1,720,957 research outputs found
Crown ether catalyzed stereospecific synthesis of Z- and E-stilbenes by Wittig reaction in a solid-liquid two-phases system
No full textPotassium hydroxide and a catalytic amount of 18-crown-6 are used, in alternative to the classical Wittig conditions, to prepare very rapidly and stereoselectively Z- and E-stilbenes. In particular, the use of benzyltriphenylphosphonium iodides always leads to a complete Z-stereospecificity, while benzyldiphenylchlorophosphonium salts give a complete E-stereospecificity. (C) 1996 Elsevier Science Lt
Lifetime of the glucosyl oxocarbenium ion and stereoselectivity in the glycosidation of phenols with 1,2-anhydro-3,4,6-tri-O-methyl-alpha-D-glucopyranose
No full textThe glycosidation of para substitued phenols with 1,2-anhydro-3,4,6-tri-O-methyl-alpha-D-glucopyranose. ZnCl2 catalysed, gives predominantly the corresponding a-anomers. In the presence of an amino base, 1.1,3,3-tetramethylguanidine. however, enhances the beta-selectivity, which becomes practically complete when the reaction is carried out under basic conditions, by addition of K2CO3 and 18-crown-6. A rationalisation based on the lifetime of the oxocarbenium ion intermediate and on the nucleophilicity of the phenols is proposed. (C) 1997 Elsevier Science Ltd
Crown ether catalyzed stereoselective synthesis of vinyl ethers in a solid liquid two-phase system
No full textPotassium hydroxide and a catalytic amount of 18-crown-6 provide a simple and stereoselective access to Z vinyl ethers in high yields by addition of primary, secondary and tertiary alcohols to phenylacetylene
Formation of bromocarbenium bromide ion pairs in the electrophilic bromination of highly reactive olefins in chlorinated aprotic solvents
No full textThe kinetics and the products of bromination of several substituted stilbenes with tetrabutylammonium tribromide (TBAT) have been investigated in aprotic solvents at different temperatures, Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products. The reactions followed a second-order rate law, and an inverse kinetic isotope effect (KIE), K-H/k(D) = 0.85(0.05), was found for the bromination of cis-stilbene. The reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides both in chloroform and 1,2-dichloroethane. The rate constants (k(Br3)-) measured for the latter olefins deviated considerably fi om the Hammett correlations, and added bromide had a significant effect on the rates. The reactions of these activated stilbenes with molecular Br-2, carried out at low Br-2 concentration, followed a mixed second/third-order rate law. The kinetic and product distribution data for the reaction, with TBAT, of stilbenes bearing electron-withdrawing or moderately electron-donating substituents are interpreted on the basis of the known mechanism involving a product- and rate-determining nucleophilic attack by bromide on the olefin-Br-2 pi-complex. The data related to the bromination of the more activated methoxystilbenes are rationalized considering that, for these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 rr-complex to a bromocarbenium bromide ion pair can compete both with the formation of a bromonium-tribromide ion pair and with the nucleophilic attack by Br-. For this second-order process (first order in Br-2), the kinetic constants and the activation parameters have been measured in chloroform and 1,2-dichloroethane and the activation parameters have been compared with those related to the third-order Br-2 addition and to the reaction with TBAT
FACIAL STEREOSELECTIVITY OF 2-STEP ADDITIONS INITIATED BY ELECTROPHILIC HALOGENS TO METHYLENECYCLOHEXANES - A COMPARISON WITH EPOXIDATION
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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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