1,1′-Binaphthyl-2-methylpyridinium-Based Peroxophosphotungstate Salts: Synthesis, Characterization, and Their Use as Oxidation Catalysts

A series of 1,1'-binaphthyl-2-methylammonium and pyridinium salts 6, 7, and 8 was synthesized through the coupling reaction of 2-(bromomethyl)-1,1'-binaphthalene (5) with the dendritic tetraallyl pyridinedicarbinol dendron 2 as well as triethylamine and 4-tert-butylpyridine. Tetraallyl pyridinedicarbinol dendron 2 was prepared by allylation of commercially available diethyl pyridine-3,5-dicarboxylate (1). The allylation of 2 with allyltrimethylsilane in the presence of boron trifluoride was unsuccessful, as tetraallyl pyridinedicarbinol trifluoroboron adduct 3 was obtained instead of expected hexaallylpyridine compound 4. The catalytic hydrogenation of allyl groups of the ammonium salt of 2, namely, tetraallyl 1,1'-binaphthyl-2-methylpyridinium salt 6, successfully led to the corresponding tetra-n-propyl 1,1'-binaphthyl-2-methylpyridinium salt 9. The reaction between salts 7, 8, and 9 and the heteropolyacid H3PW12O40 in the presence of a large excess of hydrogen peroxide afforded the corresponding 1,1'-binaphthyl-2-methylaramonium-based polyoxometalate salts 10, 11, and 12, which contain a catalytically active trianionic [H3PW12O40](3-) in the core. These binaphthyl-POM salts are soluble in commonly used organic solvents, and their IR and P-31 NMR spectroscopic and elemental analysis data indicate the presence of the POM unit in the frameworks. These POM hybrids are efficient, recoverable, and reusable catalysts in the oxidation of thioanisole, cyclooctene, and cyclohexanol, with H2O2 as the oxidant. A study of the countercation effects indicated that the reaction kinetics and the selectivity are sensitive to the structure of the cation. Two cycles of catalytic reactions were performed without a discernible loss in activity. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)