1,721,170 research outputs found
Oxygen-Bearing Lithium Compounds in Modern Synthesis
No full textThis chapter addresses the genesis, reactivity and applications of oxygen-bearing lithium compounds in organic synthesis, focusing in particular on the most recent inspiring contributions in this area over the past decade. Key selected examples dealing with stereochemical aspects of asymmetric transformations and natural product synthesis are herein presented, and special attention is also deserved to mechanistic, structural, and spectroscopic studies with the aim of providing readers, wherever applicable, information about the structure-reactivity relationship which is central to the organolithium field. The chapter begins with an overview of α-lithiated oxygen-substituted compounds split into two subsections: acyclic (nonstabilized and dipole-stabilized) and cyclic α-alkoxy organolithiums, discussing their chemical and configurational stability as well as synthetic applications. Next, a section describes the effectiveness and advantages of ortho-lithiation for regioselective functionalization/transformations of oxygen-bearing aromatic compounds. Finally, new synthetic opportunities associated with remote or "through-space"-induced lithiation are also highlighted. The chapter concludes with an outlook section concentrating on new directions and opportunities in the area of oxygenated compounds which, hopefully, may contribute to the future growth of the field
Modern Lithium Carbenoid Chemistry (Chapter 11)
No full textThis chapter addresses the reactivity of a-lithiated organohalogens, ethers, and amines. Special emphasis is placed on their structure, configurational stability, and applications to synthetic chemistry in light of the recent advances that have appeared in the literature over the past decade. The chapter highlights the role played by aggregation and solvation on the structure-reactivity relationship. The characteristic features of a-lithiated halogens (Li/Hal) and a-lithiated ethers (Li/OR) carbenoids are discussed. These include: (i) the electrophilic reactivity, as observed in reactions with nucleophiles; (ii) the deshielding of the C atom in the NMR spectrum; and (iii) the C-X bond length in the corresponding solid state structures. The chapter concludes with a discussion on lithium-nitrogen carbenoids
Reductive Lithiation and Multilithiated Compounds in Synthesis
No full textThe reductive metalation procedure is a powerful approach to the generation of non-functionalized and functionalized organolithium reagents. As an alternative and a complement to the widely employed deprotonation procedure, reductive lithiation strongly extends the number of precursors which can be transformed into the desired organometals and, in several instances, shows distinct advantages in terms of chemo- regio- and stereo-selectivities as well as mildness of reaction conditions. In this chapter it is reported a selection of the relevant literature on this topic published during the period 2000 – 2012, focusing on significant applications of this methodology in organic synthesi
Function of the photosynthetic reaction center from Rhodobacter sphaeroides in deep eutectic solvents
No full textDeep eutectic solvents (DESs) are emerging as a new class of green solvents, with the potentiality of replacing organic solvents in many applications both at industrial and laboratory scale. The intriguing possibility of carrying out bio-catalytic reactions has been recently explored using hydrolases with promising results [1]. In this work we aim to offer a complete characterization of the behavior of the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides in a series of choline chloride-based DESs. The stable charge-separated state of RC, attained under illumination, represents the first step of light energy conversion into chemical energy in phototrophic organisms and a number of reports have recently enlightened the possibility of effectively exploiting RC photo-activity in bio-devices. The employment of non-aqueous solvents would then open the way to a wider range of technological applications. Moreover, the fully characterized RC is the ideal model for carrying out basic studies of protein-solvent interactions, due to its information-rich optical spectrum and its light-triggered enzymatic activity.
Herein we report that RC (a) is stable in all the DESs tested, (b) is able to generate the charge-separated state under illumination, and (c) even to perform its natural photocycle. We have indeed demonstrated in DES environment that RC can effectively promote under light the reduction of quinone molecules by withdrawing electrons from cytochrome c. Finally, as an example of biotechnological application, a photo-electrochemical cell based on DES-dissolved RC has been designed and successfully employed to generate photocurrents arising from the reduction of the electron-donor ferrocene-methanol
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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