Selective oxidation of a pyrimidine thioether using supported tantalum catalysts

Ta2O5–SiO2 catalysts were prepared by a sol–gel method using tetraethyl orthosilicate (TEOS) and tantalum (V) ethoxide as the sources of silicon and tantalum, and two families of quaternary ammonium salts, [CnH2n+1(CH3)3N]Br (image, 16, 18) and [(CnH2n+1)4N]Br (image, 12, 16, 18) as surfactants. The catalysts were compared for the selective sulfoxidation of 4,6-dimethyl-2-thiomethylpyrimidine using peroxide as an oxidising agent in a range of ionic liquids and organic solvents. The sol–gel catalysts were also compared with tantalum on MCM-41 prepared by grafting. The catalysts were characterized from adsorption–desorption isotherms of N2, XRD patterns, small-angle X-ray scattering, IR spectra from adsorbed pyridine and CDCl3, XPS spectra, and 29Si magic angle spinning (MAS) NMR experiments. The effect of recycling on the catalyst leaching and selectivity/activity was also studied. High activities and selectivities were found in [NTf2]− based ionic liquids and organic solvents with good recyclability of the catalyst. Tantalum was found in the solution after reaction; however, this was determined to be due to entrapment of catalyst particulates, as opposed to leaching of the active metal.