1,720,990 research outputs found
Divergent Pathways Involving 1,3-Dipolar Addition and N−N Bond Splitting of an Organic Azide across a Zirconium Methylidene
No full textThe zirconium methylidene (PNP)Zr=CH2(OAr) (1) reacts with N3Ad to give two products (PNP)Zr=NAd(OAr) (2) and (PNP)Zr(η2-N=NAd)(N=CH2)(OAr) (3), both resulting from a common cycloaddition intermediate (PNP)Zr(CH2N3Ad)(OAr) (A). Using a series of control experiments in combination with DFT calculations, it was found that 2 results from a nitrene by a carbene metathesis reaction in which N2 acts as a delivery vehicle and forms N2CH2 as a side product. In the case of 3, N−N bond splitting of the azide at the α-position allowed the isolation of a rare example of a parent ketimide complex of zirconium. Isotopic labeling studies and solid-state X-ray analysis are presented for 2 and 3, in addition to an independent synthesis for the former. T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei
Gold(I)-Catalyzed Hydroxy Group Assisted C(sp<sup>2</sup>)–H Alkylation of Enaminones with Diazo Compounds To Access 3‑Alkyl Chromones
No full textA strategy for expedient
synthesis of 3-substituted chromones from
easily available o-hydroxyarylenaminones and
diazo compounds has been developed. Carefully conducted experimental
and computational studies led us to propose an uncommon mechanistic
pathway involving the hydroxyl group assisted alkylation of enaminones
with in situ generated gold carbenes
Schrock vs Fischer carbenes: A quantum chemical perspective
No full textEarly and late transition metal-carbon multiple bonds that have been widely used for many catalytic processes, organic transformations, and olefin metathesis reactions are described. Especially, the development of Schrock and Fischer type olefin metathesis catalysts aided by computational studies is discussed, focusing on work that aims at improving the reactivity, stability, and regioselectivity. The intriguing electronic feature and reactivity of a titanium alkylidyne, which leads to many unique transformations of organic molecules, are summarized. The development of Fischer type olefin metathesis catalysts to control the regioselectivity in cyclopolymerization of diynes with RuII-alkylidene catalysts employing quantum chemical studies is summarized. © 2019 Elsevier Inc.11sci
The Mechanism of Copper-Catalyzed Trifunctionalization of Terminal Allenes
Full text linkA highly selective copper-catalyzed trifunctionalization of allenes has been established based on diborylation/cyanation with bis(pinacolato)diboron (B 2 pin 2 ) and N-cyano-N-phenyl-p-toluenesulfonamide (NCTS). The Cu-catalyzed trifunctionalization of terminal allenes is composed of three catalytic reactions (first borocupration, electrophilic cyanation, and second borocupration) that provide a densely functionalized product with regio-, chemo- and diastereoselectivity. Allene substrates have multiple reaction-sites, and the selectivities are determined by the suitable interactions (e.g., electronic and steric demands) between the catalyst and substrates. We employed DFT calculations to understand the cascade copper-catalyzed trifunctionalization of terminal allenes, providing densely-functionalized organic molecules with outstanding regio-, chemo- and diastereoselectivity in high yields. The selectivity challenges presented by cumulated π-systems are addressed by systematic computational studies; these give insight to the catalytic multiple-functionalization strategies and explain the high selectivities that we see for these reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei
Experimental and Computational Study of the (<i>Z</i>)‑Selective Formation of Trisubstituted Olefins and Benzo-Fused Oxacycles from the Ruthenium-Catalyzed Dehydrative C–H Coupling of Phenols with Ketones
Full text linkThe
cationic Ru–H complex was found to be an effective catalyst
for the dehydrative C–H coupling of phenols with ketones to
form the trisubstituted olefin products. The coupling of phenol with
linear ketones led to highly stereoselective formation of the (Z)-olefin products. The dehydrative coupling of phenol with
enones and diones efficiently formed the benzopyrene and related oxacyclic
derivatives. The reaction of 3,5-dimethoxyphenol with cyclohexanone-2,2,6,6-d4 showed a significant H/D exchange to both
vinyl and α-CH2 positions on the olefin product (72–75%
D). A significant carbon isotope effect was observed on the ortho-arene carbon of the olefin product. The free energies
of intermediate species for the entire catalytic cycle were successfully
computed by using the DFT method. The DFT study revealed that the E/Z stereoselectivity is a result of the
energy difference in the insertion step of ortho-metalated
phenol to an enol form of the ketone substrate (ΔΔE = 9.6 kcal/mol). The coupling method provides a direct
catalytic C–H olefination method for ketones to form trisubstituted
olefins without employing any reactive reagents or forming any wasteful
byproducts
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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