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The Effect of the Fluorine Substitution on the Enantioselective Oxidation of Sulfides with Chiral Titanium Catalysts: A Combined Computational and Experimental Investigation
No full textThe results of a combined computational-experimental study of the oxidation of various fluorinated aryl benzyl sulfides using tert-butyl hydroperoxide (TBHP) in the presence of a complex of titanium and (S,S)-hydrobenzoin are presented. As observed in previous studies for other aryl benzyl sulfides, the reaction leads to enantiopure sulfoxides (ee>98 %) in good isolated yields (81–96 %) except the case of pentafluorobenzyl pentafluorophenyl sulfide for which a lower ee (61 %) is observed. DFT computations on a model-system formed by the substrate, the oxidant TBHP and the [(S,S)-hydrobenzoin]2-Ti complex satisfactorily explain this unexpected item. The enantioselectivity is governed by the relative energy of the two diastereomeric octahedral complexes that form if TBHP approaches the initial complex between substrate and [(S,S)-hydrobenzoin]2-Ti before the oxygen transfer. For pentafluorobenzyl pentafluorophenyl sulfide, the two octahedral complexes are almost degenerate and, thus, they form in similar amounts. As the two corresponding diastereomeric transition states are similar in energy, the probability to follow one or the other diastereomeric reaction channel becomes comparable, which leads to the lower enantiomeric excess experimentally observed. Our computations indicate that the particular “folded conformation”, adopted by the substrate only if both phenyl rings are fluorinated, is the key factor that determines the near degeneracy of the two diastereomeric octahedral complexes
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
A combined theoretical and crystallographic investigation of racemic and enantiopure aryl benzyl sulfoxides
No full textThe separation of enantiomers by crystallization is of great interest to the pharmaceutical industry. The existence of a stable conglomerate is a necessary (but not sufficient) condition for the application of a preferential crystallization [1]. So, the comparison between the crystal structure of an enantiopure molecule and the corresponding racemate is an important point. In the past, it was generally accepted that the crystal structure of a racemate is tightly and denser than that of pure enantiomers (a statement sometimes referred as the “Wallach rule”). This result had been based upon the inspection of a limited number of pairs from the literature [2]. On the other hand, a slightly larger comparison in the crystallographic database showed that the validity of this empirical rule can be questioned [3]. Further analysis confirmed the complexity of the question [4-6] and also found some counterintuitive results, such as a racemic mixture that crystallises in a non centro-symmetric space group [4]. Research in this field is discouraged by the fact that it is not easy to make efficient queries to the Cambridge Crystallographic Database to extract information about pairs of racemic and enantiopure compounds. Some relevant active pharmaceutical ingredients are chiral sulfoxides, such as (S)-omeprazole one of the world's best-selling drugs. This drug is an aryl benzyl sulfoxide. In this communication, a series of 7 simple racemic and enantiopure aryl benzyl sulfoxides was investigated. The main interactions that make up the crystal structures were identified, and the energetic calculations were performed. This research was aided by the recent release of the CrystalExplorer17 program [7], that is able also to estimate reliable lattice energies, starting from the experimental coordinates deriving from the X-ray diffraction experiments It is interesting to observe that these molecules can organise themselves in both a gauche- or in an anti- conformation depending on the interactions that form the crystals. As a final result, in the majority of cases, the energetic calculations seem to confirm the “Wallach rule”, but interesting exceptions emerge
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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