Photocatalytic oxidation of cyclohexane by (nBu(4)N)(4)W10O32/Fe(II) porphyrins integrated systems

The oxidation of cyclohexane by photoexcited (nBu(4)N)(4)W10O32 has been investigated in the presence of iron meso-tetraarylporphyrins bearing different substituents in the beta-pyrrole positions and/or in the meso-aryl groups. Irradiation at 325 nm leads to the reduction of the polyoxotungstate with the simultaneous oxidation of cyclohexane to cyclohexyl radicals which can be detected by the ESR spin trapping technique. In oxygen-free solutions, the photoreduced polyoxotungstate is able to transfer one electron to the Fe(III)porphyrin to give the ferrous complex. The subsequent reaction between this species and cyclohexyl radicals leads to the formation of sigma-alkyl-Fe(III)porphyrin complexes, as demonstrated by UV-VIS and NMR spectroscopy. In the presence of oxygen, the photoreduced polyoxotungstate has the role of initiating the activation of O-2 through its reduction to O-2(.) and H2O2. As a consequence, cyclohexane is converted to cyclohexanone and cyclohexanol in a cyclic way. The Fe(III)porphyrin complex strongly affects the product distribution probably through its hydrogen peroxide and alkyl-hydroperoxide-dependent oxidation. In comparison with (nBu(4)N)(4)W10O32 alone, a higher selectivity is obtained with the formation of cyclohexanol as a major product.