OXIDATION OF CYCLOHEXANE BY MOLECULAR-OXYGEN PHOTOASSISTED BY MESO-TETRAARYLPORPHYRIN IRON(III)-HYDROXO COMPLEXES

The photochemical and photocatalytic properties of iron meso-tetraaryl porphyrins bearing an OH- axial ligand and different substituents in t he beta-positions of the porphyrin ring are reported. Irradiation (lam bda = 365 nm) in the absence of dioxygen leads to the reduction of Fe- III to Fe-II with the formation of OH radicals. Substituents at the p yrrole P-positions are found to markedly affect the photoreduction qua ntum yields. Under aerobic conditions, this photoreaction can induce t he subsequent oxidation of cyclohexane to cyclohexanone and cyclohexan ol by O-2 itself. The process occurs under mild conditions (22 degrees C; 760 Torr of Or) and without the consumption of a reducing agent. T he polarity of the solvent and the nature of the porphyrin ring have a remarkable effect on the selectivity of the photooxidation process, l ikely controlling the cleavage of O-O bonds of possible iron peroxoalk yl intermediates. In particular, in pure cyclohexane, oxidation occurs with the selective formation of cyclohexanone; in contrast, in dichlo romethane/cyclohexane mixed solvent, the main oxidation product is cyc lohexanol. Phenyl-tert-butylnitrone (pbn) has been found to quench the radical chain autooxidation of the substrate thus increasing the yiel d of cyclohexanol. This becomes the only oxidation product when iron 5 ,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin hydroxide (Fe-III(TDCP P)(OH)) is used as photocatalyst.