10 research outputs found
New systems for catalytic asymmetric epoxidation
This thesis describes the catalytic asymmetric epoxidation of olefins mediated by
oxaziridinium salts. The introduction highlights some of the most successful methods
for preparing chiral oxiranes and hints at the synthetic utility of this versatile
molecule.
The second chapter is dedicated to our efforts to synthesize chiral iminium salts as
catalysts for asymmetric epoxidation. The first part of this chapter describes previous
Page group findings and leads on to the current author's efforts in this area. Initial
results from modified amino acids, cyclo-condensed to form dihydroisoquinolinium
salts, showed that an aromatic unit at C4 of our catalysts was vital for asymmetric
induction. Following this, several catalysts substituted at the 4-position of the
aromatic functionality were tested, and found to be effective mediators for asymmetric
epoxidation. Triphenylethylene was epoxidized with up to 71% ee when using oxone
as the stochieometric oxidant. [Continues.
Continuous flow ozonolysis using atmospheric plasma
Ozonolysis is widely used in organic synthesis to obtain aldehydes and ketones from
alkenes, a process of great interest, for example, for the pharmaceutical industry. This
reaction is more environmentally accepted than other alternative oxidations and it has
good atom efficiency. Ozonolysis, however, has an important drawback; the ozonides
generated as intermediates in the process are unstable and pose a risk of explosion. To
minimize this risk, continuous flow processing can be used, as this eliminates the
accumulation of large amounts of hazardous intermediates, thereby offering an
alternative to batch processing that greatly enhances the control and safety of the
ozonolysis process.1,2 Here we report on the results obtained with an air plasma-driven
continuous-flow ozonolysis system. (... continues
Observations on the Modified Wenker Synthesis of Aziridines and the Development of a Biphasic System
A cheap and reliable process for the modified Wenker
cyclization
to afford aziridines has been achieved using biphasic conditions for
a range of amino alcohol starting materials. A 100 mmol āone-potā
process has also been devised, and the enantiopurity of the starting
amino alcohol is retained in the aziridine product
āCopper-in-Charcoalā Revisited: Delineating the Nature of the Copper Species and Its Role in Catalysis
āCopper-in-charcoalā
has been shown to be a versatile
catalytic source of supported copper for a variety of important synthetic
transformations, as well as in other fields such as energy. We herein
report the characterization of this material and the implications
that its preparation has on catalysis, thus providing a greater understanding
of the scope and limitations of this catalyst system
Controlling the Assembly of C2-Symmetric Molecular Tectons Using a Thiocarbamate Appended Carbocyclic Cleft Molecule Analogous to TroĢgerās Base
By way of appending the C<sub>2</sub>-symmetric carbocyclic cleft
diol with thiocarbamates with varying substituents, significant control
of the hydrogen bonded network can be achieved. Smaller alkyl substituents
lead to the formation of stacked columns of components with the apex
of one molecule suitably aligned in the cleft of a second. Aryl substituents,
however, lead to the formation of ribbons via an H-bonding network.
Additionally, the packing of these ribbons into networks is considerably
different between the enantiopure and racemic clefts, with the latter
giving rise to channels within the crystal structure
Phosphino-Triazole Ligands for Palladium-Catalysed Cross-Coupling
<div>Twelve 1,5-disubtituted and fourteen 5-substituted 1,2,3-triazole derivatives bearing diaryl or dialkyl phosphines at the 5-position were synthesised and used as ligands for palladium-catalysed Suzuki-Miyaura cross-coupling reactions. Bulky substrates were tested, and lead-like product formation was demonstrated. The online tool SambVca 2.0 was used to assess steric parameters of ligands and preliminary buried volume determination using XRD obtained data in a small number of cases proved to be informative. Two modelling approaches were compared for the determination of</div><div>the buried volume of ligands where XRD data was not available. An approach with imposed steric restrictions was found to be superior in leading to buried volume determinations that closely correlate with observed reaction conversions. The online tool LLAMA was used to determine lead-likeness of potential Suzuki-Miyaura cross-coupling products, from which ten of the most lead-like were successfully synthesised. Thus, confirming these readily accessible triazole-containing phosphines as highly suitable ligands for reaction screening and optimisation in drug discovery campaigns.</div
Simultaneous Clicknetic Resolution Coetaneous catalytic kinetic resolution of alkynes and azides through asymmetric triazole formation
A non-enzymatic simultaneous kinetic resolution of racemic alkynes and racemic azides utilising an asymmetric CuAAC reaction is reported. The use of a CuCl (R,R)Ph-Pybox catalyst system effects a simultaneous kinetic resolution of two racemic starting materials to give one major triazolic diasteroisomer in the ratio 74:12:4:10 (dr 84:16, 90% ee maj). The corresponding control reaction using an achiral copper catalyst gives the four possible diastereoisomers in a 23:27:23:27 ratio, demonstrating minimal inherent substrate control.<br
Kinetic Resolution in Asymmetric Epoxidation using Iminium Salt Catalysis
The
first reported examples of kinetic resolution in epoxidation
reactions using iminium salt catalysis are described, providing up
to 99% ee in the epoxidation of racemic <i>cis</i>-chromenes
Asymmetric Epoxidation Using Iminium Salt Organocatalysts Featuring Dynamically Controlled Atropoisomerism
Introduction of a pseudoaxial substituent at a stereogenic
center adjacent to the nitrogen atom in binaphthyl- and biphenyl-derived
azepinium salt organocatalysts affords improved enantioselectivities
and yields in the epoxidation of unfunctionalized alkenes. In the
biphenyl-derived catalysts, the atropoisomerism at the biphenyl axis
is controlled by the interaction of this substituent with the chiral
substituent at nitrogen
Asymmetric Epoxidation Using Iminium Salt Organocatalysts Featuring Dynamically Controlled Atropoisomerism
Introduction of a pseudoaxial substituent at a stereogenic
center adjacent to the nitrogen atom in binaphthyl- and biphenyl-derived
azepinium salt organocatalysts affords improved enantioselectivities
and yields in the epoxidation of unfunctionalized alkenes. In the
biphenyl-derived catalysts, the atropoisomerism at the biphenyl axis
is controlled by the interaction of this substituent with the chiral
substituent at nitrogen