Photooxidation of Cyclohexane with Dioxygen in Acetonitrile in the Presence of Transition Metal Chlorides

Abstract

Irradiation of cyclohexane solution in acetonitrile (λ = 300, 365, or 436 nm) in the presence of dioxygen and catalytic amounts of FeCl₃, CuCl₃, Na[AuCl₄] or H₂[PtCl]₆ gives rise to cyclohexanol and cyclohexanone (after GLC). The quantum yields with respect to the products lie within the range of 0.01 to 0.05. The most efficient of the catalysts used is FeCl₃, and the most efficient radiation wavelength is λ = 365 nm. A reaction pathway is suggested; it is assumed that the first step is photoexcitation of M-Cl followed by electron transfer from the ligand to the metal associated with the formation of the Cl^{<strong>.</strong>} radical, which reacts (probably after solvation by the solvent) with RH giving R^{<strong>.</strong>}. The Fe(II) species is oxidized by ROO^{<strong>.</strong>}, and the photocatalytically active complex is restored. Complexes with metals in low oxidation states (particularly in the case of Cu, Au and Pt), however, can react with molecular oxygen to give peroxo or oxo complexes, which can directly hydroxylate alkanes.