Alkene epoxidation by iodosylbenzene catalysed by iron(III) 5, 10, 15, 20-tetra(2,6-dichlorophenyl)porphyrin coordinated to pyridine-modified silica

Abstract

Six silicas, with different surface areas, pore sizes and pore volumes, have been surface modified with pyridine groups and used to support iron(III) 5, 10, 15, 20-tetra(2,6-dichlorophenyl)porphyrin (Fe^IIITDCPP) by ligation to the metal ion. Comparison of these materials (SiPy–Fe^IIITDCPP) as catalysts for the epoxidation of (Z)-cyclooctene by iodosylbenzene in dichloromethane reveals that all have essentially the same catalytic activity and that the heterogeneous reactions are significantly slower (ca. ten-fold) than the homogeneous analogue. In small scale repeat-use experiments the catalysts achieve 2200 catalyst turnovers without loss of activity, however, larger scale reactions (7900 turnovers) lead to catalytic oxidative bleaching and > 50% reduction in activity.

A Hammett study of the epoxidation of styrene and 4-substituted styrenes gives ρ values (vs. σ⁺) of –0.95 and –0.98 for oxidations catalysed by Fe^IIITDCPP and a supported catalyst SiPy–Fe^IIITDCPP, respectively. The similarity of these values to each other and to values from other iron(III) porphyrin-catalysed epoxidations is discussed. A minor pathway in the oxidation of the styrenes in air that leads to benzaldehydes is attributed to dioxygen intercepting an intermediate in the reaction.

Oxidations of (Z)- and (E)-4-methylpent-2-ene catalysed by Fe^IIITDCPP and by SiPy–Fe^IIITDCPP show a marked difference. With the former the relative reactivity of (Z)- to (E)-alkene is 13:1, whereas the latter fails to catalyse the epoxidation of the (E)-alkene and instead is converted into a green material; this is possibly an iron N-alkylporphyrin.