Enantioselective sulfoxidation as a probe for a metal-based mechanism in H₂O₂-dependent oxidations catalyzed by a diiron complex

The catalytic properties of the diiron complex 1, Fe₂OL₄(H₂O)₂(ClO₄) ₄ with L = (-)-4,5-pinenebipyridine, a chiral bipyridine derivative, have been investigated. Complex 1 efficiently catalyzes the oxidation of aryl sulfides to the corresponding sulfoxides by hydrogen peroxide, with yields ranging from 45 to 90% based on the oxidant. Furthermore the reactions were enantioselective and produced a mixture of sulfoxide enantiomers with significant enantiomeric excesses. The largest ee value (40%) was found in the case of p-bromophenyl methyl sulfide. Optimal ee's were obtained in polar solvents and at low temperature (below 0 °C), when the excess of the oxidant was limited. The observation of (i) a saturation kinetics with respect to both sulfide and H₂O₂ concentrations, (ii) a linear Hammett correlation of the initial V_max values with σ_p values, for a series of p-substituted aryl methyl sulfides, (iii) iron-peroxo complexes, characterized by light absorption and Raman resonance spectroscopies, during reaction of complex 1 with H₂O₂, and (iv) a saturation kinetics with respect to sulfide during oxidation of sulfide into sulfoxide by the iron-peroxo complexes led us to propose that the iron-peroxo moiety is the actual oxygen atom donor to the substrate, thus explaining the enantioselective control of the catalytic reaction. These data demonstrate that oxidations by non heme diiron complexes can proceed through metal-based pathways and can thus be made stereoselective.