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(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.