1,720,974 research outputs found
Sustainable Methodologies for the Synthesis of High-Value Organic Compounds: A Journey Through Photochemistry, XAT and Flow Chemistry
No full textNegli ultimi anni, le crescenti preoccupazioni riguardo all'impatto ambientale e ai problemi di sostenibilità associati ai processi chimici tradizionali hanno spinto la comunità scientifica ad adottare metodologie volte a ridurre gli sprechi, utilizzare condizioni di reazione più blande ed atte ad un miglioramento dell'economia atomica. Questi fattori hanno stimolato lo sviluppo del concetto di chimica verde, portando a significativi progressi nel campo della chimica. Il settore della chimica organica ha recentemente subito profonde trasformazioni grazie all'impiego di metodologie innovative progettate per migliorare l'efficienza, la selettività e la sostenibilità. Tra questi approcci, questa tesi affronta specificamente la fotochimica, l’halogen atom transfer (XAT) e la chimica in flusso continuo, includendo processi in cui queste strategie operano simultaneamente per promuovere trasformazioni chimiche innovative per la produzione di molecole ad alto valore biologico e industriale. In particolare, il primo capitolo esplora l'uso della luce come fonte di energia per promuovere reazioni chimiche. La luce rappresenta una fonte di energia infinita ed ecocompatibile, che consente di promuovere reazioni agendo direttamente su uno o più reagenti coinvolti nella reazione chimica (fotochimica diretta) o attraverso l'uso di un fotosensibilizzatore (fotocatalisi). In questo contesto, vengono trattati due progetti. Il primo riguarda l'accoppiamento cross-deidrogenativo mediato da fotocatalisi utilizzando il nitruro di carbonio grafitico mesoporoso (mpg-CN), un fotocatalizzatore eterogeneo privo di metalli, per la sintesi di amminoacidi non proteinogenici. Il secondo progetto, condotto durante il mio periodo di ricerca presso la RWTH Aachen, nel gruppo del Prof. Leonori, si concentra sulla foto-permutazione degli isossazoli nei corrispondenti ossazoli mediante irradiazione diretta del mezzo di reazione. Nel secondo capitolo viene discusso l’halogen atom transfer (XAT) come via per la conversione diretta degli alogenuri alchilici in radicali centrati sul carbonio, intermedi altamente reattivi utili per i processi chimici. In particolare, vengono riportati due diversi progetti. Il primo esplora una strategia fotocatalizzata per la formazione di analoghi dell'α-C-glicosil alanina tramite una reazione di Giese. Il secondo, condotto presso la RWTH Aachen, studia una strategia basata sui radicali per l'alchilazione delle ammidi con alogenuri alchilici, combinando halogen atom transfer, mediato da un radicale borilico, con la catalisi a base di rame. Il terzo capitolo discute la chimica in flusso continuo come strategia per migliorare le condizioni di reazione e la sicurezza nella produzione di composti di rilevanza biologica e industriale. Nel corso degli anni, questa metodologia si è dimostrata un approccio eccellente per migliorare le condizioni di lavoro e aumentare le rese dei prodotti, offrendo significativi vantaggi quando anche applicata da una prospettiva industriale. Relativamente a questo argomento vengono discussi due progetti. Il primo si concentra sulla sintesi di amminoacidi non proteinogenici tramite accoppiamento cross-deidrogenativo tra derivati della glicina e chetoni, utilizzando un approccio flusso continuo in due step in serie, che impiega fotocatalisi eterogenea e organocatalisi. Il secondo progetto riguarda la sintesi di carbonati ciclici mediante inserzione di anidride carbonica in epossidi, utilizzando un approccio a flusso continuo con organocatalisi omogenea.In recent years, growing concerns about the environmental impact and sustainability issues associated with traditional chemical processes have led the scientific community to adopt methodologies aimed at reducing waste, utilizing milder reaction conditions, and improving atom economy. These factors have sparked the development of the concept of green chemistry, bringing significant advancements in the field of chemistry. The domain of organic chemistry has recently undergone profound transformations through the use of innovative methodologies designed to improve efficiency, selectivity, and sustainability. Among these approaches, this thesis specifically addresses photochemistry, halogen atom transfer, and flow chemistry, including processes where these strategies operate simultaneously to promote innovative chemical transformation for the production of high-value biological and industrial molecules. Specifically, the first chapter explores the use of light as an energy source to drive chemical reactions. Light represents an infinite and environmentally friendly energy source that enables the promotion of reactions by acting directly on one or more of the reactants involved in the chemical reaction (direct photochemistry) or through the use of a photosensitizer (photocatalysis). In this context, two projects are addressed. The first involves cross-dehydrogenative coupling mediated by photocatalysis using mesoporous graphitic carbon nitride (mpg-CN), a metal-free heterogeneous photocatalyst, for the synthesis of non-proteinogenic amino acids, and the second project, conducted during my research stay at the RWTH Aachen, in the Prof. Leonori’s group, focuses on the photo-permutation of isoxazoles into their corresponding oxazoles via direct irradiation of the reaction medium. In the second chapter, halogen atom transfer (XAT) is discussed as a pathway for the direct conversion of alkyl halides into carbon-centered radicals, which are highly reactive intermediates useful for chemical processes. Specifically, two different projects are reported. The first one explores a photocatalyzed strategy for the formation of α-C-glycosyl alanine analogues via a Giese reaction, and the second one, conducted at the RWTH Aachen, explores a radical-based strategy for the alkylation of amides with alkyl halides, combining halogen atom transfer, mediated by a boryl radical, and copper catalysis. The third chapter discusses flow chemistry as a strategy to enhance reaction conditions and safety in the production of biologically and industrially relevant compounds. Over the years, this methodology has been widely demonstrated to be an excellent approach for improving working conditions and increasing product yields, offering significant advantages when applied from an industrial perspective. Two projects regarding this topic are discussed. The first one focuses on the synthesis of non-proteinogenic amino acids via cross-dehydrogenative coupling between glycine derivatives and ketones, utilizing a telescopic continuous flow approach that employs heterogeneous photocatalysis and organocatalysis. The second project addresses the synthesis of cyclic carbonates via carbon dioxide insertion into epoxides using a plug-flow approach with homogeneous organocatalysis
A Radical Strategy for the Alkylation of Amides with Alkyl Halides by Merging Boryl Radical-Mediated Halogen-Atom Transfer and Copper Catalysis
No full textAmide alkylation is a fundamental process in organic chemistry. However, the low nucleophilicity of amides means that divergent coupling with alkyl electrophiles is often not achievable. To circumvent this reactivity challenge, individual amine synthesis followed by amidation with standard coupling agents is generally required. Herein, we demonstrate a radical solution to this challenge by using an amine–borane complex and copper catalysis under oxidative conditions. While borohydride reagents are generally used as reducing agents in ionic chemistry, their conversion into amine-ligated boryl radicals diverts their reactivity toward halogen-atom transfer. This enables the conversion of alkyl halides into the corresponding alkyl radicals for amide functionalization via copper catalysis. The process is applicable to the N-alkylation of primary amides employing unactivated alkyl iodides and bromides, and it was also showcased in the late-state functionalization of both complex amide- and halide-containing drugs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Exploring the Potential of High Molar Mass Poly(hydroxymethylfuroate): Synthesis, Properties, and Recyclability Studies
No full textThe synthesis of the fully furan-based high-molar-mass poly(hydroxymethylfuroate) (HMM-PHMF) is herein presented along with the investigation of thermal properties, filmability, water contact angle (WCA), and recyclability. The polymer (Mn > 200000 g mol-1) has been produced on a multigram scale by N-heterocyclic carbene (NHC)-promoted chain extension of PHMF (1 h, 190 °C), which can be directly synthesized from 5-(hydroxymethyl)furfural (HMF) or 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The polyester is thermally highly stable up to 280 °C, showing a semicrystalline behavior with Tm ∼ 194 °C when processed from solvent and an amorphous character with Tg ∼ 90 °C when processed from the melt. HMM-PHMF has been efficiently filmed by compression molding and submitted to WCA analyses (86 ± 2°), behaving as a potential biobased food packaging material. Finally, recyclability has been assessed through alkaline hydrolysis producing HMFCA and cyclodepolymerization affording the starting hydroxymethylfuroate macrocyclic oligoesters c(HMF)n, which can be converted again through entropically driven ring-opening polymerization (ED-ROP), in a closed-loop cycle
Exploiting cooperative photoredox and asymmetric organocatalysis in cross-dehydrogenative coupling of glycine analogues with ketones
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