1,720,971 research outputs found
Oxidative functionalization of organic compounds promoted by nonheme Fe and Mn complexes
Full text linkNonheme iron and manganese complexes are versatile and environmentally sustainable catalysts able to promote the oxidation of a broad range of organic compounds with interesting synthetic application. Careful simple modification in the design of this catalytic systems, can contribute to change significantly their selectivity and efficiency.
In this Ph.D. thesis, some aspects related to the mechanistic analysis and catalytic activity of three different nonheme catalytic systems: aminopyridine iron complexes, imine iron complexes and aminopyridine supramolecular iron and manganese complexes, have been investigated in detail.
The occurrence of an electron transfer-oxygen rebound process in the oxidation of aryl sulfides promoted by two tetradentate ([(PDP)FeII] and [(BPMCN)FeII]) and two pentadentate ([(N4Py)FeII] and [(Bn-TPEN)FeII]) aminopyridine iron complexes, has been highlighted through detailed product and kinetic studies of the oxidation of two series of aromatic sulfides (aryl 1-methyl-1-phenylethyl sulfides and aryl diphenylmethyl sulfides) whose corresponding radical cations are characterized by high C-S fragmentation rate constants. Using aryl 1-methyl-1-phenylethyl sulfides as substrates, it has been possible, moreover, to estimate the rate constants of the oxygen rebound process (kOT) from the reduced nonheme iron-oxo complexes to the sulfide radical cations.
A bioinspired supramolecular version of the aminopyridine iron and manganese complexes, [(PDP)FeII] and [(PDP)MnII] containing benzo-18-crown-6 ether as receptor that reversibly bind the substrate, has been prepared. The ditopic catalysts have been applied with success to achieve high site-selective C-H functionalization on C8 and C9 methylene positions of a series of linear protonated primary amines thanks to reversible pre-association between ammonium moiety and the benzo-18-crown-6 ether recognition sites.
Imine based iron complexes represent another interesting class of nonheme catalysts. Iminopyridine iron(II) complex, easily prepared in situ by self-assembly of cheap and commercially available 2 picolylaldehyde, 2 picolylamine, and Fe(OTf)2, in a 2:2:1 ratio, displayed good-to-moderate efficiency in the oxidation of aliphatic alcohols with H2O2. Quite surprisingly, benzylic alcohols are oxidized to aromatic ketones with low efficiency due to strong competitive aromatic hydroxylation. The remarkable selectivity for the oxidation of the aromatic ring in benzylic alcohols, has been exploited in the oxidation of alkyl aromatic compounds. In the presence of an aliphatic chain, the catalyst is highly selective for the aromatic nucleus hydroxylation with a selectivity pattern that closely matched that of electrophilic aromatic substitutions with a metal–based SEAr pathway, without a significant involvement of free diffusing radical pathways.
A low efficiency in the oxidation of electron rich benzylic alcohols has been also observed, moreover, for the MnII/pyridine carboxylic acid/butanedione catalytic system. The analysis of the inhibition process of this catalytic system by catechol and guaiacol substrates indicated that the Mn/inhibitor ratio is a key parameter for the activity/inactivity of the system. The right proportion between them may prevents an irreversible deactivation of the complex.
Finally, the fundamental importance of the identification of the active intermediates involved in the catalytic cycles, has prompted us to project an innovative approach consisting in the simultaneously use of time-resolved Energy Dispersive X-Ray Absorption (EDXAS) and UV/Vis spectroscopies with millisecond resolution to follow the time evolution of the catalyst activation processes. The excellent results obtained for the evolution of [(TPA)FeII(CH3CN)2]2+ with H2O2 and CH3COOOH, could give the opportunity to follow the evolution of less know systems and to detect and quantitatively monitor some elusive species, such as FeV intermediates
A dynamical model of genetic networks for cell differentiation.
No full textA mathematical model is proposed which is able to describe the most important features of cell differentiation, without requiring specific detailed assumptions concerning the interactions which drive the phenomenon. On the contrary, cell differentiation is described here as an emergent property of a generic model of the underlying gene regulatory network, and it can therefore be applied to a variety of different organisms. The model points to a peculiar role of cellular noise in differentiation and leads to non trivial predictions which could be subject to experimental testing. Moreover, a single model proves able to describe several different phenomena observed in various differentiation processes
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Extended notion of attractors in noisy random Boolean networks
No full textSince real networks are noisy systems, in this work we investigate thedynamics of the random Boolean networks affected by different size of smallrandom fluctuations. In this case jumps among different attractors are possible,thereby leading to an asymptotic dynamics different from that of the underlyingdeterministic model. The significance of the jumps among attractors is investigated.The notion of “ergodic set” is discussed and generalized in terms of“threshold ergodic set”, a concept that take into account the system lifetime. Inorder to evaluate possible differences due to the topology of the nets the experimentsare effectuated on Erdos-Renyi and scale-free topologies, showing similarbehaviour
Alcohol oxidation with H2O2 catalyzed by a cheap and promptly available imine based iron complex
No full textWe previously reported that the iminopyridine iron(ii) complex 1, easily and quantitatively obtainable in situ, can activate H2O2 to form a powerful oxidant, capable of aliphatic C-H bond hydroxylation. In the present study we expand the application of this catalyst to the oxidation of a series of alcohols to the corresponding carbonyl compounds. The oxidation of aliphatic alcohols proceeds smoothly, while that of benzylic alcohols is shown to be challenging. Some collected pieces of evidence suggest a preference of the oxidizing species for the aromatic ring instead for the alcoholic moiety. The decrease of the electron density in the aromatic ring shifts the oxidation from the aromatic towards the alcoholic moiety. Quite surprisingly, preferential oxidation of cyclohexanol versus benzylic alcohol was achieved, showing unprecedented selectivity
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