1,721,280 research outputs found
Reactions of Diazo Carbonyls with [PtX(CH3) (chiral diphosphine)] (X = Cl, Br, I): Chemoselectivity and Diastereoselectivity of Pt-C and Pt-X Carbene Insertion
No full textIn polar solvents (e.g. MeCN), ethyl diazoacetate reacts with [PtXMe(S,S-diop)], where X = Cl, Br, I, to give the corresponding [PtX(CHMeCO2Et)(S,S-diop)] as a 2:1 mixture of diastereoisomers in high yields. The major diastereoisomer of [PtCl(CHMeCO2Et)(S,S-diop)] is readily separated in crystalline form, and its crystal structure reveals that the configuration at the α-carbon is R; it is configurationally stable in CDCl3 for at least 14 days. The factors that influence the diastereoselectivity have been examined by comparing (by 31P NMR spectroscopy) the ratio of diastereoisomers formed in the reactions between [PtXMe(diphos*)] and N2CHCOR: X = Cl, Br, I; diphos* = S,S-diop, R,R-diop, S,S-skewphos, S,S-chiraphos; R = OEt, O(l-menthyl), Ph. In MeCN, the diastereoselectivity is independent of halogen but is a sensitive function of the chiral diphosphine and diazo carbonyl, though no systematic correlations have been divined. In solvents of lower polarity (e.g. CH2Cl2), diazo carbonyls react with [PtXMe(diphos*)] to give the products derived from Pt-X insertion as well as Pt-C insertion. When C6H6 is used as the solvent, the compounds [PtMe(CHICOR)(S,S-diop)], where R = OEt, O(l-menthyl), Ph, are formed in high yields and have been isolated. Redissolving these compounds in MeCN did not lead to isomerization to the Pt-C insertion species [PtI(CHMeCOR)(S,S-diop)]. Several trends have been found relating the extent of Pt-C insertion to the nature of the solvent and the structure of the reagents: the proportion of Pt-C insertion increases with (i) increasing polarity of the solvent (C6H6< CHCl3< CH2-Cl2< (CH3)2SO), (ii) increasing nucleofugacity of the halogen (I < Br < Cl), (iii) decreasing bite angle of the diphosphine (diop < chiraphos, skewphos), and (iv) diazo ketone < diazo ester. A mechanism which is consistent with these observations is discussed. Many of the compounds discussed here have been observed in solution only by 31P NMR, but representative species have been isolated and fully characterized by a combination of elemental analysis, IR spectroscopy, and 1H, 13C, 31P, and 195Pt NMR spectroscopy. © 1995, American Chemical Society. All rights reserved
Anchimeric assistance by platinum(II) in the epimerizations of [PtX(CHXSiMe(3))(R,R-chiraphos)]
No full textThe crystal structures of solvates of [PtCl(S-CHClSiMe3)(R,R-chiraphos)], [PtBr(S-CHBrSiMe3)(S,S-chiraphos)], and [PtCl(R-CHClSiMe3)(R,R-chiraphos)] (chiraphos = 2,3-bis(diphenylphosphino)butane) have been determined allowing assignment of the absolute stereochemistry for all the species [PtX(CHXSiMe3)(chiraphos)], 1a-c and 2a-c (X = Cl, Br, I). Inversion at the α-carbon in [PtX(CHXSiMe3)(chiraphos)] has been observed at ambient temperature and below for X = Br or I and at elevated temperatures for X = Cl. The rates of these epimerizations have been monitored by 31P NMR spectroscopy (X = Cl or Br) or polarimetry (X = Br or I). All the epimerizations follow first order kinetics and the rate constants kobs have been calculated. From these values the following can be deduced: (i) The rates increase in the order Cl < Br < I. (ii) Addition of large amounts of halide salts has only a small effect on the rate of epimerization. (iii) The reactions are significantly slower in MeCN than in CHCl3 or CH2Cl2. A mechanism consistent with these observations is proposed involving α-halogen migration to the platinum(II)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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