REMARKABLE EFFICIENCY OF IRON(III) VERSUS MANGANESE(III) TETRAPHENYLPORPHYRINS AS CATALYSTS FOR FAST AND QUANTITATIVE OXIDATION OF SULFIDESINTO SULFONES BY HYDROGEN-PEROXIDE

The efficiency of various metallo-phtalocyanines (Pht) and -tetrapheny lporphyrins (TPP) as catalysts for the H2O2 oxidations of dibenzylsulf ide, phenylchloroethylsulfide, and thioanisole is investigated in etha nol and acetonitrile, using imidazole as a cocatalyst. Neither PhtNi(I I) nor TPPCoII exhibits any catalytic activity. PhtMn(II) and (TPPMnCl )-Cl-III accelerate markedly these reactions but do not promote quanti tative 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, disulfid e in particular, suggests that a free sulfenium radical cation is not an active intermediate in these reactions. The marked differences in t he behaviour of (TPPMnCl)-Cl-III and TPPFeTIII Cl are analyzed by comp aring the rates of the catalyst decomposition, of the sulfoxide and su lfone formation as a function of the hydrogen peroxide concentration. The results are discussed in terms of a competition between the severa l oxidative pathways and a possible mechanism for the oxygen transfer to sulfides.