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https://www.readbyqxmd.com/read/29019361/copper-i-catalysed-asymmetric-allylic-reductions-with-hydrosilanes
#1
T N Thanh Nguyen, Niklas O Thiel, Johannes F Teichert
A copper(i)-catalysed asymmetric allylic reduction enables a regio- and stereoselective transfer of a hydride nucleophile in an SN2'-fashion onto allylic bromides. This transformation represents a conceptually orthogonal approach to allylic substitution reactions with carbon nucleophiles. A copper(i) complex based upon a chiral N-heterocyclic carbene (NHC) ligand allows for stereoselectivity reaching 99% ee. The catalyst enables a stereoconvergent reaction irrespective of the double bond configuration of the starting materials...
October 11, 2017: Chemical Communications: Chem Comm
https://www.readbyqxmd.com/read/28990768/reactions-of-germylenes-and-stannylenes-with-halo-hydrocarbyl-and-chloro-amino-phosphines-oxidative-addition-versus-ligand-transfer
#2
Joseph K West, Lothar Stahl
Oxidative addition (OA) is an important elementary step in chemistry, but it has been studied mainly in the context of transition-metal-catalyzed reactions and mainly with carbon-X substrates (X = halogen, H). Reports of main-group metal compounds undergoing OA are rare by comparison, and those involving phosphorus-halogen substrates are rarer still. Acyclic and cyclic diazagermylenes and -stannylenes react with chloro(hydrocarbyl)phosphines with the intermediacy of oxidative addition products. Stannylenes react faster than germylenes, and these reactions are first-order in both reactants and slowed by steric bulk...
October 9, 2017: Inorganic Chemistry
https://www.readbyqxmd.com/read/28985079/dynamics-and-novel-mechanisms-of-sn2-reactions-on-ab-initio-analytical-potential-energy-surfaces
#3
Istvan Szabo, Gabor Czako
We describe a novel theoretical approach to the bimolecular nucleophilic substitution (SN2) reactions which is based on analytical potential energy surfaces (PESs) obtained by fitting a few tens of thousands high-level ab initio energy points. These PESs allow computing millions of quasi-classical trajectories thereby providing unprecedented statistical accuracy for SN2 reactions, as well as performing high-dimensional quantum dynamics computations. We developed full-dimensional ab initio PESs for the F- + CH3Y [Y = F, Cl, I] systems, which describe the direct and indirect, complex-forming Walden-inversion, the front-side attack, and the new double-inversion pathways as well as the proton-transfer channels...
October 6, 2017: Journal of Physical Chemistry. A
https://www.readbyqxmd.com/read/28977796/efficacy-of-stannous-ions-on-enamel-demineralization-under-normal-and-hyposalivatory-conditions-a-controlled-randomized-in-situ-pilot-trial
#4
Núbia Inocencya Pavesi Pini, Nadine Schlueter, Daniel Sundfeld, Wiebke Semper Hogg, Alan Roger Santos-Silva, Marcio Ajudarte Lopes, Flávio H B Aguiar, J R Lovadino, D A N L Lima
The study aim was to investigate the effect of antierosive agents on enamel under normal and hyposalivatory conditions. This double-blind crossover in situ pilot study evaluated 4 toothpastes: placebo (0 ppm F), sodium fluoride (NaF, 1,450 ppm), stannous/sodium fluoride (SnF/NaF, 1,450 ppm F-, 1,090 ppm Sn2+), and sodium fluoride, stannous chloride and chitosan (NaF/Sn/Ch, 1,450 ppm F-, 3,500 ppm Sn2+, 0.5% Ch). Twenty participants were assigned to 2 groups (n = 10 each): normal and low salivary flow. Participants wore palatal appliances holding 4 bovine enamel specimens previously eroded in vitro (D1) for 20 min prior to an in situ phase after which they were eroded again (D2)...
January 5, 2017: Caries Research
https://www.readbyqxmd.com/read/28956039/a-study-on-an-unusual-sn2-mechanism-in-the-methylation-of-benzyne-through-nickel-complexation
#5
Makoto Hatakeyama, Yuki Sakamoto, Koji Ogata, Yuto Sumida, Tomoe Sumida, Takamitsu Hosoya, Shinichiro Nakamura
In this study, three reaction mechanisms of a benzyne-nickel (Ni) complex ([Ni(C6H4)(dcpe)]) with iodomethane during the methylation process were investigated, namely (a) SN2 reaction of the benzyne-Ni complex with iodomethane, (b) concerted σ-bond metathesis during the bond breaking/forming processes, and (c) oxidative addition of iodomethane to the Ni-center and the subsequent reductive elimination process. DFT calculations revealed that the reaction barrier of the SN2 reaction is slightly lower than those of the other mechanisms...
October 11, 2017: Physical Chemistry Chemical Physics: PCCP
https://www.readbyqxmd.com/read/28946325/fast-cleavage-of-phycocyanobilin-from-phycocyanin-for-use-in-food-colouring
#6
Maria Cinta Roda-Serrat, Knud Villy Christensen, Rime Bahij El-Houri, Xavier Fretté, Lars Porskjær Christensen
Phycocyanins from cyanobacteria are possible sources for new natural blue colourants. Their chromophore, phycocyanobilin (PCB), was cleaved from the apoprotein by solvolysis in alcohols and alcoholic aqueous solutions. In all cases two PCB isomers were obtained, while different solvent adducts were formed upon the use of different reagents. The reaction is believed to take place via two competing pathways, a concerted E2 elimination and a SN2 nucleophilic substitution. Three cleavage methods were compared in terms of yield and purity: conventional reflux, sealed vessel heated in an oil bath, and microwave assisted reaction...
February 1, 2018: Food Chemistry
https://www.readbyqxmd.com/read/28930437/c-and-n-metalated-nitriles-the-relationship-between-structure-and-selectivity
#7
Xun Yang, Fraser F Fleming
Metalated nitriles are exceptional nucleophiles capable of forging highly hindered stereocenters in cases where enolates are unreactive. The excellent nucleophilicity emanates from the powerful inductive stabilization of adjacent negative charge by the nitrile, which has a miniscule steric demand. Inductive stabilization is the key to understanding the reactivity of metalated nitriles because this permits a continuum of structures that range from N-metalated ketenimines to nitrile anions. Solution and solid-state analyses reveal two different metal coordination sites, the formally anionic carbon and the nitrile nitrogen, with the site of metalation depending intimately on the solvent, counterion, temperature, and ligands...
October 17, 2017: Accounts of Chemical Research
https://www.readbyqxmd.com/read/28926128/copper-catalysed-aminoboration-of-vinylarenes-with-hydroxylamine-esters-a-computational-mechanistic-study
#8
Sven Tobisch
This first comprehensive computational study of the copper-mediated formal aminoboration utilising an electrophilic strategy has identified the most accessible pathway for the productive catalysis. The mechanistic picture derived from smooth energy profiles acquired by employing a reliable computational protocol applied to a realistic catalyst model conforms to all available experimental data. The high degree of chemo- and stereoselectivity achieved in syn-borylcupration and Umpolung electrophilic amination is instrumental to the exclusive generation of the (syn)-beta-aminoalkylborane product...
September 19, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28924216/organocatalytic-synthesis-of-chiral-tetrasubstituted-allenes-from-racemic-propargylic-alcohols
#9
Deyun Qian, LinLin Wu, Zhenyang Lin, Jianwei Sun
Although chiral allene preparation via formal SN2' nucleophilic substitutions of enantioenriched propargylic derivatives or metal-catalyzed reactions of racemic propargylic derivatives has attracted considerable attention and found applications in many areas of research, direct use of propargylic alcohols instead of propargylic derivatives for catalytic asymmetric allene synthesis is unknown. Here, we show that a highly enantioselective synthesis of tetrasubstituted allenes from racemic propargylic alcohols has been realized by organocatalysis with good efficiency (up to 96% yield and 97% ee)...
September 18, 2017: Nature Communications
https://www.readbyqxmd.com/read/28923355/in-vitro-anticancer-activity-of-new-gold-iii-porphyrin-complexes-in-colon-cancer-cells
#10
Fatima Dandash, David Yannick Léger, Chloë Fidanzi-Dugas, Soumaya Nasri, Frédérique Brégier, Robert Granet, Walid Karam, Mona Diab-Assaf, Vincent Sol, Bertrand Liagre
Colorectal cancer (CRC) is the third most common cancer diagnosed worldwide. The limitations of cisplatin-based chemotherapy have prompted intense interest among scientists to search for alternative metal-based anticancer medicines. Gold(III) complexes have been among the most widely investigated since they showed higher cytotoxicity than cisplatin and promising in vitro and in vivo anticancer activities in CRC but their clinical usefulness has been limited by their poor stability under physiological conditions...
September 6, 2017: Journal of Inorganic Biochemistry
https://www.readbyqxmd.com/read/28921953/new-antimony-selenide-nickel-oxide-photocathode-boosts-the-efficiency-of-graphene-quantum-dot-co-sensitized-solar-cells
#11
Ankita Kolay, Ramesh K Kokal, Ankarao Kalluri, Isaac Macwan, Prabir K Patra, Partha Ghosal, Melepurath Deepa
A novel assembly of a photocathode and a photoanode is investigated to explore their complementary effects in enhancing the photovoltaic performance of a quantum-dot solar cell (QDSC). While p-type nickel oxide (NiO) has been used previously, antimony selenide (Sb2Se3) has not been used in a QDSC, especially as a component of a counter electrode (CE) architecture that doubles as the photocathode. Here, near-infrared (NIR) light-absorbing Sb2Se3 nanoparticles (NPs) coated over electrodeposited NiO nanofibers on a carbon (C) fabric substrate was employed as the highly efficient photocathode...
October 11, 2017: ACS Applied Materials & Interfaces
https://www.readbyqxmd.com/read/28920682/carboxylate-assisted-formation-of-aryl-co-iii-masked-carbenes-in-cobalt-catalyzed-c-h-functionalization-with-diazo-esters
#12
Oriol Planas, Steven Roldán-Gómez, Vlad Martin-Diaconescu, Teodor Parella, Josep M Luis, Anna Company, Xavi Ribas
Herein we describe the synthesis of a family of aryl-Co(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co(III) masked-carbenes. Moreover, additional evidence for an unprecedented Co(III)-mediated intramolecular SN2-type C-C bond formation in which the carboxylate moiety acts as a relay is disclosed...
October 3, 2017: Journal of the American Chemical Society
https://www.readbyqxmd.com/read/28914473/ethers-on-si-001-a-prime-example-for-the-common-ground-between-surface-science-and-molecular-organic-chemistry
#13
Lisa Pecher, Slimane Laref, Marc Raupach, Ralf Ewald Tonner
Using computational chemistry, we show that the adsorption of ether molecules on Si(001) under ultra-high vacuum conditions can be understood with textbook organic chemistry. The two-step reaction mechanism of (1) dative bond formation between the ether oxygen and a Lewis acidic surface atom and (2) a nucleophilic attack of a nearby Lewis basic surface atom is analysed in detail and found to mirror the acid-catalysed ether cleavage in solution. The O-Si dative bond is found to be the strongest of its kind and reactivity from this state defies the Bell-Evans-Polanyi principle...
September 15, 2017: Angewandte Chemie
https://www.readbyqxmd.com/read/28882041/dynamics-of-silver-nanoparticles-in-aqueous-solution-in-the-presence-of-metal-ions
#14
Kamonwad Ngamchuea, Christopher Batchelor-McAuley, Stanislav V Sokolov, Richard G Compton
Using a combined UV-vis, DLS, and electrochemical approach, this work experimentally studies the physical origin of the observed colorimetric sensitivity of aqueous silver nanoparticles toward divalent metal ions. In the presence of Pb(2+), AgNPs are slow to reversibly form agglomerates (the time scale of the reverse deagglomeration process is of the order of hours). This agglomeration is shown to be induced by complex formation between Pb(2+) and citrate groups localized on the AgNPs, reducing surface charges (zeta-potential) and hence electrostatic repulsion between the AgNPs...
September 21, 2017: Analytical Chemistry
https://www.readbyqxmd.com/read/28881399/heavy-atom-tunneling-calculations-in-thirteen-organic-reactions-tunneling-contributions-are-substantial-and-bell-s-formula-closely-approximates-multidimensional-tunneling-at-%C3%A2-250%C3%A2-k
#15
Charles Doubleday, Randy Armas, Dana Walker, Christopher V Cosgriff, Edyta M Greer
Multidimensional tunneling calculations are carried out for 13 reactions, to test the scope of heavy-atom tunneling in organic chemistry, and to check the accuracy of one-dimensional tunneling models. The reactions include pericyclic, cycloaromatization, radical cyclization and ring opening, and SN 2. When compared at the temperatures that give the same effective rate constant of 3×10(-5)  s(-1) , tunneling accounts for 25-95 % of the rate in 8 of the 13 reactions. Values of transmission coefficients predicted by Bell's formula, κBell  , agree well with multidimensional tunneling (canonical variational transition state theory with small curvature tunneling), κSCT ...
September 7, 2017: Angewandte Chemie
https://www.readbyqxmd.com/read/28880543/the-transition-state-structure-for-human-mat2a-from-isotope-effects
#16
Ross S Firestone, Vern L Schramm
Human methionine S-adenosyltransferase (MAT2A) catalyzes the formation of S-adenosylmethionine (SAM) from ATP and methionine. Synthetic lethal genetic analysis has identified MAT2A as an anticancer target in tumor cells lacking expression of 5'-methylthioadenosine phosphorylase (MTAP). Approximately 15% of human cancers are MTAP(-/-). The remainder can be rendered MTAP(-) through MTAP inhibitors. We used kinetic isotope effect (KIE), commitment factor (Cf), and binding isotope effect (BIE) measurements combined with quantum mechanical (QM) calculations to solve the transition state structure of human MAT2A...
October 4, 2017: Journal of the American Chemical Society
https://www.readbyqxmd.com/read/28880094/enantiospecific-synthesis-of-%C3%AE-substituted-tryptamines
#17
Heather N Rubin, Kinney Van Hecke, Jonathan J Mills, Jennifer Cockrell, Jeremy B Morgan
Functionalized tryptamines are targets of interest for development as small molecule therapeutics. The ring opening of aziridines with indoles is a powerful method for tryptamine synthesis where isomer formation can be controlled. 3,5-Dinitrobenzoyl (DNB)-protected aziridines undergo regioselective, enantiospecific ring opening to produce β-substituted tryptamines for a series of indoles. Attack at the more substituted aziridine carbon occurs in an SN2-like fashion to generate DNB-tryptamine products as synthetic precursors...
September 7, 2017: Organic Letters
https://www.readbyqxmd.com/read/28876944/polycyclic-azetidines-and-pyrrolidines-via-palladium-catalyzed-intramolecular-amination-of-unactivated-c-sp3-h-bonds
#18
Jie Zhao, Xiao-Jing Zhao, Pei Cao, Ji-Kai Liu, Bin Wu
A novel strategy to construct complex polycyclic nitrogen-containing heterocycles from aliphatic amines via picolinamide-assisted palladium-catalyzed C-H bond activation reaction was reported. The reaction exhibits broad substrate scope for the synthesis of various azabicyclic scaffolds, including azetidines and tropane-class alkaloids. Application of this method to naturally occurring (-)-cis-myrtanylamine, an unprecedented type of carbon-carbon bond activation, in which the electron-pair involved initiates an intramolecular "SN2-like" displacement of a cyclopalladium-fragment from a tertiary center, is described...
September 6, 2017: Organic Letters
https://www.readbyqxmd.com/read/28861584/synthetic-route-to-chiral-indolines-via-cu-oac-2-catalyzed-ring-opening-c-sp-2-h-activation-of-activated-aziridines
#19
Abhijit Mal, Gaurav Goswami, Imtiyaz Ahmad Wani, Manas K Ghorai
A simple strategy for the synthesis of highly functionalized indolines via Lewis acid catalyzed ring-opening of activated aziridines with various nucleophiles followed by Cu(OAc)2-mediated intramolecular C-H amination in one-pot has been developed with excellent enantio- and diastereospecificity (ee 99%; de >99%). The reaction proceeds via Cu(OAc)2-catalyzed SN2-type ring-opening of 2-phenyl-N-(2-pyridinesulfonyl)aziridine with alcohols and arene, followed by copper-mediated pyridine-2-sulfonamide directed intramolecular C(sp(2))-H activation/cyclization in a stepwise fashion to furnish the indoline derivatives in excellent yields (up to 91%)...
September 14, 2017: Chemical Communications: Chem Comm
https://www.readbyqxmd.com/read/28858515/direct-access-to-2-3-4-6-tetrasubstituted-tetrahydro-2h-pyrans-via-tandem-sn2-prins-cyclization
#20
Jimena Scoccia, Sixto J Pérez, Victoria Sinka, Daniel A Cruz, Juan M López-Soria, Israel Fernández, Víctor S Martín, Pedro O Miranda, Juan I Padrón
A new, direct, and diastereoselective synthesis of activated 2,3,4,6-tetrasubstituted tetrahydro-2H-pyrans is described. In this reaction, iron(III) catalyzed an SN2'-Prins cyclization tandem process leading to the creation of three new stereocenters in one single step. These activated tetrahydro-2H-pyran units are easily derivatizable through CuAAC conjugations in order to generate multifunctionalized complex molecules. DFT calculations support the in situ SN2' reaction as a preliminary step in the Prins cyclization...
August 31, 2017: Organic Letters
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