Hydrocarbon oxidation with Ph4PHSO5 catalyzed by manganese(III) porphyrins in homogeneous solution

Article abstract of DOI:10.1002/(SICI)1099-1395(199608)9:8<539::AID-POC814>3.0.CO;2-Y

The oxidation of ethylbenzene with Ph₄PHSO₅ catalyzed by Mn(TMP)Cl in the presence of 4-terr-butylpyridine was studied in 1,2-dichloroethane. The reaction affords acetophenone together with minor amounts of 1-phenylethyl alcohol. The oxidation of 1-phenylethyl alcohol to acetophenone was studied under identical experimental conditions. In both cases, the oxidation rates are independent of the concentration of the substrates whereas they depend linearly on catalyst concentration. By increasing the concentration of 4-tert-butylpyridine, which acts as an axial ligand of the catalyst, saturation behavior is observed. The observations reported above, together with the chemoselectivity observed, the fact that the reactivities of the alkane and of the alcohol are similar and the activation parameters of ethylbenzene oxidation (ΔH^? = 57 kJ mol^-1, ΔS^? = -134 J K^-1 mol^-1) are rationalized on the basis of the formation of an alkane-oxo-porphyrinato complex. The ketone is generated by further oxidation of the alcohol, produced from the intermediate, still present in the coordination sphere of the metal. Furthermore, it is suggested that the hydroxylation proceeds through a concerted insertion of the oxo-oxygen on the hydrogen-carbon bond rather than through a recombination of radicals. The formation of the ketone appears to be better accommodated in a reaction scheme in which the product is formed via an oxygenation of the alcohol to give a geminal diol rather than through dehydrogenation.