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Modifications of Adams' platinum catalyst ; I The hydrogenation of benzoic acidEarle, Ralph Hervey 12 1900 (has links)
No description available.
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A Mechanistic Study in Methanol: Cleavage of RNA Models and Highly Stable Phosphodiesters with Dinuclear Zn(II) ComplexesMelnychuk, Stephanie 15 September 2008 (has links)
Phosphoryl transfer reactions are vital to life. In response to the slow intrinsic
rates of phosphoryl transfer, Nature has evolved a series of enzymes designed to
accelerate these reactions and allow them to occur at biologically relevant rates. These
metallo-enzymes are largely characterized by bi- or tri-nuclear active sites with effective dielectric constants that more closely resemble those of organic solvents than water. This project was designed to better understand the mechanisms by which metalloenzymes cleave phosphodiesters with poor leaving groups. The stability of the phosphodiester is central to the storage of genetic information in DNA and RNA. The cleavage of a series of more reactive RNA models, 2-hydroxylpropyl aryl phosphates 1a-g, catalyzed by a dinuclear Zn(II)2 complex of 53 in methanol was explored. A solution of 53:Zn(II)2:(-OCH3) was observed to accelerate the decomposition of 1a-g with rates that were 10^11-10^12-fold greater than the methoxidepromoted reaction at ss pH
9.47, approaching rate accelerations achieved by natural enzymes. The remarkable activity of 53:Zn(II)2:(-OCH3) and 36:Zn(II)2:(-OCH3) towards the cleavage of 1a-g probed the study of the decomposition of diribonucleotides(3'-> 5')UpU and (3'-> 5')ApC in methanol. The 53:Zn(II)2:(-OCH3)- and 36:Zn(II)2:(-OCH3)-catalyzed decomposition of UpU achieved k2 values of 1.21 ± 0.17 and (7.04 ± 0.99) x 10^-2 M^-1s^-1. The reactivity of ApC in the presence of these systems was unimpressive, however Zn(II) ions in ethanol resulted in the isomerization of
3'-> 5')ApC to (2'-> 5')ApC providing support for the existence of a pentacoordinate
phosphorane intermediate. The pentacoordinate phosphorane was further explored through the reaction of
36:Zn(II)2:(-OCH3) with the cyclic phosphate 58 and 2-hydroxylpropyl methyl phosphate
(59). In the presence of 36:Zn(II)2:(-OCH3) the rate of isomerization of 59/59a (kobs =
(4.7 ± 0.5) x 10^-3 s^-1) exceeded that of expulsion of the methoxy group (kobs = 1.62 x 10^-3 s^-1), thus confirming the existence of a pentacoordinate phosphorane intermediate (60)and providing support for a two-step phosphodiester cleavage reaction. The catalytic efficiency of 36:Zn(II)2:(-OCH3) towards the cleavage of stable phosphodiesters probed its application towards the decomposition of dimethyl phosphate (2) in methanol-d4. The exchange of OCH3 for OCD3 occurred with kcatmax = (2.27 ± 0.03) x 10^-6 s^-1. / Thesis (Master, Chemistry) -- Queen's University, 2008-09-12 13:09:42.427
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Design and evaluation of a new Lewis acid-assisted Lewis acid catalyst system and further applications of a double-allylation reagentSivasubramaniam, Umakanthan Unknown Date
No description available.
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Mechanistic Investigation, Development and Synthetic Applications of a Catalytic Enantioselective and Diastereoselective Allylboration MethodologyRauniyar, Vivek Unknown Date
No description available.
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Reaction of some chelating thioesters with chlorotris (triphenylphosphine) rhodium (I) : a model of the initial stages of hydrodesulfurizationUhm, Hae Won January 1986 (has links)
No description available.
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Supramolecular resorcin [4] arene-capped porphyrins : ligands towards homogeneous catalysisJanuary 2008 (has links)
The synthesis of cavitand-capped porphyrin ligands, with a view towards their potential as
ligands in homogeneous catalysis, is described. The ligand apertures, one of which is outlined
in the figure below, are focal with the aim of synthesising a ligand which can control access to
the active site of the porphyrin via these apertures Synthesis of the target ligand (where R' = CH2 in the figure presented) was attempted via two
pathways. Synthesis commenced by using an in situ protocol, which used successive
functionalisation of the cavitand structure towards the required aldehyde precursor for porphyrin
formation. It was found that subsequent in situ cyclisation and porphyrin formation was
hindered by steric factors, arising directly from the short -CH2O- bridges used to link the
cavitand to the porphyrin. Ligand synthesis was thus unsuccessful.
In a second approach, the porphyrin was synthesised in isolation before being coupled with the
cavitand in a direct capping protocol, which gave more promising results. In the case of R =
C11H23 (in the figure above), preliminary UV-Vis analysis indicated a successful synthesis.
Subsequent analysis of the reaction product by NMR techniques and mass spectrometry could
not conclusively confirm the synthesis of the target ligand. The synthesis could therefore not be
deemed a success; conceivably the short bridge length being the decisive factor once more. Computational chemistry was used to investigate synthetic results, and therefore the viability of
using the -CH2O- bridges to afford limited access to the porphyrin active site. By using
molecular mechanics, -CH2O- bridges were found to be too short, giving an aperture of
insufficient size to enable only the terminus of a linear paraffin to gain access to the inner cavity
of the ligand. Further investigation using molecular dynamics indicated that a ligand bearing
bridges four or five atoms in length would afford an aperture of the desired size to accommodate
the terminus of a paraffin exclusively.
Consequently, synthesis was redesigned towards the preparation of two new ligands, bearing -
O(CH2)2O- (four atom, R' = O(CH2)2 in the figure above) and -O(CH2)3O- (five atom, R' =
O(CH2)3 in the figure above) bridges. Using 2-phenylethyl feet (R = CH2CH2C6H5 in the figure
presented) and adopting the in situ synthetic protocol, both ligands were successfully
synthesised. Characterisation using UV-Vis and NMR spectroscopic techniques, as well as
mass spectrometry confirmed that both ligands had been obtained pure. Additionally, the in situ
cyclisation (in both ligands) was performed via the use of microwave heating, a technique
hitherto unreported.
A viable synthetic route was thus established for the preparation of two new cavitand-capped
porphyrin ligands towards their use in size-selective catalysis.
In addition, a number of crystal structures of synthetic intermediates are described, five of
which are newly reported. These illustrated notable structural features regarding
resorcin[4]arene cavitands and their abilities as host molecules. In particular, the structure of
the aldehyde precursor to capped porphyrin formation following the (initial) in situ synthetic
protocol was significant in illustrating the reason as to why in situ cyclisation was unsuccessful
for the synthesis involving -CH2O- bridges. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2008.
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Activation of n-hexane using vanadium-exchanged zeolites.Naicker, Thirusha. January 2010 (has links)
The influence of the form of the ZSM-5 zeolite, vanadium content and the elimination of the exterior surface acidity on the activity and selectivity of n-hexane oxidation was studied using a fixed bed reactor. Blank reactor studies (carborundum packed reactor) showed no conversion below 450°C with the highest conversion (8%) at 500°C. The dominant products were found to be carbon oxides (Sel./% = 90) with minor selectivities to the hexene isomers (7%) and the remainder being cracked products, THF and benzene. H-ZSM-5 with different SiO2/Al2O3 ratios (100 and 320) and Na-ZSM-5 (SiO2/Al2O3 ratio of 100) were tested under non-oxidative and oxidative conditions. Under oxidative conditions as the ratio of the SiO2/Al2O3 increased, the aluminium content decreased and so too did the cracking ability of the zeolite (i.e. yield of cracked products dropped from 36% to 8%). However, the use of the Na- form of ZSM-5 completely eliminated acid cracking. Under oxidative conditions H-ZSM-5 (100) was found to be more active and resulted in higher formation of cyclic and aromatic compounds. With increasing time on-stream and higher temperatures the catalyst was found to deactivate. Evidence of this was seen by a decrease in surface area and pore volumes of the spent catalyst. The Na-ZSM-5 (100) showed lower activity, but deactivation was shown to be lower. These findings led to the investigation of vanadium ion-exchanged Na-ZSM-5 catalyst for n-hexane activation. Catalysts with different vanadium loadings were prepared using the solid state ion-exchange method. Catalysts were characterised using various methods. These techniques showed that vanadium was successful loaded onto the catalyst and that the highest vanadium loading that could be achieved was 2.5%. The lower loadings were not found to alter the catalyst structure while the highest loading of 2.5% was found to show some pore blockage and to possibly alter the structural environment of the zeolite. Time on stream experiments were conducted and temperature (350, 400 and 450°C), contact time (0.5, 0.8, 1.1 and 1.5 s) and fuel-air ratios (0.7, 1.3 and 2) were varied. The optimum conditions (Conv./% = 39) for terminally activated products were found using the Na-V-ZSM-5 (0.9%) at a temperature of 400°C, a contact time of 1.1 s and fuel-air ratio of 1.3. With the lower fuel-air ratio of 0.7 (oxygen rich conditions), hexanal formation was favoured. The Na-V-ZSM-5 catalyst could be regenerated with initial activity and selectivity being regained. Silanisation was found to be possible, however, the extent and degree of silanisation was difficult to control. Pore blockage was possibly responsible for the lower activity and selectivity obtained using the silanised Na-V-ZSM-5. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2010.
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A study of nickel molybdenum oxide catalysts for the oxidative dehydrogenation of n-hexane.Pillay, Bavani. January 2009 (has links)
Nickel molybdenum oxide catalysts with different chemical compositions have been synthesized
and tested for the oxidative dehydrogenation of n-hexane. The co-precipitation method was used
for the synthesis and several methods were used to characterize these catalysts. These include
inductively-coupled plasma optical emission spectroscopy, Raman spectroscopy, infrared
spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, temperature
programmed reduction, temperature programmed desorption, X-ray photo-electron spectroscopy
and X-ray diffraction spectroscopy techniques as well as the Brauner-Emmet-Teller technique for
surface area determination. The phase composition of the catalysts was largely dependent on the
chemical composition.
Catalyst testing on n-hexane feed was done with a fixed bed continuous flow reactor and
experiments were performed with feed/air ratios above and below the flammability limit. Varied
reaction conditions were used for the catalytic testing. Prior to the catalytic testing, blank
experiments were performed. Analysis of the products were done both online and offline in
conjunction with gas chromatography employing FID and TCD detectors.
The influence of the catalyst on the conversion of n-hexane and selectivity to dehydrogenation
products is reported. Products observed were the carbon oxides (CO and CO2), isomers of hexene
(1-hexene, 2-hexene and 3-hexene), cyclic C6 products (cyclohexene and benzene), cracked
products: alkanes/alkenes (propane/ene, butane/ene) and oxygenates (ethanal, acetic acid and
propanoic acid). B-NiMoO4 was most selective to the hexenes, especially, 1-hexene and a reaction
scheme is proposed. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2009.
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Synthesis and evaluation of novel tetrahydroisoquinoline organocatalysts in asymmetric catalysis.Naicker, Tricia. January 2012 (has links)
Organocatalysis has rapidly expanded in the last decade to encompass a wide variety of small
organic molecules that are capable of either activating substrates or transforming them into
more reactive forms. The aim of this study was to develop novel chiral organocatalysts based
on the tetrahydroisoquinoline backbone and evaluate them on asymmetric reactions. Three
organocatalytic modes of activation have been investigated for C-C bond forming
asymmetric reactions. In chapter 2, for the first time organocatalysts bearing a secondary
nitrogen within a cyclohexane ring were evaluated in the asymmetric Diels–Alder reaction.
These catalysts were tested over a range of dienes and dienophiles and displayed promising
chemical conversions of up to 100 % with up to 64 % ee when triflic acid was employed as
the cocatalyst. Density functional theory computational studies and 2D NMR spectroscopy
were used to determine the structure of the intermediate iminium ion formed between the
most efficient catalyst and cinnamaldehyde. Chapter 3 includes a series of novel
tetrahydroisoquinoline chiral N-oxide organocatalysts and their evaluation in the asymmetric
allylation reaction of aromatic and α-β-unsaturated aldehydes with allyltrichlorosilane. The
chiral homoallyl products were obtained with good chemical efficiency (up to 93 % yield)
and high enantioselectivity (up to 91 % ee) under mild reaction conditions (23 °C). Chapter 4
is the simple and practical microwave-assisted synthesis of new tetrahydroisquinoline
guanidine organocatalysts and their evaluation in the asymmetric Michael addition reaction
of malonates and β-ketoesters with nitro-olefins. In addition, a novel microwave assisted
procedure of introducing the guanidine unit onto amino amide derivatives is reported. The
chiral products were obtained with quantitative chemical efficiency (up to 99 % yield) and
excellent enantioselectivity (up to 97 % ee). Chapter 5 is a collection of all X-ray crystal
structures that were published from novel compounds synthesized pertaining to Chapters 2-4,
it contains 15 published crystal structures while Chapters 3-4 contain 3 other X-ray crystal
structures.
It should be noted that with the exception of the introduction and Chapter 4 (submitted for
publication), the remaining chapters of this thesis have been published in international peer
reviewed journals. In the next section (DECLARATION 2 – PUBLICATIONS) a precise
description of my contribution to each of the publications/chapters is provided. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2012.
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Phase-transfer catalysis in supercritical fluid solventsWheeler, Theresa Christy 05 1900 (has links)
No description available.
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