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Back to Natural Cinchona Alkaloids: Highly Enantioselective Michael Addition of Malononitrile to Enones
No full textAn efficient and convenient highly enantioselective
Michael addition of malononitrile to
enones has been developed by using quinine as the
organocatalyst. The adducts were isolated in excellent
yield and high asymmetric induction (up to
95% ee). An easy route to difficultly accessible
ester derivatives has been also disclosed
Methyl and phenyl substituent effects on the catalytic behavior of NHC ruthenium complexes
No full textNew second-generation ruthenium benzylidene and isopropoxybenzylidene catalysts bearing N-heterocyclic carbene (NHC) ligands with o-biphenyl groups at the N-atoms and syn methyl or phenyl groups on the backbone were obtained and their catalytic behaviors were compared to those of analogous N-o-tolyl catalysts in standard ring-closing metathesis (RCM) reactions. A pronounced difference in catalyst efficiency was observed depending on the nature of ortho-N-aryl substituents (methyl or phenyl). Notably, very impressive catalytic performances were exhibited by N-o-biphenyl complexes with a syn dimethyl backbone in the formation of di- and trisubstituted cycloalkenes. To rationalize catalytic results, methyl and phenyl substituent effects on the steric and electronic properties of NHC ligands were assessed through experimental and theoretical investigations involving ruthenium complexes as well as newly developed rhodium derivatives. Despite the different electron donor capacities of the examined carbenes, the steric differences shown by N-o-biphenyl and N-o-tolyl NHCs, although subtle, were found to be the key factor in addressing catalyst behavior
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
Olefin Metathesis Reactions Promoted by Ru-Catalysts with a Syn Substituted N-heterocyclic Carbene Backbone
Get PDF2012 - 2013The development of efficient catalytic systems dedicated to the formation of C−C double bonds from simple to highly functionalized alkenes represents a great challenge in modern organic synthesis. In addressing this challenge, olefin metathesis has become an extremely versatile tool simplifying synthetic routes to numerous and valuable natural and unnatural products dramatically. Breakthroughs made in the last two decades have applied mechanistic understanding in the design of innovative and well-defined homogeneous pre-catalysts, increasing the appeal of this highly atom efficient reaction methodology.
Ruthenium based metathesis catalysts have received considerable attention because of their tolerance to oxygen, moisture and many functional group, but even more because it is possible to control catalyst activity by fine tuning of the steric and electronic properties of the ligands.
Major improvements in Ru-based pre-catalysts were achieved through the incorporation of N-heterocyclic carbene (NHC) as ancillary ligand. In general, N-aryl bulk was found to increase activity, whereas increased backbone substitution decreased activity but increased catalyst lifetime. The development of this class of complexes has broadened the scope and the utility of the olefin metathesis reaction in both organic synthesis and polymer science.
The research group in which I developed my PhD thesis, recently focused on the investigation of NHC ligand symmetry in ruthenium based complexes underlining the pivotal role played by NHC backbone substitution in the design of highly active pre-catalyst for the RCM of hindered di-olefins. This thesis was aimed to furnish a deeper understanding of the effect of syn backbone substitution on catalyst activity by creating a properly designed catalyst library. In particular, besides testing their activity toward many different metathesis reactions, we performed conformational studies of these newly synthesized complexes (via NMR and DFT calculations)... [edited by Author]XII n.s
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