Réaction de Wittig

Résumé

La réaction de Wittig est un cas particulier des additions nucléophiles. Il s'agit de l'addition d'un ylure de phosphore (phosphorane) (souvent appelé réactif de Wittig) sur un composé carbonylé (aldéhydes et cétones) pour former des composés éthyléniques (éthylène substitué). Georg Wittig a reçu en 1979 le prix Nobel de chimie pour la découverte de cette nouvelle méthode de synthèse d'alcènes. Réaction center Cette réaction est ainsi très utilisée en synthèse organique pour former des dérivés éthyléniques peu substitués, difficile à former par des réactions d'élimination, qui forment majoritairement les composés les plus substitués. Elle a de plus un très bon rendement, qu'on peut expliquer thermodynamiquement par la formation de l'oxyde de triphénylphosphine qui possède une liaison P=O très forte (de l'ordre de ). center On notera par la suite la partie triphénylphosphine - pour cette molécule ainsi que pour toutes celles qui possèdent cette partie. Cette réaction

Source officielle

https://fr.wikipedia.org/wiki/R%C3%A9action_de_Wittig

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alt=Structure atomique de l'aldéhyde.|vignette|Structure atomique de l'aldéhyde. Un aldéhyde est un composé organique, faisant partie de la famille des composés carbonylés, dont l'un des atomes de ca

Un ylure est une espèce chimique moléculaire neutre et dipolaire dont un atome possède une charge formelle négative (il s'agit souvent d'un atome de carbone, la molécule étant dans ce cas un carbanion

Les alcènes sont des hydrocarbures insaturés, caractérisés par la présence d'au moins une double liaison covalente entre deux atomes de carbone. Ces liaisons sont toujours de types covalentes normal

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In many plant species, the acetate fraction is the fourth most prominent fraction by weight after cellulose, hemicellulose and lignin, and can be easily extracted as a single stable molecule, acetic acid, at high yields. Despite this, upgrading the acetate fraction of biomass has received very limited attention. Here, we demonstrate a valorization route for the acetate fraction as well as mixtures of acetic acid and other volatile fatty acids produced from the polysaccharide fraction. Aqueous solutions of acetic acid, including solutions produced during steam explosion pretreatment and subsequently purified can be upgraded at high selectivity to a valuable mixture of aromatics, substituted cycloalkenes and gas olefins in a single step using Cu/ZrO2. The catalyst displays remarkable stability despite the presence of acids, water and other biomass-derived impurities. We also show that acetic acid can be further valorized over the same catalyst by converting it in the presence of butanoic acid that was produced in a consolidated bioprocess from the same pretreated wood that was the source of the acetic acid. In this case, the acetic acid rapidly ketonizes with the butanoic acid and the resulting beta-ketones further condense to form aromatics and cycloalkenes with a higher average carbon number than those produced solely from acetic acid. Overall, our process yields a biomass-derived organic oil consisting of aromatics and cycloalkenes that spontaneously separates from water, can be tuned by varying the incoming mixture of carboxylic acids and has suitable properties for being used as a direct blend with aviation fuel.

2019

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Exploring Synthetic Strategies for Nanocrystal/Reticular Framework Hybrids and Their Potential as CO2 Reduction Electrocatalysts

Yannick Thomas Guntern

The electrochemical CO2 reduction reaction (CO2RR) into valuable chemicals has the potential to realize a carbon-neutral energy cycle. Developing catalysts that can achieve high selectivity towards one of the possible reduction products is one of the biggest challenges in this field. Catalysts possessing one single type of active sites cannot fulfil this need, yet to establish design rules for bifunctional catalysts is not trivial. To have material platforms which allow to study the sensitivity of the catalytic behaviour to structural and compositional features of the catalyst is of the uttermost importance to inform catalyst design. This thesis aims at combining well-defined colloidal nanocrystals (NCs) and porous reticular frameworks as hybrid materials to explore their synergistic interactions as CO2RR catalysts. These classes of materials were chosen because they possess complementary chemical properties which are appealing towards eventually construct an ideal catalyst with high performance. However, one of the challenges is to obtain these hybrids with tunable structural features in an unrestricted compositional range. The overall objective is to design and explore synthetic strategies for the formation of new multifunctional composites consisting of colloidal NCs and metal-organic/covalent organic frameworks (MOFs/COFs). First, a synthetic approach for the formation of NC/MOF hybrid thin films is proposed, which enables their application as a novel catalytic platform for CO2RR. We demonstrate that a combination of colloidal chemistry, atomic-layer deposition and solvothermal synthesis allows to form an intimate contact between Ag NCs and an Al-PMOF matrix. Our tuneable Ag@Al-PMOF hybrid catalysts show promoted CO2RR which is attributed to electronic changes in the Ag NC surface and minor mass-transport effects. Moreover, the hybrids improve the morphological stability of the Ag NCs under CO2RR conditions. We show that the synthetic approach can be further extended to form hybrids with other metallic NCs, thereby representing a new tool for the preparation of composite catalysts for CO2RR. One of the limitations of thin film composites is that synthesis conditions might need to be adapted for different substrates. To prepare hybrid materials in the form of dispensible colloidal solutions is highly appealing and better suited for mechanistic studies. The latter are important because understanding the chemistry driving the formation of such composites is eventually the key for achieving compositional and structural tunability. In the second part, we introduce a new strategy to encapsulate various colloidal NCs in the imine-linked COF LZU1. The synthetic approach allows us to tune the shell thickness and crystallization of the COF in the presence of the NCs. After understanding the nucleation and growth mechanisms of the hybrids, we extend the developed synthesis to obtain multi-layered structures with controlled spatial distribution of various NCs with different compositions. Altogether, we demonstrate that our approach can be applied to prepare hybrids with new functionalities that derive from the encapsulated NCs. Overall, this thesis proposes two different approaches which combine colloidal and reticular chemistry to synthesize tunable material platforms which might aid the design of CO2RR catalysts as well as contributing to other applications which require multifunctional porous materials.

EPFL2021

Chargement

Engineered peptide macrocycles can inhibit matrix metalloproteinases with high selectivity

Alice Baumann, Christian Heinis, Xudong Kong, Michal Sabisz, Jeremy Touati, Jonas Alfred Karl Wilbs

Matrix metalloproteinases (MMPs) are zinc‐dependent endopeptidases at the intersection of health and disease due to their involvement in processes such as tissue repair and immunity as well as cancer and inflammation. Because of the high structural conservation in the catalytic domains and shallow substrate binding sites, selective, small‐molecule inhibitors of MMPs have remained elusive. In a tour‐de‐force peptide engineering approach combining phage‐display selections, rational design of enhanced zinc chelation, and d‐amino acid screening, we succeeded in developing a first synthetic MMP‐2 inhibitor that combines high potency (Ki=1.9±0.5 nm), high target selectivity, and proteolytic stability, and thus fulfills all the required qualities for in cell culture and in vivo application. Our work suggests that selective MMP inhibition is achievable with peptide macrocycles and paves the way for developing specific inhibitors for application as chemical probes and potentially therapeutics.

2019

Chargement

Exploring Synthetic Strategies for Nanocrystal/Reticular Framework Hybrids and Their Potential as CO2 Reduction Electrocatalysts

Yannick Thomas Guntern

EPFL2021