295 research outputs found
Direct functionalization of heterocyclic and non-heterocyclic arenes
Lâapplication des mĂ©taux de transition Ă la fonctionnalisation directe a ouvert la voie Ă une nouvelle classe de rĂ©actions pour la formation de liens carbone-carbone. De par l'omniprĂ©sence des liaisons CâH, lâintroduction de nouvelles fonctionnalitĂ©s chimiques par voie directe et prĂ©-activation minimale sâimpose comme une stratĂ©gie de synthĂšse trĂšs attrayante. Ainsi, il est envisageable de prĂ©parer de maniĂšre rapide et efficace des supports complexes menant Ă des molĂ©cules complexes, qui pourraient ĂȘtre utiles dans divers domaines de la chimie.
L'objectif principal de la prĂ©sente thĂšse vise la fonctionnalisation directe des arĂšnes hĂ©tĂ©rocycliques et non hĂ©tĂ©rocycliques et, plus prĂ©cisĂ©ment, les techniques dâarylation. Dans un premier temps, nous allons aborder le thĂšme de lâarylation directe tout en mettant lâaccent sur les pyridines (Chapitre 1). Ces molĂ©cules sont Ă la base d'une multitude de composĂ©s biologiquement actifs et jouent un rĂŽle important dans le domaine des sciences des matĂ©riaux, de lâagrochimie et de la synthĂšse des produits naturels. Dans un deuxiĂšme temps, nous discuterons de nos travaux sur lâarylation directe catalysĂ© par un complex de palladium sur des ylures de N-iminopyridinium en soulignant la dĂ©rivatisation du sel de pyridinium aprĂšs une phĂ©nylation sp2 (Chapitre 2). LâĂ©tude de ce procĂ©dĂ© nous a permis de mettre en lumiĂšre plusieurs dĂ©couvertes importantes, que nous expliquerons en dĂ©tails une Ă une : lâarylation benzylique directe lorsque des ylures N-iminopyridinium substituĂ©es avec un groupement alkyl Ă la position 2 sont utilisĂ©s comme partenaires dans la rĂ©action; les allylations Tsuji-Trost catalysĂ©e par un complex de palladium; et lâalkylation directe et sans mĂ©tal via une catalyse par transfert de phase.
Plusieurs dĂ©fis restent Ă relever pour le dĂ©veloppement de procĂ©dĂ©s directs utilisant des mĂ©taux de transition peu coĂ»teux, dâautant plus que la synthĂšse par transformation directe des pyridines 2-alcĂ©nyles, lesquelles sont pertinentes sur le plan pharmacologique, nâa pas encore Ă©tĂ© rapportĂ©e Ă ce jour. Avec cette problĂ©matique en tĂȘte, nous avons rĂ©ussi Ă mettre au point une alcĂ©nylation directe catalysĂ© par un complex de cuivre sur des ylures de N-iminopyridinium. Nous discuterons Ă©galement dâune nouvelle mĂ©thode pour la prĂ©paration des iodures de vinyle utilisĂ©s dans les couplages. Ces rĂ©actions sont non seulement remarquablement chimiosĂ©lectives, mais sont aussi applicables Ă plusieurs substrats (Chapitre 3). En optimisant ce procĂ©dĂ© direct, nous avons dĂ©couvert une façon unique de synthĂ©tiser les pyrazolo[1,5-a]pyridines 2-substituĂ©es (Chapitre 4). Le mĂ©canisme global met en jeu une sĂ©quence tandem de fonctionnalisation-cyclisation directe et un procĂ©dĂ© direct en cascade, qui nâavais jamais Ă©tĂ© rapportĂ©. Cela simplifie ansi la synthĂšse autrement compliquĂ©e de ces substrats en y apportant une solution Ă un problĂšme de longue date.
Dans les deux derniers chapitres, nous examinerons en dĂ©tail les techniques dâarylation directe qui n'impliquent pas les partenaires de couplage hĂ©tĂ©rocycliques. Entre autres, au Chapitre 5, nous soulignerons notre dĂ©couverte dâun umpolung dirigĂ© et catalysĂ© par un complexe de palladium du benzĂšne et de quelques autres dĂ©rivĂ©s arĂšnes. Il sâagit lĂ du premier cas de fonctionnalisation directe dans laquelle le groupe directeur se trouve sur le partenaire halogĂšne et il sâajoute Ă la courte liste dâexemples connus dans la littĂ©rature rapportant une arylation directe du benzĂšne. Finalement, au Chapitre 6, nous passerons en revue une nouvelle arylation directe catalysĂ©e au fer, qui se veut un procĂ©dĂ© peu coĂ»teux, durable et prĂ©sentant une Ă©conomie dâatomes. Nous discutons des substrats possibles ainsi des Ă©tudes mĂ©canistiques rĂ©alisĂ©s.The application of transition metals towards direct functionalization processes has exposed an opportunistic new class of carbon-carbon bond forming reactions. Given the undeniable ubiquity of CâH bonds, the possibility of introducing functionality through direct means with minimal preactivation is an irresistible strategy in synthesis. As such one can envision rapidly and efficiently building up complex scaffolds towards complex molecules of interest in a plethora of chemical fields.
The focus of this thesis is on the direct functionalization of heterocyclic and non-heterocyclic arenes, focusing on arylation technologies. First, the topic of direct arylation will be introduced, with special emphasis being on pyridines (Chapter 1). These molecules comprise the backbone of a myriad of biologically active compounds, and are also relevant in material sciences, agrochemicals, and natural products synthesis. This will be followed by a discussion of work on the palladium-catalyzed direct arylation of N-iminopyridinium ylides with accent on the derivatization of the pyridinium following the sp2 phenylation (Chapter 2). The exploration of this process led to the discovery of direct benzylic arylation when 2-alkyl NÂŹ-iminopyridinium ylides are employed as reacting partners, in addition to palladium-catalyzed Tsuji-Trost allylations, and metal-free direct alkylation via phase transfer catalysis. All of these findings will be discussed in detail.
There remains a significant challenge in developing direct processes utilizing inexpensive transition metals. Furthermore, the synthesis of pharmacologically relevant 2-alkenyl pyridines through direct transformations had not yet been reported. We focused on these challenges and developed a copper-catalyzed direct alkenylation of N-iminopyridinium ylides. A novel method to prepare the vinyl iodide coupling partners will also be discussed. The scopes of these reactions are quite large and remarkably chemoselective (Chapter 3). Through the optimization of this direct process we uncovered an unique means of synthesizing 2-substituted pyrazolo[1,5-a]pyridines (Chapter 4). The global process involved a tandem direct functionalization/cyclization sequence, and may be the first account of a direct process used in a cascade. This work also solves an important problem, as the synthesis of these substrates through alternate means is not straightforward.
The last two chapters will detail direct arylation technologies that do not involve heterocyclic coupling partners. Chapter 5 will highlight our uncovering of a palladium-catalyzed, directed, umpolung arylation of benzene and other arene derivatives. This was the first account of a direct functionalization whereby the directing group is situated on the pseudo electrophile. Also, it adds to the few examples of direct benzene arylation exisiting in the literature. Finally, a discussion of an atom economical, inexpensive, sustainable iron-catalyzed direct arylation process will be presented with special emphasis on substrate scope and mechanistic investigations (Chapter 6)
A Dioxane Template for Highly Selective Epoxy Alcohol Cyclizations
Ladder polyether natural products are a class of natural products denoted by their high functional-group density and large number of well-defined stereocenters. They comprise the toxic component of harmful algal blooms (HABs), having significant negative economic and environmental ramifications. However, their mode of action, namely blocking various cellular ion channels, also denotes their promise as potential anticancer agents. Understanding their potential mode of biosynthesis will not only help with developing ways to limit the damage of HABs, but would also facilitate the synthesis of a range of analogs with interesting biological activity. 1,3-Dioxan-5-ol substrates display remarkable âenhanced template effectsâ in water-promoted epoxide cyclization processes en route to the synthesis of these ladder polyether natural products. In many cases, they provide near complete endo-to-exo selectivity in the cyclization of epoxy alcohols, thereby strongly favoring the formation of tetrahydropyran (THP) over tetrahydrofuran (THF) rings. The effects of various BrĂžnsted and Lewis acidic and basic conditions are explored to demonstrate the superior selectivity of the template over the previously reported THP-based epoxy alcohols. In addition, the consideration of other synthetic routes are also considered with the goal of gaining rapid access to a plethora of potential starting materials applicable towards the synthesis of ladder polyethers. Finally, cascade sequences with polyepoxides are investigated, further demonstrating the versatility of this new reaction template.National Institute of General Medical Sciences (U.S.) (Grant GM72566)Natural Sciences and Engineering Research Council of CanadaFonds queÌbeÌcois de la recherche sur la nature et les technologie
α-Amino bicycloalkylation through organophotoredox catalysis â
Bridged bicycloalkanes such as bicyclo[1.1.1]pentanes (BCPs) and bicyclo[3.1.1]heptanes (BCHeps) are important motifs in contemporary drug design due to their potential to act as bioisosteres of disubstituted benzene rings, often resulting in compounds with improved physicochemical and pharmacokinetic properties. Access to such motifs with proximal nitrogen atoms (i.e. α-amino/amido bicycloalkanes) is highly desirable for drug discovery applications, but their synthesis is challenging. Here we report an approach to α-amino BCPs and BCHeps through the visible-light enabled addition of α-amino radicals to the interbridgehead CâC bonds of [1.1.1] and [3.1.1]propellane respectively. The reaction proceeds under exceptionally mild conditions and displays broad substrate scope, providing access to an array of medicinally-relevant BCP and BCHep products. Experimental and computational mechanistic studies provide evidence for a radical chain pathway which depends critically on the stability of the α-amino radical, as well as effective catalyst turnover
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The Pandora multi-algorithm approach to automated pattern recognition of cosmic-ray muon and neutrino events in the MicroBooNE detector.
The development and operation of liquid-argon time-projection chambers for neutrino physics has created a need for new approaches to pattern recognition in order to fully exploit the imaging capabilities offered by this technology. Whereas the human brain can excel at identifying features in the recorded events, it is a significant challenge to develop an automated, algorithmic solution. The Pandora Software Development Kit provides functionality to aid the design and implementation of pattern-recognition algorithms. It promotes the use of a multi-algorithm approach to pattern recognition, in which individual algorithms each address a specific task in a particular topology. Many tens of algorithms then carefully build up a picture of the event and, together, provide a robust automated pattern-recognition solution. This paper describes details of the chain of over one hundred Pandora algorithms and tools used to reconstruct cosmic-ray muon and neutrino events in the MicroBooNE detector. Metrics that assess the current pattern-recognition performance are presented for simulated MicroBooNE events, using a selection of final-state event topologies
Convolutional Neural Networks Applied to Neutrino Events in a Liquid Argon Time Projection Chamber
We present several studies of convolutional neural networks applied to data
coming from the MicroBooNE detector, a liquid argon time projection chamber
(LArTPC). The algorithms studied include the classification of single particle
images, the localization of single particle and neutrino interactions in an
image, and the detection of a simulated neutrino event overlaid with cosmic ray
backgrounds taken from real detector data. These studies demonstrate the
potential of convolutional neural networks for particle identification or event
detection on simulated neutrino interactions. We also address technical issues
that arise when applying this technique to data from a large LArTPC at or near
ground level
The Pandora multi-algorithm approach to automated pattern recognition of cosmic-ray muon and neutrino events in the MicroBooNE detector
The development and operation of Liquid-Argon Time-Projection Chambers for
neutrino physics has created a need for new approaches to pattern recognition
in order to fully exploit the imaging capabilities offered by this technology.
Whereas the human brain can excel at identifying features in the recorded
events, it is a significant challenge to develop an automated, algorithmic
solution. The Pandora Software Development Kit provides functionality to aid
the design and implementation of pattern-recognition algorithms. It promotes
the use of a multi-algorithm approach to pattern recognition, in which
individual algorithms each address a specific task in a particular topology.
Many tens of algorithms then carefully build up a picture of the event and,
together, provide a robust automated pattern-recognition solution. This paper
describes details of the chain of over one hundred Pandora algorithms and tools
used to reconstruct cosmic-ray muon and neutrino events in the MicroBooNE
detector. Metrics that assess the current pattern-recognition performance are
presented for simulated MicroBooNE events, using a selection of final-state
event topologies.Comment: Preprint to be submitted to The European Physical Journal
Determination of muon momentum in the MicroBooNE LArTPC using an improved model of multiple Coulomb scattering
We discuss a technique for measuring a charged particle's momentum by means
of multiple Coulomb scattering (MCS) in the MicroBooNE liquid argon time
projection chamber (LArTPC). This method does not require the full particle
ionization track to be contained inside of the detector volume as other track
momentum reconstruction methods do (range-based momentum reconstruction and
calorimetric momentum reconstruction). We motivate use of this technique,
describe a tuning of the underlying phenomenological formula, quantify its
performance on fully contained beam-neutrino-induced muon tracks both in
simulation and in data, and quantify its performance on exiting muon tracks in
simulation. Using simulation, we have shown that the standard Highland formula
should be re-tuned specifically for scattering in liquid argon, which
significantly improves the bias and resolution of the momentum measurement.
With the tuned formula, we find agreement between data and simulation for
contained tracks, with a small bias in the momentum reconstruction and with
resolutions that vary as a function of track length, improving from about 10%
for the shortest (one meter long) tracks to 5% for longer (several meter)
tracks. For simulated exiting muons with at least one meter of track contained,
we find a similarly small bias, and a resolution which is less than 15% for
muons with momentum below 2 GeV/c. Above 2 GeV/c, results are given as a first
estimate of the MCS momentum measurement capabilities of MicroBooNE for high
momentum exiting tracks
Design and construction of the MicroBooNE Cosmic Ray Tagger system
The MicroBooNE detector utilizes a liquid argon time projection chamber
(LArTPC) with an 85 t active mass to study neutrino interactions along the
Booster Neutrino Beam (BNB) at Fermilab. With a deployment location near ground
level, the detector records many cosmic muon tracks in each beam-related
detector trigger that can be misidentified as signals of interest. To reduce
these cosmogenic backgrounds, we have designed and constructed a TPC-external
Cosmic Ray Tagger (CRT). This sub-system was developed by the Laboratory for
High Energy Physics (LHEP), Albert Einstein center for fundamental physics,
University of Bern. The system utilizes plastic scintillation modules to
provide precise time and position information for TPC-traversing particles.
Successful matching of TPC tracks and CRT data will allow us to reduce
cosmogenic background and better characterize the light collection system and
LArTPC data using cosmic muons. In this paper we describe the design and
installation of the MicroBooNE CRT system and provide an overview of a series
of tests done to verify the proper operation of the system and its components
during installation, commissioning, and physics data-taking
Noise Characterization and Filtering in the MicroBooNE Liquid Argon TPC
The low-noise operation of readout electronics in a liquid argon time
projection chamber (LArTPC) is critical to properly extract the distribution of
ionization charge deposited on the wire planes of the TPC, especially for the
induction planes. This paper describes the characteristics and mitigation of
the observed noise in the MicroBooNE detector. The MicroBooNE's single-phase
LArTPC comprises two induction planes and one collection sense wire plane with
a total of 8256 wires. Current induced on each TPC wire is amplified and shaped
by custom low-power, low-noise ASICs immersed in the liquid argon. The
digitization of the signal waveform occurs outside the cryostat. Using data
from the first year of MicroBooNE operations, several excess noise sources in
the TPC were identified and mitigated. The residual equivalent noise charge
(ENC) after noise filtering varies with wire length and is found to be below
400 electrons for the longest wires (4.7 m). The response is consistent with
the cold electronics design expectations and is found to be stable with time
and uniform over the functioning channels. This noise level is significantly
lower than previous experiments utilizing warm front-end electronics.Comment: 36 pages, 20 figure
Ionization Electron Signal Processing in Single Phase LArTPCs II. Data/Simulation Comparison and Performance in MicroBooNE
The single-phase liquid argon time projection chamber (LArTPC) provides a
large amount of detailed information in the form of fine-grained drifted
ionization charge from particle traces. To fully utilize this information, the
deposited charge must be accurately extracted from the raw digitized waveforms
via a robust signal processing chain. Enabled by the ultra-low noise levels
associated with cryogenic electronics in the MicroBooNE detector, the precise
extraction of ionization charge from the induction wire planes in a
single-phase LArTPC is qualitatively demonstrated on MicroBooNE data with event
display images, and quantitatively demonstrated via waveform-level and
track-level metrics. Improved performance of induction plane calorimetry is
demonstrated through the agreement of extracted ionization charge measurements
across different wire planes for various event topologies. In addition to the
comprehensive waveform-level comparison of data and simulation, a calibration
of the cryogenic electronics response is presented and solutions to various
MicroBooNE-specific TPC issues are discussed. This work presents an important
improvement in LArTPC signal processing, the foundation of reconstruction and
therefore physics analyses in MicroBooNE.Comment: 54 pages, 36 figures; the first part of this work can be found at
arXiv:1802.0870
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