Inherently chiral calixarenes; methodology and applications

Thesis (PhD)--Stellenbosch University, 2016.ENGLISH ABSTRACT: The use of chiral directing groups has provided an efficient route to meta-functionalised inherently chiral calixarenes. Previously reported ortholithiation methods, incorporating chiral oxazolines have been reexamined, with the aim of revising the individual roles of the three major components of the reaction. The potential mechanistic ramifications of the solvent, additive and alkyllithium structure on the reaction outcomes were individually evaluated. The overarching complexity inherent in this chemistry, coupled with a wide scope of experimental results, point to a number of substrate, solvent and also reagent dependent reaction mechanisms. In addition to the oxazolines, the tert-butyl sulfoxide functional group has also been established as an effective chiral auxiliary for this ortholithiation strategy, yielding enriched diastereomeric mixtures of inherently chiral sulfoxide calixarenes. The absolute stereochemistry of these major and minor products were determined crystallographically. Despite restrictions in electrophile choice, an efficient desulfurization method afforded a route to a new class of meta-functionalised inherently chiral compounds. With the view to evaluating the potential of inherent chirality, oxazoline-directed ortholithiation methods were used to synthesize a new class of inherently chiral phosphine oxazoline ligands. The results from the application studies of these compounds, to both the Tsuji-Trost allylation and asymmetric Suzuki coupling reactions, suggested a definitive relationship between the configurations of the inherent chirality of the calixarene, and central chirality of the oxazoline auxiliary. A model of the -allyl palladium intermediate responsible for the enantioselection seen in the Tsuji-Trost reaction has been proposed for these calixarene ligands. The model was constructed using previously reported calixarene crystallographic data, in conjunction with numerous solid-state and computational studies of simpler Phox ligand systems. The experimental results obtained for this reaction were found to be in good correlation with the proposed model for the inherently chiral calixarene ligands. To confirm these observations, two additional inherently chiral Phox ligands, lacking a stereocenter on the oxazoline, were synthesized. Their preliminary application to the Tsuji-Trost allylation also yielded enantiomerically enriched product mixtures. These results point to a significant contribution made by the inherent chirality, toward the overall asymmetric induction observed in the reaction. These findings serve as a positive affirmation of the viability of inherently chiral calixarenes in the field of asymmetric catalysis, and create a platform for further development of new ligands containing this structural property.AFRIKAANSE OPSOMMING: Die gebruik van chirale regie groepe verskaf ‘n doeltreffende roete tot meta-gefunksionaliseerde inherente chirale calixarene. Ortolitiëring metodes wat chirale oksasoliene inkorporeer, waarop daar voorheen verslag gelewer is, is weer ondersoek. Hierdie ondersoek is onderneem om die individuele rolle van die drie hoofkomponente van die reaksie te hersien. Die potensiële meganistiese effek van die oplosmiddel, ligande, en alkiellitiumstruktuur op die reaksie uitkomstes (not sure about uitkomstes, maybe produkte?), is individueel ondersoek. Die oorkoepelende inherente kompleksiteit van hierdie chemie, tesame met die breë omvang van die eksperimentele resultate, dui daarop dat daar ‘n hele paar substraat-, oplosmiddel-, en reagensafhanklike reaksiemeganismes teenwoordig is. Tersiêre-butielsulfoksied funksionele groepe is ook bevestig as ‘n effektiewe chirale hulpsgroep vir hierdie ortolitiasiestrategie, wat ‘n opbrengs van verrykde diastereomeriese mengsels van inherente chirale sulfoksied calixerene lewer. Die absolute stereochemie van die hoof- en byprodukte is bepaal deur die gebruik van kristallografie. Ten spyte van beperkte keuse van elektrofiele, is ‘n doeltreffende ontswawelingsmetode gebruik om ‘n nuwe klas van meta-gefunksionaliseerde inherent chirale molekule te produseer. Oksasolien-gerigte ortolitiasie metodes is gebruik om ‘n reeks nuwe inherente chirale fosfiene ligande te sintetiseer, en om sodoende die potensiaal van inherente chiraliteit te evalueer. Die resultate van die toepassingstudies van hierdie molekules op beide die Tsuji-Trost allilering en die asimetriese Suzuki koppelingsreaksies, dui op ‘n definitiewe verhouding tussen die konfigurasie van die inherente chiraliteit van die calixareen en die sentrale chiraliteit van die oksasoliene hulpgroep. ‘n Hipotetiese model van die - alliel palladium intermediêr wat verantwoordelik is vir die enantiomeer seleksie wat tydens Tsuji-Trost reaksie waargeneem word, is vir die calixareen ligande voorgestel. Die model is gebou deur die gebruik van voorheen gerapporteerde kristallografiese data, talle vastetoestand studies en berekeningsstudies van eenvoudige fosfienoksasolienligandsisteme. Die eksperimentele resultate wat vir hierdie reaksie verkry is, het goed gekorreleer met die voorgestelde model vir die inherente chirale calixareen ligande. Twee addisionele inherent chirale fosfienoksasolienligande waar die stereosentrum op die oksasolien ontbreek, is gesintetiseer om hierdie waarnemings te bevestig. Hul voorlopige toepassing op die Tsuji-Trost allilasie het ook enantiomeriese verrykde produkmengsels opgelewer. Hierdie resultate dui op ‘n betekenisvolle bydrae deur die inherente chiraliteit tot die algehele asimetriese induksie wat in die reaksie waargeneem word. Hierdie bevindinge dien as ‘n positiewe bevestiging van die moontlike sukses van inherente chirale calixarene in die gebied van asimetriese katalise, en skep ‘n fondasie waarop verdere ontwikkeling gedoen kan word van nuwe ligande wat hierdie strukturele eienskap besit

Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence : Implications for Remote Sensing of Photosynthesis

Novel satellite measurements of solar-induced chlorophyll fluorescence (SIF) can improve our understanding of global photosynthesis; however, little is known about how to interpret the controls on its spectral variability. To address this, we disentangle simultaneous drivers of fluorescence spectra by coupling active and passive fluorescence measurements with photosynthesis. We show empirical and mechanistic evidence for where, why, and to what extent leaf fluorescence spectra change. Three distinct components explain more than 95% of the variance in leaf fluorescence spectra under both steady-state and changing illumination conditions. A single spectral shape of fluorescence explains 84% of the variance across a wide range of species. The magnitude of this shape responds to absorbed light and photosynthetic up/down regulation; meanwhile, chlorophyll concentration and nonphotochemical quenching control 9% and 3% of the remaining spectral variance, respectively. The spectral shape of fluorescence is remarkably stable where most current satellite retrievals occur (far-red, >740nm), and dynamic downregulation of photosynthesis reduces fluorescence magnitude similarly across the 670- to 850-nm range. We conduct an exploratory analysis of hourly red and far-red canopy SIF in soybean, which shows a subtle change in red:far-red fluorescence coincident with photosynthetic downregulation but is overshadowed by longer-term changes in canopy chlorophyll and structure. Based on our leaf and canopy analysis, caution should be taken when attributing large changes in the spectral shape of remotely sensed SIF to plant stress, particularly if data acquisition is temporally sparse. Ultimately, changes in SIF magnitude at wavelengths greater than 740 nm alone may prove sufficient for tracking photosynthetic dynamics. Plain Language Summary Satellite remote sensing provides a global picture of photosynthetic activity-allowing us to see when, where, and how much CO2 plants are assimilating. To do this, satellites measure a small emission of energy from the plants called chlorophyll fluorescence. However, this measurement is typically made across a narrow wavelength range, while the emission spectrum (650-850 nm) is quite dynamic. We show where, why, and to what extent leaf fluorescence spectra change across a diverse range of species and conditions, ultimately informing canopy remote sensing measurements. Results suggest that wavelengths currently used by satellites are stable enough to track the downregulation of photosynthesis resulting from stress, while spectral shape changes respond more strongly to dynamics in canopy structure and chlorophyll concentration.Peer reviewe

Warmer spring alleviated the impacts of 2018 European summer heatwave and drought on vegetation photosynthesis

Future projections of climate extremes are expected to become more frequent. Parts of Europe experienced an extensive heatwave and drought during 2018. However, its impacts on terrestrial carbon cycle remain elusive. Here we investigated the vegetation responses to the heatwave and drought during 2018 based on satellite solar-induced chlorophyll fluorescence (SIF) and near-infrared reflectance (NIRv) data, which were used to estimate gross primary productivity (GPP). Results showed that there were no significant (p= 0.60) reductions in GPP across most of Europe during April-August of 2018. The higher temperatures in spring enhanced vegetation GPP, largely alleviated the negative impacts of heatwave and drought on vegetation photosynthesis during the subsequent summer, which resulted in evident compensation effects. Concurrently, warmer spring also had lagged effects by diminishing soil moisture, accompanied by scarce precipitation, leading to water stress on plant growth during summer. This observation-based study highlights the need for more considerations of seasonal compensation and lagged effects on the interactions between climate extreme events and biosphere.Peer reviewe

Towards a Quality Indicator for Research Data publications and Research Software publications -- A vision from the Helmholtz Association

Research data and software are widely accepted as an outcome of scientific work. However, in comparison to text-based publications, there is not yet an established process to assess and evaluate quality of research data and research software publications. This paper presents an attempt to fill this gap. Initiated by the Working Group Open Science of the Helmholtz Association the Task Group Helmholtz Quality Indicators for Data and Software Publications currently develops a quality indicator for research data and research software publications to be used within the Association. This report summarizes the vision of the group of what all contributes to such an indicator. The proposed approach relies on generic well-established concepts for quality criteria, such as the FAIR Principles and the COBIT Maturity Model. It does - on purpose - not limit itself to technical implementation possibilities to avoid using an existing metric for a new purpose. The intention of this paper is to share the current state for further discussion with all stakeholders, particularly with other groups also working on similar metrics but also with entities that use the metrics.Comment: 21 pages, 1 figur

Efficient low-loaded ternary Pd-In2O3-Al2O3 catalysts for methanol production

Pd-In2O3 catalysts are among the most promising alternatives to Cu-ZnO-Al2O3 for synthesis of CH3OH from CO2. However, the intrinsic activity and stability of In2O3 per unit mass should be increased to reduce the content of this scarcely available element and to enhance the catalyst lifetime. Herein, we propose and demonstrate a strategy for obtaining highly dispersed Pd and In2O3 nanoparticles onto an Al2O3 matrix by a one-step coprecipitation followed by calcination and activation. The activity of this catalyst is comparable with that of a Pd-In2O3 catalyst (0.52 vs 0.55 gMeOH h−1 gcat-1 at 300 \ub0C, 30 bar, 40,800 mL h−1 gcat-1) but the In2O3 loading decreases from 98 to 12 wt% while improving the long-term stability by threefold at 30 bar. In the new Pd-In2O3-Al2O3 system, the intrinsic activity of In2O3 is highly increased both in terms of STY normalized to In specific surface area and In2O3 mass (4.32 vs 0.56 g gMeOH h−1 gIn2O3-1 of a Pd- In2O3 catalyst operating at 300 \ub0C, 30 bar, 40,800 mL h−1 gcat-1).The combination of ex situ and in situ catalyst characterizations during reduction provides insights into the interaction between Pd and In and with the support. The enhanced activity is likely related to the close proximity of Pd and In2O3, wherein the H2 splitting activity of Pd promotes, in combination with CO2 activation over highly dispersed In2O3 particles, facile formation of CH3OH

Integrating Data Science and Earth Science

This open access book presents the results of three years collaboration between earth scientists and data scientists, in developing and applying data science methods for scientific discovery. The book will be highly beneficial for other researchers at senior and graduate level, interested in applying visual data exploration, computational approaches and scientifc workflows

Nuevas oportunidades ante un desafío sistémico

Los Informes C son documentos breves sobre los temas seleccionados por la Mesa del Congreso que contextualizan y resumen la evidencia científica disponible para el tema de análisis. Además, recogen las áreas de consenso, disenso, las incógnitas y los debates en curso. El proceso de elaboración de los informes se basa en una exhaustiva revisión bibliográfica que se complementa con entrevistas a personas expertas en la materia y dos rondas de revisión posterior por su parte. La Oficina C colabora con la Dirección de Documentación, Biblioteca y Archivo del Congreso de los Diputados en este proceso. Para la redacción del presente informe la Oficina C ha referenciado 530 documentos y consultado a un total de 21 personas expertas en la materia. Se trata de un grupo multidisciplinar en el que el 43 % se agrupa en torno a ciencias de la vida (medicina, geología, química, farmacia, biología, ciencias e ingeniería medioambientales), el 48 % pertenece a ciencias físicas e ingeniería (ingeniería química, industrial y del medio ambiente, de caminos canales y puertos, físicas) y el 10 % a ciencias sociales (sociología, ciencia política y administración pública). El 57 % trabaja en centros o instituciones españolas mientras que el 43 % está afiliado al menos a una institución en el extranjero. La Oficina C es la responsable editorial de este informe.La pérdida de calidad del aire derivada de la contaminación atmosférica es uno de los principales problemas de salud pública en Europa y España. La evidencia científica pone de relieve graves impactos sobre la salud humana, el medio ambiente y la economía que afectan de forma desigual a la población. Esta se muestra mayoritariamente preocupada ante la amenaza y, además, en muchos casos dispone de opciones limitadas para autoprotegerse. En las últimas décadas se ha consolidado un amplio marco de políticas públicas a nivel nacional y europeo que persiguen reducir la contaminación atmosférica y sus efectos adversos. Se trata de una herramienta útil, como refleja la mejora progresiva de la calidad del aire en España y el resto de los países comunitarios, pero en base a los datos disponibles, insuficiente. Así lo señala la Unión Europea que trabaja actualmente en la adecuación y mejora del marco regulatorio. Este informe ofrece una visión general sobre el estado, impacto y principales desafíos entorno a la calidad del aire ambiente en el marco español y repasa las principales estrategias de mitigación que pueden articular una mejora de esta problemática en las diversas áreas que marcan la amplia interseccionalidad del desafío.Peer reviewe