Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing HO Activation Pathways: Unusual Effect of Base.

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (BuN)[{PWOZr(μ-OH)(HO)}] () efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous HO as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (BuNOH) in the amount of 1 equiv per dimer suppress HO 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 HO 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 HO degradation have been rationalized in terms of their impact on hydrolysis of to form monomeric species, [PWOZr(OH)(HO)] (, = 1 or 2), which favors HO homolytic decomposition. The interaction of with HO has been investigated by HR-ESI-MS, ATR-FT-IR, and P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr(μ-η:η-O){PWO}(HO)]] and [{Zr(μ-η-O)}(PWO)(HO)], along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of HO. Both dimeric μ-η-peroxo intermediates are inert toward alkenes under stoichiometric conditions. V-shape Hammett plots obtained for epoxidation of -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 [{PWOZr}(μ-O)(μ-OOH)], which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.