Polymer and Silica Supported Tridentate Schiff Base Vanadium Catalysts for the Asymmetric Oxidation of Ethyl Mandelate - Activity, Stability and Recyclability

Homogeneous tridentate Schiff base vanadium catalysts derived from salicylaldehydes and tert-leucinol or tert-leucine are known to be excellent catalysts for the asymmetric oxidation of alpha-hydroxy esters including ethyl mandelate. Herein, new analogous supported, semi-soluble and insoluble catalysts are synthesized and their activities relative to the homogeneous catalyst are reported. The new catalysts are characterized by H-1 and C-13 NMR spectroscopy, mass spectrometry (EI, ESI, FAB), X-ray crystallography, elemental analysis, gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) spectroscopy, and nitrogen physisorption. The effects of support material, synthesis procedure, and reaction solvent are examined to probe the utility of these catalysts. Linear poly(styrene) supported catalysts are partially soluble under the reaction conditions, and it is shown that the soluble species contribute significantly to the catalytic reactivity. Insoluble catalysts based on the same vanadyl complexes supported on cross-linked poly(styrene) resin or mesoporous silica allow for catalyst recovery and recycle, showing equivalent selectivities over multiple reaction cycles. The mesoporous silica supported catalyst exhibits greater selectivity than the analogous homogeneous and polymer supported catalysts. Rigorous recycle studies show a loss of activity in each recycle, which is attributed to the decomposition of some portion of the vanadyl complexes in each cycle.