Enantiopure 24-armed dendritic polyoxometalates: Synthesis and evaluation as recoverable catalysts for asymmetric sulfide oxidation

Enantiopure 24-armed dendritic polyoxometalate (DENDRI-POM) hybrids were prepared by ionic coupling of enantiopure 8-armed n-propyl dendritic ammoniums with a catalytically active peroxophosphotungstate trianion {PO4[WO(O-2)(2)](4)}(3). The catalytic properties of these DENDRI-POM hybrids were evaluated in the oxidation of thioanisole as a model reaction and compared to those of the 12-armed n-propyl analogous previously reported in our group. Up to 10% enantiomeric excess (ee) was obtained, indicating a negative effect on the reaction rate and the enantioselectivity, whereas the selectivity to the chiral sulfoxide versus sulfone was improved. This study aids understanding of how the structure and size of the dendritic wedge around the POM can influence its catalytic properties, especially regarding enantioselectivity. Four catalytic cycles were performed without any obvious change in the activity, selectivity and enantioselectivity. (C) 2016 Elsevier B.V. All rights reserved

Optically Active Tripodal Dendritic Polyoxometalates: Synthesis, Characterization and Their Use in Asymmetric Sulfide Oxidation with Hydrogen Peroxide

A series of structurally well-defined enantiopure tripodal allyl dendritic structures bearing three amine groups have been synthesized. The hydrogenation of the allyl groups in the presence of a Pd/C catalyst gave the corresponding enantiopure n-propyl counterparts. Treatment of these n-propyl amino dendrimers with heteropolyacid H3PW12O40 and excess H2O2 gave the enantiopure n-propyl {PO4[WO(O2)2]4}3– salts. Characterization of these dendritic POM hybrids in solution by NMR spectroscopy, elemental analysis, UV/Vis spectrophotometry, circular dichroism (CD), vibrational circular dichroism (VCD) and fluorimetry indicates the presence of POM–ligand interactions and confirms their optical and chiroptical properties. The hybrid compounds selectively oxidized sulfides to the corresponding chiral sulfoxides with up to 13 % enantiomeric excess (ee), highlighting the transfer of chirality from the dendritic wedges to the inorganic cluster. The properties of the POM anion, especially its solubility and regio- and stereoselectivity, are sensitive to the structure of the cation. The catalyst was recovered by precipitation without any discernible loss in activity, selectivity or enantioselectivity over three catalytic cycles at –50 °C. Interestingly, a dendritic effect was noted in the enantioselectivity as the dendritic-POM hybrids are more selective than their non-dendritic counterparts. The ee resulting from chirality transfer to the anionic POM unit is comparable to that obtained in our previous work with monopodal dendritic polyoxometalates (14 %) despite the polyvalency of the highly charged tripodal ligand, which is rationalized by different spectroscopic methods

Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries

International audienceLarge supramolecular metallodendrimers can now be synthesised rapidly, reaching a high number of branches in only a few generations, and approaching the de Gennes steric limit. They are characterised by their MALDI TOF mass spectra, in particular by the molecular peak, and by H-1, C-13 and P-31 NMR. Following rational molecular engineering, they can be designed to achieve essential functions such as molecular batteries, catalysts and sensors for the recognition of various anions. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

Chirality induction to achiral molecules by silica-coated chiral molecular assemblies

Hybrid silica-organic nanohelices are used to organize a large variety of nonchiral small organic molecules or inorganic anions to nanometer-sized assemblies. Such chiral organization of achiral molecules induces chiroptical properties as detected by vibrational or electronic circular dichroism (CD), as well as from circularly polarized luminescence (CPL)