1,721,028 research outputs found
NChloroNsodio-carbamates as a Practical Amidating Reagent for Scalable and Sustainable Amidation of Aldehydes under Visible Light
No full text© 2021 American Chemical Society. All rights reserved.Herein, we describe the scalability and sustainability investigations toward the visible light-mediated amidation of aldehydes with N-chloro-N-sodio-carbamates. The practicality credentials of N-chloro-N-sodio-carbamates for their use in multigram scale amidation reactions are proven by scale-up productions and thermogravimetric analysis. The described amidation, which was previously hampered in sustainability perspectives by the use of α,α,α-trifluorotoluene solvent, is now improved through re-optimizations using ethyl acetate as an alternative green solvent. The generality of multigram scale amidations is demonstrated by the conversion of a selected list of aldehydes, in which the corresponding amides can be isolated in high purity without the need for special purification procedures.11Nsciescopu
Ring-opening functionalizations of unstrained cyclic amines enabled by difluorocarbene transfer
No full textChemical synthesis based on the skeletal variation has been prolifically utilized as an attractive approach for modification of molecular properties. Given the ubiquity of unstrained cyclic amines, the ability to directly alter such motifs would grant an efficient platform to access unique chemical space. Here, we report a highly efficient and practical strategy that enables the selective ring-opening functionalization of unstrained cyclic amines. The use of difluorocarbene leads to a wide variety of multifaceted acyclic architectures, which can be further diversified to a range of distinctive homologative cyclic scaffolds. The virtue of this deconstructive strategy is demonstrated by successful modification of several natural products and pharmaceutical analogues. Cyclic amines are commonly present in natural products and synthetic compounds, but methods for their skeletal diversification are limited. Here, the authors report a strategy for selective ring-opening functionalization of unstrained cyclic amines to pluripotent products that can be further diversified.11sciescopu
Photoinduced α-C−H Amination of Cyclic Amine Scaffolds Enabled by Polar-Radical Relay
No full text© 2022 Wiley-VCH GmbH.Herein, we report a polar-radical relay strategy for α-C−H amination of cyclic amines with N-chloro-N-sodio-carbamates. The relay is initiated by in situ generation of cyclic iminium intermediate using N-iodosuccinimide (NIS) oxidant as an initiator, which then operates through a series of polar (addition and elimination) and radical (homolysis, hydrogen- and halogen atom transfer) reactions to enable the challenging C−N bond formation in a controlled manner. A broad range of α-amino cyclic amines were readily accessed with excellent regioselectivity, and the superb applicability was further demonstrated by functionalization of biologically relevant compounds.11Nsciescopu
Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect
No full text© XXXX The Authors. Published by American Chemical Society.Herein, we report a photoinduced transition-metal-free C(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronic acids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amide coupling partner. Chloride anion, either generated in situ by photodissociation of chlorinated solvent molecules or added separately as an additive, was found to play a critical cooperative role, thereby giving convenient access to a wide range of synthetically versatile N-arylamides under mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scale reactions, synthesis of 15N-labeled arylamides, and applicability toward biologically relevant compounds. On the basis of mechanistic investigations, two distinctive photoexcitations are proposed to function in the current process, in which the first excitation involving chloro-boron adduct facilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables the otherwise-inoperative 1,2-aryl migration of the thus-formed N-chloroamido-borate adduct.11Nsciescopu
Intramolecular hydroamidation of alkenes enabling asymmetric synthesis of β-lactams via transposed NiH catalysis
No full textSynthetic methods for constructing enantioenriched β-lactams are highly valuable given their ubiquity in bioactive compounds, most notably in antibiotics such as penicillins and carbapenems. Intramolecular hydroamidation of β,γ-unsaturated amides would provide a convenient means to reach this alluring chemical space, yet it remains limited due to the regioselectivity issue arising from the difficulty associated with the formation of strained four-membered rings. Here we describe a NiH-catalysed strategy that addresses this challenge through the use of readily accessible alkenyl dioxazolone derivatives. The reaction transcends the conventional NiH operation mode via a transposed mechanism initiated by N-activation, thus allowing for proximal C–N bond formation with excellent regioselectivity, regardless of the electronic properties of substituents. This mechanistic platform is also highly effective for the enantioselective intramolecular hydroamidation of alkenes to enable a convenient access to enantioenriched β-lactams. [Figure not available: see fulltext.] © 2023, The Author(s), under exclusive licence to Springer Nature Limited.11Nsciescopu
Merging NiH Catalysis and Inner-Sphere Metal-Nitrenoid Transfer for Hydroamidation of Alkynes
No full textThe formal hydroamination/hydroamidation utilizing metal hydride is an appealing synthetic tool for the construction of valuable nitrogen-containing compounds from unsaturated hydrocarbons. While significant advances have been made for the functionalizations of alkenes in this realm, the direct hydroamidation of alkynes remains rather limited due to the high feasibility of the key metal-alkenyl intermediate to choose other reaction pathways. Herein, we report a NiH-catalyzed strategy for the hydroamidation of alkynes with dioxazolones, which allows convenient access to synthetically useful secondary enamides in (E)-anti-Markovnikov or Markovnikov selectivity. The reaction is viable for both terminal and internal alkynes and is also tolerant with a range of subtle functional groups. With H2O found as an essential component for high catalyst turnovers, the involvement of inner-sphere nitrenoid transfer is proposed that outcompetes an undesired semireduction process, thus representing the first example to show the competence of Ni catalysis for metal-nitrenoid formation from dioxazolones.11Nsciescopu
Endo-Selective Intramolecular Alkyne Hydroamidation Enabled by NiH Catalysis Incorporating Alkenylnickel Isomerization
No full text© Intramolecular alkyne hydroamidation represents a straightforward approach for the access to synthetically valuable cyclic enamides. Despite some advances made in this realm, the ability to attain a precise regiocontrol still remains challenging, especially for endo cyclization that leads to six-membered and larger azacyclic rings. Herein, we report a NiH-catalyzed intramolecular hydroamidation of alkynyl dioxazolones that allows for an excellent endo selectivity, thus affording a range of six- to eight-membered endocyclic enamides with a broad scope. Mechanistic investigations revealed that Ni(I) catalysis is operative in the current system, proceeding via regioselective syn-hydronickelation, alkenylnickel E/Z isomerization, and Ni-centered inner-sphere nitrenoid transfer. In particular, the key alkenylnickel isomerization step, which previously lacked mechanistic understandings, was found to take place through the η2-vinyl transition state. The synthetic value of this protocol was demonstrated by diastereoselective modifications of the obtained endocyclic enamides to highly functionalized δ-lactam scaffolds.11Nsciescopu
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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