Highly Selective Thioether Oxidation with H2O2 Catalyzed by Zr‐Substituted Keggin Phosphotungstate: Mechanistic Insights

Abstract The Zr‐monosubstituted phosphotungstate (Bu 4 N) 8 [{PW 11 O 39 Zr(μ‐OH)(H 2 O)} 2 ] ( 1 ) is a highly active and selective catalyst for thioether sulfoxidation with aqueous H 2 O 2 in acetonitrile or acetonitrile/ethanol. The reaction proceeds unprecedentedly fast and completes within seconds at room temperature. Various sulfides produce corresponding sulfoxides with selectivity of 98–99 % and oxidant utilization efficiency of 92–99 %. The reaction mechanism was investigated using test substrates, kinetic, isotopic labeling, and spectroscopic tools. The oxidation of thianthrene‐5‐oxide and competitive sulfide‐sulfoxide oxidation point to an electrophilic oxidation mechanism, but a V‐shaped Hammett plot found for oxidation of p ‐substituted thioanisoles may suggest a biphilic nature of active species. Kinetic studies revealed that the rate‐limiting step of the reaction is the interaction of a peroxo zirconium intermediate with organic substrates. ATR‐FT‐IR and 31 P NMR spectroscopic studies coupled with stoichiometric experiments indicated that peroxo species capable of transferring an oxygen atom to thioethers are dimeric side‐on mono‐ and diperoxo complexes, [Zr 2 (μ‐η 2 : η 2 ‐O 2 ){PW 11 O 39 } 2 (H 2 O) x ]] 8− ( I ) and [{Zr(μ‐η 2 ‐O 2 )} 2 (PW 11 O 39 ) 2 (H 2 O) y ] 10− ( II ), and monomeric Zr‐hydroperoxo [Zr(η 2 ‐OOH)PW 11 O 39 ] 4− ( III ) and peroxo [Zr(η 2 ‐O 2 )PW 11 O 39 ] 5− ( IV ) species, which dominate at high excess of H 2 O 2 . While I and III are endowed with electrophilic oxygen, which can be transferred to electron‐rich sulfides, more negatively charged II and IV preferably react with thioethers comprising electron‐withdrawing substituents.