203 research outputs found
Exploring iminium ion catalysis
This thesis is composed of two central themes of research Chapters 2-6 describe efforts to understand and increase the activity of iminium ion catalysts. Chapters 7-9 are free-standing investigations exploring concepts and observations that were encountered through the course of the research. Chapter 1 briefly introduces iminium ion catalysis before discussing the experimental and theoretical techniques that are routinely applied to investigate reaction mechanism. The discussion of techniques is divided into three sections structural, kinetic and theoretical methods. This is followed by a passage that highlights the reported techniques that have been applied to understand mechanisms of iminium ion catalysed processes. Chapter 2 highlights the work previously conducted within the group developing catalysts for the iminium ion catalysed Diels-Alder reaction and describes a SAR study designed to understand the relationship between the cc-effect and P-EWG components of catalysts to aid future catalyst design. The study found that the components work independently. Chapter 3 describes a further SAR study conducted to provide evidence for the role of the P-EWG in increasing catalyst activity. The important conclusions drawn were that P-EWG was not acting as a proton shuttle as previously hypothesised and that EWG's that do not contain a carbonyl group could be exploited to increase the activity of a catalyst. Chapter 4 describes investigations into mechanistic aspects of the catalytic cycle for the iminium ion catalysed Diels-Alder reaction. The isolation of key iminium ion intermediate allowed for structural studies and kinetic investigations of the individual steps of the catalytic cycle. The Diels-Alder cycloaddition was found to be the RDS and the physical reasons for this were understood. The hypothesis was formed that a lowering in the LUMO energy of the dienophile by including a strong p-EWG into the catalyst would accelerate the overall catalytic cycle. Chapter 5 describes the application of our findings to the design and synthesis of more active catalysts based around the scaffold of MacMillans imidazolidinone catalyst. The inclusion of an additional p-EWG within the catalyst scaffold provided unprecedented levels of activity supporting our hypothesis. The development and evaluation of a predictive theoretical tool for catalytic activity is also discussed. Chapter 6 shows the preliminary development of piperazinones as catalyst for the iminium ion catalysed Diels-Alder reaction of aldehydes and ketones. Chapter 1 describes our efforts to develop a chiral dynamic resolution procedure for the iminium ion catalysed Michael addition reaction of nitroalkanes to cc,p-unsaturated ketones. Chapter 8 reports the development of a one-pot monocarboxymethylation procedure for primary amines and diamines using glyoxylic acid under mild conditions. Chapter 9 describes the first aminocatalytic method for the preparation of non-natural and natural bis-indolyl alkanes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Solution phase, solid state, and theoretical investigations on the MacMillan imidazolidinone
A combination of soln. phase NMR, X-ray crystallog. studies, and DFT calcns. provide a consistent structural conformation for iminium ions derived from the MacMillan imidazolidinone
The alpha-effect in cyclic secondary amines: new scaffolds for iminium ion accelerated transformations
Five-membered secondary amine heterocycles containing an α-heteroatom were prepared and shown to be ineffective as catalysts for the iminium ion catalysed DielsâAlder reaction between cinnamaldehyde and cyclopentadiene. Their six-membered counterparts proved to be highly active catalysts. In stark contrast, the catalytic activity observed when comparing the non α-heteroatom cyclic amines proline methyl ester and methyl pipecolinate showed the five-membered ring amine was significantly more active. Concurrent density functional theoretical calculations suggest a rationale for the observed trends in reactivity, highlighting that LUMO activation through an iminium ion intermediate plays a key role in catalytic activity
Improving catalyst activity in secondary amine catalysed transformations
The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increases the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed
Design and operation of the Chicago air shower array
The Chicago Air Shower Array (CASA) is a largeâarea surface scintillator array designed to study PeV sources of cosmic rays. The complete detector will consist of 1089 detector stations, distributed on a square 15 m grid. We have operated an array of 49 stations for much of the 1989 calendar year, an array of 529 stations for much of 1990, and the balance of the 1089 stations will be operating in early 1991. This surface array, together with the University of Michigan underground muon detectors (MIA), and the University of Utah atmospheric Äerenkov telescopes and Flyâs Eye air fluorescence detector, constitute a uniquely powerful instrument, dubbed the Utah Michigan Chicago (UMC) experiment, for the study of PeV sources. We report here the performance and current status of these detectors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87431/2/111_1.pd
Preliminary results from the Chicago air shower array and the Michigan muon array
The Chicago Air Shower Array (CASA) is a large area surface array designed to detect extensive air showers (EAS) produced by primaries with energy âŒ100 TeV. It operates in coincidence with the underground Michigan Muon Array (MIA). Preliminary results are presented from a search for steady emission and daily emission from three astrophysical sources: Cygnus Xâ3, Hercules Xâ1, and the Crab nebula and pulsar. There is no evidence for a significant signal from any of these sources in the 1989 data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87432/2/122_1.pd
CASAâMIA: A ââprecisionââ EAS detector
The CASAâMIA detector was constructed to search for sources of UHE neutral radiation. As such it has established limits well below those of previously reported observations and of most contemporaneous detectors. In addition to its primary mission, however, CASAâMIA measures the lateral distribution of both electrons and muons in EAS throughout a range of energies and with a degree of sampling not previously available.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87669/2/207_1.pd
A search for diffuse gamma rays with energies above 1014 eV from molecular clouds in the galaxy
Diffuse gammaârays from molecular clouds are excellent tracers of cosmic rays in the galaxy over a wide range of energies. For example, diffuse emission detected by EGRET already places significant constraints on the spectrum and origin of galactic cosmic rays at GeV energies. Likewise, by measuring diffuse gamma rays with groundâbased air shower experiments, we can probe the galactic distribution of cosmic rays in the energy regime above 100 TeV.The Chicago Air Shower Array (CASA) which operates in coincidence with the Michigan muon array (MIA) is the worldâs most sensitive experiment to gammaârays with energies âł100 TeV, and is wellâsuited for studies of diffuse sources based upon the muon content of air showers. We describe a search for diffuse gammaâarray emission from molecular cloud regions observed by CASAâMIA. If we assume that the flux of cosmic rays is uniform in the galaxy, then we predict that diffuse emission will probably be detectable by CASAâMIA within the lifetime of the experiment. Furthermore, if there are sources of cosmic rays in close proximity to certain molecular clouds, then the spectrum of gammaârays from these clouds will be stronger and harder. By searching for such enhancements in the diffuse emission, and by correlating the CASAâMIA results with emission detected at lower energies by EGRET, we may identify or constrain the nature of cosmic rays sources in both energy regimes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87646/2/499_1.pd
Giant Superfluorescent Bursts from a Semiconductor Magnetoplasma
Currently, considerable resurgent interest exists in the concept of
superradiance (SR), i.e., accelerated relaxation of excited dipoles due to
cooperative spontaneous emission, first proposed by Dicke in 1954. Recent
authors have discussed SR in diverse contexts, including cavity quantum
electrodynamics, quantum phase transitions, and plasmonics. At the heart of
these various experiments lies the coherent coupling of constituent particles
to each other via their radiation field that cooperatively governs the dynamics
of the whole system. In the most exciting form of SR, called superfluorescence
(SF), macroscopic coherence spontaneously builds up out of an initially
incoherent ensemble of excited dipoles and then decays abruptly. Here, we
demonstrate the emergence of this photon-mediated, cooperative, many-body state
in a very unlikely system: an ultradense electron-hole plasma in a
semiconductor. We observe intense, delayed pulses, or bursts, of coherent
radiation from highly photo-excited semiconductor quantum wells with a
concomitant sudden decrease in population from total inversion to zero. Unlike
previously reported SF in atomic and molecular systems that occur on nanosecond
time scales, these intense SF bursts have picosecond pulse-widths and are
delayed in time by tens of picoseconds with respect to the excitation pulse.
They appear only at sufficiently high excitation powers and magnetic fields and
sufficiently low temperatures - where various interactions causing decoherence
are suppressed. We present theoretical simulations based on the relaxation and
recombination dynamics of ultrahigh-density electron-hole pairs in a quantizing
magnetic field, which successfully capture the salient features of the
experimental observations.Comment: 21 pages, 4 figure
A search for astrophysical point sources of 100 TeV gamma rays by the UMC collaboration
The CASAâMIA experiment is a very large extensive air shower detector with good angular resolution. This instrument has been used to search the entire visible sky for astrophysical point sources of 100 TeV gamma rays. Approximately 90% of the isotropic charged cosmic ray background is rejected by measuring the muon content of the showers. Stringent limits are placed on the possible flux of 100 TeV sources across a large part of the Northern sky.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87711/2/1203_1.pd
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