Activation of tert ‐Butyl Hydroperoxide by Zr(IV) Stabilized by Polyoxotungstate Scaffolds

Abstract Zr‐monosubstituted polyoxometalates (Zr‐POMs) of the Keggin (Bu 4 N) 8 [{PW 11 O 39 Zr(μ‐OH)} 2 ] ( Zr‐K ), Lindqvist (Bu 4 N) 6 [{W 5 O 18 Zr(μ‐OH)} 2 ] ( Zr‐L ), and Wells‐Dawson (Bu 4 N) 11 H 3 [{P 2 W 17 O 61 Zr(μ‐OH)} 2 ] ( Zr‐WD ) structures are capable of heterolytic activation of the environmentally benign oxidant tert ‐butyl hydroperoxide (TBHP) and catalyze epoxidation of alkenes and oxidation of alcohols to carbonyl compounds. Catalytic activity of corresponding Ti‐POMs is much lower. Among Zr‐POMs, Zr‐K revealed higher epoxide yields. All Zr‐POMs do not catalyze unproductive TBHP degradation, and epoxide yields with both aqueous and anhydrous TBHP are generally higher than with aqueous H 2 O 2 . Regioselectivity of the Zr‐K‐ catalyzed limonene epoxidation with TBHP is different from that with H 2 O 2 : the more substituted and nucleophilic double bond is preferably epoxidized, pointing to an electrophilic oxygen transfer mechanism. The oxidation rates are first order in catalyst ( Zr‐K ) and substrate (cyclooctene or cyclohexanol) and show a saturation behavior with increasing concentration of TBHP. Studies by HR‐ESI‐MS, ATR‐FT‐IR, and 31 P NMR spectroscopic techniques implicated the formation of zirconium alkylperoxo species upon interaction of Zr‐POMs with TBHP. HR‐ESI‐MS revealed the existence of monomeric and dimeric alkylperoxo complexes, [{PW 11 O 39 Zr}((CH 3 ) 3 COO)] 4− and [{PW 11 O 39 Zr((CH 3 ) 3 COO)} 2 ] 8− , with predomination of the former, which is most likely the active species responsible for the selective oxidations.