1,720,957 research outputs found
Stereoselective catalytic sulfoxidations mediated by new titanium and zirconium C-3 trialkanolamine complexes
No full textMonomeric Ti(IV)/C3 trialkanolamine complexes are effective catalysts in the
stereoselective sulfoxidation of alkyl aryl sulfides (ee’s up to 84%, 0.1% of catalyst). Such
complexes were shown to have a biphilic nature, behaving as electrophilic oxidants towards
sulfides while a nucleophilic pathway dominates the oxidation of sulfoxides. The analogous
Zr(IV) complexes, likely dimeric, are even better and more general stereoselective sulfoxidation
catalysts (ee’s up to 91%, 50 T.O.), affording sulfoxides with the opposite absolute
configuratio
Effective Oxidation of Secondary Amines to Nitrones with Alkyl Hydroperoxides Catalysed by (Trialkanolaminato)titanium(IV) Complexes
No full textThe effective catalytic oxidation of secondary amines to nitrones with alkyl hydroperoxides as the primary oxidants is described. The titanium alkoxide catalysts are protected from the water co-product by the combined use of a tightly binding trialkanolamine ligand and molecular sieves. Nitrones can be obtained in high yields (up to 98%) under homogeneous, anhydrous conditions and even in the absence of solvent. The reactions are fast (2-7 h) and good selectivity and complete conversion can be achieved with as little as 1% catalys
Stereoselective Control by Face-to-Face Versus Edge-to-Face Aromatic Interactions: The Case of C-3-Ti-IV Amino Trialkolate Sulfoxidation Catalysts
No full textThe stereoselective oxidation of differently functionalised benzyl phenyl sulfides has been examined by using enantiopure TiIV trialkanolamine complexes. These complexes efficiently catalyse the sulfoxidation with good stereoselectivities. The data highlight the contribution to the stereoselectivity of steric effects and non-covalent π–π interactions between the aromatic rings of the TiIV complex and those pertaining to the substrates. Enantiomeric excesses have been correlated with the electrostatic potential surfaces (EPS) of the reacting sulfides. The overall study leads to a mechanistic interpretation that explains the stereoselectivity of the system and dissects the role of aromatic and steric interactions in the stereoselective process
The first chiral zirconium(IV) catalyst for highly stereoselective sulfoxidation
No full textPartially-hydrolized chiral Zr(IV) catalysts, based on C3 symmetric trialkanolamine ligands 1a-c in the presence of cumyl hydroperoxide, provide a new homogeneous system for the enantioselective sulfoxidation of a wide range of aryl alkyl sulfides. Ee’s in the range 80-90% are obtained, through the cooperative effect of two independent stereoselective processes, namely the direct asymmetric oxidation of the sulfide to the sulfoxide and its subsequent kinetic resolution to sulfone, allowing the recover of the desired product with yields up to 35%. In every reaction the peroxide is converted quantitatively with as little as 2% of catalys
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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