Aerobic oxidation catalyzed by polyoxometalates associated to an artificial reductase at room temperature and in water

Abstract

Four polyoxometalates (POMs) were combined with an artificial reductase based on polyethyleneimine (PEI) and flavin mononucleotide (FMN) which is capable of delivering single electrons upon addition of nicotinamide adenine dinucleotide (NADH). The efficiency of this system for sulfoxidation reactions in water, at room temperature, under atmospheric pressure and with dioxygen as an oxidant was demonstrated, with yields up to 82% in sulfoxide. Mechanistic investigations allowed proposing two different catalytic pathways, depending on the nature of the POM. For the polyoxotungstates Na₁₂[(ZnW₉O₃₄)₂WZn₃(H₂O)₂]·46H₂O and Na₇K₇[{(PW₉O₃₄)Ni₃(OH)(H₂O)₂(Ale)}₂Ni]·34H₂O (Ale = alendronate = [O₃PC(O)(C₃H₆NH₃)PO₃]⁴⁻), the in situ production of H₂O₂ by the artificial reductase that activates the POM was evidenced. Alternatively, for the polyoxomolybdates H₃[PMo₁₂O₄₀]·14H₂O and H₄[PMo₁₁VO₄₀]·14H₂O, the reduction of the POM by the artificial reductase, followed by the reductive activation of O₂, led to the sulfoxidation reaction.