Kinetic study of the epoxidation of alkenes by iodosylbenzene catalysed by iron(III) tetra(4-N-methylpyridyl)porphyrin in methanol

Abstract

A kinetic analysis of the epoxidation of alkenes by iodosylbenzene catalysed by iron(III) tetra(4-N-methylpyridyl)porphyrin (Fe^IIIT4MPyP) in methanol solution is presented. Using the initial rate method, the rates of epoxidation are found to be first-order in Fe^IIIT4MPyP and to show saturation kinetics in both the oxidant and the substrate. Interestingly, for the two most reactive styrenes, 4-methoxy- and 4-methylstyrene, the rate of epoxidation is first-order in the substrate: saturation kinetics in substrate are not observed. A Michaelis–Menten-like rate equation is derived for the system, under conditions where the catalyst is saturated in oxidant, and is used to compare the rates of epoxidation of a selection of aliphatic alkenes and styrenes.

The reactivity of an alkene is found to be determined by the influence of the substituent on the formation of a complex with the active oxidant and on the subsequent breakdown of this intermediate to give products. The results from the less reactive styrenes are analysed using the Hammett equation and the mechanism of the epoxidation is discussed.