1,721,150 research outputs found
Recent Developments in the Direct Methylation of Electron Deficient N-Heteroarenes
No full textThe direct introduction of methyl groups into electron-deficient N-heteroarenes has recently emerged as an efficient and promising strategy to afford methylated N-heteroarenes, which are important motifs in a variety of pharmaceuticals and biologically active molecules. Herein, three general state-of-the-art approaches including radical-mediated, transition-metal-catalyzed, or transition-metal-free direct C-H methylations of electron-deficient N-heteroarenes are highlighted.111310sciescopu
Copper-Catalyzed Diastereoselective Addition of Diborylmethane to N-tert-Butanesulfinyl Aldimines: Synthesis of β-Aminoboronates
No full textWe have developed a highly chemo- and diastereoselective alkylation of N-tert-butanesulfinyl aldimines with diborylmethane. Whereas the addition of diborylmethane under metal-free conditions shows poor diastereoselectivity, the use of a copper catalyst and a bidentate phosphine ligand significantly enhances the diastereoselectivity, providing chiral beta-aminoboronates in good yields. On the basis of the stereochemical outcome, we propose that the reaction likely proceeds via a boron-chelating six-membered chairlike transition state.112816sciescopu
Synthesis of Branched Alkylboronates by Copper-Catalyzed Allylic Substitution Reactions of Allylic Chlorides with 1,1-Diborylalkanes
No full textReported herein is a copper-catalyzed S(N)2'-selective allylic substitution reaction using readily accessible allylic chlorides and 1,1-diborylalkanes, a reaction which proceeds with chemoselective C-B bond activation of the 1,1-diborylalkanes. In the presence of a catalytic amount of [Cu(IMes) Cl] [IMes=1,3-bis(2,4,6-trimethylphenyl) imidazole-2-ylidene] and LiOtBu as a base, a range of primary and secondary allylic chlorides undergo the S(N)2'-selective allylic substitution reaction to produce branched alkylboronates. The synthetic utilities of the obtained alkylboronates are also presented.115036Nscopu
Transition-Metal-Free Regioselective Alkylation of Pyridine N-Oxides Using 1,1-Diborylalkanes as Alkylating Reagents
No full textReported herein is an unprecedented base-promoted deborylative alkylation of pyridine N-oxides using 1,1-diborylalkanes as alkyl sources. The reaction proceeds efficiently for a wide range of pyridine N-oxides and 1,1-diborylalkanes with excellent regioselectivity. The utility of the developed method is demonstrated by the sequential C-H arylation and methylation of pyridine N-oxides. The reaction also can be applied for the direct introduction of a methyl group to 9-O-methylquinine N-oxide, thus it can serve as a powerful method for late-stage functionalization.115835Nsciescopu
Pitch carbon-coated lithium sulfide electrode for advanced, lithium-metal free-sulfur batteries
No full textIn this work we report the properties and characteristics of a pitch carbon-coated lithium sulfide material. We show that this material behaves as a very promising electrode in liquid glyme-based and in solid poly(ethylene oxide) PEO-based electrolyte lithium cells. Particular relevant is the response obtained in the solid-state configuration where a capacity approaching the theoretical value of the full Li2S conversion process, i.e. 1200 mAh gLi2S -1 is obtained at a C/10 rate with no signature of ohmic polarization. The solid-state cell cycles at a C/3 rate with a stable capacity of 500 mAh gLi2 S -1, i.e., a value about three times higher than that offered by conventional intercalation lithium metal oxide electrodes. This unique performance makes the pitch carbon-coated lithium sulfide electrode a good candidate for the development of high energy density, lithium-metal free-sulfur superbatteries to be addressed to the electric vehicle market. Copyright © 2011 De Gruyter
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
- …
