The influence of substrate composition on the kinetics of olefin epoxidation by hydrogen peroxide catalyzed by iron(III) [tetrakis(pentafluorophenyl)] porphyrin

We have recently proposed a mechanism for the epoxidation of cyclooctene by hydrogen peroxide catalyzed by iron(III) [tetrakis(pentafluorophenyl)] porphyrin chloride. The expressions for the rate of hydrogen peroxide consumption and the yield of epoxide derived from this mechanism are in excellent agreement with experimental observations for a wide range of reaction conditions. An interesting feature of the expressions for the apparent rate coefficient and the yield of epoxide relative to peroxide consumed is that they are independent of the properties of the olefin. The present work was undertaken with the aim of determining whether this is a general result applicable to the epoxidation of other olefins. To this end, the rates of epoxidation of cyclooctene, styrene, cis-stilbene, cyclohexene, and norbornene were measured under identical conditions. For cyclooctene, styrene, and cis-stilbene, the observed kinetics and yield of epoxide were independent of the substrate, and no evidence was found by either UV–visible or 1H NMR spectroscopy for olefin coordination to the iron cation of the porphyrin. By contrast, the rates of cyclohexene and norbornene epoxidation and the corresponding epoxide yields were significantly lower than those of the other three olefins. The lower catalyst activity and lower epoxide yield correlated with both UV–visible and 1H NMR evidence for olefin coordination to the iron cation of the porphyrin. Olefin coordination increases the electron density on the iron cation and promotes homolytic cleavage relative to heterolytic cleavage of the oxygen-oxygen bond of coordinated hydrogen peroxide. This has the effect of reducing both the apparent rate coefficient for hydrogen peroxide consumption and the epoxide yield.

UV–visible and NMR spectroscopies indicate that norbornene and cyclohexene coordinate axially to iron(III) [tetrakis(pentafluorophenyl)] porphyrin, while cyclooctene, cis-stilbene, and styrene do not. The observed reaction rate and final yield for the iron(III) [tetrakis(pentafluorophenyl)] porphyrin chloride catalyzed epoxidation of olefins by H2O2 is independent of the substrate if the substrate does not coordinate to the iron center of the porphyrin. Figure optionsDownload as PowerPoint slide