Grafted Ta-calixarenes: Tunable, selective catalysts for direct olefin epoxidation with aqueous hydrogen peroxide

Supported Ta(V) oxide catalysts were prepared by grafting calixarene-Ta(V) and TaX₅ complexes on SiO₂ at coverages less than 0.25 Ta nm^-2. Thermogravimetric and elemental analyses and UV-visible spectroscopy indicate that catalysts consist of isolated, 1:1 Ta:ligand surface sites. Catalysts obtained by this one-pot procedure were studied in cyclohexene and cyclooctene epoxidation with H₂O₂. In sharp contrast with bare oxides, calixarene-containing catalysts had initial cyclohexene direct epoxidation turnover rates of 3.9 ± 0.1 × 10^-2 s ^-1 unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3× more active than the corresponding TaCl₅-based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus <65% selectivity for TaCl₅-based catalysts or only ∼20% selectivity for grafted calixarene-Ti(IV) catalysts. Capping silanols with octanol reduced epoxide hydrolysis from >50% to <30%. These catalysts demonstrate the utility of ligand-protected, supported Ta catalysts for epoxidation with H₂O₂ and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways.