene)

Beavan, S. W. January 1975 (has links)
No description available.
17

Tropolone and sorbicillactone biosynthesis in fungi

Fahad, Ahmed Al January 2014 (has links)
Stipitatic acid 12 is an aromatic seven-membered ring produced by Talaromyces stipitatus (previously known as Penicillium stipitatus). The identification of this secondary metabolite changed the concept of aromaticity and paved the way for the structural elucidation of many non-benzenoid aromatic natural products called tropolones. Stipitatic acid has undergone one of the most detailed isotopic labelling studies to understand how nature could form such an aromatic seven-membered ring under biological conditions. The incorporation of labelled acetate units into the carbon skeleton of stipitatic acid pointed to a polyketide origin for this fungal tropolonoid metabolite. Further labelling study using radioactive molecules of 3-methylocinaldehyde 29 suggested an oxidative ring expansion rearrangement for the formation of the tropolone seven-membered ring.
18

Catalytic stereoselective synthesis of 2-deoxyglycosides

Balmond, Edward Iain January 2014 (has links)
The work described in this thesis deals with the development of catalytic methods for the stereoselective synthesis of 2-deoxyglycosides A thiourea has been successfully demonstrated as an efficient and mild catalyst for glycosylations of various protected galactals with a series of glycoside acceptors to afford 2- deoxy-a-D-galactosides, in yields of 72-98%. Complete a-selectivity was observed in all cases. The high a-selectivity is independent of the glycoside substitution pattern and reactivity profile. The method is tolerant of most common protecting groups and is orthogonal to thioglycosylation type reactions; this has allowed for a one-pot tandem glycosylation reaction to afford a trisaccharide in 58% yield with complete stereocontrol. Details of mechanistic investigations are described. Subjection of glucal substrates to the thiourea-catalysed conditions did not afford the same success, due to significant reactivity differences. Attempts to find a new, more active organocatalyst were also unsuccessful. The use of a cyclic protecting group on glucal substrates was found, under acid catalysis, to afford 2-deoxY-D-glucosides in high yields (81-89%), with high (20:1) to complete astereoselectivity. The 3,4-0-disiloxane cyclic protecting group was shown to work for a wide range of acceptors, which included monosaccharides, amino acids and a natural product. This protecting group was also shown to be successful for the synthesis of 2,6-dideoxY-Lglucosides in high yields (81-95%), with moderate (3.5:1) to good a-selectivity. Details of mechanistic and computational investigations are described. The activation of 2-nitro-o-galactals towards nucleophilic attack by alcohols via thiourea catalysis was found to be possible. Initial studies show that a thiourea bearing two cinchona alkaloid substituents is an effective catalyst; details of reaction optimisation will be described.
19

Iterative reagent-controlled homologation of boronic esters

Burns, Matthew January 2014 (has links)
This thesis is concerned with developing a protocol for the reagent-controlled iterative homologation of boronic esters. Initial work focused on the synthesis of the highly enantioenriched Grignard reagent (5)-261 generated from sulfoxide (-)-anti-250 using a sulfoxide ligand exchange reaction. Grignard reagent (5)-261 has been successfully applied in the homologation of boronic esters 175 and (R)-176. However, it was found that Grignard reagent (5)-261 did not possess sufficiently high reactivity to homologate the sterically more hindered boronic ester 177. We therefore explored the use of highly enantioenriched Iithio derivatives as reagents for the homologation of boronic esters. Methods to generate Iithiated benzoates through a sulfoxide ligand exc~ange reaction proved fruitless as competing deprotonation occurred instead. A successful protocol was ultimately found in utilising the tin - lithium exchange reaction. A twostep procedure was developed that enabled the synthesis of either enantiomer of stannane 328 in 99.9 : 0.1 e.r.. Subjecting stannane 328 to a tin - lithium exchange generated Iithiated benzoate 272 which proved to be a highly effective reagent for the iterative homologation of boronic esters. Using Iithiated benzoate 272 we were able to rapidly construct molecules with ten contiguous stereocentres through an iterative homologation sequence. By choosing to use either (5)-272 or (R)-272 we have developed a system where any diastereoisomer can be synthesised. To demonstrate the potential of the methodology we have synthesised boronic esters 339,350 and 353. We have shown that by controlling. the configuration of the stereoecentres along a branched carbon chain we are able to control the conformation of the carbon backbone in the solid state. The alternating syn-anti diastereoisomer 350 causes the molecule to adopt a linear conformation in the solid state in order to minimise destabilising syn-pentane interactions. The all syn diastereoisomer, boronic ester 353 was successfully synthesised using our methodology. Additionally, we have shown that the crystalline derivative 355, adopts a helical conformation in the solid state, again to minimise destabilising syn-pentane interactions.
20

The synthesis of (-)-blepharocalyxin D and other oxygen heterocycles

Cons, Benjamin David January 2014 (has links)
This thesis describes the total synthesis of (-)-blepharocalyxin D and related analogues via a novel Prins-type cyclisation that allows the core bicyclic stlUcture to be synthesised in a single step, generating both rings and four new stereocenters. A variety of synthetic routes to y,8-unsaturated alcohols such as 305 were investigated, the key steps in the final route including Wittig olefination, rhodium catalysed 1,4-addition and Chugaev elimination to give 305 which was taken fOlward in the synthesis of 35. In the final chapter, further y,8-unsaturated alcohols were prepared and used in the synthesis of bicyclic lactone 329. Alkene 328 was modified post cyclisation to give a series of novel compounds including protected amine 345. It was found that the reaction of y,8-unsaturated alcohol 106 with aldehydes gave trisubstituted tetrahydropyrans with good stereocontrol. By varying the reaction conditions either an axial or equatorial side-chain was introduced at C-3.

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