Remarkable efficiency of iron(III) versus manganese(III) tetraphenylporphyrins as catalysts for fast and quantitative oxidation of sulfides into sulfones by hydrogen peroxide

摘要

The efficiency of various metallo-phtalocyanines (Pht) and -tetraphenylporphyrins (TPP) as catalysts for the H2O2 oxidations of dibenzylsulfide, phenylchloroethylsulfide, and thioanisole is investigated in ethanol and acetonitrile, using imidazole as a cocatalyst. Neither PhtNi(II) nor TPPCoII exhibits any catalytic activity. PhtMn(II) and (TPPMnCl)-Cl-III accelerate markedly these reactions but do not promote quantitative oxidations, at most 70% of the sulfides being transformed into sulfoxides. in contrast, with PhtFe(II) sulfoxides are obtained with a 100% yield from sulfides. Finally, the only catalyst able to oxidize sulfides rapidly (<5 min), completely and quantitatively (100% sulfone) is (TPPFeCl)-Cl-III in EtOH. The absence of any by-product, disulfide in particular, suggests that a free sulfenium radical cation is not an active intermediate in these reactions. The marked differences in the behaviour of (TPPMnCl)-Cl-III and TPPFeTIII Cl are analyzed by comparing the rates of the catalyst decomposition, of the sulfoxide and sulfone formation as a function of the hydrogen peroxide concentration. The results are discussed in terms of a competition between the several oxidative pathways and a possible mechanism for the oxygen transfer to sulfides. [References: 23]