The oxidation of ethylbenzene and other alkylaromatics by dioxygen catalysed by iron(III) tetrakis(pentafluorophenyl)porphyrin and related iron porphyrins

The oxidation of ethylbenzene with dioxygen catalysed by iron(III) porphyri ns in a solvent free system has been studied over the temperature range 30- 110 degrees C. The time dependence of the formation of the three main produ cts, 1-phenylethanol, acetophenone and 1-phenylethyl hydroperoxide, and the fate of the iron porphyrin are interpreted in terms of a free radical auto xidation mechanism. The yields of the oxidation products are determined by the rate of reaction and by the lifetime of the catalyst. Catalyst degradat ion is shown to involve reaction of the porphyrin ligand with 1-phenylethox yl and 1-phenylethylperoxyl radicals. The disadvantages of increased induct ion periods and longer reaction times of the oxidations observed at lower r eaction temperatures are counter balanced by increased catalyst turnovers. Less extensive studies on the oxidations of toluene, cumene, (2-methylpropy l)benzene and tert-butylbenzene support the overall mechanism proposed for ethylbenzene. A comparative study using the catalysts iron(III) 2,3,7,8,12,13,17,18-octac hloro-5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin and iron(III) tetrak is(pentafluorophenyl)porphyrin and five of its derivatives reveals that hal ogenation of the beta-pyrrole positions markedly increases the activity of the catalysts but not the stability of the porphyrin towards degradation. T he highest yields were obtained with the mu-oxodimer of iron(III) tetrakis( pentafluorophenyl)porphyrin and iron(III) tetrakis(4-dimethylamino-2,3,5,6- tetrafluorophenyl)porphyrin.