1,720,992 research outputs found
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
Aggregation Dynamics Of Lithium Diisopropylamide And Their Mechanistic Influence On Reactivity
Get PDFLithium diisopropylamide (LDA) is the premier base in organic chemistry ever since its development in 1950 by Hammel and Levine. In a comprehensive survey of reagents in approximately 500 natural product syntheses, LDA emerged at the top attesting to its wide range of synthetic applications and efficacy. Significant efforts to the understanding of its complex coordination chemistry and affiliated mode of reactivity has led to a review by Collum that painted a seemingly coherent and general picture of LDA-mediated reactions. This mechanistic view, however, was gleaned under conditions in which LDA aggregate equilibria proceeded very quickly compared to the rate of the reaction. The kinetics of LDA aggregation, therefore, were inconsequential and hidden to the investigator. While shifting the focus to more reactive substrates that required a reduction in temperature to -78 °C to maintain convenient time scales for monitoring reaction rates, something odd happened. Instead of conventional first-order substrate decays that had been observed for almost twenty years, we began to observe nonstandard curvatures with evidence of substrate-independent rates, autocatalysis and lithium salt catalysis. The mechanistic intricacy was eventually traced to one common culprit: the rate of LDA aggregation at -78 °C in tetrahydrofuran (THF) had become rate-limiting with the rate of substrate reaction now being post-rate-limiting. The work described herein presents three experimental accounts that peer into the mechanism of three LDA-aggregation limited reactions: LDA-mediated ortholithiations of fluoroand trifluoromethyl arenes and the 1,4-addition to unsaturated esters. Although the studies are internally consistent, a chaotic mechanistic picture emerged that appeared to lack coherency. A theoretical treatise of LDA aggregation concludes this work in an attempt to comprehend the source of this complexity and garner a more general understanding. Dozens of reactive forms of LDA emerged as part of a potential energy surface that began to explain our experimental findings. Despite significant advances, we have only scratched the surface of the mechanistic diversity of LDA aggregation. The kinetic tools and knowledge, however, are now in place to peer ever deeper into this mechanistic extravaganza
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
Stereoselective Epoxide Polymerization: Exploring Isoselective Bimetallic Catalysts, Ionic Cocatalysts, And The Use Of Alcohols As Chain Transfer Agents
Get PDFFor over 50 years, researchers have attempted to create catalysts for stereoselectively polymerizing epoxides. Until very recently, catalysts for the polymerization of racemic epoxides gave mixtures of atactic and isotactic material or moderate tacticity polyethers. In 2008, the Coates Research Group reported a bimetallic catalyst system for enantioselectively polymerizing racemic epoxides to highly isotactic enantiopure polyethers. Catalyst simplification led to a highly active, isoselective catalyst. This catalyst is capable of quantitatively polymerizing racemic terminal epoxides to yield isotactic polyethers. Organic ionic compounds were synthesized and investigated as cocatalyts. The identities of both the cation and anion were systematically varied and subsequent polymerization reactivity was studied. The nature of the ionic cocatalyst dramatically impacted the rate and enantioselectivity of the catalyst system. The ionic cocatalyst [P(N=P(N(CH2)4)3)4+][tBuCO2-] produced a catalyst system that exhibited the greatest activity and selectivity for a variety of mono-substituted epoxides. The addition of alcohols to these bimetallic polymerization systems leads a lowering of molecular weight due to chain transfer. Polymer end-groups can be determined by choice of alcohol, addition of diols allows for the synthesis of isotactic telechelic poly(propylene oxide) diols
Mechanistic Studies And Applications Of The Copper-Catalyzed Rearrangement Of Vinyl Heterocycles
Get PDFThe development of new synthetic methods advances the study of chemistry on many fronts, including new drugs for the treatment of human disease and new materials. The copper-catalyzed rearrangement of vinyl heterocycles is one such method, providing access to a wide array of five-membered oxygen, nitrogen, and sulfur-containing heterocycles. To harness the true potential of a new methodology, an understanding of the mechanistic underpinnings is required. Utilizing an array of mechanistic tools, the rearrangement of vinyl aziridines is studied in detail. Using mechanistically inspired metal additives, a dramatic acceleration of the rearrangement of vinyl aziridines was realized. Demonstrating the use of in situ reducing agents significantly accelerate the rearrangement, suggesting a copper(I) species is the active catalytic species. This was corroborated through the use of (COD)Cu(hfacac) as a copper(I) starting point. Detailed NMR kinetic studies revealed the relative importance of olefin and sulfonamide electronics, and that the rearrangement is first order in substrate and first order in catalyst. As a result, a new catalytic system was discovered, which provides enhanced chemoselectivity and milder reaction conditions towards five-membered heterocycles. In addition, studying the mechanism provided a new direction for future rational catalyst designs towards more active catalysts, and catalysts for chiral applications, such as asymmetric desymmetrization. To further demonstrate the utility of the new copper(I) catalytic system, an expedient and scalable approach was designed utilizing (COD)Cu(hfacac) in the first total synthesis of members of a family of heterocyclic labdane natural products. Through synthesis, conformation and clarification of the structural assignment of isolated smallmolecules was realized. The route provided access to quantities of material for biological screening purposes, and the versatility of the route could be used to provide synthetic analogues of the natural structures
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
Evans Enolates: The Influence Of Aggregation And Solvation On The Mechanism Of The Aldol Addition To Lithiated Oxazolidinone-Derived Enolates
No full textThe results of a combination of 6Li and 13C NMR spectroscopic and computational studies of oxazolidinone-based lithium enolates-Evans enolates-in tetrahydrofuran (THF) solution revealed a mixture of dimers, tetramers, and oligomers (possibly ladders). The distribution depended on the structure of the oxazolidinone auxiliary, substituent on the enolate, and THF concentration (in THF/toluene mixtures). The unsolvated tetrameric form contained a D2d-symmetric core structure, whereas the dimers were determined experimentally and computationally to be trisolvates with several isomeric forms. Aldol additions to isobutyraldehyde and cyclohexanone with lithium enolates derived from Evans enolates are described. The trisolvated dimeric enolates undergo rapid addition to isobutyraldehyde to give a 12:1 syn:syn selectivity in high yield along with small amounts of one anti isomer. The efficacy of the addition depends critically on aging effects and the reaction quench. Unsolvated tetrameric enolates that form on warming the solutions are unreactive toward isobutyraldehyde and undergo retroaldol reaction under forcing conditions. Additions to cyclohexanone are relatively slow but form a ! single isomeric adduct in >80% yield. The ketone-derived aldolates are robust. All attempts to control stereoselectivity by controlling aggregation failed. Rate studies of addition to cyclohexanone trace the lack of aggregation-dependent selectivities to a monomer-based mechanism. The synthetic implications and possible utility of lithium enolates in Evans aldol additions are discussed.
- …
