1,721,000 research outputs found
Remote amino acid recognition enables effective hydrogen peroxide activation at a manganese oxidation catalyst
Full text linkPrecise delivery of a proton plays a key role in O2 activation at iron oxygenases, enabling the crucial O−O cleavage step that generates the oxidizing high-valent metal–oxo species. Such a proton is delivered by acidic residues that may either directly bind the iron center or lie in its second coordination sphere. Herein, a supramolecular strategy for enzyme-like H2O2 activation at a biologically inspired manganese catalyst, with a nearly stoichiometric amount (1–1.5 equiv) of a carboxylic acid is disclosed. Key for this strategy is the incorporation of an α,ω-amino acid in the second coordination sphere of a chiral catalyst via remote ammonium-crown ether recognition. The properly positioned carboxylic acid function enables effective activation of hydrogen peroxide, leading to catalytic asymmetric epoxidation. Modulation of both amino acid and catalyst structure can tune the efficiency and the enantioselectivity of the reaction, and a study on the oxidative degradation pathway of the system is presented
Rational design of bioinspired catalysts for selective oxidations
No full textRecognizing Nature's unique ability to perform challenging oxygenation reactions with exquisite selectivity parameters at iron-dependent oxygenases, chemists have long sought to understand and mimic these enzymatic processes with artificial systems. In the last two decades, replication of the reactivity of non-heme iron oxygenases has become feasible even with simple coordination complexes of iron and manganese. A bona fide minimalistic functional model was the tetradentate-N4 ligand based iron complex [Fe(tpa)(CH3CN)2]2+(Fe(tpa), tpa = tris(2-methylpyridyl)amine), which activates H2O2 via a mechanism that mirrors key steps of enzymatic O2 activation processes at mononuclear iron centers: controlled O-O bond cleavage, generation of a high-valent Fe═O oxidant, and promotion of almost the full spectrum of its oxidative reactivity (C-H hydroxylation, olefin epoxidation, syn-dihydroxylation, and desaturation). These landmark discoveries set the mechanistic framework to use iron coordination complexes with nitrogen-rich ligands as catalysts for oxidizing organic substrates under synthetically relevant conditions. Due to proof-of-concept demonstrations of the potential of these catalysts in organic synthesis, this chemistry has flourished over the past decade. In parallel to the realization of the potential of this class of catalysts in diverse organic transformations, effort has been spent to manipulate the catalyst structure with the aim of tuning both the reactivity and selectivity of the oxidation reactions. This perspective provides an overview of the progress of this research. Some key features of the archetypical Fe(tpa) catalyst have stayed surprisingly true throughout this evolution, but a series of alterations that modulate its electronic, steric, or binding properties allowed a rational elicitation of a specific reactivity or selectivity. In some cases, the replacement of iron by manganese has also proven beneficial. Overall, the rational optimization of the catalyst structure has enabled the development of highly asymmetric olefin epoxidation, syn-dihydroxylation, and site-selective and even enantioselective C-H oxidation reactions
Non-heme Imine-based iron complexes as catalysts for oxidative processes
Full text linkNon-heme iron complexes are emerging as powerful and versatile catalysts in several oxidative transformations. The most investigated iron complex structures are based on aminopyridine ligands, but a number of imine-based ligands have been also tested. In this review a collection of recent results obtained in oxidation catalysis with non-heme imine-based iron complexes is presented. Their catalytic performances in C-H, C=C and -S- oxidation are spread over a wide range of efficiency, going from very low to quite high. Such performances are discussed, whenever possible, in light of the operating reaction mechanisms and of catalyst stability. In order to facilitate the discussion, an initial survey of the most useful mechanistic tools widely applied to distinguish a metal-based oxidation from a radical-chain process is also reported. Imine-based catalysts are divided into two classes: (i) salen-Fe complexes, and (ii) imine-Fe complexes. In some cases clues for free-radical oxidation mechanisms have been reported while in other cases evidence for metal-based mechanisms has been collected. The preferred mechanistic pathway is shown to be a function of catalyst structure and features. Interestingly, some imine-based iron complexes are able to perform stereospecific oxidation reactions, demonstrating that the imine functionality can be incorporated in ligands designed for oxidation catalysis. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Capitolo IV - Analisi territoriali, economiche ed urbane
No full textIl Capitolo IV affronta il tema delle analisi svolte per realizzare lo studio e i progetti sull'Isola dell'Unione a Chioggia durante il Laboratorio integrato di progettazione urbanistica dell'anno accademico 2019-2020. I contributi di docenti e studenti del Laboratorio Integrato di Progettazione urbanistica, del Corso di laurea magistrale in Architettura dell’Università degli Studi di Udine, sono integrati in questo volume da scritti di poeti, studiosi e professionisti. Il rapporto tra costoro va oltre il significato e i termini istituzionali della stesura di elaborati d’esame o di tesi di laurea. Ne emerge un vivace spaccato progettuale sulla città di Chioggia che, seppure non esaustivo, esalta le caratteristiche policentriche del sito, immaginando la stessa Chioggia come un paradigma per le città del futuro. Essa appare bisognosa di ricuciture tra differenti centri propulsori, ciascuno dotato di proprie specificità e bisognoso di integrarsi con il contesto urbano, con il paesaggio e con l’ambiente complessivo. L’isola dell’Unione, vista assieme alla piccola Laguna del Lusenzo come nuova centralità nevralgica del sistema urbano, l’Isola dei Saloni, rivalutata nelle sue funzioni trainanti, il Forte San Felice verso la Bocca di porto e l’Isola del Buon Castello, sono parti di una complessità urbana che potrebbe ritrovarsi in innumerevoli città in Italia e nel mondo, con sullo sfondo l’intento, comune agli altri progetti del LIPU, di voler ridurre secolarmente a paradigma del passato il concetto di periferia. L’interesse di questo libro va però anche oltre, poiché gran parte di questi progetti sono stati ideati e disegnati in un periodo particolarissimo, nella fase acuta della pandemia da Covid 19, o SARS-CoV-2 del 2020-2021, e si sono svolti, per la prima volta in forma obbligatoria in Italia, con didattica a distanza, attraverso lezioni e revisioni condotte interamente tramite il programma Teams messo a disposizione dall’Università degli Studi di Udine, intera¬mente via internet
Capitolo V - Progetti per un nuovo borgo urbano sull’Isola dell’Unione
No full textIl Capitolo V affronta il tema delle progettazioni strategiche svolte per realizzare lo studio e i progetti sull'Isola dell'Unione a Chioggia durante il Laboratorio integrato di progettazione urbanistica dell'anno accademico 2019-2020. A seguito di questi Diagrammi chiave sono state realizzate le planimetrie, i plastici, i progetti architettonici ed il piano urbanistico dell'Isola dell'Unione. I contributi di docenti e studenti del Laboratorio Integrato di Progettazione urbanistica, del Corso di laurea magistrale in Architettura dell’Università degli Studi di Udine, sono integrati in questo volume da scritti di poeti, studiosi e professionisti. Il rapporto tra costoro va oltre il significato e i termini istituzionali della stesura di elaborati d’esame o di tesi di laurea. Ne emerge un vivace spaccato progettuale sulla città di Chioggia che, seppure non esaustivo, esalta le caratteristiche policentriche del sito, immaginando la stessa Chioggia come un paradigma per le città del futuro. Essa appare bisognosa di ricuciture tra differenti centri propulsori, ciascuno dotato di proprie specificità e bisognoso di integrarsi con il contesto urbano, con il paesaggio e con l’ambiente complessivo. L’isola dell’Unione, vista assieme alla piccola Laguna del Lusenzo come nuova centralità nevralgica del sistema urbano, l’Isola dei Saloni, rivalutata nelle sue funzioni trainanti, il Forte San Felice verso la Bocca di porto e l’Isola del Buon Castello, sono parti di una complessità urbana che potrebbe ritrovarsi in innumerevoli città in Italia e nel mondo, con sullo sfondo l’intento, comune agli altri progetti del LIPU, di voler ridurre secolarmente a paradigma del passato il concetto di periferia. L’interesse di questo libro va però anche oltre, poiché gran parte di questi progetti sono stati ideati e disegnati in un periodo particolarissimo, nella fase acuta della pandemia da Covid 19, o SARS-CoV-2 del 2020-2021, e si sono svolti, per la prima volta in forma obbligatoria in Italia, con didattica a distanza, attraverso lezioni e revisioni condotte interamente tramite il programma Teams messo a disposizione dall’Università degli Studi di Udine, intera¬mente via internet
Electronic effect in iron catalysed C-H bond oxidation
No full textThe search for efficient and selective methods to oxidize C-H bonds has been a longstanding goal in organic chemistry. However, it is less than 5 years that this goal managed to become a useful tool in synthetic chemists hands by the use of nonheme iron catalysts. One of the most efficient catalytic systems prepared so far is White’s complex, reported in Figure 1 (R=H, the first structure on the left).
In our work we shed some light on the mechanism by which this system works, and evaluated the substituent effect on the reactivity of the catalyst in C-H bond oxidation. We report the synthesis and the characterization of the three substituted iron (II) PDP complexes shown above, two of which were never prepared before. The chosen substituents let us investigate the entire spectrum of electronic effects, from the EWGs to ERGs.
Moreover we studied the reaction of the iron (II) complexes with several oxidants in order to delucidate further the mechanism by which the active form of the catalysts is generated and subsequently oxidizes the substrate. We observed the formation and the subsequent decay of an intermediate, which can be the Fe(III) peroxide complex or the the oxoferryl. To discriminate between these two hypothesis we used a non-peroxide type oxidant, the iodosylbenzene. Finally we studied the kinetics of the formation and the subsequent decay of the intermediate with the three substituted complexes, thus comparing their reactivity. We investigated also the substituent effect on the yield and the product distribution of the reaction.
Our work let us understand further the way the Fe(II)PDP system works, and this is a step fundamental for the design of more efficient catalysts. The different oxidation activities exhibited by the substituted complexes also indicate the way to best modify the complex structure in order to improve their reactivity
Substituent Effect on the Catalytic Activity of Bipyrrolidine Based Iron Complexes
No full textThe catalytic activity and the selectivity of new bipyrrolidine based Fe(II) complexes 2●Fe(OTf)2 and 3●Fe(OTf)2 in the oxidation of a series of alkyl and alkenyl hydrocarbons as well as of an aromatic sulfide with H2O2 were tested and compared with the catalytic efficiency of White’s parent complex 1●Fe(OTf)2 in order to evaluate the sensitivity of the reaction to electronic effects
Electronic effects in iron-catalysed oxidation
No full textThe search for efficient and selective methods to oxidize C-H bonds has been a longstanding goal in organic chemistry. However, it is less than 5 years that this goal managed to become a useful tool in synthetic chemists hands by the use of nonheme iron catalysts. One of the most efficient catalytic systems prepared so far is White’s complex, reported in Figure 1 (R=H, the first structure on the left).
With the aim at shedding some light on the mechanism by which this system works, we prepared the three substituted iron (II) PDP complexes shown below. The chosen substituents allow a wide investigation of the electronic effects on the catalytic efficiency of the system.
The results obtained in the oxidation of some hydrocarbons substrates will be shown together with the full characterization of the new iron complexes
Substituent effect in iron-catalysed C-H bond oxidation
No full textThe search for efficient and selective methods to oxidize C-H bonds has been a longstanding goal in organic chemistry. However, it is less than 6 years that this goal managed to become a useful tool in synthetic chemists hands by the use of nonheme iron catalysts. One of the most efficient catalytic systems prepared so far is White’s complex,1 reported in Figure 1 (R=H, the first structure on the left).
With the aim at shedding some light on the mechanism by which this system works, we prepared the three substituted iron (II) PDP complexes shown below. The chosen substituents allow a wide investigation of the electronic effects on the catalytic efficiency of the system.
The results obtained in the oxidation of C-H bonds of some non-activated hydrocarbons substrates will be shown together with the synthetic route and a full characterization of the new iron complexes
Introduzione
No full textI contributi di docenti e studenti del Laboratorio Integrato di Progettazione urbanistica, del Corso di laurea magistrale in Architettura dell’Università degli Studi di Udine, sono integrati in questo volume da scritti di poeti, studiosi e professionisti. Il rapporto tra costoro va oltre il significato e i termini istituzionali della stesura di elaborati d’esame o di tesi di laurea. Ne emerge un vivace spaccato progettuale sulla città di Chioggia che, seppure non esaustivo, esalta le caratteristiche policentriche del sito, immaginando la stessa Chioggia come un paradigma per le città del futuro. Essa appare bisognosa di ricuciture tra differenti centri propulsori, ciascuno dotato di proprie specificità e bisognoso di integrarsi con il contesto urbano, con il paesaggio e con l’ambiente complessivo. L’isola dell’Unione, vista assieme alla piccola Laguna del Lusenzo come nuova centralità nevralgica del sistema urbano, l’Isola dei Saloni, rivalutata nelle sue funzioni trainanti, il Forte San Felice verso la Bocca di porto e l’Isola del Buon Castello, sono parti di una complessità urbana che potrebbe ritrovarsi in innumerevoli città in Italia e nel mondo, con sullo sfondo l’intento, comune agli altri progetti del LIPU, di voler ridurre secolarmente a paradigma del passato il concetto di periferia. L’interesse di questo libro va però anche oltre, poiché gran parte di questi progetti sono stati ideati e disegnati in un periodo particolarissimo, nella fase acuta della pandemia da Covid 19, o SARS-CoV-2 del 2020-2021, e si sono svolti, per la prima volta in forma obbligatoria in Italia, con didattica a distanza, attraverso lezioni e revisioni condotte interamente tramite il programma Teams messo a disposizione dall’Università degli Studi di Udine, interamente via internet
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