1,720,970 research outputs found
Homogeneous Gold Catalysis: understanding ligand and counterion effects, first steps in green chemistry
Full text linkSince the last 20 years, homogeneous gold catalysis is receiving considerable attention and great efforts have been made to disclose the mechanism of the catalytic cycle and the role of many variables: nature of the ligand, structure of the catalyst, effect of the additives, etc. Although a wealth of empirical information on ligand effects is now available on homogeneous gold catalysis, the development of new L-Au-X catalysts and reactions continues to rely upon trial and error, and the outcome of the reaction is often unpredictable. The mechanistic understanding of the gold(I)-catalysed nucleophilic addition to a carbon-carbon unsaturated bond has been pursued and appreciably extended in this thesis by experimental and theoretical investigations. We focused our study on weak interactions and counterion effects with the aim to better understand these often not considered variables. We explained that the anion is influencing each single step of the catalysis and that it is modulating its role depending on its nature (coordination power and basicity) and position. Of course, its effect is also depending on the type of reaction and on which is the rate determining step of the latter. This deep study on the ligands and counterions role in gold homogeneous catalysis, matching both experimental and theoretical studies, allow us to setup a green, room temperature, acid-free, solvent-free and sustainable methodology for the hydration of alkynes. This reaction is generally working only with acidic additives and at high temperature, thereby here is reported for the first time an innovative way to perform it. These preliminary studies open new avenues to consider and rationalize the homogeneous gold catalysis, spreading light into the weak interactions that were underestimated in this field for a long time. This study clearly demonstrates that the interplay between ligand nature and anion effect is crucial in different steps of the catalytic cycle. The multiple roles played by counterions and L-Au+ fragments in chemical transformations require more comprehensive computational and experimental studies of the ligand/anion correlatio
Toward optimizing the performance of homogeneous L-Au-X catalysts through appropriate matching of the ligand (L) and counterion (X-)
No full textThe effects of the ligand (L) and counterion (X-) are considered the two most important factors in homogeneous gold catalysis, but a rational understanding of their synergy/antagonism is still lacking. In this work, we synthesized a set of 16 gold complexes of the type L-Au-X that differ as follows: (i) L = PPh3 (L1), P(tBu)3 (L2), tris(3,5-bis(trifluoromethyl)phenyl)phosphine (PArF, L3), and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHC, L4), with the deliberate purpose of varying the electron withdrawing ability of the ligand, and (ii) X- = BF4-, OTf-, OTs-, and TFA-, which have various coordinating abilities, basicities, and hydrogen bond acceptor powers. All these catalysts were tested in two different model reactions: the cycloisomerization of N-(prop-2-ynyl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline and the methoxylation of 3-hexyne. The main results are that the choice of the most efficient L-Au-X catalyst for a given process should not be made by evaluating the properties of L and X- alone, but rather based on their best combination. For NHC-Au-X, the noncoordinating and weakly basic anions (such as BF4- and OTf-) have been recognized as the best choice for the cycloisomerization of N-(prop-2-ynyl)benzamide. On the other side, the intermediate coordinating ability and basicity of OTs- provide the best compromise for achieving an efficient methoxylation of 3-hexyne. A completely different trend is found in the case of complexes bearing phosphanes: OTs- and TFA- have been found to accelerate the cycloisomerization of N-(prop-2-ynyl)benzamide, and BF4- and OTf- are suitable for the methoxylation of 3-hexyne. A possible explanation of the observed differences between phosphane and NHC ancillary ligands might be found in the higher affinity of the counterion (especially OTs-) for the gold fragment for phosphane instead of NHC. © 2015 American Chemical Society
APPLICATORE ED APPARECCHIATURA PER ELETTROPORAZIONE
No full textElectroporation apparatus and applicator, wherein the applicator comprises a flexible support (2) which defines a plurality of housing seats (7), and a plurality of electrodes (3) each comprising a conductive needle (4) and mountable in the housing seats (7); the support (2) comprises at least a first layer (13) which comprises a sheet (15) of flexible plastic material and, at each housing seat (7), a rigid guiding element (16), made of plastic material and in which the needle (4) of one of the electrodes (3) can be slidably inserted, a first end (10) of an electric conductor (9) being aligned with each housing seat (7) to be in electrical contact with a needle (4) inserted into said housing seat (7), the electrical conductors (9) extending themselves into a second layer (14) of the support (2).
Pubblicazione PCT 21 marzo 2019 numero WO2019/05360
Ion pairing in cationic NHC and NAC gold(I) η2-alkyne complexes
No full textGold(I) cationic complexes of general formula [LAu+ X-] [L = NAC (Nitrogen-Acyclic-Carbene) or NHCs (N-Heterocyclic Carbenes), X- = weakly coordinating anion] are successfully employed as catalysts in a large variety of organic reactions involving the activation of unsaturated carbon-carbon bonds. A key role in such reactions is played by the counterion. We have studied the ion pair structure in solution of [NHC-Au(η2-3-hexyne)]BF4 and [NAC-Au(η2-3-hexyne)]BF4 by NOE NMR spectroscopy and relativistic DFT calculation. In our previous work, two main orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the 3-hexyne (B). Here we focus on the effect of different carbenes on the ion pair structure, comparing the aromatic NHC (1BF4) with 5 different ligands: a non aromatic NHC (2BF4), a polycyclic ligand with an extended aromatic system (3BF4) and three different NAC (4BF4, 5BF4 and 6BF4). For 2BF4, the A:B ratio (observed with NOE NMR spectroscopy) remains almost the same as for 1BF4, whilst the ion pair structure of the 3BF4 becomes mainly non-specific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, that shows an attractive region on the backbone of 2BF4 and a flat weak potential around the whole 3BF4. An interesting result was obtained for 4BF4 that shows a strong attractive region close to the two N-H that makes the IP 100% specific for the orientation A (figure beside). 5BF4 shows a less specific ion pair due to the presence of only one N-H on the back. For the 6BF4 the -OH group on the arm of the carbene introduce a third ion pair configuration, strongly stabilized by the hydrogen bond between the hydroxyl and the anion
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Diffusion NMR measurements on cationic linear gold(I) complexes
No full textAbstract In the last years, it has been demonstrated that the anion plays a key role in gold(I) catalysis, affecting yield, regio- and stereo-selectivity of processes. In order to perform such activity, necessarily the anion has to locate itself close to the reaction center. In this contribution, the level of aggregation of cationic linear gold complexes bearing different ligands, such as phosphines and carbenes, is studied by diffusion PGSE (Pulsed field Gradient Spin Echo) NMR spectroscopy as a function of concentration and solvent. It is found that functional groups, which establish specific interactions with the anion, such as -NH or polarized -CH moieties, strongly influence the self-aggregation of gold(I) complexes: ion pairs are the predominant species in solution, but ion quadruples also form in apolar solvents only when -NH or polarized -CH moieties are present in the ligand. In the absence of those functional groups free ions are present in solution with a small amount of ion pairs. Interestingly, the presence of an extended aromatic group on the cation leads to dicationic adducts and ion triples, which are held together by π-π stacking interactions. When more than one functional group is present, 19F, 1H HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiments and DFT coulomb potential maps are used to check which group establishes the strongest interaction with the anion. © 2015 Elsevier Ltd. All rights reserved
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