1,720,977 research outputs found
Nickel/Bis(oxazoline)-Catalyzed Asymmetric Kumada Reactions of Alkyl Electrophiles: Cross-Couplings of Racemic α-Bromoketones
No full textThe first asymmetric Kumada reactions of alkyl electrophiles are described, specifically, cross-couplings of racemic α-bromoketones with aryl Grignard reagents. Several features of this investigation are of interest. First, the couplings proceed at remarkably low temperature (−40 or −60 °C), which enables the asymmetric synthesis of racemization-prone alpha-arylketones. Second, dialkyl ketones undergo enantioselective coupling in good ee and yield. Third, readily available bis(oxazolines) are shown for the first time to be effective ligands for cross-couplings of alkyl electrophiles, thereby opening the door to new opportunities in asymmetric catalysis.National Institute of General Medical Sciences (U.S.) (Grant R01-GM62871)Merck Research LaboratoriesNovartis (Firm
Enantioselective Alkenylation via Nickel-Catalyzed Cross-Coupling with Organozirconium Reagents
No full textA new family of organometallic compounds, organozirconium reagents, are shown to serve as suitable partners in cross-coupling reactions of (activated) secondary alkyl electrophiles. Thus, the first catalytic method for coupling secondary α-bromoketones with alkenylmetal reagents has been developed, specifically, a mild, versatile, and stereoconvergent carbon−carbon bond-forming process that generates potentially labile β,γ-unsaturated ketones with good enantioselectivity.National Institute of General Medical Sciences (U.S.) (grant R01-GM62871)Novartis (Firm)Merck Research Laboratorie
Palladium-Catalyzed Alkyl-Alkyl Suzuki Cross-Couplings of Primary Alkyl Bromides at Room Temperature: (13-Chlorotridecyloxy)triethylsilane [Silane, [(13-chlorotridecyl)oxy]triethyl-]
No full textA. 1-Bromo-8-chlorooctane (1). An oven-dried, 200-mL, two-necked,
round-bottomed flask equipped with an argon inlet and a magnetic stirbar
(octagonal, molded pivot ring, 25 mm length and 6 mm diameter) is purged
with argon for 5 min and then charged through the open neck with CH2Cl2
(50 mL via syringe) (Note 1), imidazole (2.19 g, 32.1 mmol, 1.10 equiv)
(Note 2), and dichlorotriphenylphosphorane (10.4 g, 31.2 mmol, 1.07 equiv)
(Note 3). The open neck is capped with a rubber septum, and the stirred
solution is cooled in an ice bath for 5 min. A solution of 8-bromo-1-octanol
(5.0 mL, 6.11 g, 29.2 mmol, 1.00 equiv) (Note 4) in CH2Cl2 (10 mL) (Note 1)
is added via syringe over 5 min. The reaction mixture is allowed to warm to rt, and the resulting heterogeneous solution (a white precipitate formed) is
stirred for 4 h. The progress of the reaction is followed by TLC analysis on
SiO2 (10% EtOAc/hexanes as the eluent; visualization with a KMnO4 stain;
the alcohol starting material has an Rf = 0.2, and the chloride product has an
Rf = 0.9) (Note 5). After the alcohol is consumed, the reaction is diluted with
pentane (200 mL), and the mixture is filtered through a pad of SiO2 (7 cm
diameter 6 cm height) in a sintered glass funnel. The SiO2 is washed with
additional pentane (400 mL). The filtrate is concentrated by rotary
evaporation (20 mmHg, 30 °C), which furnishes the desired product as a
colorless oil (6.23–6.44 g, 94–97 % yield) (Note 6). The product is used in
the next step without further purification
Nickel-Catalyzed Asymmetric Negishi Cross-Couplings of Racemic Secondary Allylic Chlorides with Alkylzincs
No full textThe transition metal-catalyzed enantioselective coupling of allylic electrophiles with carbon nucleophiles has been the focus of intense investigation.5 Salient examples include palladium-catalyzed couplings with enolates, nickel-catalyzed couplings with Grignard reagents, and copper-catalyzed couplings with Grignard and diorganozinc reagents.6 Despite impressive progress, the development of methods that have broader scope with respect to the nucleophile, as well as improved functional-group compatibility, persist as important challenges
Synthesis of Chiral Pyridine Bis(Oxazoline) Ligands for Nickel-Catalyzed Asymmetric Negishi Cross-Couplings of Secondary Allylic Chlorides with Alkylzincs: 2,6-bis[(4R)-4,5-dihydro-4- (2-phenylethyl)-2-oxazolyl]- Pyridine
Full text link[Pyridine, 2,6-bis[(4R)-4,5-dihydro-4-(2-phenylethyl)-2-oxazolyl]-
Nickel-Catalyzed Enantioselective Negishi Cross-Couplings of Racemic Secondary α-Bromo Amides with Alkylzinc Reagents: (S)-N-Benzyl-7-cyano-2-ethyl-N-phenylheptanamide
Full text linkProcedure: A. (5-Cyanopentyl)zinc(II) bromide (1). An oven-dried, 200-mL pear-shaped Schlenk flask equipped with a magnetic stirbar (egg shaped, 25.4 × 12.7 mm) and an argon line connected to the standard taper outer joint is purged with argon for 5 min. Zinc powder (9.80 g, 150 mmol, 1.50 equiv) (Note 1) is added through the open neck, and then the flask is capped with a rubber septum and heated in an oil bath under high vacuum (0.5 mmHg) at 70 °C for 30 min. Then, the flask is refilled with argon, and anhydrous 1,3-dimethyl-2-imidazolidinone (DMI; 100 mL) (Note 2) is added via syringe. Iodine (I2) (634 mg, 2.50 mmol, 0.0250 equiv) (Note 3) is added in one portion through the neck. The neck is re-capped with a rubber septum, and the reaction mixture is stirred at 70 °C in an oil bath until the red color fades (~5 min). 6-Bromohexanenitrile (13.2 mL, 100 mmol, 1.00 equiv) (Note 4) is added via syringe over 4 min, and the reaction mixture is stirred at 70 °C for 12 h. Then, the oil bath is removed, and the mixture is allowed to cool at rt for 1 h without stirring. During this time, the unreacted zinc powder settles at the bottom of the flask. The flask is equipped with a fritted filter tube of medium porosity capped with an oven-dried, 2-necked 250-mL round-bottom flask, and the supernatant solution is filtered under argon by inverting the set-up (Note 5). The resulting clear yellow solution is employed in the next step without further purification. 1H NMR spectroscopy is used to determine that the concentration of the alkylzinc solution is 0.72 M (Note 6). This organozinc solution can be stored under argon at 0-4 °C for up to 3 weeks without deterioration
Palladium/Tris(tert-butyl)phosphine-Catalyzed Suzuki Cross-Couplings in the Presence of Water
No full textDipalladiumtris(dibenzylideneacetone)/tris(tert-butyl)phosphonium tetrafluoroborate/potassium fluoride dihydrate [Pd2(dba)3/[HP(t-Bu)3]BF4/KF⋅2 H2O] serves as a mild, robust, and user-friendly method for the efficient Suzuki cross-coupling of a diverse array of aryl and heteroaryl halides with aryl- and heteroarylboronic acids.National Institutes of Health (U.S.) (National Institute of General Medical Sciences, grant R01-GM62871)Merck Research LaboratoriesNovartis (Firm
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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