Borylation and rearrangement of alkynyloxiranes: a stereospecific route to substituted α-enynes

International audienc

Materials Exposure and Degradation Experiment (MEDET)

Material properties are of primary interest for spacecraft designers and their evolution as a consequence of exposure to the space environment must be accurately predicted. In this context, CNES, ESA, the University of Southampton and ONERA are participating in a co-operative effort to develop a test-bed called the Material Exposure and Degradation Experiment (MEDET). The experiment will be integrated on the European Technology Exposure Facility (EuTEF), and as such will be placed on the Columbus External Payload Facility, an exterior platform on the International Space Station. The objectives of the experiment are concerned with the effect of the complex space environment on the optical and thermo-optical properties of materials and the characterisation of the ISS environment. This paper describes the MEDET system configuration, the different scientific experiments, the control electronics and the overall implementation of the payload

New catalytic pathways for the synthesis of hydrosilanes from renewable reductants

L’utilisation massive de ressources fossiles telles que le charbon ou le pétrole provoque, outre l’appauvrissement de ces ressources, de fortes émissions de CO₂ anthropogéniques. Pour évoluer vers une société durable et plus respectueuse de l’environnement, une stratégie d’économie circulaire est indispensable. Dans ce cadre, la valorisation de ressources carbonées alternatives telles que le CO₂, la biomasse ou les déchets plastiques oxygénés vers des produits à haute valeur ajoutée est particulièrement attrayante. Elle nécessite néanmoins le développement de méthodes efficaces pour la réduction de liaisons C–O (σ ou π) en liaisons C–H. Les hydrosilanes et les hydroboranes sont des réducteurs adéquats pour réaliser ces transformations, mais ils ne sont pas renouvelables et leur production est très énergivore. Dans ces travaux de thèse, nous avons développé de nouvelles voies de synthèse des hydrosilanes à partir de H₂, un réducteur renouvelable. Des systèmes catalytiques à base d’iridium ou de bore ont été découverts, qui facilitent ces nouvelles voies de synthèse plus efficaces d’un point de vue énergétique. Les travaux de cette thèse ont, de plus, ouvert la voie à la synthèse d’hydroboranes à partir de H₂. Enfin, une approche électrocatalytique pour la synthèse d’hydrosilanes a également été explorée.The wide-scale use of fossil resources such as coal or oil causes, not only the depletion of these resources, but also high anthropogenic CO₂ emissions. A circular economy strategy is essential to progress towards a sustainable society, more respectful of the environment. In this context, the conversion of alternative carbon resources such as CO₂, biomass or oxygenated plastic wastes into useful chemicals is particularly attractive. It requires the development of efficient methods for the reduction of C–O bonds (σ or π) into C–H bonds. While hydrosilanes and hydroboranes are suitable reductants to perform these transformations, they are not renewable and their production is energy- intensive. In this thesis, we have developed new pathways for the synthesis of hydrosilanes from H₂, a renewable reductant. Catalytic systems based on iridium or boron have been discovered which facilitate these new, more energy-efficient synthetic routes. Moreover, this work has paved the way to the synthesis of hydroboranes from H₂. Finally, an electrocatalytic approach for the synthesis of hydrosilanes has also been explored

Nouvelles voies de synthèse catalytique d'hydrosilanes à partir de réducteurs renouvelables

The wide-scale use of fossil resources such as coal or oil causes, not only the depletion of these resources, but also high anthropogenic CO₂ emissions. A circular economy strategy is essential to progress towards a sustainable society, more respectful of the environment. In this context, the conversion of alternative carbon resources such as CO₂, biomass or oxygenated plastic wastes into useful chemicals is particularly attractive. It requires the development of efficient methods for the reduction of C–O bonds (σ or π) into C–H bonds. While hydrosilanes and hydroboranes are suitable reductants to perform these transformations, they are not renewable and their production is energy- intensive. In this thesis, we have developed new pathways for the synthesis of hydrosilanes from H₂, a renewable reductant. Catalytic systems based on iridium or boron have been discovered which facilitate these new, more energy-efficient synthetic routes. Moreover, this work has paved the way to the synthesis of hydroboranes from H₂. Finally, an electrocatalytic approach for the synthesis of hydrosilanes has also been explored.L’utilisation massive de ressources fossiles telles que le charbon ou le pétrole provoque, outre l’appauvrissement de ces ressources, de fortes émissions de CO₂ anthropogéniques. Pour évoluer vers une société durable et plus respectueuse de l’environnement, une stratégie d’économie circulaire est indispensable. Dans ce cadre, la valorisation de ressources carbonées alternatives telles que le CO₂, la biomasse ou les déchets plastiques oxygénés vers des produits à haute valeur ajoutée est particulièrement attrayante. Elle nécessite néanmoins le développement de méthodes efficaces pour la réduction de liaisons C–O (σ ou π) en liaisons C–H. Les hydrosilanes et les hydroboranes sont des réducteurs adéquats pour réaliser ces transformations, mais ils ne sont pas renouvelables et leur production est très énergivore. Dans ces travaux de thèse, nous avons développé de nouvelles voies de synthèse des hydrosilanes à partir de H₂, un réducteur renouvelable. Des systèmes catalytiques à base d’iridium ou de bore ont été découverts, qui facilitent ces nouvelles voies de synthèse plus efficaces d’un point de vue énergétique. Les travaux de cette thèse ont, de plus, ouvert la voie à la synthèse d’hydroboranes à partir de H₂. Enfin, une approche électrocatalytique pour la synthèse d’hydrosilanes a également été explorée

Experiment–Theory Synergy: Connecting the Kinetics of the Molecular Catalysis of Electrochemical Reactions with Calculated Energy Landscapes

International audienceWhile energy profiles from quantum mechanical calculations suggest mechanisms for molecular catalysis of electrochemical reactions, they frequently lack experimental kinetic validation due to limited kinetic data or ambiguities linking calculated and experimental observables. Herein, we expand the "energetic span model", traditionally applied in homogeneous systems, to molecularly catalyzed electrochemical reactions focusing on EC 1 ..C n E′-type mechanisms. We thus establish a framework for aligning theoretical turnover frequency estimates with practical cyclic voltammetry measurements in electrochemical systems, i.e., extracted rate constants accounting for diffusion-reaction layer complexities. The analysis also identifies specific kinetic zones, defining conditions under which different catalyst intermediates dominate the diffusion-reaction layer. This approach helps refine the energetic span model for electrochemical catalysis and may improve the alignment of experimental data with the theoretical calculation. It is applied to the experimentally well-studied electrochemical reduction of CO2 to CO using an iron tetraphenylporphyrin catalyst and phenol as proton donor. Previously explored theoretical pathways align partly with experimental data, but important discrepancies exist, especially regarding the reaction's dependence on CO2 binding and proton donor concentration. The findings highlight the challenges in predicting the catalyst behavior and underscore the significance of intermediate energetics in reaction modeling. Nonetheless, cross-talk between theoretical calculations and solid kinetic experimental studies should be a reasonable path toward reaching mechanistic consensus

Vers la synthèse de réducteurs renouvelables à base de silicium

International audienceFaciliter l’émergence d’une économie circulaire du carbone, dans laquelle des déchets tels que le CO2 ou les résidusde la biomasse sont des matières premières, nécessite l’utilisation de réducteurs efficaces et recyclables, capablesde convertir des liaisons C–O en liaisons C–H et C–C. À cette fin, de nouvelles méthodes de production de réducteurssilylés ont été explorées, permettant de s’affranchir de l’utilisation d’hydrures métalliques dont la production esttrès énergivore. Les formiates de silicium, obtenus à partir d’acide formique, sont une nouvelle classe de mimesd’hydrosilanes. Alternativement, la synthèse d’hydrosilanes vrais a pu être réalisée par hydrogénolyse catalytiquede chlorosilanes, en utilisant H2 comme source d’hydrure

Proton‐Coupled Electron Transfer Deoxygenation of Pyridine N‐Oxide: A Mechanistic Study

International audienceElectrochemical reductive deoxygenation of pyridine N‐oxide is investigated with particular focus on the role of proton‐coupled electron transfers. A detailed analysis of cyclic voltammograms reveals that the initial electron transfer is followed by protonation of the pyridine N‐oxide anion radical. Kinetic analysis reveals an unusual fifth‐order dependence on the concentration of the proton donor (either water or ethanol), suggesting the involvement of a proton donor cluster in the protonation step. The resulting neutral radical represents a key bottleneck in the reaction pathway, as it can proceed via either a parent‐child coupling reaction or NO bond cleavage, the latter leading to the formation of pyridine. This competition between reaction pathways allows extraction of both the rate constant for the protonation of the N‐oxide radical anion and kinetic information related to the reductive NO bond cleavage. The reductive cleavage of the protonated N‐oxide radical may proceed via two possible mechanisms: 1) homolytic bond cleavage followed by reduction of the hydroxyl radical, or 2) a concerted dissociative electron transfer. The observed hydrogen‐bonding effects, combined with the higher driving force for the concerted pathway, support the latter mechanism, where stabilization of the departing hydroxide ion facilitates the electron transfer

Unlocking the Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with Superbases

The efficient synthesis of hydrosilanes by catalytic ydrogenolysis of chlorosilanes is described, using an Iridum (III) pincer catalyst. A careful selection of a nitrogen base (incl. sterically hindered guanidines and phosphazenes) can unlock the preparation of Me3SiH, Et3SiH and Me2SiHCl in high yield (up to 98%), directly from their corresponding chlorosilanes

Unlocking the Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with Superbases

The efficient synthesis of hydrosilanes by catalytic ydrogenolysis of chlorosilanes is described, using an Iridum (III) pincer catalyst. A careful selection of a nitrogen base (incl. sterically hindered guanidines and phosphazenes) can unlock the preparation of Me&lt;sub&gt;3&lt;/sub&gt;SiH, Et&lt;sub&gt;3&lt;/sub&gt;SiH and Me&lt;sub&gt;2&lt;/sub&gt;SiHCl in high yield (up to 98%), directly from their corresponding chlorosilanes.</jats:p