Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin

Abstract

The oxygen atom transfer–electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H₂O₂ in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p′,p″,p‴-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron–oxo complex porphyrin radical cation, P⁺˙ Fe(IV)O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH₃, CH₃, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H₂¹⁸O or H₂¹⁸O₂ clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the β C–H and C–S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron–oxo complex to the sulfide. However, competitive experiments between thioanisole (E° = 1.49 V vs. NHE in H₂O) and N,N-dimethylaniline (E° = 0.97 V vs. NHE in H₂O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E° = 0.95 V vs. NHE in CH₃CN) and N,N-dimethyl-4-methylthioaniline (11, E° = 0.65 V vs. NHE in H₂O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)O (the reduced form of the iron–oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.