1,720,982 research outputs found
Synthesis of Tetrahydrofurans via Chemoselective Addition of organolithium and Grignard Reagents to γ-Chloroketones in Deep Eutectic Solvents and Water
No full textAmong saturated oxygen heterocycles, substituted tetrahydrofuran derivatives are important structural components of several natural compounds.1 Synthesis and functionalization of 2,2- disubstituted tetrahydrofurans, in particular are generally based on Williamson’s intramolecular cyclization of γ-chloroalcohols as a result of reaction with different organometallic compounds. Volatile organic solvents generally used in laboratories or in industrial processes may cause environmental problems, because their accumulation in the atmosphere, thereby contributing to ozone depletion and smog in urban areas. Thus, to further assess the potential impact of more ecofriendly solvents in organolithium chemistry, we turned our attention to the so-called Deep Eutectic Solvents (DESs)2, which were introduced by Abbott and co-workers in 2003.3 DESs are formed by mixing a hydrogen-bond donor (e.g., urea) and a hydrogen-bond acceptor (e.g., the quaternary ammonium salt choline chloride), two solid starting materials with higher melting points than the eutectic mixture.
Very recent independent contribution from Hevia4 and our group5 have shown that the reactions of simple or functionalized organolithiums can be nicely performed in DESs at room temperature and under air competitively with protonolysis. In this communication, the formation of 2,2-disubstituted tetrahydrofuran derivated through the direct addition of organolithium and Grignard reagents to 4- cholobutyrophenone, chosen as model substrate, in both DESs and water. Different eutectic mixture and different organometallic’s concentrations influence on the yields of the final products. The influence of different eutectic mixtures and of the concentration of the employed organometallic species on the yield of the final products will be tackled as well.
References:
1. (a) Wolfe, J. P.; Hay, M. B. Tetrahedron 2007, 63, 261; (b) Lorente, A.; Lamariano-Merketegi, J.; Albericio, F.;
Alvarez, M. Chem. Rev. 2013, 113, 4567;
2. Francisco, M.; van den Bruinhorst, A.; Kroon, M. C. Angew. Chem. Int. Ed. 2013, 52, 3074;
3. A. P. Abbott, G. Copper, D. L. Davies, R. K. Rasheed, V. Tambyrajah Chem. Commun. 2003, 70;
4. Hevia, Angew. Chem, Int. Ed., 2014, 53, 5969-5973;
5. Mallardo, V.; Rizzi, R.; Sassone, F.; Mansueto, R.; Perna, F. M.; Salomone, A.; Capriati. V., Chem Comm. 2014, 50,
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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
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