Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H₂O₂ Activation...

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (Bu₄N)₈[{PW₁₁O₃₉Zr(μ-OH)(H₂O)}₂] (1) efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous H₂O₂ as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (Bu₄NOH) in the amount of 1 equiv per dimer 1 suppress H₂O₂ unproductive decomposition, increase substrate conversion, improve yield of heterolytic oxidation products and oxidant utilization efficiency, and also affect regioselectivity of epoxidation, enhancing oxygen transfer to sterically hindered electron-rich C═C bonds. Acid additives produce a reverse effect on the substrate conversion and H₂O₂ efficiency. The reaction mechanism was explored using a range of test substrates, kinetic, and spectroscopic tools. The opposite effects of acid and base additives on alkene epoxidation and H₂O₂ degradation have been rationalized in terms of their impact on hydrolysis of 1 to form monomeric species, [PW₁₁O₃₉Zr(OH)(H₂O)]^4- (1-M, x = 1 or 2), which favors H₂O₂ homolytic decomposition. The interaction of 1 with H₂O₂ has been investigated by HR-ESI-MS, ATR-FT-IR, and ³¹P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr₂(μ-η²:η²-O₂){PW₁₁O₃₉}₂(H₂O)]]^8- and [{Zr(μ-η²-O₂)}₂(PW₁₁O₃₉)₂(H₂O)]^10-, along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of H₂O₂. Both dimeric μ-η²-peroxo intermediates are inert toward alkenes under stoichiometric conditions. V-shape Hammett plots obtained for epoxidation of p-substituted styrenes suggested a biphilic nature of the active oxidizing species, which are monomeric Zr-hydroperoxo and peroxo species. Small basic additives increase the electrophilicity of the catalyst and decrease its nucleophilicity. HR-ESI-MS has identified a dimeric, most likely, bridging hydroperoxo species [{PW₁₁O₃₉Zr}₂(μ-O)(μ-OOH)]^9-, which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.