L-Proline-Derived Dendritic Tetrakis(diperoxotungsto)phosphate: Synthesis and Enantioselective Oxidation Catalysis

As a part of our ongoing research towards the synthesis of chiral dendritic polyoxometalate (DENDRI-POM) hybrids that are able to catalyze oxidation of organic substrates with high selectivity and enantioselectivity, we have prepared an enantiopure DENDRI-POM salt by coupling a well-defined L-proline-derived tetrapropylammonium dendron with the tetrakis(diperoxotungsto) phosphate [PO4{WO(O-2)(2)}(4)](3-). This DENDRI-POM hybrid was characterized by infrared, NMR spectroscopy, polarimetry, and elemental analysis. The data obtained are consistent with the structure in which the trianionic POM is surrounded by three L-proline-based dendrons. This DENDRI-POM oxidized sulfides with moderate activity and very low enantioselectivities (up to 4%), whereas alkenes were efficiently oxidized with up to 37% enantiomeric excess (ee), and good yields. Although these ee values are still not satisfactory for practical asymmetric synthesis, 37% ee represents the best enantioselectivity reported to date in the oxidation of organic substrate with chiral POM hybrids. Studies of temperature, solvent, and catalyst loading effects on the outcome of the reaction indicate that the ee is sensitive to the nature of the solvent. Moreover, the catalyst was recovered at low temperature and reused while retaining its activity and enantioselectivity. These results contrast and complement our reported results in which DENDRI-POMs based on phenylethylamine-derived ligands efficiently oxidize sulfides with up to 14% ee, whereas alkenes were not oxidized with this family of chiral POM compounds. Thus, the influence of the nature of the chiral countercation on the POM properties is again observed.