Oxidation of Cyclohexane by Molecular Oxygen Photoassisted by meso-Tetraarylporphyrin Iron(III)−Hydroxo Complexes

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Abstract
The photochemical and photocatalytic properties of iron meso-tetraarylporphyrins bearing an OH- axial ligand and different substituents in the β-positions of the porphyrin ring are reported. Irradiation (λ = 365 nm) in the absence of dioxygen leads to the reduction of FeIII to FeII with the formation of OH• radicals. Substituents at the pyrrole β-positions are found to markedly affect the photoreduction quantum yields. Under aerobic conditions, this photoreaction can induce the subsequent oxidation of cyclohexane to cyclohexanone and cyclohexanol by O2 itself. The process occurs under mild conditions (22 °C; 760 Torr of O2) and without the consumption of a reducing agent. The polarity of the solvent and the nature of the porphyrin ring have a remarkable effect on the selectivity of the photooxidation process, likely controlling the cleavage of O−O bonds of possible iron peroxoalkyl intermediates. In particular, in pure cyclohexane, oxidation occurs with the selective formation of cyclohexanone; in contrast, in dichloromethane/cyclohexane mixed solvent, the main oxidation product is cyclohexanol. Phenyl-tert-butylnitrone (pbn) has been found to quench the radical chain autooxidation of the substrate thus increasing the yield of cyclohexanol. This becomes the only oxidation product when iron 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin hydroxide (FeIII(TDCPP)(OH)) is used as photocatalyst.

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