1,720,988 research outputs found
N-Heterocyclic carbene-catalyzed deaminative cross-coupling of aldehydes with Katritzky pyridinium salts
No full textThis journal is © The Royal Society of Chemistry.By employing an N-heterocyclic carbene (NHC) catalyst, we developed a versatile catalytic system that enables deaminative cross-coupling reactions of aldehydes with redox-active pyridinium salts. Katritzky pyridinium salts behave as single-electron oxidants capable of generating alkyl radicals enabled by the redox properties of the enolate form of Breslow intermediates. The resultant alkyl radical undergoes efficient recombination with the NHC-bound aldehyde-derived carbonyl carbon radical for the formation of a C-C bond. The mild and transition metal-free reaction conditions tolerate a broad range of functional groups, and its utility has been further demonstrated by the modification of a series of peptide feedstocks and application to the three-component dicarbofunctionalization of olefins11sciescopu
Functionalization of Pyridinium Derivatives with 1,4-Dihydropyridines Enabled by Photoinduced Charge Transfer
No full text© 2020 American Chemical Society. By exploiting electron donor-acceptor (EDA) complexes between 1,4-dihydropyridines and N-amidopyridinium salts under visible light irradiation, we discovered that photoinduced intermolecular charge transfer induces a single-electron transfer event without requiring a photocatalyst for the facile functionalization of pyridines. The generality of this method is amenable to various types of 1,4-dihydropyridines radical precursors to generate structurally different radicals such as alkyl, acyl, and carbamoyl radicals, ultimately providing facile access to synthetically valuable C4-functionalized pyridines. A broad range of functional groups are well accommodated under mild and metal-free conditions, and the synthetic utility of the present method is showcased by the late-stage functionalization of a variety of biologically relevant pyridine-based compounds, pharmaceuticals, and peptide feedstocks11Nsciescopu
Visible-Light-Induced Pyridylation of Remote C(sp3)?H Bonds by Radical Translocation of N-Alkoxypyridinium Salts
No full textMetal-free, visible-light-induced site-selective heteroarylation of remote C(sp3)?H bonds has been accomplished through the design of N-alkoxyheteroarenium salts serving as both alkoxy radical precursors and heteroaryl sources. The transient alkoxy radical can be generated by the single-electron reduction of an N-alkoxypyridinium substrate by a photoexcited quinolinone catalyst. Subsequent radical translocation of the alkoxy radical forms a nucleophilic alkyl radical intermediate, which undergoes addition to the substrate to achieve remote C(sp3)?H heteroarylation. This cascade strategy provides a powerful platform for remote C(sp3)?H heteroarylation in a controllable and selective manner and is well suited for late-stage functionalization of complex bioactive molecules. ⓒ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhe
Visible-light-induced cascade radical ring-closure and pyridylation for the synthesis of tetrahydrofurans
No full text© 2019 The Royal Society of Chemistry. Reported is a novel visible-light-enabled alkoxy radical ring-closure and pyridylation from N-alkenyloxypyridinium salts serving as both alkoxy radical precursors and heteroaryl sources. This strategy features a photoredox tandem radical process involving a sequential fragmentation of an N-alkoxypyridinium salt, a radical cyclization process, and a pyridylation process. This method exhibited broad substrate scope, good functional group compatibility, and metal-free mild conditions, offering a powerful synthetic tool for assembling various pyridine-tethered tetrahydrofurans and late-stage functionalization of complex biorelevant molecules. Moreover, radical cascade cyclization could be successfully achieved, leading to the synthesis of previously challenging and important pyridine-tethered bicyclic oxaspiro ring systems11sci
Direct Phosphonation of Quinolinones and Coumarins Driven by the Photochemical Activity of Substrates and Products
No full textLight-promoted phosphonation of quinolinones
and coumarins was
developed without the need for an external photocatalyst. Investigations
support a mechanism whereby both starting materials and products act
as photosensitizers upon excitation using compact fluorescent light
sources to photochemically promote the dissociation of the N–O
bond in the pyridinium salt by a single electron transfer pathway.
A wide range of quinolinone and coumarin substrates can be utilized
in the phosphonation process under mild reaction conditions
Site-Selective Functionalization of Pyridinium Derivatives via Visible-Light-Driven Photocatalysis with Quinolinone
No full text© Copyright 2019 American Chemical Society.The selective installation of phosphinoyl and carbamoyl moieties on the pyridine scaffold is an important transformation in synthetic and medicinal chemistry. By employing quinolinone as an efficient organic photocatalyst, we developed a catalytic system driven by visible light that forms phosphinoyl and carbamoyl radicals, which react with various heteroarenium derivatives under mild, transition-metal-free conditions. This straightforward and environmentally friendly synthetic method represents a new approach to site-divergent pyridine functionalization that offers considerable advantages in both simplicity and efficiency. Ambient temperature is sufficient for the formation of the reactive radicals, and the site-selectivity can be switched from C2 to C4 by changing the radical coupling sources. Under standard reaction conditions, phosphinoyl radicals give access to C4 products, while carbamoyl radicals selectively give C2 products. We found that the carbamoyl radical overcomes the intrinsic preference for forming the ortho-product by allowing the oxo functionality of the carbamoyl radical to electrostatically engage the nitrogen of the pyridinium substrate, which preferentially gives the ortho-product. The phosphinoyl radical cannot engage in the same interaction, because the phosphorus is too large. This novel synthetic route tolerates a broad range of substrates and provides a convenient and powerful synthetic tool for accessing the core structures of numerous privileged scaffold
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
- …
