Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H 2 O 2 Activation Pathways: Unusual Effect of Base.

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (Bu 4 N) 8 [{PW 11 O 39 Zr(μ-OH)(H 2 O)} 2 ] ( 1 ) efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous H 2 O 2 as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (Bu 4 NOH) in the amount of 1 equiv per dimer 1 suppress H 2 O 2 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 H 2 O 2 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 H 2 O 2 degradation have been rationalized in terms of their impact on hydrolysis of 1 to form monomeric species, [PW 11 O 39 Zr(OH)(H 2 O) x ] 4- ( 1-M , x = 1 or 2), which favors H 2 O 2 homolytic decomposition. The interaction of 1 with H 2 O 2 has been investigated by HR-ESI-MS, ATR-FT-IR, and 31 P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr 2 (μ-η 2 :η 2 -O 2 ){PW 11 O 39 } 2 (H 2 O) x ]] 8- and [{Zr(μ-η 2 -O 2 )} 2 (PW 11 O 39 ) 2 (H 2 O) y ] 10- , along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of H 2 O 2 . Both dimeric μ-η 2 -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 [{PW 11 O 39 Zr} 2 (μ-O)(μ-OOH)] 9- , which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.