1,720,967 research outputs found
Silver-Catalyzed Cyclopropanation of Alkenes Using N-Nosylhydrazones as Diazo Surrogates
No full textAn efficient silver-catalyzed [2 + 1] cyclopropanation of
sterically hindered internal alkenes with diazo compounds in which room-
temperature-decomposable N-nosylhydrazones are used as diazo surro-
gates is reported. The unexpected unique catalytic activity of silver was
ascribed to its dual role as a Lewis acid activating alkene substrates and as a
transition metal forming silver carbenoids. A wide range of internal
alkenes, including challenging diarylethenes, were suitable for this
protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions
Silver-Catalyzed Tandem C=C Bond Hydroazidation/Radical Addition/Cyclization of Biphenyl Acetylene: One-Pot Synthesis of 6-Methyl Sulfonylated Phenanthridines
Full text linkA silver-catalyzed tandem carbon!carbon triple bond hydroazidation, radical addition, and cyclization of biphenyl acetylene is described under mild conditions, leading to the formation of 6-methyl sulfonylated phenanthridines in good yields. In this novel cascade reaction, most of the atoms are incorporated into the product without cleavage of the CC bond. Mechanistic studies suggest the reaction should proceed through an iminyl radical reactive intermediate
Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis
No full textThe decomposition of N-tosylhydrazones is a safe and convenient method for the generation of donor carbenes. However, alkynyl carbenes cannot be iso- lated by this route because they readily undergo intramo- lecular cyclization to pyrazoles as soon as formed from al- kynyl N-tosylhydrazones. Here, the use of alkynyl N-nosyl- hydrazones for the in situ generation of alkynyl carbenes and their coupling reaction with boronic acids under metal-free conditions is reported, giving rise to a wide array of di- and trisubstituted allenes. Preliminary mecha- nistic investigations demonstrated that g-protodeboration of propargyl boric acid was responsible for the initial allene formation. This methodology based on the nosyl group allows for novel transformations that involve an al- kynylcarbene transient species
Silver-Based Radical Reactions: Development and Insights
Full text linkSilver-mediated reactions have recently emerged as a frontier area in organic chemistry. Indeed, the remarkable property of silver as a single- electron oxidant has led to its exploitment into the field of radical chemistry. The excellent reactivity and high selectivity exhibited by silver salts in the ac- tivation of different organic substrates place them among the most prominent reagents in free radical reactions. The present review aims to provide the readers with a critical overview of silver-based radi- cal reactions, especially emphasizing the reactivity, selectivity and a detailed description of the reaction mechanism with particular attention to the role of Ag(I)-based catalysts/reactants. The selected exam- ples have been classified according to the type of radicals generated, namely carbon-centered radicals and heteroatom (such as nitrogen, oxygen, phospho- rus, sulfur)-centered radicals. The generation of such radicals as well as their application in synthetic pro- cesses including oxidation, addition, coupling, and radical cascades have been described in detail
α-Substituted vinyl azides: An emerging functionalized alkene
Full text linkVinyl azides are highly versatile synthons that provide access to numerous N-heterocycles and other functional groups. a-Substituted vinyl azides (azido vinylidenes) are a special class that display unique reactivity, able to react not only as azides, but also as radical acceptors, enamine-type nucleophiles, and even electrophiles, thus delivering a wide range of nitrogen-containing compounds and their derivatives. An impressive variety of intermediates – such as iminodiazonium ions, nitrilium ions, iminyl radicals, and metal enaminyl radicals – can be generated from vinyl azides and exploited in cycloadditions, C–H functionalizations, hydrolysis processes, and cascade reactions under transition metal/photoredox catalysis. In addition to presenting synthetic protocols to access vinyl azides, this Review offers a comprehensive coverage of the development of their multifaceted reactivity, and highlights their potential as versatile precursors for synthetic applications
Use of trifluoroacetaldehyde N-tfsylhydrazone as a trifluorodiazoethane surrogate and its synthetic applications
Full text linkTrifluorodiazoethane (CF3CHN2), a highly reactive fluoroalkylating reagent, offers a useful means to introduce trifluoromethyl groups into organic molecules. At present, CF3CHN2 can only be generated by oxidation of trifluoroethylamine hydrochloride under acidic conditions; due to its toxic and explosive nature, its safe generation and use remains a prominent concern, hampering wider synthetic exploitation. Here we report the development of trifluoroacetaldehyde N-tfsylhydrazone (TFHZ-Tfs) as a CF3CHN2 surrogate, which is capable of generating CF3CHN2 in situ under basic conditions. The reaction conditions employed in this chemistry enabled a difluoroalkenylation of X-H bonds (X = N, O, S, Se), affording a wide range of heteroatom-substituted gem-difluoroalkenes, along with Doyle-Kirmse rearrangements and trifluoromethylcyclopropanation reactions, with superior outcomes to approaches using pre-formed CF3CHN2. Given the importance of generally applicable fluorination methodologies, the use of TFHZ-Tfs thus creates opportunities across organic and medicinal chemistry, by enabling the wider exploration of the reactivity of trifluorodiazoethane
Silver-catalyzed direct conversion of epoxides into cyclopropanes using N-triftosylhydrazones
No full textAbstract Epoxides, as a prominent small ring O-heterocyclic and the privileged pharmacophores for medicinal chemistry, have recently represented an ideal substrate for the development of single-atom replacements. The previous O-to-C replacement strategy for epoxides to date typically requires high temperatures to achieve low yields and lacks substrate range and functional group tolerance, so achieving this oxygen-carbon exchange remains a formidable challenge. Here, we report a silver-catalyzed direct conversion of epoxides into trifluoromethylcyclopropanes in a single step using trifluoromethyl N-triftosylhydrazones as carbene precursors, thereby achieving oxygen-carbon exchange via a tandem deoxygenation/[2 + 1] cycloaddition. The reaction shows broad tolerance of functional groups, allowing routine cheletropic olefin synthesis in a strategy for the net oxygen-carbon exchange reaction. The utility of this method is further showcased with the late-stage diversification of epoxides derived from bioactive natural products and drugs. Mechanistic experiments and DFT calculations elucidate the reaction mechanism and the origin of the chemo- and stereoselectivity
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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