1,721,007 research outputs found
Amines bearing tertiary substituents by tandem enantioselective carbolithiation-rearrangement of vinylureas
No full textIn the presence of (-)-sparteine or a (+)-sparteine surrogate, organolithiums add to N-alkenyl-N'-arylureas to give benzylic organolithiums in an enantioselective manner. Under the influence of DMPU, these organolithiums undergo rearrangement with migration of the N'-aryl ring from N to C, leading to the urea derivatives of enantiomerically enriched amines bearing tertiary substituents. Basic hydrolysis returns the functionalized amine, providing a new synthetic route to compounds with quaternary stereogenic centers bearing nitrogen.</p
Silylformylation as a central tool for the synthesis of complex silaheterocycles
No full textBien que le silicium soit le second élément le plus abondant de la croûte terrestre, il n’est que très peu présent chez l’Homme et est absent des molécules biologiques organiques de la vie. Les propriétés physico-chimiques (électropositivité, taille, lipophilie etc…) de cet atome ainsi que le fait qu’il soit dans la même colonne que le carbone en font pourtant un bioisostère potentiel particulièrement intéressant. Dans ce contexte, de nombreux groupes se sont intéressés à la substitution d’atomes de carbone par un atome de silicium, concept nommé le « carbon-silicon switch ». Parmi les sila-analogues étudiés, certains d’entre eux sont obtenus par inclusion d’un atome de silicium dans un cycle (les silacycles). Néanmoins, contrairement à leurs équivalents carbonés, il n’existe que peu de méthodologies générales permettant l’accès à de tels composés. Ainsi, nos travaux ont alors porté sur la synthèse de silacycles en deux étapes. La première étape est basée sur une réaction de silylformylation d’alcynes introduisant un silane aromatique et un aldéhyde de part et d’autre d’une double liaison C=C. La seconde étape repose sur une cyclisation de type Friedel-Crafts fournissant une nouvelle famille d’hétérocycles silylés dont les propriétés photophysiques ont pu être étudiées. Dans un second temps, une méthode d’annélation d’ynamides conduisant à des amido-indénones a également été mise au point.Although silicon is the second most abundant element in the earth's crust, it is only slightly present in human beings and absent from the organic biological molecules. The physico-chemical properties (electropositivity, size, lipophilicity etc...) of this atom and its presence in the same column as carbon makes it an interesting potential bioisostere. In this context, many research groups have been interested in the substitution of carbon atoms by a silicon atom ("carbon-silicon switch"). Among the sila-analogues studied, some of them are obtained by the inclusion of a silicon atom in a ring (silacycles). Nevertheless, there is a real lack of general methodologies to access such compounds. Thus, we aimed to develop a two-step synthesis of silicon containing heterocycles starting from alkyne derivatives. The first step is based on a silylformylation reaction of alkynes introducing an aromatic silane and an aldehyde on both sides of a C=C double bond. The second step is based on a Friedel-Crafts type cyclization providing a new family of silaheterocycles. The photophysical properties of these compounds were also investigated. In addition, an annulation reaction of ynamides was developed leading to amido-indenones
Versatile synthesis and application of original 2-aminoacrylonitrile derivatives
No full textLes 2-aminoacrylonitriles, qui ont connu un essor important lors des années 1970 et 1980, sont des synthons utilisés depuis longtemps en synthèse organique notamment pour la formation d’édifices cycliques. Pourtant, les voies de synthèse de composés de type 2-amidoacrylonitriles sont beaucoup moins répandues et générales. Pour répondre à ce manque de méthodologies, l’objectif de ces travaux est la synthèse de 2-amidoacrylonitriles polyfonctionnalisés de manière stéréocontrolée. Notre idée était de générer ce motif en réalisant des réactions de cyanation sur des ynamides. Une première approche se basant sur une réaction de carbosilylation à partir de TMSCN a été développée afin d’obtenir efficacement un large spectre de 3-silyl-2-amidoacrylonitriles tétrasubstitués de manière régio- et stéréocontrolée. Puis, un procédé d’hydrocyanation a pu être mis au point pour la formation de 2-amidoacrylonitriles trisubstitués géométriquement définis (E) par catalyse au zinc. Ces derniers ont pu être isomérisés sous irradiation UV afin de générer l’autre stéréoisomère (Z). Par la suite, l’applicabilité de ces synthons a été démontrée lors d’un criblage de réactions de post-fonctionnalisation variées. Ces synthons se sont révélés être des précurseurs de carbo- et hétérocycles originaux en tirant parti des différents substituants présent sur le motif 2-amidoacrylonitrile. Ainsi, ces composés font l’objet d’un grand nombre de perspectives et mettent en lumière la chimie riche qui peut découler de tels motifs.2-aminoacrylonitriles, which have experienced a major boom in the 1970s and 1980s, have long been used in organic synthesis, particularly for the formation of cyclic structures. However, the synthetic routes towards 2-amidoacrylonitriles are much less widespread and general. To address this lack of methodologies, the objective of this work is the synthesis of stereocontrolled polyfunctionalized 2-amidoacrylonitriles. Our idea was to generate this motif by performing cyanation reactions on ynamides. A first approach based on a carbosilylation reaction from TMSCN was developed to efficiently obtain a broad spectrum of tetrasubstituted 3-silyl-2-amidoacrylonitriles in a regio- and stereocontrolled manner. Then, a hydrocyanation process could be developed for the formation of geometrically defined trisubstituted 2-amidoacrylonitriles (E) by zinc catalysis. The latter could then be isomerised under UV-irradiation to generate the other stereoisomer (Z). Subsequently, the applicability of these building-blocks was demonstrated in performing follow-up chemistry on them. These compounds were also shown to be novel carbo- and heterocycle precursors by taking advantage of the different substitutions of the 2-amidoacrylonitrile unit. Thus, these compounds are the subject of many perspectives and highlight the rich chemistry that can arise from such motifs
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
(Cyclic) diaryl λ3-bromanes/chloranes : rare hypervalent compounds for the rapid synthesis of complex molecules
No full textContrairement aux λ3-iodanes diaryliques bien étudiés, la chimie des dérivés λ3-bromane et λ3-chlorane a été rarement explorée en raison de leur synthèse complexe, occultant ainsi les applications potentielles. Nous nous sommes concentrés sur le développement de méthodes de synthèse efficaces pour les λ3-bromanes/chloranes (cyliques) diaryliques, ainsi que sur l'exploration de leur potentiel unique en tant que précurseurs d'arynes novateurs et leur réactivité en tant que réactifs efficace d’arylation. Les intermédiaires aryne, générés à partir de λ3-bromanes/chloranes, ont servi de plateformes synthétiques puissantes et durables pour la construction efficace de structures moléculaires complexes. De plus, l’insertion d'arynes dans les liaisons σ N–Si d’aminosilanes avec des réactifs λ3-bromanes/chloranes (cyliques) diaryliques a permis un accès direct à des dérivés 3-amino-2-silylbiaryls. Des investigations expérimentales mécanistiques, combinées à des calculs DFT, ont été menées pour élucider la réactivité distincte et améliorée des réactifs (cyliques) diaryliques de brome(III) et de chlore(III) dans transformations chimiques.In contrast to well-studied diaryl λ3-iodanes, the chemistry of λ3-bromane and λ3-chlorane congeners has been scarcely explored due to their challenging synthesis, shadowing their potential applications. For this purpose, we focused on developing the practical and efficient synthetic methods for (cyclic) diaryl λ3-bromanes/chloranes, as well as exploring their unique as novel aryne precursors and superior reactivity as enhanced arylating reagents. The aryne intermediates, generated from (cyclic) diaryl λ3-bromanes/chloranes, served as a potent and sustainable synthetic platforms for a straightforward and efficient construction of complex molecular frameworks. In addition, aryne insertion into N–Si σ-bonds of aminosilanes with cyclic diaryl λ3-bromane/chlorane reagents enabled a direct access to 3-amino-2-silylbiaryl derivatives. Experimental mechanistic investigations, combined with DFT calculations, were further conducted to elucidate the distinct and enhanced reactivity of the rarely explored (cyclic) diaryl hypervalent bromine(III) and chlorine(III) reagents in diverse and versatile chemical transformations
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
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