Photocatalytic properties of iron porphyrins revisited in aqueous micellar environment: oxygenation of alkenes and reductive degradation of carbon tetrachloride

Abstract

N,N-Dimethyltetradecylamine N-oxide (DTAO) is an appropriate surfactant to form micelles able to host the iron(III) meso-tetrakis(2,6-dichlorophenyl)porphyrin [Fe(III)(TDCPP)]. The so obtained microheterogeneous catalyst can induce biomimetic redox processes on organic substrates in aqueous medium, using sunlight and oxygen as clean reagents. The primary photochemical process consists in the photoinduced reduction of Fe(III) to Fe(II) with the contemporaneous oxidation of the axial ligand to radical species. The micelle environment may control some main parameters affecting the reactivity of these intermediates and, therefore, the chemoselectivity of the hydrocarbon oxidation processes. In contrast to what is observed in homogeneous organic solution, both cyclohexene and cyclooctene can be oxidised to the corresponding epoxides, with a selectivity higher than 90% in the case of cyclooctene. On the other hand, the main oxidation product of cyclohexene is cyclohex-2-en-1-one as expected in a hydrophobic micellar environment. The Fe(III)(TDCPP)/DTAO photocatalyst is very promising also in view of obtaining catalytic systems capable of converting small amounts of toxic halogenated alkanes present in water into less dangerous products. In particular, CCl₄ can be reduced by ethanol or cyclohexanol with high quantum yields (>10⁻¹), with good conversion (ca. 75%) and turnover values (>1500).