Thioether oxidation with tert-butyl hydroperoxide catalysed by Zr(iv)-substituted polyoxometalates

Abstract

Zr-substituted polyoxometalates (Zr-POMs) with Lindqvist, Keggin and Wells–Dawson structures, (Bu₄N)₆[{W₅O₁₈Zr(μ-OH)}₂] (Zr-L), (Bu₄N)₈[{PW₁₁O₃₉Zr(μ-OH)}₂] (Zr-K) and (Bu₄N)₁₁H₃[{P₂W₁₇O₆₁Zr(μ-OH)}₂] (Zr-WD), can activate the environmentally friendly oxidant tert-butyl hydroperoxide (TBHP) and efficiently catalyse the oxidation of various thioethers to produce sulfoxides and sulfones. Ti- and Nb-substituted POMs are significantly less active than Zr-POMs. The activity and selectivity of Zr-POMs strongly depend on their structural type. The reaction rate decreases when moving from Zr-K and Zr-WD to Zr-L. With 1 equiv. of TBHP, the main product of Zr-K is sulfoxide, whereas sulfone predominates with Zr-L. The reaction mechanism was probed using test substrates, and kinetic, isotopic (H₂¹⁸O), and spectroscopic methods. Monomeric alkylperoxo complexes bearing one tert-butyl peroxo moiety per Zr-POM were obtained by the reaction of Zr-K and Zr-L with TBHP and characterized by elemental analysis, HR-ESI-MS, ATR-FT-IR and multinuclear NMR spectroscopy. Both Zr-K and Zr-L alkylperoxo complexes revealed activity toward thioethers under stoichiometric conditions and produced predominantly sulfoxides, pointing to an electrophilic oxygen transfer mechanism. Under turnover conditions, binding of sulfoxide to Zr-L masks the electrophilic character of oxidation and increases the yield of sulfone. The solvent nature strongly affects the oxygen transfer mechanism and oxidation selectivity.