1,720,968 research outputs found
Mechanistic Studies on the Palladium-Catalyzed Direct C-5 Arylation of Imidazoles: The Fundamental Role of the Azole as a Ligand for Palladium
No full textAn in-depth mechanistic study on the palladium-catalyzed direct arylation of imidazoles at the C-5 position is presented. The interactions of triphenylphosphine (PPh3)-ligated aryl-Pd species with 1,2-dimethyl-1H-imidazole (dmim) have been studied in detail. In contrast with previous suggestions, phosphine-ligated organo-Pd species are not active and the reaction proceeds through imidazole-ligated organo-Pd intermediates. The kinetics of the oxidative addition of aryl halides with dmim-ligated Pd(0) species have been characterized in a Pd(dba)2/dmim model system. A thorough study of the equilibria involving novel [ArPd(dmim)2X] complexes (X=I, OAc) and the unexpected cationic [ArPd(dmim)3]+is also reported. The ability of these species to effect the C-H arylation of dmim at room temperature in the presence of acetate is also demonstrated
Photochemical C H Hydroxyalkylation of Quinolines and Isoquinolines
No full textWe report herein a visible light‐mediated C−H hydroxyalkylation of quinolines and isoquinolines that proceeds via a radical path. The process exploits the excited‐state reactivity of 4‐acyl‐1,4‐dihydropyridines, which can readily generate acyl radicals upon blue light absorption. By avoiding the need for external oxidants, this radical‐generating strategy enables a departure from the classical, oxidative Minisci‐type pattern and unlocks a unique reactivity, leading to hydroxyalkylated heteroarenes. Mechanistic investigations provide evidence that a radical‐mediated spin‐center shift is the key step of the process. The method's mild reaction conditions and high functional group tolerance accounted for the late‐stage functionalization of active pharmaceutical ingredients and natural products
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
Réactions catalysées par des complexes métalliques : études mécanistiques et développements méthodologiques
No full textIn this thesis, the mechanism of three organic reactions catalyzed by palladium and copper complexes has been elucidated by the use of both experimental and theoretical methods. The first reaction is the synthesis of amides from haloarenes, isocyanides and water as an example of the broad family of palladium-catalyzed imidoylative couplings. Multiple roles of the isocyanide as both a ligand and a substrate in the different steps of the catalytic cycle have been disclosed. The second transformation is the palladium-catalyzed ring opening of benzofurans leading to indoles. Optimal conditions for this transformation have been found and the key aspects of its mechanism clarified. The last reaction, which is the main topic of this thesis, is the addition of amines to allenes catalyzed by copper salts (hydroamination). A characterization of the catalytically active copper(I) species and insight from theoretical calculations suggested how to extend this reaction to other substrates (allenamides, N-allenylazoles, N-allenylsulfonamides) under mild and efficient conditions.Dans cette thèse les mécanismes de trois réactions catalysées par des complexes de palladium et de cuivre ont été étudiés en utilisant des méthodes expérimentales et théoriques. La première réaction est la synthèse d’amides à partir d’halogénoarènes, d’isonitriles et d’eau, qui est un exemple de couplage catalysé par le palladium impliquant l’insertion d’un isonitrile. Cette dernière molécule sert à la fois de ligand et de substrat, et son influence sur chaque étape du cycle catalytique a été mise en évidence. La deuxième réaction est l’ouverture des benzofuranes conduisant à des dérivés indoliques catalysée par des sels de palladium. Les conditions opératoires ont été optimisées et les étapes clés du mécanisme ont été élucidées.La dernière réaction étudiée, qui est le sujet principal de cette thèse, est l’addition d’amines sur des allènes catalysée par des sels de cuivre (hydroamination). La caractérisation des espèces catalytiques de cuivre(I) et l’étude théorique du mécanisme ont permis d’étendre cette réaction à différents substrats (allénamides, N-allénylazoles, N-allénylsulfamides) dans des conditions particulièrement douces et efficaces
Mechanistic Studies on the Palladium-Catalyzed Direct Arylation of Imidazoles
No full textWe have studied some mechanistic aspects of the palladium-catalyzed direct arylation of imidazole derivatives at the C5 position with aryl halides. Differently from previously reported studies on similar transformations, our focus was not only on the C-H functionalization step, but on the whole catalytic cycle.
We have shown that PPh3-ligated organopalladium species are unlikely intermediates of the catalytic cycle and the interactions within 1,2-dimethylimidazole (dmim) and PPh3-ligated organopalladium complexes have been studied in detail. PPh3, when used as a supporting ligand in conjunction with Pd(OAc)2, is oxidized to PPh3O under the reaction conditions, thus the reaction proceeds through imidazole-ligated organopalladium intermediates. The kinetics of the oxidative addition of aryl halides to dmim-ligated Pd(0) species has been characterized.
Deuterium kinetic isotope effect (KIE) demonstrated that C-H bond cleavage is involved in the turnover limiting step of the catalytic cycle. Complexes of formula ArPd(dmim)3I have been isolated and characterized for the first time. They behave as a mixture of trans-[ArPd(dmim)2I] and dmim in apolar solvent, while in polar solvents they partially dissociate to give the [ArPd(dmim)3]+. The latter cationic species is active towards the direct arylation of dmim at room temperature in the presence of AcO- as a base. Preliminary kinetic data point out that C-H bond cleavage occurs by a concertated metalation-deprotonation mechanism with AcO- acting as an outer-sphere base. Reactivity studies on 1-aryl-2-methylimidazole showed that the reaction under study is faster for electron-poor imidazole substrates.
We propose a revised catalytic cycle, consistent with all the experimental data. We are confident that our results will be useful for the development of enhanced catalytic systems for C-H activation
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