1,721,036 research outputs found
Reactivity of Alpha-Diketones in Umpolung Catalysis and Enantioselective Sulfoxidation by Novel Titanium Complexes
No full textNella continua ricerca di metodologie atte alla formazione di nuovi legami carbonio-carbonio, l’organocatalisi svolge un ruolo sempre più fondamentale; in questo contesto, la catalisi di Umpolung, che prevede l’inversione di reattività di un dato substrato, rappresenta un’importante via per accedere a nuove strategie sintetiche. All’interno di quel gruppo di molecole in grado di fornire reattività di umpolung, gli α-dichetoni rappresentano senza dubbio substrati molto interessati ed innovativi.
Alla luce di queste premesse, la reattività di α-dichetoni di-arilici è stata oggetto dei nostri studi. Inizialmente è stata valutata la reattività in reazioni di tipo cross-benzoinico con aldeidi, introducendo un nuovo catalizzatore di umpolung, il metil solfonil (dimsile) carbanione, con ottimi risultati in termini di efficienza di reazione e di chemoselettività.
La messa a punto di questa metodologia ha inspirato lo sviluppo di un nuova strategia sintetica per l’accesso a 1,2-dichetoni asimmetrici, che ha previsto una prima reazione cross-benzoinica catalizzata dall’anione dimsile , seguita da un’ossidazione in situ con riscaldamento a microonde.
Successivamente, si è passati all’impiego di α-dichetoni di-arilici in reazioni di addizione nucleofila con accettori di Michael; in questo caso è stata sviluppata una nuova reazione di doppia aroilazione catalizzata dal metil solfonil anione. E’ stato infine conferito un ulteriore valore aggiunto ai prodotti di reazione grazie ad un successivo stadio ossidativo, che ha consentito l’accesso ai corrispettivi alcheni tetrasostituiti.
Inoltre, uno degli aspetti più rilevanti della nostra attività di ricerca è consistito nell’indagine meccanicistica supportata da misure di tipo NMR ed ESI-MS. In questo contesto, è stato recentemente condotto lo studio ESI-MS della reazione di ossidazione aerobica di aldeidi catalizzata, da N-eterociclo carbeni (NHC). Mediante l’utilizzo di un NHC opportunamente funzionalizzato è stato possibile individuare le diverse specie e gli equilibri coinvolti nel processo catalitico.
Alla luce dell’esperienza pregressa del mio gruppo di ricerca nel campo della chimica in flusso, diversi processi di umpolung, quali reazioni di tipo benzoinico, aciloinico e di Stetter, sono stati studiati utilizzando microreattori monolitici tiazolio-funzionalizzati in regime di flusso.
Infine, durante il periodo svolto presso il gruppo del Prof. Berkessel, l’attività di ricerca si è incentrata sulla sintesi e l’impiego di ligandi cis-DACH-salalen in reazioni di solfossidazione stereoselettiva catalizzate da complessi di Titanio, utilizzando H2O2 come ossidante; durante questo lavoro è stato
messo a punto un solido protocollo per lo step di solfossidazione enantioselettiva della sintesi dell’esomeprazolo.In the investigation of effective chemical transformations, organocatalyzed carbon-carbon bond forming reactions play an outstanding role. Particularly, the inversion of the typical reactivity of a given substrate (umpolung) opens up new and unconventional synthetic pathways. In the continuous research of new acyl anion precursors, α-diketones actually represent innovative and efficient substrates.
In this regard, the it was studied the umpolung reactivity of α-diketones confirming their crucial role as acyl anion precursors; after previous investigations by our group on dialkyl diketones, we focused our research on diaryl diketones. First, their reactivity in the cross-benzoin reaction with aldehydes was evaluated, introducing a new umpolung catalyst, the methyl sulfinyl (dimsyl) anion, with great results in terms of yields and chemoselectivity.
This new optimized procedure opened a new synthetic route towards the synthesis of unsymmetrical αdiketones, thanks to a one-pot two-step process that involved a first cross-benzoin reaction catalyzed by the dimsyl anion, followed by in situ oxidation performed under microwave conditions.
As a logical development of our research, the reactivity of diaryl 1,2-diketones with Michael acceptors in “Stetter-like” reactions was investigated; a new double aroylation reaction catalyzed by the methyl sulfinyl carbanion was discovered. The utility of the resulting 2-benzoyl-1,4-diones was also demonstrated by their facile conversion into the corresponding tetrasubstituted olefins.
Investigation of reaction mechanisms by NMR and ESI-MS analysis represented an important part of our work. In this regard, a charge-tagged N-heterocyclic carbene (NHC) was employed as mass spectrometric probe to identify the species involved in the different oxidation steps of the aerobic NHC-catalyzed oxidation of aldehydes. Moreover, in continuation of the reserach of our group in the flow chemistry field, we prepared thiazolium-functionalized monolithic microreactors and demonstrated their synthetic potential in continuous-flow umpolung transformations such as benzoin, acyloin-type, and Stetter reactions. Finally, during my period in Prof. Berkessel group, a series of cis-DACH-derived salalen ligands were synthetized and their activity evaluated in the titanium-catalyzed asymmetric sulfoxidation of thioethers with aqueous hydrogen peroxide; a solid methodology for the last enantioselective sulfoxidation step of Esomeprazole synthesis was developed
Ordini molteplici della costruzione nell'architettura di Peter Märkli
No full textL’indagine che qui si presenta ha la finalità di esplicitare le domande essenziali su cui l’opera dell’architetto svizzero Peter Märkli riflette nell’ambito del rapporto tra Architettura e Costru- zione.
I caratteri costitutivi del modus operandi di questo autore così singolare nel panorama architettonico contemporaneo sono forma ed espressione.
È sostanzialmente dal reciproco contrasto tra questi due termini durante il processo progettuale che Märkli inventa le proprie opere, distanziandosi da ogni modalità preconcetta o ideologica di concepirle. Una postura sempre aperta al costante dialogo con le pratiche dell’arte e con la storia delle forme dell’architettura, dalla quale risulta complessivamente un eclettismo creativo di difficile inquadramento ed interpretazione che si manifesta nell’adozione di linguaggi che spaziano dall’evocazione del linguaggio classico alla pura astrazione.
L’opera nel suo complesso è un contributo critico fondamentale ed una posizione estremamente radicale nel dibattito architettonico contemporaneo, avvicinando questo autore ai suoi maestri d’elezione: Le Corbusier, Sigurd Lewerentz, ma soprattutto Mies van der Rohe.
Analogamente alla riflessione maturata dal maestro tedesco, Märkli afferma che l’architettura è un antico linguaggio che ha una sua grammatica, rivendicando il suo essere primariamente Baukunst, e cioè, arte del costruire.The investigation presented here aims to clarify the essential questions on which the work of the Swiss architect Peter Märkli reflects in the context of the relationship between Architecture and Construction.
The constitutive characters of the modus operandi of this author so singular in the contemporary architectural panorama are form and expression.
It is substantially from the mutual contrast between these two terms during the design process that Märkli invents his own works, distancing himself from any preconceived or ideological way of conceiving them. A posture always open to constant dialogue with the practices of art and with the history of architectural forms, from which an overall creative eclecticism emerges that is difficult to classify and interpret, which manifests itself in the adoption of languages that range from the evocation of classical language to pure abstraction.
The work as a whole is a fundamental critical contribution and an extremely radical position in the contemporary architectural debate, bringing this author closer to his chosen masters: Le Corbusier, Sigurd Lewerentz, but above all Mies van der Rohe. Similarly to the reflection matured by the German master, Märkli states that architecture is an ancient language that has its own grammar, claiming its being primarily Baukunst, that is, the art of building
Methylsulfinyl (Dimsyl) anion as valuable replacement of CN- in benzoin- and Stetter-like reactions
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A Visible‐Light‐Powered Polymerization Method for the Immobilization of Enantioselective Organocatalysts into Microreactors
No full textA versatile one-step photopolymerization approach for the immobilization of enantioselective organocatalysts is presented. Chiral organocatalyst-containing monoliths based on polystyrene divinylbenzene copolymer were generated inside channels of microfluidic chips. Exemplary performance tests were performed for the monolithic Hayashi–Jørgensen catalyst in continuous flow, which showed good results for the Michael addition of aldehydes to nitroalkenes in terms of stereoselectivity and catalyst stability with minimal consumption of reagents and solvents
Rheological response of polylactic acid dispersions in water with xanthan gum
Full text linkIn this work, the rheological behaviour of stable PLA dispersions in water, intended for coating applications, was investigated. The newly prepared dispersions consist of PLA particles with average diameter of 221.9 ± 2.0 nm based on DLS and SEM analyses, at concentrations varying in the 5-22 wt % range. Xanthan gum (XG), a bacterial polysaccharide, was used as a thickening agent to modulate the viscosity of the formulations. The rheological properties of the PLA dispersions with different XG and PLA contents were studied in steady shear, amplitude sweep and frequency sweep experiments. Under steady shear conditions, the viscosity of all the formulations showed a shear-thinning behaviour similar to XG solutions in the whole investigated 1-1000 s-1 range, with values dependent on both PLA particles and XG concentrations.
Amplitude and frequency sweep data revealed a weak-gel behaviour except in the case of the most diluted sample, with moduli dependent on both PLA and XG content. A unified scaling parameter was identified in the volume fraction (Φ) of the PLA particles, calculated by considering the dependence of the continuous phase density on the XG concentration. Accordingly, a mastercurve at different volume fractions was built through the Time-Concentration Superposition (TCS) approach. The mastercurve describes the rheological response of the system over a wider frequency window than the experimentally accessible one, and reveals the presence of a superimposed relaxation process in the high-frequency region
Exploring the Potential of High Molar Mass Poly(hydroxymethylfuroate): Synthesis, Properties, and Recyclability Studies
No full textThe synthesis of the fully furan-based high-molar-mass poly(hydroxymethylfuroate) (HMM-PHMF) is herein presented along with the investigation of thermal properties, filmability, water contact angle (WCA), and recyclability. The polymer (Mn > 200000 g mol-1) has been produced on a multigram scale by N-heterocyclic carbene (NHC)-promoted chain extension of PHMF (1 h, 190 °C), which can be directly synthesized from 5-(hydroxymethyl)furfural (HMF) or 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The polyester is thermally highly stable up to 280 °C, showing a semicrystalline behavior with Tm ∼ 194 °C when processed from solvent and an amorphous character with Tg ∼ 90 °C when processed from the melt. HMM-PHMF has been efficiently filmed by compression molding and submitted to WCA analyses (86 ± 2°), behaving as a potential biobased food packaging material. Finally, recyclability has been assessed through alkaline hydrolysis producing HMFCA and cyclodepolymerization affording the starting hydroxymethylfuroate macrocyclic oligoesters c(HMF)n, which can be converted again through entropically driven ring-opening polymerization (ED-ROP), in a closed-loop cycle
Dimethyl Sulfoxide and N,N-dimethylformamide as Umpolung Pre-catalysts in Cross Benzoin Reactions of alpha-Diketones with Aldehydes
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Exploiting cooperative photoredox and asymmetric organocatalysis in cross-dehydrogenative coupling of glycine analogues with ketones
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