1,720,958 research outputs found
Electrocatalytic Formal C(sp2)–H Alkylations via Nickel‐Catalyzed Cross‐Electrophile Coupling with Versatile Arylsulfonium Salts
No full textProducing sp3‐hybridized carbon‐enriched molecules is of particular interest due to their high success rate in clinical trials. The installation of aliphatic chains onto aromatic scaffolds was accomplished by nickel‐catalyzed C(sp2)–C(sp3) cross‐electrophile coupling with arylsulfonium salts. Thus, simple non‐prefunctionalized arenes could be alkylated through the formation of aryldibenzothiophenium salts. The reaction employs an electrochemical approach to avoid potentially hazardous chemical redox agents, and importantly, the one‐pot alkylation proved also viable, highlighting the robustness of our approach
Weak-coordination-auxiliary aminocatalysis enables directed [3 + 2] cyclization for 2-acylindolizines
No full textA weak-coordination-auxiliary amino-catalyzed approach enables directed [3 + 2] cyclization of α,β-unsaturated ketones and N-heteroaryl ketones for the desired 2-acylindolizines
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an iminium ion/enamine tandem sequence.The synthesis of 2-acylindolizines, possessing a readily modifiable ketone group, is of significant importance as it provides versatile precursors for the preparation of various indolizines. However, due to the electronically less active and more sterically demanding nature of α,β-unsaturated ketones toward iminium formation with an aminocatalyst, the efficient one-pot transformation of α,β-unsaturated ketones for distinct 2-acylindolizines bearing sensitive groups represents a challenge for synthetic chemists. Herein, we report a weak-coordination-auxiliary amino-catalyzed approach that enables directed [3 + 2] cyclization of α,β-unsaturated ketones and N-heteroaryl ketones for the desired 2-acylindolizines
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an iminium ion/enamine tandem sequence. A highly broad range of commercially available α,β-unsaturated ketones (internal, terminal, and cyclic enones) can act as coupling partners for readily accessible 2-acylindolizines relative to the existing state-of-the-art methods. Control experiments and in-depth DFT calculations highlight the importance of weakly coordinated glycine's carboxylic group in promoting the intramolecular cyclization and 1,5-proton transfer processes.A weak-coordination-auxiliary amino-catalyzed approach enables directed [3 + 2] cyclization of α,β-unsaturated ketones and N-heteroaryl ketones for the desired 2-acylindolizines
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an iminium ion/enamine tandem sequence.The synthesis of 2-acylindolizines, possessing a readily modifiable ketone group, is of significant importance as it provides versatile precursors for the preparation of various indolizines. However, due to the electronically less active and more sterically demanding nature of α,β-unsaturated ketones toward iminium formation with an aminocatalyst, the efficient one-pot transformation of α,β-unsaturated ketones for distinct 2-acylindolizines bearing sensitive groups represents a challenge for synthetic chemists. Herein, we report a weak-coordination-auxiliary amino-catalyzed approach that enables directed [3 + 2] cyclization of α,β-unsaturated ketones and N-heteroaryl ketones for the desired 2-acylindolizines
via
an iminium ion/enamine tandem sequence. A highly broad range of commercially available α,β-unsaturated ketones (internal, terminal, and cyclic enones) can act as coupling partners for readily accessible 2-acylindolizines relative to the existing state-of-the-art methods. Control experiments and in-depth DFT calculations highlight the importance of weakly coordinated glycine's carboxylic group in promoting the intramolecular cyclization and 1,5-proton transfer processes.Georg-August-Universität Göttingen https://doi.org/10.13039/501100003385China Scholarship Council https://doi.org/10.13039/501100004543H2020 European Research Council https://doi.org/10.13039/100010663dDeutsche Forschungsgemeinschaft https://doi.org/10.13039/501100001659Deutsches Zentrum für Herz-Kreislaufforschung https://doi.org/10.13039/10001044
Iron‐catalyzed C−H Alkylation/Ring Opening with vinylbenzofurans Enabled by Triazoles
No full textWe report an unprecedented iron‐catalyzed C−H annulation using readily available 2‐vinylbenzofurans as the reaction pattern. The redox‐neutral strategy, based on cheap, non‐toxic and earth abundant iron catalysts, exploits triazole assistance to promote a cascade C−H alkylation, benzofuran ring‐opening and insertion into a Fe−N bond, to form highly functionalized isoquinolones. Detailed mechanistic studies supported by DFT calculations fully disclosed the manifold of the iron catalysis
Enantioselective C–H annulations enabled by either nickel- or cobalt-electrocatalysed C–H activation for catalyst-controlled chemodivergence
No full textAbstract Enantioselective electrocatalysis shows unique potential for the sustainable assembly of enantiomerically enriched molecules. This approach allows electro-oxidative C–H activation to be performed paired to the hydrogen evolution reaction. Recent progress has featured scarce transition metals with limited availability. Here we reveal that the earth-abundant 3 d transition metals nickel and cobalt exhibit distinctive performance for enantioselective electrocatalysis with chemodivergent reactivity patterns. Enantioselective desymmetrizations of strained bicyclic alkenes were achieved through C–H annulations. A data-driven optimization of chiral N , O -bidentate salicyloxazoline-type ligands was crucial for enhancing enantioselectivity in nickel electrocatalysis. Notably, in the transition state of the enantio-determining step, secondary weak attractive π – π and CH– π interactions were identified, reflecting the informed adaptations in the ligand design. Detailed mechanistic investigations by experimental and computational studies revealed for the nickel electrocatalysis a C–N bond-forming reductive elimination from nickel(III) and for the cobalt electrocatalysis a C–C bond-forming nucleophilic addition from cobalt(III) as the product-determining steps
Parametrization of κ 2 - N , O -Oxazoline Preligands for Enantioselective Cobaltaelectro-Catalyzed C–H Activations
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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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