1,720,977 research outputs found
Mechanistic Insights into Direct Amide Bond Formation Catalyzed by Boronic Acids: Halogens as Lewis Bases
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Origin of Enantioselectivity in the Organocatalytic Reductive Amination of α-Branched Aldehydes
No full textThe reason for enantioselectivity in thereductive amination of α-branched aldehydes wasinvestigated. The relative energies of all the diastereomeric transition states for hydride transfer of a suitable computational model were calculated at the B3LYP/6-311+(2d,2p) level of theory. Our calculations successfully reproduce and rationalize the experimentally observed stereochemical outcome of the reaction.</p
Cinchona Alkaloids in Asymmetric Organocatalysis
No full textThis article reviews the applications of cinchona alkaloids as asymmetric catalysts. In the last few years, characterized by the resurgence of interest in asymmetric organocatalysis, cinchona derivatives have been shown to catalyze an outstanding array of chemical reactions, often with remarkable stereoselectivity. This work presents an overview of the transformations developed in the period from 2001 through 2009, highlighting applications in the synthesis of bioactive molecules and natural products
Theoretical Study of Mechanism and Stereoselectivity of Catalytic Kinugasa Reaction
No full textThe mechanism of the catalytic Kinugasa reaction is investigated by means of density functional theory calculations. Different possible mechanistic scenarios are presented using phenanthroline as a ligand, and it is shown that the most reasonable one in terms of energy barriers involves two copper ions. The reaction starts with the formation of a dicopper-acetylide that undergoes a stepwise cycloaddition with the nitrone, generating a five-membered ring intermediate. Protonation of the nitrogen of the metalated isoxazoline intermediate results in ring opening and the formation of a ketene intermediate. This then undergoes a copper-catalyzed cyclization by an intramolecular nucleophilic attack of the nitrogen on the ketene, affording a cyclic copper enolate. Catalyst release and tautomerization gives the final beta-lactamic product. A comprehensive study of the enantioselective reaction was also performed with a chiral bis(azaferrocene) ligand. In this case, two different reaction mechanisms, involving either the scenario with the two copper ions or a direct cycloaddition of the parent alkyne using one copper ion, were found to have quite similar barriers. Both mechanisms reproduced the experimental enantioselectivity, and the current calculations can therefore not distinguish between the two possibilities.</p
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
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