1,720,970 research outputs found
STEREOSELECTIVITY OF INTRAMOLECULAR NITRILE OXIDE CYCLOADDITIONS TO Z-CHIRAL ALKENE AND E-CHIRAL ALKENES
No full textIntramolecular nitrile oxide cycloaddition (INOC) reactions on chiral alkenes were studied in order to evaluate
the influence of the double bond configuration on the stereochemical outcome of the process. Oximes 11-16
were prepared, starting from aldehydes 1-4, via Wittig reaction, isomerization of the double bond for the E
derivatives, Swern oxidation, and reaction with hydroxylamine. Treatment of oximes 11-16 with sodium hypochlorite
gave the nitrile oxides, which were trapped in situ by intramolecular cycloaddition to give the corresponding
isoxazolines 17-22 as mixtures of diastereoisomers (Table I). From (2)-alkenyl oximes C-4/C-5 syn
products and from (E)-alkenyl oximes C-4/C-5 anti products were obtained, while the relative stereochemistry
at C-5/C-5‘ of the predominant isomers was found to be anti in all cases. The assignment of relative stereochemistry
was based on ‘H and 13C NMR spectroscopic evidence and on chemical correlations. With Houk‘s approach,
MM2 calculations were performed to evaluate the relative energies of the transition structures. The CNO-ethylene
fragment was frozen in the ab initio HCNO-ethylene transition structure model geometry, and the substituents
were fully optimized by MM2. With the (2)-alkenes, the “small” group of the allylic stereocenter prefers the
inside position, the “medium” the anti, and the “large” the outside, with respect to the forming C-0 bond, and
the factors controlling the stereoselectivity are mainly steric. On the contrary, with the (E)-alkenes the “medium”
group will be inside, the “large” anti, and the “small” outside. In the case of allyl ethers this model is mainly
ruled by electronic factors. Quite good stereoselectivities were achieved in the INOC reactions using the allyl
ethers derived from glyceraldehyde. A rationale for this result has been proposed
Enantioselective Direct Aldol Reaction “on water” Promoted by Chiral Organic Catalysts
No full text1,1¢-Binaphthyl-2,2¢-diamine-based (S)-prolinamides in the presence of stearic acid were able to promote the direct aldol condensation of
cyclohexanone and other ketones with different aldehydes in the presence of a massive amount of water in very good yields, high
diastereoselectivity, and up to 99% ee. The behavior of both C2- and C1-symmetric catalysts in combination with different additives was
investigated, and a preliminary experiment of recovering and recycling of the catalytic system was also attempted
Novel carbohydrate-based bifunctional organocatalysts for nucleophilic addition to nitroolefins and imines
Full text linkGlucosamine has been selected as a cheap and readily available chiral scaffold for the synthesis of a series of novel enantiomerically pure bifunctional organocatalysts bearing a tertiary amino group in proximity to a (thio)urea group. The catalytic behaviour of these compounds, both as neutral and N-protonated species, was investigated using the addition of acetylacetone to β-nitrostyrene as a model reaction. Under optimized experimental conditions, chemical yields up to 93% and enantioselectivities up to 89% were obtained. Semiempirical (AM1) computational studies allowed to find a theoretical rationale for the chemical and stereochemical behaviour of the catalyst of choice. These catalysts were also preliminarily investigated as promoters in the addition of diethyl malonate to the N-Boc imine of benzaldehyde, affording the product in up to 81% ee
Organic-inorganic hybrid: the role played by surface energy and structure of the hydrophobizing molecules
No full textThe functionalization of nano-TiO2 by siloxanes gives rise to robust covalent bonds by mild reactions upon hydrolysis of –OR’ moieties producing a very promising class of materials, in which siloxanes may also serve as powerful linkers for different functionalities. These linkages bear technological relevance to widely different fields, from the promotion of self-cleaning processes, to the prevention of clotting in artificial blood vessels, the protection of outdoor cultural heritage or in enhancing the electron transfer in photovoltaic devices. In this work the features of hydrophobic layers deposited onto rough TiO2 films composed by home-made nanometric particles are investigated. Three different kinds of siloxanes were chosen, as functionalizing agents, both commercial and laboratory made showing different end-groups. For the three molecules the surface energy, together with their polar and disperse components, is calculated on the grounds of contact angle measurements of different solvents on thick siloxane coatings deposited onto smooth glass slides. The morphology of the hybrid TiO2-siloxane films is analyzed by AFM and EDX and their wettability by water is evaluated by receding and advancing contact angle determinations. The structure of the surface layer and the modes of attachment of Si atoms are investigated by combining data of contact angle hysteresis, surface mapping by EDX and CP/MAS NMR analyses performed on samples with increasing siloxane content. The chemical specificity introduced in the TiO2 hybrids by the different siloxane end-groups is discussed on the grounds of the composite interactions with different solvents. Finally self cleaning tests by using Alizarine Red dye are performed on the siloxane-clad TiO2 layers to evaluate the actual hydrophobicity of the hybrid materials
Surface Free Energy of Bare and Fluorinated Siloxanes: Comparing Experimental Evaluation and Dipole Moments Calculations
No full textThe surface free energy (SFE) is a key parameter in evaluating the physico-chemical features of solid surfaces. Studies on the composition, structure and interfacial phenomena of a surface layer of materials are of special importance. Its assessment allows the interpretation of several phenomena, such as adsorption mechanisms and surface wettability behaviors, occurring at solid-liquid and solid-gas interfaces; hence the knowledge of such properties is fundamental for both technological and more fundamental point of view. Here, the surface free energy of three non-fluorinated and three fluorinated siloxanes is evaluated. All siloxanes were synthesized and deposited in thick films onto a glass substrate by spin coating. Experimental static contact angle data elaborated by empirical models (Zisman, EOS, OWRK), together with theoretical dipole moment calculations and wetting envelope (WE) elaborations allow us to predict the wetting behaviors towards the most common solvents. The effect of the addition of fluorinated -CF3 end-groups in imposing the final wetting features of the siloxane films is fully investigated on the grounds of the above mentioned experimental and theoretical investigations
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
Interplay between Chemistry and Texture in Hydrophobic TiO2Hybrids
No full textOrganic-nano-TiO(2) hybrids are successfully employed in different fields, from biomaterials to self-cleaning processes and from sensing to photovoltaics. The specific wetting properties of these materials bear relevance to any application. Here, nano-TiO(2) films with a multiscale roughness, as shown by AFM, are derivatized with tri- and bifunctional siloxanes, bearing either alkyl or aromatic end groups. Dynamic contact angle measurements show a marked dependence of the wettability on the siloxane nature, leading to different self-cleaning properties, as assessed by ATR-FTIR. The different behavior is rationalized on the grounds of the structure of the functionalized layer, by employing solid state (29)Si CP-MAS NMR and EDX mapping. Semiempirical calculations of hybrids dipole moments in conjunction with solvent surface energy components allow us to explain the strongly different solvents wettability. This work brings attention to the importance of the hydrophobizing molecule nature and its chemical interactions for the interpretation of wettability versus surface texture
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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