Stereoisomerism | EPFL Graph Search

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer. Enantiomers Chirality (chemistry) and Enantiomers Enantiomers, also known as optical isomers, are two stereoisomers that are related to each other by a reflection: they are s of each other that are non-superposable. Human hands are a macroscopic analog of this. Every stereogenic center in one has the opposite configuration in the other. Two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact

Influences des variables géologiques sur la structure de communautés bactériennes impliquées dans la déhalogenation réductrice des éthènes chlores dans un aquifère contaminé

Romain Coulon

Le tetrachloroéthène (PCE), le trichloroéthène (TCE), le dichloroéthène (DCE) et le chlorure de vinyle (VC) appartiennent aux éthènes chlorés. Le tetrachloréthène (PCE) et le trichloroéthène (TCE) sont utilisés comme solvant, notamment pour le nettoyage à sec des tissus et le dégrassage des pièces métalliques. Le dichloroéthène (DCE) et le chlorure de vinyle (VC), produits de dégradation du PCE et du TCE, sont également utilisés dans l'industrie. Utilisés intensivement durant le siècle dernier, surtout comme agent dégraissant, ce sont les polluants les plus abondants dans les nappes et les sols. Les éthènes chlorés ont été désignés dans le cadre de directives européennes (The water framework directive) et dans le cadre des législations nationales des différents membres de l'UE comme des substances prioritaires dans l'assainissement des sites contaminés. D'où l'intérêt d'évaluer et de prédire les processus d'atténuation de ces milieux contaminés. La déhlorespiration est un processus microbiologique où les éthènes chlorés sont utilisés comme accepteur d'électrons dans la chaîne respiratoire. Alors que les communautés impliquées dans la déholorespiration et leurs dynamiques durant le processus d'atténuation ont largement été étudiées, l'influence des variables environnementales est encore peu connue. Parmi les variables environnementales qui pourraient influencer ces communautés, nous pensons que la géologie aurait un rôle déterminant. Nous allons donc étudier l'influence de la granulométrie et de la géochimie sur une communauté bactérienne présente dans un aquifère anoxique contaminé par des éthènes chlorés.

2008

Access to Chiral Cyclopentane Scaffolds by Enantioselective Nickel-Catalyzed Annulations

Joachim Sven Ernst Ahlin

The cyclopentane scaffold has attracted much attention due to its ubiquity in numerous bioactive natural products such as prostaglandins and polyquinanes. However, despite a plethora of available procedures to access this motif, a lack of methods that rival the generality of the Diels-Alder reaction for the synthesis of six-membered rings represents an important gap. Thus, the development of protocols providing efficient access to functionalized chiral cyclopentanes from simple and readily accessible starting materials is highly desirable. In this respect, the tremendous advances in the development of nickel-catalyzed carbon-carbon bond-forming reactions, combined with the unique properties of N-heterocyclic carbenes have led to the development of new approaches to five-membered carbocyclic rings. These include nickel-catalyzed reductive couplings and reductive [3+2] cycloadditions. These strategies are attractive, as functionalized five-membered carbocyclic rings can be accessed from two simple pi-components without any changes in atom connectivity. Despite the recent development of numerous chiral N-heterocyclic carbenes and their successful application in various transition metal-catalyzed reactions, applications in asymmetric nickel catalysis remain scarce. This thesis describes the development of efficient enantioselective nickel-catalyzed methodologies for the formation of five-membered carbocycles from readily accessible and inexpensive starting materials, enabled by two novel chiral N-heterocyclic carbene ligands. First, the development of an intermolecular enantioselective nickel-catalyzed reductive [3+2] cycloaddition of unsaturated aromatic esters and internal alkynes for the synthesis of cyclopentenones is described. For this scalable methodology, a bulky chiral C1-symmetric N-heterocyclic carbene ligand was shown to facilitate the efficient asymmetric synthesis of cyclopentenones from a wide range of mesityl enoates and internal alkynes under mild conditions. Unsymmetrically substituted alkynes were incorporated in a regioselective fashion and the cyclopentenones were synthesized in high yields and enantioselectivity. As a second methodology for the construction of chiral cyclopentane scaffolds, an intermolecular diastereoselective and enantioselective nickel-catalyzed reductive three-component coupling of various aryl aldehydes, norbornene derivatives and silanes is described, which gives access to synthetically valuable silyl-protected indan-1-ols via an aromatic C(sp2)-H functionalization. A bulky chiral C2-symmetric N-heterocyclic carbene ligand possessing a 1,2-(dinaphthalen-1-yl)ethylene diamine scaffold provided the silylated indan-1-ols under mild and scalable conditions, as a single diastereoisomer, in high enantioselectivity and good regioselectivity in the case of meta-substituted aryl aldehydes.

EPFL2016

Mechanistic Insight into Supramolecular Polymerization in Water Tunable by Molecular Geometry

Lukas Pfeifer, Fan Xu

Supramolecular self-assembly in water based on non-covalent bonding is attracting major attention due to the potential of hydrogels and aqueous polymers in biomedical applications. Although supramolecular polymerization in organic solvents is well established, the key design features, the assembly mechanisms in water and achieving control over the aggregate structures remain challenging. Here, we present the assembly and disassembly of geometrical isomers of a stiff-stilbene bis-urea amphiphile (SA) in pure water. A remarkable feature of this system is that the (E)-isomer forms supramolecular polymers in both pure water and organic solvents. Taking advantage of this unique property, the hydrophobic effect was studied by comparing the supramolecular assembly in both systems. The assembly process in water follows an enthalpy-driven nucleation-elongation (cooperative) supramolecular polymerization mechanism with a standard Gibbs free energy (Delta G degrees = -53 kJ mol(-1)) double the value of the one found in toluene. We attributed this distinctive feature to the hydrophobic effect in water. Furthermore, we discovered an isomer-dependent assembly process, which can be used to control aggregation in aqueous media. Due to the substantial geometric difference between (E)-SA and (Z)-SA, we compared their assembly in water to study the influence of different driving forces involved in the process. The supramolecular polymerization of (E)-SA was cooperatively influenced by hydrogen bonding, pi-stacking, and hydrophobic effects, whereas the assembly of (Z)-SA was mainly driven by hydrophobic effects. As a result, the fiber length of (E)-SA in water is much longer than that of (Z)-SA, presenting opportunities for geometrical control of aggregation in aqueous media. [GRAPHICS]

CHINESE CHEMICAL SOC2022

Access to Chiral Cyclopentane Scaffolds by Enantioselective Nickel-Catalyzed Annulations

Joachim Sven Ernst Ahlin

EPFL2016