Optically Active Polyoxotungstates Bearing Chiral Organophosphonate Substituents

Divacant Keggin-type polyoxotungstates [gamma-XW10O36](8-) with X = Si or Ge, were functionalized with chiral phosphoryl groups. The hybrid compounds [(R*PO)(2)(gamma-XW10O36)](4-) with R = N-protected aminoalkyl groups or O-protected amino acid derivatives, were isolated. The solution characterization of the products was performed by different techniques: (183)w, (31)p, C-13, and H-1 NMR spectroscopy, electrospray ionization mass spectrometry, UV/Vis spectroscopy, and circular dichroism (CD). The experimental data confirm the covalent grafting of the organic moieties onto the polyanionic surface. A chirality transfer, from the pendant organic arm to the inorganic framework is apparent from CD studies. Multiple Cotton effects were observed in the region of charge-transfer transitions pertaining to W-O bonds. Furthermore, the W-183 NMR spectra are consistent with the expected C-2 symmetry, resulting from introduction of two organic stereocenters. The title complexes were used in the presence of hydrogen peroxide to perform the oxidation of methyl p-tolyl sulfide. Implications for the design of enantioselective catalysts based on these derivatives are discussed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)