Slow kinetic evolution of nanohelices based on gemini surfactant self-assemblies with various enantiomeric excess

Very slow kinetic evolution (days) of chiral nanoribbons based on amphiphilic molecular assemblies is investigated for various enantiomeric excess. In the early stage of ribbon formation, those formed for low enantiomeric excess (0 <= ee <= 0.5) show chiral segregation, and are made with a mixture of short-pitched right-handed and left-handed twisted ribbons. With time, these nano-ribbons merge to form ribbons with larger pitch and single-handedness, which is determined by the majority enantiomer.Initiative d'excellence de l'Université de Bordeau

Chem. Commun.

Hybrid nanometric helical structures formed by the molecular assemblies of dicationic gemini surfactants with tartrate counterions covered with helical silica walls interact differently with matching or mismatching enantiomers of the tartrate. The difference of the interaction is based on the cooperativity between the chiral crystalline gemini surfactant molecular organization/conformation and the rigid chiral nanospace formed by the helical silica wall

Catalyseurs polyoxométallates dendritiques recyclables pour les réactions d'oxydation

Les métallodendrimères constituent une catégorie de catalyseurs relativement nouveaux, dont la propriété essentielle est la recyclabilité. Ils sont synthétisés de façon à bénéficier de la sélectivité et des performances des catlyseurs homogènes, tout en profitant de la recyclabilité des catalyseurs hétérogènes. L'association des polyoxométallates et des dendrimères permet d'accèder à un nouveau type de métallodendrimères efficaces et recyclables pour la catalyse d'oxydation. Plusieurs séries de dendrimères fonctionnaliés à la périphérie, au coeur et à l'intersection des branches par les polyoxométallates ont été synthétisés. Cette variation de la position de l'espèce active permet d'étudier l'influence de son environnement sur ses propriétés (solubilité, efficacité en catalyse, recyclage).BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Polyoxométallates dendritiques énantiopurs recyclables pour la catalyse asymétrique

Les polyoxométallates (POMs) dendritiques constituent une catégorie de catalyseursrelativement nouvelle. L association d un dendrimère avec un polyoxométallate permet debénéficier des propriétés catalytiques intéressantes des POMs et de pouvoir modifier cespropriétés en variant la structure du dendrimère. Une variété de composés hybrides POMsdendritiques a été préparée par formation de liaisons ioniques entre une unité POM anioniqueachirale et des cations dendritiques énantiopurs. Ces composés se sont montrés très efficacespour la catalyse d oxydation asymétrique, avec une stéréosélectivité significative. Cesrésultats ont démontré pour la première fois un transfert de chiralité d une structure organiqueénantiopure vers une unité POM achirale. Ces composés sont stables et recyclables dans desconditions douces, sans perte significative de réactivité et de stéréosélectivité.Polyoxometalates (POMs) represent a new generation of catalysts. Association of dendrimersand POMs allows modification of POM catalytic properties by modifying the dendrimerstructure. Several dendritic POM hybrid compounds have been prepared by ionic bondformation between an anionic POM unit and enantiopure dendritic cations. These compoundsare efficient for asymmetric oxidation catalysis with significant stereoselectivity. Theseresults have shown, for the first time, a chirality transfer from an enantiopure organicstructure to an achiral POM unit. These compounds are stable and recoverable under mildconditions without any significant loss of reactivity and stereoselectivity.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Synthèse et recyclage de catalyseurs métallodendritiques par leur greffage sur des nanoparticules magnétiques

La récupération et le recyclage des catalyseurs représentent un véritable challenge économique, sanitaire et environnemental. Depuis quelques années, l utilisation des nanoparticules magnétiques comme support de catalyseurs a émergé comme une voie alternative pour leur recyclage. En effet, les catalyseurs supportés sur des nanoparticules magnétiques peuvent être facilement isolés et recyclés par décantation magnétique avec un simple aimant. Dans ce travail, nous décrivons la synthèse de métallodendrons porteurs de site catalytiques diphosphinopalladium et leur greffage sur des nanoparticules magnétiques cœur-écorce. L intérêt majeur de ce travail a été de montrer la grande efficacité du recyclage des catalyseurs notamment en milieu aqueux. L utilisation judicieuse des structures dendritiques a permis d augmenter la fonctionnalisation de la surface des MNPs. Ceci nous a permis de préparer des catalyseurs supportés très performants dans des réactions de couplage de Suzuki et de SonogashiraThe recovery and recycling of catalysts represent a real challenge for economic, health and environmental reasons. Since few years, the use of magnetic nanoparticles as catalysts supports has emerged as an alternative for their recovery. Indeed, magnetic nanoparticles-supported catalysts could be easily isolated and recycled from the reaction medium by magnetization with a simple magnet. In this work, we report the synthesis of metallodendrons bearing pallado phosphine catalysts and their grafting onto core-shell magnetic nanoparticles. The main interest of this work was to demonstrate the efficiency of the recycling especially in aqueous media. Judicious use of dendritic structures has increased the surface functionalization of nanoparticles. Therefore, it was possible to prepare highly performant catalysts for Suzuki and Sonogashira cross-coupling reactions.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Dendritic stars by ring-opening-metathesis polymerization from ruthenium-carbene initiators

Dendrimers are a rich and appealing field of polymer chemistry. Organic polymerization form dendritic initiators also offers a route to highly branched polymers, and indeed these have been obtained, amongest other methods, ..

Synthesis, Chemistry, DFT Calculations, and ROMP Activity of Monomeric Benzylidene Complexes Containing a Chelating Diphosphine and of Four Generations of Metallodendritic Analogues. Positive and Negative Dendritic Effects and Formation of Dendritic Ruthenium−Polynorbornene Stars

The reaction of Hoveyda's catalyst [Ru{η2-(CHAr)}(PCy3)Cl2] (1; Ar = o-O-i-Pr-C6H4) with the diphosphine PhCH2N(CH2PCy2)2 (2) gives the new air-stable green ruthenium carbene complex [Ru{η1-(CHAr)}{η2-(Cy2PCH2)2N(CH2Ph)}Cl2] (3A), in which 2 models a dendritic branch of poly(diphosphine) dendrimers DAB-dendr-[N(CH2PCy2)2]n (G1, n = 4; G2, n = 8; G3, n = 16; G4, n = 32). The complex 3A reversibly dimerizes in concentrated solution, a trend favored at low temperature. The structure of 3A was also confirmed by DFT calculations, which also establish the dimeric structure of 3B and the fact that the dimerization energy of 3A is small. Facile halide abstraction is shown by MALDI-TOF mass spectroscopy, and reaction with AgPF6 gives the air-stable green dicationic dimer 5, whose structure has been confirmed by DFT calculations and whose reactions with ligands (I- and DMSO) gives monomeric alkylidene complexes. The diiodo analogue of 3A, 7A, is also synthesized by addition of NaI to either 3A or 5 and dimerizes more readily than the dichloro analogue 3A. On the basis of this chemistry, metallodendrimers DAB-dendr-[PCy2CH2NCH2PCy2Ru(CHAr)(PPh3)(Cl)2]n (8−11) derived from the four first generations of DAB polyamines containing, respectively, 4, 8, 16, and 32 ruthenium branches have been synthesized and characterized by elemental and standard spectroscopic analysis. Dimerization of the ruthenium alkylidene species of these dendrimers is found to increase upon dilution, which is taken into account by intradendritic dimerization and larger extension of the branches, providing more freedom for dimerization in dilute solution. These dendritic ruthenium−carbene complexes are shown to initiate the ROMP of norbornene at room temperature to form star-shaped metallodendritic polymers. Interestingly, the metallodendrimer G1 initiates the ROMP of norbornene much faster than the model ruthenium complex 3, the overall rate order being G1 > G2 > G3 > model. The dramatic positive dendritic effect is rationalized in terms of the labilization of a ruthenium−phosphorus bond at each Ru within the dendrimers. Such a speculative dissociative metathesis mechanism (3A, 16e → 14e) would be in accord with the limited ROMP activity, the lack of RCM activity, the instability in air, and the DFT calculations showing that the interaction of 3A with ethylene is repulsive. The second dendritic effect, negative among the generations, is taken into account by the increasing bulk as the generation number increases, slowing down the approach of Ru by norbornene. Cleavage of the polynorbornene branches of these metallodendritic polymer stars using ethyl vinyl ether followed by SEC analysis shows that the observed masses are close to the theoretical ones, indicating that dendritic-star polymers have formed in the ROMP process

Enhanced catalyst recovery in an aqueous copper-free Sonogashira cross-coupling reaction

We report the synthesis of a superparamagnetic nanoparticle MNP (c-Fe2O3/polymer) supported dendritic catalyst based on a bulky electron-rich phosphine Pd(II) complex. The high reactivity of this catalyst is described in a copper-free Sonogashira C–C cross-coupling reaction in water, and the significant role of surfactant additives is highlighted in the recovery study

Efficient strategy to increase the surface functionalization of core-shell superparamagnetic nanoparticles using dendron grafting

Core-shell c-Fe2O3/polymer 300 nm superparamagnetic nanoparticles, grafted by fluorescent dendrons using a convergent approach, showed an increase in their surface functionalization compared to grafting using a linear analogue