Enhanced catalytic activity and unexpected products from the oxidation of cyclohexene by organic nanoparticles of 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinatoiron(III) in water by using O2

Abstract

The catalytic oxidation of alkenes by most iron porphyrins using a variety of oxygen sources, but generally not dioxygen, yields the epoxide with minor quantities of other products. The turnover numbers for these catalysts are modest, ranging from a few hundred to a few thousand depending on the porphyrin structure, axial ligands, and other reaction conditions. Halogenation of substituents increases the activity of the metalloporphyrin catalyst and/or makes it more robust to oxidative degradation. Oxidation of cyclohexene by 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)porphyrinato iron(III), ([Fe(III)(tppf(20))]) and H(2)O(2) is typical of the latter: the epoxide is 99 % of the product and turnover numbers are about 350.1-4 Herein, we report that dynamic organic nanoparticles (ONPs) of [Fe(III)(tppf(20))] with a diameter of 10 nm, formed by host-guest solvent methods, catalytically oxidize cyclohexene with O(2) to yield only 2-cyclohexene-1-one and 2-cyclohexene-1-ol with approximately 10-fold greater turnover numbers compared to the non-aggregated metalloporphyrin in acetonitrile/methanol. These ONPs facilitate a greener reaction because the reaction solvent is 89 % water and O(2) is the oxidant in place of synthetic oxygen sources. This reactivity is unexpected because the metalloporphyrins are in close proximity and oxidative degradation of the catalyst should be enhanced, thus causing a significant decrease in catalytic turnovers. The allylic products suggest a different oxidative mechanism compared to that of the solvated metalloporphyrins. These results illustrate the unique properties of some ONPs relative to the component molecules or those attached to supports.

Figure 1

Preparation of ONPs. Water (5.0 mL) is added to a solution of [Fe^IIICl(tppf₂₀)] in THF (0.4 mL, 1.0 mm) and PEG (0.2 mL) while stirring or sonicating to yield ONPs with a diameter of 80 nm and 10 nm, respectively.

Scheme 1

Oxidation of cyclohexene.