1,720,966 research outputs found
Zirconium-Assisted Aldol Condensation Reactions of Amido Enolates: Structural and Kinetic Analysis of the Reaction of N,N-Diphenylacetamide and N,N-Diphenylpropionamide Enolates with Benzaldehyde and p-Substituted Acetophenones
No full textThe deprotonation of N,N-diphenyl amides with LDA and subsequent reaction with [(cp)(2)ZrCl2] (cp = eta(5)-C5H5) allowed the enolate complexes [{Ph(2)NC(CH2)O}Zr(cp)(2)(Cl)] (5) and [{Ph(2)NC(CHCH3)O}Zr(cp)(2)(Cl)] (6) to be isolated. The crystal structure of 6 is reported. Reaction of 5 with [Cr(CO)(5)]. THF gave [Ph(2)NC{CH2Cr(CO)(5)}{OZr(Cl)(cp)(2)}] (7), an 0- and C-bonded dimetallic amido enolate. Reaction of 5 and 6 with benzaldehyde gave the corresponding aldol products 8 and 9; according to previous reports, the conversion of 6 into 9 is syn selective. Reaction of 6 with acetophenone followed by addition of silver triflate gave the complex [Ph(2)NC(=O)CH(CH3)C(Me)(Ph)OZr(cp)(2)(Cl)(OSO2CF3)] (11) as a diastereoisomeric mixture in the ratio anti:syn = 65:35. The crystal structure of sym-11 is reported. This cyclic complex mimics the postulated cyclic transition state of the aldol reaction mediated by zirconium amide enolates. A kinetic investigation of the reaction of 6 with acetophenone was performed and gave the following activation parameters: Delta H-double dagger = 38.3 +/- 0.9 kJ mol(-1); Delta S-double dagger greater than or equal to -181 +/- 3 J mol(-1) K-1; Delta G(298)(double dagger) = 92.2 +/- 1.2 kJ mol(-1). A similar study with 4-fluoroacetophenone gave Delta H-double dagger = 43.5 +/- 1.3 kJ mol(-1), Delta S-double dagger greater than or equal to -167 +/- 4 J mol(-1) K-1, and Delta G(298)(double dagger) = 93.3 +/- 1.8 kJ mol(-1). The reaction rate at 320 K determined with 4-chloro-, 4-methyl-, and 4-nitroacetophenone allowed the determination of a Hammett plot with rho = 0.41. This value is implicit of a carbon-carbon bond-forming, rate-limiting step
Titanium and Zirconium Ferrocene-Substituted Enolates and Their Reaction Products with Benzaldehyde and Acetophenone: Structural and Kinetic Studies of the Aldol Condensation Pathway
No full textThe reaction of acetyl- and propionylferrocene, [(RCOcp)(cp)Fe] (R = Me (1), Et (2)), with KH led to the isolation of the corresponding ion-pair enolates 3 and a in the solid state. When the deprotonation of 1 is carried out in the presence of 18-crown-6, the naked enolate 5, [(CH(2)COcp)(cp)Fe]K--(18-crown-6)(+)], was isolated. The potassium enolate 3 has been converted into the corresponding titanium and zirconium enolates via the reaction with (cp)(2)MCl(2) (M = Ti, Zr), while only 4 was converted into the analogous zirconium enolate. The following metal enolates have been isolated in good yield and as crystalline solids: [(cp)-Fe(cpC(CH2)O)M(cp)(2)(Cl)] (M = Ti (6), Zr (7)) and [(cp)Fe(cpC(CHMe)O)Zr(cp)(2)(Cl)] (8). Compounds 6 and 7 have been characterized by H-1 and C-13 NMR, and an X-ray crystal structure of 7 was obtained; compound 8 was characterized by H-1 NMR. The aldol reaction of 6 and 7 with benzaldehyde led to the corresponding metal aldol derivatives [(cp)Fe(cpC-(O)CH2C(H)(Ph)O)M(cp)(2)(Cl)] (M = Ti (9), Zr(10)). For compound 9 the solid-state structure and solution data are reported. Complex 9 undergoes a facile Ti-Cl ionization, leading to the cationic complex [(cp)Fe(cpC(O)CH2C(H)(Ph)O)Ti(cp)(2)](+)BPh(4)(-) (11). In complex 11, the aldol fragment forms a metallacycle where both oxygens are bonded to titanium. This structure mimics the bond connectivity of the generally proposed ''transition state'' of aldol reactions. The isolation of structurally well-defined titanium and zirconium enolates allowed us to carry out a kinetic investigation into the reaction of 7 with acetophenone. The reaction was carried out in C6D6 at temperatures from 283 to 340 K. The reaction is second order (first order in each reactant), and the following activation parameters have been obtained: Delta H-double dagger = 44.4 +/- 1.7 kJ mol(-1), Delta S-double dagger greater than or equal to -150 +/- 6 J mol(-1) K-1, and Delta G(298)(double dagger) = 89.0 +/- 2.4 kJ mol(-1). A similar study with 4-fluoroacetophenone gave Delta H-double dagger = 33.0 +/- 4.6 kJ mol(-1), Delta S-double dagger greater than or equal to -189 +/- 15 J mol(-1) K-1, and Delta G(298)(double dagger) = 89.2 +/- 6.4 kJ mol(-1). The reaction rate at 320 K determined with 4-chloro-, 4-methyl-, and 4-nitroacetophenone allowed the determination of a Hammett plot with rho = 0.42 +/- 0.9. This value is implicit for a carbon-carbon bond-forming, rate-limiting step. Complexes 7 and 9 have been characterized by X-ray analysis
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
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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