Hydroxylation of aliphatic hydrocarbons with m-chloroperbenzoic acid catalyzed by electron-deficient iron(III) porphyrin complexes

Abstract

The catalytic hydroxylation of aliphatic hydrocarbons by m- chloroperbenzoic acid (MCPBA) has been studied in the presence of electron- deficient iron(III) porphyrin complexes. High yields of alcohol products were obtained with small amounts of ketone formation under mild reaction conditions. The stereospecificity and regioselectivity of the iron porphyrin complexes have been investigated in hydroxylation reactions as well. The hydroxylation of alkanes has been performed in the presence of isotopically 18O-labeled water, H2 18O, in order to understand the effects of the electronic nature of iron porphyrin complexes, the concentration of H2 18O, the C-H bond strength of alkanes, and the reaction temperature on the 18O- incorporation from the labeled water into alcohols. We found that the amounts of 18O incorporated into the alcohol products varied in the reactions; these results were interpreted with that the reaction of oxygen atom transfer from a high-valent iron oxoporphyrin complex to alkanes competes with that of oxygen atom exchange between the intermediate and labeled water that leads to 18O-incorporation from H2 18O into the alcohol products. Deuterium kinetic isotope effects (KIEs) in the alkane hydroxylations by the iron porphyrin complexes and MCPBA have been studied with a mixture of cyclohexane and cyclohexane-d12. The KIE values obtained in the reactions were found to depend significantly on the nature to the iron porphyrin complexes. The temperature dependence of k(H)/k(D) was also studied from -40 to 25 °C and the parameters of Arrhenius equation (i.e., the pre-exponential factor ratio, A(H)/A(D), and the isotopic difference of C-H and C-D bond activation energies, E(a)(D)-E(a)(H)) were determined.