Journal
EUROPEAN JOURNAL OF INORGANIC CHEMISTRY
Volume -, Issue 5, Pages 727-738Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejic.201001111
Keywords
Asymmetric catalysis; Chirality; Dendrimers; Polyoxometalates; Oxidation
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Funding
- Agence National de la Recherche [ANR-06-BLAN-0215]
- University of Bordeaux
- Centre National de la Recherche Scientifique (CNRS)
- European Science Foundation (ESF)
- European Cooperation in Science and Technology
- University of Padova
- European Research Council under the European Community [FP7/2007-2013/ERC, 208702]
- Region Aquitaine
- Agence Nationale de la Recherche (ANR) [ANR-06-BLAN-0215] Funding Source: Agence Nationale de la Recherche (ANR)
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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(O-2)(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 degrees 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.
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